Dr Tanzina Haque
email: t.haque@medsch.ucl.ac.uk

Turren Chaggar

Jenna Schafers

Claire Atkinson

Karen A Mcaulay

Dorothy H Crawford

Claire Mcaulay

serum marker for Epstein-Barr virusassociated lymphoproliferative diseases

Keywords: Soluble CD30, EBV, Acute IM, PTLD, Serum marker Soluble CD30, EBV, Acute IM, PTLD, serum marker

come

The soluble form of CD30 (sCD30), a member of tumour necrosis factor receptor superfamily, has been used as a marker of disease activity in various lymphomas. Epstein-Barr virus (EBV) is a potent stimulator of CD30 expression. The study aims to evaluate whether sCD30 can be used as a diagnostic marker for EBV-associated infectious mononucleosis (IM) and post-transplant lymphoproliferative disease (PTLD). Plasma from EBV seropositive healthy controls (N=90), acute IM patients (n=90), non-PTLD heart/lung transplant recipients (N=30) and EBV-positive PTLD patients (N=23) was tested for sCD30 using a commercially available ELISA kit. EBV DNA was tested by real time quantitative polymerase chain reaction assay. Significantly higher sCD30 levels were observed in acute IM patients (median 242.9 ng/mL) compared to EBV seropositive controls (median 15.7 ng/mL; p<0.0001). These levels were highest in IM patients within 14 days of onset of illness.

PTLD patients had significantly higher sCD30 levels (median 94 ng/mL) than healthy controls (p<0.0001) and transplant patients (median 27 ng/mL; p=0.0007). EBV DNA was detected mostly in acute IM and PTLD patients. In both cases there was a significant correlation between sCD30 and EBV DNA levels in plasma (p<0.0001). This study demonstrates that sCD30 and EBV DNA levels can be used as potential markers for diagnosis of IM and PTLD.

Introduction

The membrane glycoprotein CD30, a member of the tumour necrosis factor receptor (TNFR) superfamily was originally identified as a marker for Hodgkin's and Reed-Sternberg cells in Hodgkin's lymphoma [START_REF] Smith | CD30 antigen, a marker for Hodgkin's lymphoma, is a receptor whose ligand defines an emerging family of cytokines with homology to TNF[END_REF][START_REF] Falini | CD30 (Ki-1) molecule: a new cytokine receptor of the tumor necrosis factor receptor superfamily as a tool for diagnosis and immunotherapy[END_REF]. Subsequently the expression of CD30 has been seen in some lymphoid tumours and on a small proportion of activated B cells and activated CD8+ and CD4+ T cells [START_REF] Romagnani | CD30 and type 2 T helper (Th2) responses[END_REF]. CD30 is believed to act as a co-stimulatory molecule in signal transduction pathways with a preference for creating a T helper 2 (Th2) environment for B-cell activation, differentiation and antibody class switching [START_REF] Romagnani | CD30 and type 2 T helper (Th2) responses[END_REF][START_REF] Gruss | Pleiotropic effects of the CD30 ligand on CD30-expressing cells and lymphoma cell lines[END_REF][START_REF] Horie | CD30: expression and function in health and disease[END_REF].

A soluble form of CD30 (sCD30) is generated by proteolytic cleavage of the extracellular portion of this transmembrane molecule and can be measured in serum or plasma by enzyme linked immunosorbent assays (ELISA) [START_REF] Josimovic-Alasevic | Ki-1 (CD30) antigen is released by Ki-1-positive tumor cells in vitro and in vivo. I. Partial characterization of soluble Ki-1 antigen and detection of the antigen in cell culture supernatants and in serum by an enzyme-linked immunosorbent assay[END_REF][START_REF] Hansen | The ectodomain shedding of CD30 is specifically regulated by peptide motifs in its cysteine-rich domains 2 and 5[END_REF]. Soluble CD30 levels have been correlated with disease activity in autoimmune diseases such as systemic lupus erythematosus and rheumatoid arthritis [START_REF] Ciferska | The levels of sCD30 and of sCD40L in a group of patients with systemic lupus erythematodes and their diagnostic value[END_REF][START_REF] Oflazoglu | Targeting CD30/CD30L in oncology and autoimmune and inflammatory diseases[END_REF]. In Hodgkin's lymphoma, the level of serum sCD30 correlates with disease activity and response to therapy [START_REF] Visco | Very high levels of soluble CD30 recognize the patients with classical Hodgkin's lymphoma retaining a very poor prognosis[END_REF]. It has recently been shown that a high pretransplant level of sCD30 in kidney recipients is associated with an increase incidence of graft failure post-transplant [START_REF] Heinemann | Association of elevated pretransplant sCD30 levels with graft loss in 206 patients treated with modern immunosuppressive therapies after renal transplantation[END_REF][START_REF] Sengul | Identification of patients at risk of acute rejection by pretransplantation and posttransplantation monitoring of soluble CD30 levels in kidney transplantation[END_REF]. In patients infected with human immunodeficiency virus (HIV)-1, decreasing levels of sCD30 is a good prognostic factor for effective response to antiviral HIV therapy [START_REF] Sadeghi | Short communication: decreasing soluble CD30 and increasing IFN-gamma plasma levels are indicators of effective highly active antiretroviral therapy[END_REF].

Epstein-Barr virus (EBV), a human herpesvirus that establishes a life-long infection in circulating B lymphocytes in vivo, is also an inducer of CD30 expression on B cells in vitro [START_REF] Gregory | Isolation of a normal B cell subset with a Burkitt-like phenotype and transformation in vitro with Epstein-Barr virus[END_REF][START_REF] Siemer | EBV transformation overrides gene expression patterns of B cell differentiation stages[END_REF]. EBV is the causative agent of infectious [START_REF] Niederman | Prevalence, incidence and persistence of EB virus antibody in young adults[END_REF]. Increased serum levels of sCD30 have been found in IM patients compared to the healthy controls [START_REF] Vinante | Soluble forms of p55-IL-2R alpha, CD8, and CD30 molecules as markers of lymphoid cell activation in infectious mononucleosis[END_REF]. EBV is also associated with lymphoid tumours such as a subset of Hodgkin's lymphoma and lymphoproliferative diseases in immunocompromised patients, in particular in transplant patients (Post-transplant lymphoproliferative disease (PTLD) [START_REF] Emery | Immunotherapy and vaccination after transplant: the present, the future[END_REF]. PTLD is a recognised complication of immunosuppression affecting up to 10% of transplant recipients. The majority of PTLD are positive for EBV, and some PTLD tumour cells also express CD30 on their surface [START_REF] Adams | Clinical, phenotypic and genetic similarities and disparities between post-transplant and classical Hodgkin lymphomas with respect to therapeutic targets[END_REF][START_REF] Schwend | Rapidly growing Epstein-Barr virus-associated pulmonary lymphoma after heart transplantation[END_REF][START_REF] Lok | Post-transplant cutaneous T-cell lymphomas[END_REF]. The diagnosis of PTLD is based on clinical features and histological findings of the tumour biopsy. Currently there is no serum marker available to predict PTLD development or aid PTLD diagnosis in transplant patients. Monitoring EBV DNA levels in blood is not always helpful since there are some patients with EBV-associated diseases without detectable DNA in blood [START_REF] Hopwood | Persistent Epstein-Barr virus infection: unrestricted latent and lytic viral gene expression in healthy immunosuppressed transplant recipients[END_REF]. The prognosis for PTLD is also variable with only up to 50% demonstrating a response to treatment. Moreover, the response to treatment is often difficult to assess, particularly when the tumour is not easily measurable or accessible.

The present study was undertaken to ascertain whether sCD30 can be used as a serum marker for EBV-associated lymphoproliferation such as acute IM and PTLD and whether the levels of sCD30 correlate with EBV DNA levels in the plasma of these patients.

PATIENTS, MATERIALS AND METHODS

Study Cohort

The study population consisted of 90 EBV seropositive healthy individuals (Group A), 90 individuals with clinically diagnosed and serologically confirmed acute infectious mononucleosis (IM; Group B), 30 adult heart and/or lung transplant recipients (Group C) and 23 patients with biopsy proven EBV-positive post-transplant lymphoproliferative disease (PTLD; Group D). All these patients were enrolled into previous ethically approved clinical trials. All patients were anonomysed at source, given unique identifying numbers and aliquots of plasma samples were collected.

Detection of soluble CD30 (sCD30)

The levels of sCD30 in plasma were determined using a commercially available enzyme linked immunoassay kit (Bender MedSystems®, A-1030 Vienna, Austria) as directed by the manufacturer's instructions. Briefly, microwell strips pre-coated with monoclonal antibody against soluble CD30 were used. A tenfold dilution of positive control material was prepared from reconstituted sCD30 standard supplied by the manufacturer. Anti-human horse radish peroxidise conjugate was used as detection antibody. The plate was read using an ELISA plate reader (ASYS Expert 96, Scientific Laboratory Supplies) and the OD levels detected for each sample were used to estimate the sCD30 values off the standard curve.

Detection of EBV DNA using a quantitative polymerase chain reaction assay

EBV DNA levels in plasma were measured by real time quantitative polymerase chain reaction (qPCR) amplification of EBNA 1 gene using an ABI Prism 7000 system (Taqman®, Applied Biosystem, Warrington, UK) [START_REF] Jabs | Normalized quantification by real-time PCR of Epstein-Barr virus load in patients at risk for posttransplant lymphoproliferative disorders[END_REF]. Briefly, DNA was extracted from plasma using EasyMag® as per manufacturer's instructions (bioMerieux, Basingstoke, Standard curves were created by plotting crossing threshold (CT) values against known EBV-DNA concentrations in triplicates using Sequence Detection System (version 1.2.3, Applied Biosystem). The lower limit of detection of the assay was 100 EBV copies/mL of blood.

Statistical Analysis

GraphPad Prism 5 was used for data analysis and to create graphs. Soluble CD30 values and EBV viral loads in different study groups were compared using Mann-Whitney U test (unpaired non-parametric) and the association between sCD30 and EBV was examined by Spearman Rank test. P values of less than 0.05 were regarded as statistically significant.

Results

Study Population

Serum samples used in the study were collected from 233 individuals who were classified into 4 groups: A) EBV seropositive healthy controls (n=90), B) Acute infectious mononucleosis (IM; n=90), C) Adult heart and/or lung transplant patients (n=30) and D) Biopsy proven EBV-positive PTLD (n=23). The transplant types included: 1 lung, 3 heart, 2 liver and small bowel, 10 liver and 7 kidney. The demographic details of PTLD patients are shown in Table 1.

Soluble CD30 (sCD30) as a diagnostic marker for EBV-driven lymphoproliferation

All plasma samples were tested for the presence and levels of sCD30 using a commercially available ELISA.

Group A-EBV seropositive healthy controls

There were 54 females and 36 males (age range 18 to 24 years) in this group. The median level of sCD30 detected was 15.7 ng/mL (range: 0 -178 ng/mL; Figure 1). There was no significant difference in sCD30 levels in male and female individuals (p=0.4).

Group B-Acute infectious mononucleosis (IM) patients

Ninety patients (age range: 15-27 years) with clinically and serologically confirmed IM (all positive for IgM antibodies against EBV viral capsid antigen) were enrolled of which 48 were females and 43 males. sCD30 was detected in all patients with IM (range 6.8 -712 ng/mL), the median value was 242.9 ng/mL. The sCD30 levels in acute IM patients were significantly higher when compared to the EBV seropositive healthy controls (Figure 1; p value < 0.0001); The date of onset of symptoms and signs of acute IM was available in 40 out of 90 IM patients. The levels of sCD30 were significantly higher in IM patients when the onset of illness was <14 days at the time of sample collection compared to those with an onset of illness ≥14 days (median values 169.8 and 38.9 ng/mL respectively, p=0.034). This indicates that sCD30 levels were at their highest during the early stage of the disease.

Group C -Adult transplant recipients

All heart and/or lung transplant patients had a median sCD30 level of 27 ng/mL (range between 2.16 -669 ng/mL). This level was significantly higher than that of the EBV seropositive healthy controls (p=0.003) indicating that sCD30 levels was increased in transplant patients even though they had no EBV-related disease.

Group D -EBV-positive Post-transplant lymphoproliferative diseases (PTLD)

All patients with EBV+ PTLD had detectable sCD30 levels in plasma ranging from 26 to 572 ng/mL (median 118 ng/mL). The sCD30 levels in these patients were significantly higher than those seen in healthy controls (Group A, p <0.0001) and in non-PTLD transplant patients (Group C; p =0.0007). These results indicate that sCD30 levels were increased in patients with EBV-driven lymphoproliferation.

There was no significant difference in the sCD30 levels and gender, age (children versus adults) and the histological types of PTLD tumours (data not shown). Information on immunostaining for CD30 marker on tumour biopsy material at PTLD diagnosis was 9 available in 10 cases, seven (70%) of these PTLD biopsies contained some tumour cells that stained positive for CD30 (Table 1).

EBV DNA levels

EBV DNA was detected in 66 (56%) out of 118 plasma samples tested (Figure 2). Only 1 out of 23 EBV seropositive healthy controls (Group A) had a low level EBV DNA in plasma (229 copies/mL). In contrast, 39 (76%) of 51 acute IM patients (Group B) had detectable EBV DNA in plasma (Range 399 -219192 copies/mL, median 6585 copies/mL). Unsurprisingly, acute IM samples had significantly higher EBV DNA levels when compared to healthy controls (p<0.0001). Thirty seven per cent (9/24) of non-PTLD transplant patients (Group C) had low levels of EBV DNA with a range of 206 -1698 copies/mL (median 482 copies/mL).

Of the 20 patients with EBV-positive PTLD that were tested, EBV DNA was detected in 17 (85%) patients (Group D) with levels ranging between 103 to 324755 copies/mL (median 6069 copies/mL). EBV DNA levels were higher in PTLD patients than those in healthy controls (p <0.0001) and transplant patients (p=0.0002).

Correlation between soluble CD30 and EBV DNA levels

In order to determine an association between sCD30 and EBV DNA in plasma, sCD30 values were plotted against the EBV DNA levels in each sample where EBV DNA was detected by quantitative PCR (n=66) and analysed by Spearman Rank correlation tests. The results are shown in Figure 3. There was significant correlation between these 2 parameters whereby high sCD30 concentrations were associated with high EBV DNA levels (r 2 =0.42; p<0.0001; Spearman Rank). This trend was seen when the analyses were done separately on acute IM and PTLD samples (p values were <0.001 and 0.02 respectively).

Discussion

We examined the levels of sCD30 and EBV DNA in the plasma of patients with EBV-driven lymphoproliferations (acute IM and PTLD), and compared these with healthy controls and transplant recipients respectively. We also investigated any association between these two markers.

High level of serum sCD30 is generally considered to be an indicator of increased immune activation, particularly of a Th2 type response [START_REF] Pellegrini | CD30 antigen: not a physiological marker for TH2 cells but an important costimulator molecule in the regulation of the balance between TH1/TH2 response[END_REF]. Our results show that sCD30 levels are significantly higher in patients with acute IM when compared with EBVseropositive healthy individuals (p<0.0001). This finding is in line with a previous report by [START_REF] Vinante | Soluble forms of p55-IL-2R alpha, CD8, and CD30 molecules as markers of lymphoid cell activation in infectious mononucleosis[END_REF], showing high sCD30 levels in 55 IM patients (477 U/mL compared to 4.9 U/mL in healthy controls). IM is an immunopathological condition resulting from a primary infection with EBV. EBV infects B lymphocytes leading to B cell activation and proliferation. This in turn stimulates a rigorous T cell response that consists of activation and expansion of EBV-specific cytotoxic CD8+ and CD4+ T cells. These T cells control the acute infection via lysis of the virus-carrying B lymphoblasts. Some infected B cells escape this T cell surveillance by initiating down-regulation of viral genes leading to limited expression of viral proteins, thereby establishing a life-long persistence in the host. The expression of CD30 is seen on EBV-infected B cells and activated T cells in vitro [START_REF] Vinante | Soluble forms of p55-IL-2R alpha, CD8, and CD30 molecules as markers of lymphoid cell activation in infectious mononucleosis[END_REF], and some CD30 molecules are cleaved and can be detected in blood. Although IM is mostly a Th1driven phenomenon, an increase in sCD30, a marker of Th2 differentiation, seen in our IM cohort may represent a measure to counteract the initial extensive Th1 activities during acute infection. This may account for sCD30 levels being significantly higher during the first 2 weeks of onset of illness in a subset of our IM patients. [START_REF] Vinante | Soluble forms of p55-IL-2R alpha, CD8, and CD30 molecules as markers of lymphoid cell activation in infectious mononucleosis[END_REF] have also shown that sCD30 levels in sequential samples from 15 IM patients fall to the near control levels to PTLD. PTLD lesions usually consist of EBV-infected B cells and some of these PTLD cells express CD30 on their surface which can be cleaved from the cell membrane (70% PTLD biopsies in our cohort). So it is not surprising that sCD30 levels and EBV DNA levels were found to be significantly higher in our patient cohorts with lymphoproliferative conditions such as acute IM and PTLD.

The exact mechanisms of signal transduction by CD30 remain to be elucidated. CD30 interacts with its ligand, CD153, a member of tumor necrosis factor (TNF) superfamily which is present on activated T cells and resting B cells [START_REF] Gruss | Pleiotropic effects of the CD30 ligand on CD30-expressing cells and lymphoma cell lines[END_REF]. It has been shown in Hodgkin's Reed-Sternberg (HRS) cells that the interaction between CD30 with its ligand leads to activation of various signal transducers of TNF receptor family (such as TNF receptor-associated factors, TRAF1, 2, 3 and 5, NF-kB, and cFLIP) which promote survival of HRS cells [START_REF] Horie | CD30: expression and function in health and disease[END_REF][START_REF] Molin | Mast cells express functional CD30 ligand and are the predominant CD30L-positive cells in Hodgkin's disease[END_REF]. EBV oncoprotein latent membrane protein (LMP) 1 is essential for transforming EBV-infected B lymphocytes into lymphoblastoid cell lines, and subsequently for the long-term survival of these cells. LMP1 interacts with TRAFs causing activation of nuclear transcription factor, NF-kB which is important for cell survival. LMP1 is expressed in all EBV-associated tumour cells and its interaction with the TNF receptor family members may play a role in increased CD30 expression on tumour cells and consequently, an increase sCD30 level in blood. In a previous study we have shown that the level of TNF-α, a member of the TNF family that promoted B cell proliferation, is increased in PTLD patients compared to healthy controls and transplant patients [START_REF] Mcaulay | Tumour necrosis factor gene polymorphism: a predictive factor for the development of post-transplant lymphoproliferative disease[END_REF]. EBV DNA detected in the plasma in our patients may have come from free virions arising from viral replication during a primary infection (in acute IM) The data provide evidence that sCD30 can be used as a marker for diagnosing EBVassociated diseases; particularly in the context of acute IM and PTLD. Follow-up samples were not available to assess changes in sCD30 levels in this cohort. Prospective monitoring of sCD30 levels and correlation with EBV DNA may provide a useful tool for prediction and early diagnosis of PTLD and other EBV-associated diseases and may have a prognostic role; however further research in a larger cohort of patients and serial sampling is required.

  Figure 1 185x118mm (300 x 300 DPI)

  ), an acute, self-limiting lymphoproliferative disease

  Ten microlitres (µl) of extract were then amplified in a 50µl reaction volume. Samples were run in duplicates, and negative and positive controls were included in every run.

  sCD30 level was increased in acute IM patients. There was no significant difference in sCD30 levels in males and females (p=0.25).

  of IM diagnosis. In patients with suppressed immune function, for example, transplant patients, EBV-carrying B cells can proliferate in an uncontrolled fashion giving rise

  -associated virus released from apoptotic or dead B cells or tumour cells (in acute IM or PTLD). A high degree of correlations between EBV DNA and sCD30 levels seen in our cohort indicate immune activation and increased signal transduction activities during EBV-driven lymphoproliferation.

Figure 1 :

 1 Figure 1: Plasma samples were tested for the presence and levels of sCD30 using a

Figure 2 :

 2 Figure 2: EBV DNA was measured in plasma by a quantitative PCR from healthy controls

Figure 3 :

 3 Figure 3: sCD30 values were plotted against EBV DNA levels in each sample where EBV

Table 1 :

 1 Demography of patients with EBV-positive PTLD PTLD, post transplant lymphoproliferative disease; nd, not done.

	Sex	Age	Type of transplant PTLD Type	CD30 staining on PTLD tissue	sCD30 levels (ng/mL)
	F	1	liver	Monomorphic	nd	571.76
	M	3	liver + small bowel	Hyperplastic	nd	308.84
	F	25	heart	Monomorphic	Positive	187.88
	M	53	kidney	Polymorphic	nd	413.72
	M	8	lung	Polymorphic	Negative	355.04
	M	60	liver	Polymorphic	nd	131.56
	F	13	kidney	Hyperplastic	nd	398.28
	M	14	liver	Hodgkin's	Positive	118.04
	M	51	heart	Monomorphic	nd	34.76
	F	48	kidney	Monomorphic	nd	66.28
	M	61	kidney	Monomorphic	nd	133.20
	F	49	liver	Monomorphic	nd	67.08
	F	67	liver	Hodgkin's	Positive	26.28
	F	35	liver	Monomorphic	nd	19.48
	M	11	kidney	Polymorphic	Positive	31.24
	M	34	heart	Hyperplastic	nd	56.92
	M	1	liver + small bowel	Hodgkin's	Negative	152.28
	M	61	liver	Monomorphic	Negative	246.56
	F	7	liver	Hyperplastic	Positive	29.48
	F	50	liver	Hodgkin's	Positive	93.56
	F	32	kidney	Monomorphic	Positive	36.32
	M	50	liver	Polymorphic	nd	42.36
	M	59	kidney	Monomorphic	nd	52.24

Journal of Medical Virology

Acknowledgements

The authors will like to thank Dr Jill Douglas at Edinburgh University for her help with sample collection, Mrs. Reeya Shudra at Royal Free Hospital for her technical help with ELISA and Ms Carol Tate at Royal Free Hospital for secretarial help. The authors have no commercial or other association that may pose a conflict of interest.


Ke Chen

Charles Gumiaux

Romain Augier

Yan Chen

Qingchen Wang

Wei Lin

Shengli Wang

The Mesozoic palaeorelief of the northern Tian Shan (China)

Keywords: palaeorelief, Mesozoic, regional onlap, reactivation, India-Asia collision Tian Shan, paleorelief, Mesozoic, regional onlap, reactivation, India-Asia collision Tian Shan, paleorelief, Mesozoic, regional onlap, reactivation, India-Asia collision

The Tian Shan range offers a natural laboratory to study orogenic processes. Most of the previous studies focused on either the Paleozoic evolution of the range or its Cenozoic intracontinental evolution linked with the India-Asia collision. In this study, detailed field investigations on the relationship between sedimentary cover and basement constrain the Mesozoic evolution of the northern Tian Shan. Sedimentological observations argue for limited transport distance for Lower and Uppermost Jurassic deposits. Geological sections presented in this paper show that, in preserved locations, Triassic to Jurassic sedimentary series present a continuous onlap type sedimentary unconformity on the top of the basement.

At different scales, observations clearly evidence the existence of a major palaeorelief during Mesozoic. According to the present study, the current Tian Shan range topography and the associated movements along its northern front structures cannot be considered as the consequence of Cenozoic reactivation alone.

Introduction

The modern Tian Shan is one of the major mountain ranges in Central Asia. The current structure of Tian Shan mainly results from a combination of two main tectonic events: i) subductions, arc-accretions and continental collision during Paleozoic (e.g. Carroll et al., 1990;Wang et al., 1990;Windley et al., 1990;Sengör et al. 1993;Carroll et al., 1995;Gao et al., 1998;Laurent-Charvet et al., 2002;Charvet et al., 2007;Wang et al., 2007) and ii) intracontinental reactivation linked to the India-Asia collision, during Cenozoic (e.g. Tapponnier and Molnar, 1977;Avouac et al., 1993). At first order, the modern Tian Shan corresponds to a raised ~east-west trending Paleozoic "basement" range, flanked by two closed intracontinental basins whose infill is a particularly accurate record for both tectonic and climate evolutions (Fig. 1). The present-day high topography of Tian Shan, with a mean altitude of ~2500 m and summits of more than 7000 m, is traditionally related to the latest intracontinental deformation of the range (e.g. Tapponnier and Molnar, 1977;Avouac et al., 1993;Burchfiel et al., 1999). Age constraints for the onset of this intracontinental reactivation range from 10 to 24 Ma (e.g. Avouac et al., 1993;Hendrix et al., 1994;Abdrakhmatov et al., 1996;Métivier and Gaudemer, 1997;Sobel et al., 1997;Dumitru et al., 2001;Charreau et al., 2009). In spite of relative age discrepancies (mostly related to the method applied), all ages estimated for the deformation and the associated relief erosion suggest that the mountain range reactivation and relief creation began during Early Miocene at oldest. However, recent Fission-track analyses suggest that uplifting may have existed well before the onset of the Tertiary reactivation through Tian Shan, with exhumation ages such as 160-120 Ma in central Tian Shan, 200-250 Ma detritus cooling age in the Manas River valley (Dumitru et al., 2001;Jolivet et al., 2010) or 140-120 Ma from Bayanbulak intracontinental basin, in the southern Tian Shan (Wang et al., 2009). Moreover, Triassic to Cretaceous sediments are composed of thick continental series characterized by massive coarse-grained formations within both basins, suggesting rather high erosion rate at that time (Hendrix, 2000;Carroll et al., 2010).

This study considers the Mesozoic tectonic and morphologic evolution of Tian Shan through field structural and sedimentological analyses of Mesozoic deposits. It focuses on the northern piedmont of the range, along a ~280 km west-east trending segment (Fig. 1). Thanks to deep incisions of seasonal rivers, contacts between Paleozoic basement and Mesozoic-Cenozoic sedimentary cover are particularly well exposed along several north-south trending sections in this region (Fig. 1). Such new constraints on Mesozoic evolution of the Tian Shan area would lead to reconsider the contribution of Cenozoic tectonics in the modern mountain range building as well as to better quantify the net shortening amount during that period.

Stratigraphy of the Mesozoic sediments within the study area

Sedimentary series can be extensively observed and have already been studied within the Junggar basin, as summarized below (Figs. 1 and 2;Hendrix et al., 1992 and references therein;XBGMR, 1993). Basement units of the north Tian Shan mainly consist of Paleozoic volcanic and sedimentary rocks. From bottom to top, the entire Mesozoic series is composed of ~5 km thick continental clastic deposits highlighting high sedimentation/preservation rates (Fig. 2). Thanks to numerous studies aimed at oil and gas exploration and to 1:200.000 mapping projects hold in this area, ages of the Mesozoic sequences were mainly constrained by floral and sporopollen assemblage analyses (Li et al., 1978;Yang et al., 1982;Liu, 1990; At first order, to the north of the Hutubi He area, Triassic to Neogene sedimentary series form a rather simple monocline structure, dipping 15-20° to the north (Fig. 5a). The southernmost fold-and-thrust belt is composed of one gentle upright anticline and associated syncline with only very limited thrusts developed along the hinge of the fold (Fig. 5a). This belt can be regarded as a fault-propagation fold structure according to the available seismic profile [START_REF] Review | Analysis of the fold-thrust zone in the southern Junggar Basin , north western China[END_REF]; Fig. 5b). Further south, entering within the interior parts of the range, sub-horizontal Lower Jurassic strata lie on top of the Carboniferous units (Fig. 5b).

While basement is strongly deformed, as evidenced by sub-vertical cleavage, its sedimentary cover is only gently folded as illustrated on the geological map, few kilometers west of the river (Fig. 5a; XBGMR, 1978XBGMR, , 1993)). No décollement structure has been observed between the basement top and the sedimentary cover. Besides, a southward backthrust of basement rocks above sediments can be observed to the south of the section, but the net slip must be limited as it rapidly dies out, the fault laterally evolving to a fold toward the west (Fig. 5a).

Unconformity of the basal contact of Mesozoic sediments on top of basement units is observed in the southernmost part of the section (Figs. 5a and6a). As a whole, the successive sub-horizontal and monocline segments define a hinge fold at regional scale (Fig. 5b).

Nevertheless, while ~900 m thick of Triassic sediments have been drilled along the W1 well in the basin (Fig. 5b), no Triassic series have been observed further south in the range, in the section area (Fig. 5a). Indeed, Lowermost to Middle Jurassic levels have been observed and mapped directly on top of the basement units (Fig. 5a; XBGMR, 1978XBGMR, , 1993)). Similarly to the Houxia area, basal contact of the Mesozoic series suggests its onlap deposit, from basin to range, at least during Triassic and Lower-Middle Jurassic.

More local-scale evidences for onlap sedimentation can also be observed in the Hutubi He area. First, to the south end of the section, Lower Jurassic strata display a kilometer scale onlap type structure on top of basement units, i.e. with younger sediments progressively covering basement, from north to south (Figs. 5a and6a). Second, ~10 km west of the Hutubi River, subhorizontal layers of the Lower Jurassic formation, which can be continuously followed in few kilometers in this small valley, structurally onlap on the deformed Paleozoic basement units (Fig. 6b). There, stratification planes are markedly oblique with the top of the Paleozoic units and no deformation has been observed along this contact. In the landscape, younger Jurassic sediments progressively extend to the south, on top of the Carboniferous units. Sediments progressively overlie and cover a ~150 m high step of the basement top which implies that a significant relief existed during Jurassic sediments deposition.

To the north of the Wusu section, first-order structure of the Mesozoic-Cenozoic series displays a 40-45° north dipping monocline (Figs. 7a and7b). To the south, bedding is subhorizontal and structure of the sedimentary series is marked by a large hinge fold. As drawn on the geological map, thrust faults are observed at the surface, but seismic data show that they do not extend downward into the Mesozoic series (Fig. 7b; suppl. material), which is compatible with the limited throw produced by these faults as deduced from map data analysis (Fig. 7a). While ~400 m of Triassic sediments have been found in a drilled well in the basin, close to this section (see W2 on Fig. 7a), Triassic series becomes much thinner to the south and Jurassic sandstones, sometimes, directly lie on top of the Carboniferous rocks with an erosional undeformed contact (Fig. 7c;XBGMR, 1973).

Discussion and conclusions

In parallel to rather classical view of frontal thrust systems, this study highlights that the northern piedmont of Tian Shan also displays segments where Mesozoic series are preserved unconformably onto the basement with onlap-type relationships. Such features are pointed out along two distant cross-sections and have also been clearly shown at outcrop-scale.

Sedimentary breccias, showing very limited transport distances, have been found within Lower Jurassic sediments of the internal parts of the range. Whatever the scale, all these observations clearly show the existence of proximal relief when Triassic/Jurassic sediments were deposited. Our field observations are consistent with previously published sedimentological datapaleocurrent measurements, heavy mineral composition, sedimentological source analysesin the Junggar and Tarim basins which suggest that Tian Shan could already have existed as a "positive" geomorphologic feature during Mesozoic times (Hendrix et al., 1992, Graham et al., 1993;Hendrix, 2000;Li et al., 2004).

In the field, the local-scale observation of onlap sedimentary architecture reveals that the paleo-altitude difference can reach 100-150 m in a hectometer horizontal distance, which highlights local steep slopes (cf. Fig. 6b). Together with the sedimentological observations in the Houxia valley, this suggests that small-scale ridges with intercalated intra-mountainous basins have probably existed during Mesozoic (Fig. 8a).

Drill wells W1 and W1" (Figs. 4a and4b) display ~1200 m of Lower Jurassic sediments (e.g. Badaowan and Shangonghe Fm., Fig. 2). A comparable thickness for Lower Jurassic has also been observed in the Manas area, 50 km west of the Hutubi section (Fig. 2; Hendrix et al., 1992). Following the geological maps (XBGMR;1978, 1993), Middle Jurassic series directly lie unconformably onto the Carboniferous basement, without Triassic and Lower Jurassic sediments in the southern part of the Hutubi River (see location A in Fig. 4a).

The thickness difference of Jurassic sediments reaches ~1200 m between the location A and the drill well location, for a horizontal distance of about 50 km (Fig. 4a). Such thickness difference may result from subsidence mechanisms. However, whatever the scale considered in this study, Triassic and mostly Jurassic series, up to the late Jurassic Kalaza formation deposit, display parallel bedding with no growth strata observed (Figs. 5b,6 and 7b). Such feature shows that, at the scale of the studied area, significant vertical differential movements may have hardly occurred during Triassic to Upper Jurassic sedimentation. In addition, as described above, Jurassic strata often overlie directly on Carboniferous by onlap, without fault or syn-sedimentary deformations. Seismic profiles across the southern margin of the Junggar basin do not display major fault that could have controlled sedimentation for the Early Jurassic period (Allen et al., 1991;[START_REF] Review | Analysis of the fold-thrust zone in the southern Junggar Basin , north western China[END_REF]. No evidence for structural control on local subsidence could thus be argued here. At larger scale, flexureeither thermal or tectoniccould also have controlled the subsidence for Mesozoic series accumulation.

If considering thermal control, the latest regional tectono-magmatic event occurring before Mesozoic in the Central Tian Shan area ended during early Permian (see compilation by Han et al., 1999), which is about 100 Ma earlier than Jurassic sedimentation. Therefore, according to Hendrix et al. (1992), it seems that thermal driving mechanism may only play a weak role for the Early and Middle Jurassic sedimentation. If considering tectonic control, compressional flexure may also be hard to explain as, for comparable vertical differential movements, buckling of a stable continental lithosphere would result in much larger wavelength (200-300 km) than observed in this study (~50 km) as shown by mechanical modelling (Martinod andDavy, 1992, 1994;Burov et al., 1993;Cloetingh et al., 1999).

At the scale of the present study, Jurassic sedimentation seems hardly controlled by either tectonic or thermal subsidence across northern Tian Shan. Accordingly, the 1200-1300 m thickness difference may be mainly produced by infilling of a long-lasting remnant relief of, at least, 1200-1300 m high at that period (i.e. from Triassic/Lowermost Jurassic; Fig. 8a). During Middle Jurassic, finer-grained sediments were conformably deposited on top of the Lower Jurassic strata within the basin, and were deposited onlap on the Paleozoic basement toward the range (Figs. 5a and8b). Middle Jurassic sediments of the Xishanyao and Toutunhe formations extend to a larger area with respect to the Lower Jurassic and, as a whole, the paleorelief lateral extension was certainly less important during Middle Jurassic than during Triassic/Early Jurassic (Fig. 8b). During latest Jurassic to Early Cretaceous, thick breccias were deposited onto the Middle Jurassic sediments through an angular unconformity (cf. Kalaza Fm.). These breccias are restricted to the front of the modern Tian Shan and change rapidly to finer-grained deposits in lateral equivalents. This configuration is reminiscent of the general architecture of current fans deposited along the modern Tian Shan range that originate from the present day high relief to the south, but show limited northward transport of sediments (Fig. 8c). In turn, location of the main morphological northern front of the paleo-Tian Shan, during Latest Jurassic to Early Cretaceous, would approximately coincide with the one of the current range front (Fig. 5). The uplift documented by these coarse-grained sedimentary breccias is also recorded by AFT chronology studies; Fissiontrack modeling indicates a ca. 160 and 120 Ma cooling age along a Dushanzi-Kuqa corridor, in the mountain interior (Dumitru et al., 2001;Jolivet et al., 2010). Apatite sampled from Bayanbulak area yields the earliest cooling ages of Early Cretaceous (Dumitru et al., 2001;Wang et al. 2009).

Because no major tectonic event occurred in the Northern Tian Shan during Mesozoic times (e.g. De Grave et al., 2007), the Paleozoic subduction-collision orogeny (Gao et al., 1998;Laurent-Charvet et al., 2002;Charvet et al., 2007) appears as the most probable origin of the Mesozoic paleorelief above described. In such case, the Tian Shan would persist as remnant relieves for several tens of Ma after the end of the orogenic events which are progressively and unconformably overlain by Triassic and then Jurassic sedimentation (e.g. onlap structures, mesoscale paleorelief infilling). This scenario implies extremely low erosion rates to allow the preservation of a significant relief; such feature has already been reported for other regions of Central Asia from thermochronological (AFT) and field data analyses (Jolivet et al., 2007). In parallel, Central Asia Mesozoic tectonic history is marked by the polyphased Cimmerian orogeny in Tibet area (e.g. Hendrix et al., 1992;Sobel and Dumitru, 1997;De Grave et al., 2007). Low amplitude, far-field effects can thus be recognized in the Mesozoic northern Tian Shan: one early collision stage -Qiantang/Kunlun blocks during Late Triassic/Middle Jurassicwould correspond to the drastic coarsening of the Lower Jurassic sedimentation. In turn, a later collision stage -Lhasa/Qiantang blocks during Late Jurassic/Early Cretaceouswell corresponds to the deposits of the thick and coarse breccias of the Kalaza Fm. at that time. In this case, a tectonic origin of the relief rejuvenation is attested by a major angular unconformity observed regionally.

As unambiguously demonstrated, the physiography of the modern Tian Shan dominantly results from the recent tectonic evolution of the area (e.g. Tapponnier and Molnar, 1977;Avouac et al., 1993;Métivier and Gaudemer, 1997;Charreau et al., 2009). However, this study put forward that the current Tian Shan topography can probably not be considered as the consequence of the Cenozoic intracontinental reactivation alone, but as a combination of the Cenozoic deformations superimposed on a reminiscent Mesozoic paleo-range. These new results question on the amplitude of the movements that could be estimated along the northern front structures during Cenozoic intracontinental reactivation. zone in the southern Junggar Basin , north western China. Earth Science Frontiers, 12, 411-421.

Wang, Z., Wu, J., Lu, X., Liu, C., and Zhang, J., 1990, Polycyclic tectonic The modern Tian Shan is one of the major mountain ranges in Central Asia. The current structure of Tian Shan mainly results from a combination of two main tectonic events: i) subductions, arc-accretions and continental collision during Paleozoic (e.g. Carroll et al., 1990;Wang et al., 1990;Windley et al., 1990;Sengör et al. 1993;Carroll et al., 1995;Gao et al., 1998;Laurent-Charvet et al., 2002;Charvet et al., 2007;Wang et al., 2007) and ii)

Figure captions

intracontinental reactivation linked to the India-Asia collision, during Cenozoic (e.g. Tapponnier and Molnar, 1977;Avouac et al., 1993). At first order, the modern Tian Shan corresponds to a raised ~east-west trending Paleozoic 'basement' range, flanked by two closed intracontinental basins whose infill is a particularly accurate record for both tectonic and climate evolutions (Fig. 1). The present-day high topography of Tian Shan, with a mean altitude of ~2500 m and summits of more than 7000 m, is traditionally related to the latest intracontinental deformation of the range (e.g. Tapponnier and Molnar, 1977;Avouac et al., 1993;Burchfiel et al., 1999). Age constraints for the onset of this intracontinental reactivation range from 10 to 24 Ma (e.g. Avouac et al., 1993;Hendrix et al., 1994;Abdrakhmatov et al., 1996;Métivier and Gaudemer, 1997;Sobel et al., 1997;Dumitru et al., 2001;Charreau et al., 2009). In spite of relative age discrepancies (mostly related to the method applied), all ages estimated for the deformation and the associated relief erosion suggest that the mountain range reactivation and relief creation began during Early Miocene at oldest.

However, recent Fission-track analyses suggest that uplifting may have existed well before the onset of the Tertiary reactivation through Tian Shan, with exhumation ages such as 160-120 Ma in central Tian Shan, 200-250 Ma detritus cooling age in the Manas River valley (Dumitru et al., 2001;Jolivet et al., 2010) or 140-120 Ma from Bayanbulak intracontinental basin, in the southern Tian Shan (Wang et al., 2009). Moreover, Triassic to Cretaceous sediments are composed of thick continental series characterized by massive coarse-grained formations within both basins, suggesting rather high erosion rate at that time (Hendrix, 2000;Carroll et al., 2010).

This study considers the Mesozoic tectonic and morphologic evolution of Tian Shan through field structural and sedimentological analyses of Mesozoic deposits. It focuses on the northern piedmont of the range, along a ~280 km west-east trending segment (Fig. 1). Thanks to deep incisions of seasonal rivers, contacts between Paleozoic basement and Mesozoic-Cenozoic sedimentary cover are particularly well exposed along several north-south trending sections in this region (Fig. 1). Such new constraints on Mesozoic evolution of the Tian Shan area would lead to reconsider the contribution of Cenozoic tectonics in the modern mountain range building as well as to better quantify the net shortening amount during that period.

Stratigraphy of the Mesozoic sediments within the study area

Sedimentary series can be extensively observed and have already been studied within the Junggar basin, as summarized below (Figs. 1 and 2; Hendrix et al., 1992 and references therein;XBGMR, 1993). Basement units of the north Tian Shan mainly consist of Paleozoic volcanic and sedimentary rocks. From bottom to top, the entire Mesozoic series is composed of ~5 km thick continental clastic deposits highlighting high sedimentation/preservation rates (Fig. 2). Thanks to numerous studies aimed at oil and gas exploration and to 1:200.000 mapping projects hold in this area, ages of the Mesozoic sequences were mainly constrained by floral and sporopollen assemblage analyses (Li et al., 1978;Yang et al., 1982;Liu, 1990;XBGMR, 1993;Lu and Deng, 2005;Yang et al., 2005;Huang and Li, 2007). In the study area, the Triassic series are mostly missing along the piedmont though locally exposed (Fig. 1).

Where it crops out, Triassic displays alluvial sandstones and conglomerates intercalated coarse conglomeratic deposits, as exposed in the south of Qingshui He (Fig. 3a). Thickness of Lower Jurassic reaches 1000 m over the area extending from Manasi to Urumqi, and decreases westwardly and eastwardly. Sediments become progressively finer upward, grading into ~3 km thick series of Lower/Middle Jurassic gray sandstones, silty shales (Fig. 2).

Middle Jurassic sandstones and mudstones locally contain coal-rich layers consistent with a lacustrine depositional facies (Hendrix et al., 1992). Upper Jurassic comprises typical finegrained red beds where conglomerates remain scarce. The transition from Jurassic to Cretaceous series is marked by up to 800 m thick sedimentary breccias (Kalaza formation -J3k; Fig. 3b) made of coarse-grained angular clasts (Fig. 3c). It rapidly dies laterally and grades to sandstones within the Junggar basin itself (XBGMR, 1978;1993). The marked unconformity between the Kalaza formation and the underlying finer-grained Upper Jurassic red beds (Fig. 3b) suggests a short transport distance of material before deposition and nearby relief uplifting.

In this study, Mesozoic sediments have also been extensively observed within internal parts of the range, and more particularly in the eastern part of the studied area (Fig. 1).

Triassic to Jurassic sediments are encountered on top of the Paleozoic basement units, at rather high altitudes (2000 to 3000 m) in comparison with the mean altitude of the foreland fold-and-thrust belts (1000 to 1500 m high). In the Houxia valley for instance -65 km SSW of Urumqi -a Lower to Upper Jurassic continuous sedimentary series is preserved in a syncline structure (Fig. 4a). According to the geological map (Fig. 4a; XBGMR, 1977), Lower Jurassic formations -i.e. Shangonghe Fm. to the northwest and Lowermost Jurassic Badaowan Fm. to the east -directly overlie on top of Carboniferous basement units, without intercalated Triassic sediments. Besides, south and north verging thrusts developed along parts of the northern and southern limbs of the fold, respectively (Fig. 4a). A section made across the north limb of the syncline displays continuous Lower to Middle Jurassic series with no fault contact observed (Fig. 4b). From south to north, bedding planes progressively straighten up from ~30° south dipping to vertical, and even overturned just below the south verging thrust (Figs. 4a ,4b and 4d). From a sedimentological point of view, the upper and main part of Jurassic series is made of fine-grained sandstones and mudstones while the lowermost deposits of the series is characterized by intercalations of coarse-grained sedimentary breccias mostly made of highly angular clasts of Carboniferous tuffaceous sandstones (Fig. 4c). Local abundance of these breccias and the common lithological nature of the clasts with the adjacent basement unit strongly suggest a very short transport distance.

Structural analysis of the Mesozoic basal contact

Deformation of the northern Tian Shan has already been studied through the structural analysis of sedimentary series, within the Cenozoic foreland basin (e.g. Avouac et al., 1993;Deng et al., 1996;Burchfiel et al., 1999). This part focuses on the basal contact of the Mesozoic sediments, in the northern "front" area of Tian Shan.

As shown on the map (Fig. 1), some parts of the piedmont display northward thrusts that mark out the boundary in between the Paleozoic uplifted basement units and relatively subsiding proximal foreland basin. Yet, as described above, Mesozoic series are not solely restricted to the piedmont fold-and-thrust belts but are also well preserved on top of the basement units, within the range (Fig. 1). Then, some other segments of the piedmont display perfectly continuous Mesozoic sedimentary series from basin until internal and higher parts of the range, without being strained by any faulting or folding in the range front area (Fig. 1).

Structural analysis of such unstrained basal contact of Mesozoic series is presented for two of the sections constructed from field work combined with available seismic profiles and drillholes data: the Hutubi He and the Wusu sections (Fig. 1).

Page 13 of 50 For Review Only

At first order, to the north of the Hutubi He area, Triassic to Neogene sedimentary series form a rather simple monocline structure, dipping 15-20° to the north (Fig. 5a). The southernmost fold-and-thrust belt is composed of one gentle upright anticline and associated syncline with only very limited thrusts developed along the hinge of the fold (Fig. 5a). This belt can be regarded as a fault-propagation fold structure according to the available seismic profile [START_REF] Review | Analysis of the fold-thrust zone in the southern Junggar Basin , north western China[END_REF]; Fig. 5b). Further south, entering within the interior parts of the range, sub-horizontal Lower Jurassic strata lie on top of the Carboniferous units (Fig. 5b).

While basement is strongly deformed, as evidenced by sub-vertical cleavage, its sedimentary cover is only gently folded as illustrated on the geological map, few kilometers west of the river (Fig. 5a; XBGMR, 1978XBGMR, , 1993)). No décollement structure has been observed between the basement top and the sedimentary cover. Besides, a southward backthrust of basement rocks above sediments can be observed to the south of the section, but the net slip must be limited as it rapidly dies out, the fault laterally evolving to a fold toward the west (Fig. 5a).

Unconformity of the basal contact of Mesozoic sediments on top of basement units is observed in the southernmost part of the section (Figs. 5a and6a). As a whole, the successive sub-horizontal and monocline segments define a hinge fold at regional scale (Fig. 5b).

Nevertheless, while ~900 m thick of Triassic sediments have been drilled along the W1 well in the basin (Fig. 5b), no Triassic series have been observed further south in the range, in the section area (Fig. 5a). Indeed, Lowermost to Middle Jurassic levels have been observed and mapped directly on top of the basement units (Fig. 5a; XBGMR, 1978XBGMR, , 1993)). Similarly to the Houxia area, basal contact of the Mesozoic series suggests its onlap deposit, from basin to range, at least during Triassic and Lower-Middle Jurassic.

More local-scale evidences for onlap sedimentation can also be observed in the Hutubi He area. First, to the south end of the section, Lower Jurassic strata display a kilometer scale onlap type structure on top of basement units, i.e. with younger sediments progressively covering basement, from north to south (Figs. 5a and6a). Second, ~10 km west of the Hutubi River, subhorizontal layers of the Lower Jurassic formation, which can be continuously followed in few kilometers in this small valley, structurally onlap on the deformed Paleozoic basement units (Fig. 6b). There, stratification planes are markedly oblique with the top of the Paleozoic units and no deformation has been observed along this contact. In the landscape, younger Jurassic sediments progressively extend to the south, on top of the Carboniferous units. Sediments progressively overlie and cover a ~150 m high step of the basement top which implies that a significant relief existed during Jurassic sediments deposition.

To the north of the Wusu section, first-order structure of the Mesozoic-Cenozoic series displays a 40-45° north dipping monocline (Figs. 7a and7b). To the south, bedding is subhorizontal and structure of the sedimentary series is marked by a large hinge fold. As drawn on the geological map, thrust faults are observed at the surface, but seismic data show that they do not extend downward into the Mesozoic series (Fig. 7b; suppl. material), which is compatible with the limited throw produced by these faults as deduced from map data analysis (Fig. 7a). While ~400 m of Triassic sediments have been found in a drilled well in the basin, close to this section (see W2 on Fig. 7a), Triassic series becomes much thinner to the south and Jurassic sandstones, sometimes, directly lie on top of the Carboniferous rocks with an erosional undeformed contact (Fig. 7c; XBGMR, 1973).

Discussion and conclusions

In parallel to rather classical view of frontal thrust systems, this study highlights that the northern piedmont of Tian Shan also displays segments where Mesozoic series are preserved unconformably onto the basement with onlap-type relationships. Such features are pointed out along two distant cross-sections and have also been clearly shown at outcrop-scale.

Sedimentary breccias, showing very limited transport distances, have been found within Lower Jurassic sediments of the internal parts of the range. Whatever the scale, all these observations clearly show the existence of proximal relief when Triassic/Jurassic sediments were deposited. Our field observations are consistent with previously published sedimentological data -paleocurrent measurements, heavy mineral composition, sedimentological source analyses -in the Junggar and Tarim basins which suggest that Tian Shan could already have existed as a "positive" geomorphologic feature during Mesozoic times (Hendrix et al., 1992, Graham et al., 1993;Hendrix, 2000;Li et al., 2004).

In the field, the local-scale observation of onlap sedimentary architecture reveals that the paleo-altitude difference can reach 100-150 m in a hectometer horizontal distance, which highlights local steep slopes (cf. Fig. 6b). Together with the sedimentological observations in the Houxia valley, this suggests that small-scale ridges with intercalated intra-mountainous basins have probably existed during Mesozoic (Fig. 8a).

Drill wells W1 and W1' (Figs. 4a and4b) display ~1200 m of Lower Jurassic sediments (e.g. Badaowan and Shangonghe Fm., Fig. 2). A comparable thickness for Lower Jurassic has also been observed in the Manas area, 50 km west of the Hutubi section (Fig. 2; Hendrix et al., 1992). Following the geological maps (XBGMR;1978, 1993), Middle Jurassic series directly lie unconformably onto the Carboniferous basement, without Triassic and Lower Jurassic sediments in the southern part of the Hutubi River (see location A in Fig. 4a).

The thickness difference of Jurassic sediments reaches ~1200 m between the location A and the drill well location, for a horizontal distance of about 50 km (Fig. 4a). Such thickness difference may result from subsidence mechanisms. However, whatever the scale considered in this study, Triassic and mostly Jurassic series, up to the late Jurassic Kalaza formation deposit, display parallel bedding with no growth strata observed (Figs. 5b, 6 and 7b). Such feature shows that, at the scale of the studied area, significant vertical differential movements may have hardly occurred during Triassic to Upper Jurassic sedimentation. In addition, as described above, Jurassic strata often overlie directly on Carboniferous by onlap, without fault or syn-sedimentary deformations. Seismic profiles across the southern margin of the Junggar basin do not display major fault that could have controlled sedimentation for the Early Jurassic period (Allen et al., 1991;[START_REF] Review | Analysis of the fold-thrust zone in the southern Junggar Basin , north western China[END_REF]. No evidence for structural control on local subsidence could thus be argued here. At larger scale, flexure -either thermal or tectonic -could also have controlled the subsidence for Mesozoic series accumulation.

If considering thermal control, the latest regional tectono-magmatic event occurring before Mesozoic in the Central Tian Shan area ended during early Permian (see compilation by Han et al., 1999), which is about 100 Ma earlier than Jurassic sedimentation. Therefore, according to Hendrix et al. (1992), it seems that thermal driving mechanism may only play a weak role for the Early and Middle Jurassic sedimentation. If considering tectonic control, compressional flexure may also be hard to explain as, for comparable vertical differential movements, buckling of a stable continental lithosphere would result in much larger wavelength (200-300 km) than observed in this study (~50 km) as shown by mechanical modelling (Martinod andDavy, 1992, 1994;Burov et al., 1993;Cloetingh et al., 1999).

At the scale of the present study, Jurassic sedimentation seems hardly controlled by either tectonic or thermal subsidence across northern Tian Shan. Accordingly, the 1200-1300 m thickness difference may be mainly produced by infilling of a long-lasting remnant relief of, at least, 1200-1300 m high at that period (i.e. from Triassic/Lowermost Jurassic; Fig. 8a). During Middle Jurassic, finer-grained sediments were conformably deposited on top of the Lower Jurassic strata within the basin, and were deposited onlap on the Paleozoic basement toward the range (Figs. 5a and8b). Middle Jurassic sediments of the Xishanyao and Toutunhe formations extend to a larger area with respect to the Lower Jurassic and, as a whole, the paleorelief lateral extension was certainly less important during Middle Jurassic than during Triassic/Early Jurassic (Fig. 8b). During latest Jurassic to Early Cretaceous, thick breccias were deposited onto the Middle Jurassic sediments through an angular unconformity (cf. Kalaza Fm.). These breccias are restricted to the front of the modern Tian Shan and change rapidly to finer-grained deposits in lateral equivalents. This configuration is reminiscent of the general architecture of current fans deposited along the modern Tian Shan range that originate from the present day high relief to the south, but show limited northward transport of sediments (Fig. 8c). In turn, location of the main morphological northern front of the paleo-Tian Shan, during Latest Jurassic to Early Cretaceous, would approximately coincide with the one of the current range front (Fig. 5). The uplift documented by these coarse-grained sedimentary breccias is also recorded by AFT chronology studies; Fissiontrack modeling indicates a ca. 160 and 120 Ma cooling age along a Dushanzi-Kuqa corridor, in the mountain interior (Dumitru et al., 2001;Jolivet et al., 2010). Apatite sampled from Bayanbulak area yields the earliest cooling ages of Early Cretaceous (Dumitru et al., 2001;Wang et al. 2009).

Because no major tectonic event occurred in the Northern Tian Shan during Mesozoic times (e.g. De Grave et al., 2007), the Paleozoic subduction-collision orogeny (Gao et al., 1998;Laurent-Charvet et al., 2002;Charvet et al., 2007) appears as the most probable origin of the Mesozoic paleorelief above described. In such case, the Tian Shan would persist as remnant relieves for several tens of Ma after the end of the orogenic events which are progressively and unconformably overlain by Triassic and then Jurassic sedimentation (e.g. onlap structures, mesoscale paleorelief infilling). This scenario implies extremely low erosion rates to allow the preservation of a significant relief; such feature has already been reported

for other regions of Central Asia from thermochronological (AFT) and field data analyses (Jolivet et al., 2007). In parallel, Central Asia Mesozoic tectonic history is marked by the polyphased Cimmerian orogeny in Tibet area (e.g. Hendrix et al., 1992;Sobel and Dumitru, 1997;De Grave et al., 2007). Low amplitude, far-field effects can thus be recognized in the Mesozoic northern Tian Shan: one early collision stage -Qiantang/Kunlun blocks during Late Triassic/Middle Jurassic -would correspond to the drastic coarsening of the Lower Jurassic sedimentation. In turn, a later collision stage -Lhasa/Qiantang blocks during Late Jurassic/Early Cretaceous -well corresponds to the deposits of the thick and coarse breccias of the Kalaza Fm. at that time. In this case, a tectonic origin of the relief rejuvenation is attested by a major angular unconformity observed regionally.

As unambiguously demonstrated, the physiography of the modern Tian Shan dominantly results from the recent tectonic evolution of the area (e.g. Tapponnier and Molnar, 1977;Avouac et al., 1993;Métivier and Gaudemer, 1997;Charreau et al., 2009). However, this study put forward that the current Tian Shan topography can probably not be considered as the consequence of the Cenozoic intracontinental reactivation alone, but as a combination of the Cenozoic deformations superimposed on a reminiscent Mesozoic paleo-range. These new results question on the amplitude of the movements that could be estimated along the northern front structures during Cenozoic intracontinental reactivation. The modern Tian Shan is one of the major mountain ranges in Central Asia. The current structure of Tian Shan mainly results from a combination of two main tectonic events: i) subductions, arc-accretions and continental collision during Paleozoic (e.g. Carroll et al., 1990;Wang et al., 1990;Windley et al., 1990;Sengör et al. 1993;Carroll et al., 1995;Gao et al., 1998;Laurent-Charvet et al., 2002;Charvet et al., 2007;Wang et al., 2007) and ii)

Figure captions

intracontinental reactivation linked to the India-Asia collision, during Cenozoic (e.g. Tapponnier and Molnar, 1977;Avouac et al., 1993). At first order, the modern Tian Shan corresponds to a raised ~east-west trending Paleozoic 'basement' range, flanked by two closed intracontinental basins whose infill is a particularly accurate record for both tectonic and climate evolutions (Fig. 1). The present-day high topography of Tian Shan, with a mean altitude of ~2500 m and summits of more than 7000 m, is traditionally related to the latest intracontinental deformation of the range (e.g. Tapponnier and Molnar, 1977;Avouac et al., 1993;Burchfiel et al., 1999). Age constraints for the onset of this intracontinental reactivation range from 10 to 24 Ma (e.g. Avouac et al., 1993;Hendrix et al., 1994;Abdrakhmatov et al., 1996;Métivier and Gaudemer, 1997;Sobel et al., 1997;Dumitru et al., 2001;Charreau et al., 2009). In spite of relative age discrepancies (mostly related to the method applied), all ages estimated for the deformation and the associated relief erosion suggest that the mountain range reactivation and relief creation began during Early Miocene at oldest.

However, recent Fission-track analyses suggest that uplifting may have existed well before the onset of the Tertiary reactivation through Tian Shan, with exhumation ages such as 160-120 Ma in central Tian Shan, 200-250 Ma detritus cooling age in the Manas River valley (Dumitru et al., 2001;Jolivet et al., 2010) or 140-120 Ma from Bayanbulak intracontinental basin, in the southern Tian Shan (Wang et al., 2009). Moreover, Triassic to Cretaceous sediments are composed of thick continental series characterized by massive coarse-grained formations within both basins, suggesting rather high erosion rate at that time (Hendrix, 2000;Carroll et al., 2010).

This study considers the Mesozoic tectonic and morphologic evolution of Tian Shan through field structural and sedimentological analyses of Mesozoic deposits. It focuses on the northern piedmont of the range, along a ~280 km west-east trending segment (Fig. 1). Thanks to deep incisions of seasonal rivers, contacts between Paleozoic basement and Mesozoic-Cenozoic sedimentary cover are particularly well exposed along several north-south trending sections in this region (Fig. 1). Such new constraints on Mesozoic evolution of the Tian Shan area would lead to reconsider the contribution of Cenozoic tectonics in the modern mountain range building as well as to better quantify the net shortening amount during that period.

Stratigraphy of the Mesozoic sediments within the study area

Sedimentary series can be extensively observed and have already been studied within the Junggar basin, as summarized below (Figs. 1 and 2; Hendrix et al., 1992 and references therein;XBGMR, 1993). Basement units of the north Tian Shan mainly consist of Paleozoic volcanic and sedimentary rocks. From bottom to top, the entire Mesozoic series is composed of ~5 km thick continental clastic deposits highlighting high sedimentation/preservation rates (Fig. 2). Thanks to numerous studies aimed at oil and gas exploration and to 1:200.000 mapping projects hold in this area, ages of the Mesozoic sequences were mainly constrained by floral and sporopollen assemblage analyses (Li et al., 1978;Yang et al., 1982;Liu, 1990;XBGMR, 1993;Lu and Deng, 2005;Yang et al., 2005;Huang and Li, 2007). In the study area, the Triassic series are mostly missing along the piedmont though locally exposed (Fig. 1).

Where it crops out, Triassic displays alluvial sandstones and conglomerates intercalated within and silty mudstones (red-beds). At the scale of the northern Tian Shan piedmont, Lowermost Jurassic layers indicate a change in the sedimentation as they consist in thick and no fault contact observed (Fig. 4b). From south to north, bedding planes progressively straighten up from ~30° south dipping to vertical, and even overturned just below the south verging thrust (Figs. 4a ,4b and 4d). From a sedimentological point of view, the upper and main part of Jurassic series is made of fine-grained sandstones and mudstones while the lowermost deposits of the series is characterized by intercalations of coarse-grained sedimentary breccias mostly made of highly angular clasts of Carboniferous tuffaceous sandstones (Fig. 4c). Local abundance of these breccias and the common lithological nature of the clasts with the adjacent basement unit strongly suggest a very short transport distance.

Structural analysis of the Mesozoic basal contact

Deformation of the northern Tian Shan has already been studied through the structural analysis of sedimentary series, within the Cenozoic foreland basin (e.g. Avouac et al., 1993;Deng et al., 1996;Burchfiel et al., 1999). This part focuses on the basal contact of the Mesozoic sediments, in the northern "front" area of Tian Shan.

As shown on the map (Fig. 1), some parts of the piedmont display northward thrusts that mark out the boundary in between the Paleozoic uplifted basement units and relatively subsiding proximal foreland basin. Yet, as described above, Mesozoic series are not solely restricted to the piedmont fold-and-thrust belts but are also well preserved on top of the basement units, within the range (Fig. 1). Then, some other segments of the piedmont display perfectly continuous Mesozoic sedimentary series from basin until internal and higher parts of the range, without being strained by any faulting or folding in the range front area (Fig. 1).

Structural analysis of such unstrained basal contact of Mesozoic series is presented for two of the sections constructed from field work combined with available seismic profiles and drillholes data: the Hutubi He and the Wusu sections (Fig. 1).

At first order, to the north of the Hutubi He area, Triassic to Neogene sedimentary series form a rather simple monocline structure, dipping 15-20° to the north (Fig. 5a). The southernmost fold-and-thrust belt is composed of one gentle upright anticline and associated syncline with only very limited thrusts developed along the hinge of the fold (Fig. 5a). This belt can be regarded as a fault-propagation fold structure according to the available seismic profile [START_REF] Review | Analysis of the fold-thrust zone in the southern Junggar Basin , north western China[END_REF]; Fig. 5b). Further south, entering within the interior parts of the range, sub-horizontal Lower Jurassic strata lie on top of the Carboniferous units (Fig. 5b).

While basement is strongly deformed, as evidenced by sub-vertical cleavage, its sedimentary cover is only gently folded as illustrated on the geological map, few kilometers west of the river (Fig. 5a; XBGMR, 1978XBGMR, , 1993)). No décollement structure has been observed between the basement top and the sedimentary cover. Besides, a southward backthrust of basement rocks above sediments can be observed to the south of the section, but the net slip must be limited as it rapidly dies out, the fault laterally evolving to a fold toward the west (Fig. 5a).

Unconformity of the basal contact of Mesozoic sediments on top of basement units is observed in the southernmost part of the section (Figs. 5a and6a). As a whole, the successive sub-horizontal and monocline segments define a hinge fold at regional scale (Fig. 5b).

Nevertheless, while ~900 m thick of Triassic sediments have been drilled along the W1 well in the basin (Fig. 5b), no Triassic series have been observed further south in the range, in the section area (Fig. 5a). Indeed, Lowermost to Middle Jurassic levels have been observed and mapped directly on top of the basement units (Fig. 5a; XBGMR, 1978XBGMR, , 1993)). Similarly to the Houxia area, basal contact of the Mesozoic series suggests its onlap deposit, from basin to range, at least during Triassic and Lower-Middle Jurassic.

More local-scale evidences for onlap sedimentation can also be observed in the Hutubi He area. First, to the south end of the section, Lower Jurassic strata display a kilometer scale onlap type structure on top of basement units, i.e. with younger sediments progressively covering basement, from north to south (Figs. 5a and6a). Second, ~10 km west of the Hutubi River, subhorizontal layers of the Lower Jurassic formation, which can be continuously followed in few kilometers in this small valley, structurally onlap on the deformed Paleozoic basement units (Fig. 6b). There, stratification planes are markedly oblique with the top of the Paleozoic units and no deformation has been observed along this contact. In the landscape, younger Jurassic sediments progressively extend to the south, on top of the Carboniferous units. Sediments progressively overlie and cover a ~150 m high step of the basement top which implies that a significant relief existed during Jurassic sediments deposition.

To the north of the Wusu section, first-order structure of the Mesozoic-Cenozoic series displays a 40-45° north dipping monocline (Figs. 7a and7b). To the south, bedding is subhorizontal and structure of the sedimentary series is marked by a large hinge fold. As drawn on the geological map, thrust faults are observed at the surface, but seismic data show that they do not extend downward into the Mesozoic series (Fig. 7b; suppl. material), which is compatible with the limited throw produced by these faults as deduced from map data analysis (Fig. 7a). While ~400 m of Triassic sediments have been found in a drilled well in the basin, close to this section (see W2 on Fig. 7a), Triassic series becomes much thinner to the south and Jurassic sandstones, sometimes, directly lie on top of the Carboniferous rocks with an erosional undeformed contact (Fig. 7c;XBGMR, 1973).

Discussion and conclusions

In parallel to rather classical view of frontal thrust systems, this study highlights that the northern piedmont of Tian Shan also displays segments where Mesozoic series are preserved unconformably onto the basement with onlap-type relationships. Such features are pointed out along two distant cross-sections and have also been clearly shown at outcrop-scale.

Sedimentary breccias, showing very limited transport distances, have been found within Lower Jurassic sediments of the internal parts of the range. Whatever the scale, all these observations clearly show the existence of proximal relief when Triassic/Jurassic sediments were deposited. Our field observations are consistent with previously published sedimentological data -paleocurrent measurements, heavy mineral composition, sedimentological source analyses -in the Junggar and Tarim basins which suggest that Tian Shan could already have existed as a "positive" geomorphologic feature during Mesozoic times (Hendrix et al., 1992, Graham et al., 1993;Hendrix, 2000;Li et al., 2004).

In the field, the local-scale observation of onlap sedimentary architecture reveals that the paleo-altitude difference can reach 100-150 m in a hectometer horizontal distance, which highlights local steep slopes (cf. Fig. 6b). Together with the sedimentological observations in the Houxia valley, this suggests that small-scale ridges with intercalated intra-mountainous basins have probably existed during Mesozoic (Fig. 8a).

Drill wells W1 and W1' (Figs. 4a and4b) display ~1200 m of Lower Jurassic sediments (e.g. Badaowan and Shangonghe Fm., Fig. 2). A comparable thickness for Lower Jurassic has also been observed in the Manas area, 50 km west of the Hutubi section (Fig. 2; Hendrix et al., 1992). Following the geological maps (XBGMR;1978, 1993), Middle Jurassic series directly lie unconformably onto the Carboniferous basement, without Triassic and Lower Jurassic sediments in the southern part of the Hutubi River (see location A in Fig. 4a).

The thickness difference of Jurassic sediments reaches ~1200 m between the location A and the drill well location, for a horizontal distance of about 50 km (Fig. 4a). Such thickness difference may result from subsidence mechanisms. However, whatever the scale considered in this study, Triassic and mostly Jurassic series, up to the late Jurassic Kalaza formation deposit, display parallel bedding with no growth strata observed (Figs. 5b,6 and 7b). Such feature shows that, at the scale of the studied area, significant vertical differential movements may have hardly occurred during Triassic to Upper Jurassic sedimentation. In addition, as described above, Jurassic strata often overlie directly on Carboniferous by onlap, without fault or syn-sedimentary deformations. Seismic profiles across the southern margin of the Junggar basin do not display major fault that could have controlled sedimentation for the Early Jurassic period (Allen et al., 1991;[START_REF] Review | Analysis of the fold-thrust zone in the southern Junggar Basin , north western China[END_REF]. No evidence for structural control on local subsidence could thus be argued here. At larger scale, flexure -either thermal or tectonic -could also have controlled the subsidence for Mesozoic series accumulation.

If considering thermal control, the latest regional tectono-magmatic event occurring before Mesozoic in the Central Tian Shan area ended during early Permian (see compilation by Han et al., 1999), which is about 100 Ma earlier than Jurassic sedimentation. Therefore, according to Hendrix et al. (1992), it seems that thermal driving mechanism may only play a weak role for the Early and Middle Jurassic sedimentation. If considering tectonic control, compressional flexure may also be hard to explain as, for comparable vertical differential movements, buckling of a stable continental lithosphere would result in much larger wavelength (200-300 km) than observed in this study (~50 km) as shown by mechanical modelling (Martinod andDavy, 1992, 1994;Burov et al., 1993;Cloetingh et al., 1999).

At the scale of the present study, Jurassic sedimentation seems hardly controlled by either tectonic or thermal subsidence across northern Tian Shan. Accordingly, the 1200-1300 m thickness difference may be mainly produced by infilling of a long-lasting remnant relief of, at least, 1200-1300 m high at that period (i.e. from Triassic/Lowermost Jurassic; Fig. 8a). During Middle Jurassic, finer-grained sediments were conformably deposited on top of the Lower Jurassic strata within the basin, and were deposited onlap on the Paleozoic basement toward the range (Figs. 5a and8b). Middle Jurassic sediments of the Xishanyao and Toutunhe formations extend to a larger area with respect to the Lower Jurassic and, as a whole, the paleorelief lateral extension was certainly less important during Middle Jurassic than during Triassic/Early Jurassic (Fig. 8b). During latest Jurassic to Early Cretaceous, thick breccias were deposited onto the Middle Jurassic sediments through an angular unconformity (cf. Kalaza Fm.). These breccias are restricted to the front of the modern Tian Shan and change rapidly to finer-grained deposits in lateral equivalents. This configuration is reminiscent of the general architecture of current fans deposited along the modern Tian Shan range that originate from the present day high relief to the south, but show limited northward transport of sediments (Fig. 8c). In turn, location of the main morphological northern front of the paleo-Tian Shan, during Latest Jurassic to Early Cretaceous, would approximately coincide with the one of the current range front (Fig. 5). The uplift documented by these coarse-grained sedimentary breccias is also recorded by AFT chronology studies; Fissiontrack modeling indicates a ca. 160 and 120 Ma cooling age along a Dushanzi-Kuqa corridor, in the mountain interior (Dumitru et al., 2001;Jolivet et al., 2010). Apatite sampled from Bayanbulak area yields the earliest cooling ages of Early Cretaceous (Dumitru et al., 2001;Wang et al. 2009).

Because no major tectonic event occurred in the Northern Tian Shan during Mesozoic times (e.g. De Grave et al., 2007), the Paleozoic subduction-collision orogeny (Gao et al., 1998;Laurent-Charvet et al., 2002;Charvet et al., 2007) appears as the most probable origin of the Mesozoic paleorelief above described. In such case, the Tian Shan would persist as remnant relieves for several tens of Ma after the end of the orogenic events which are progressively and unconformably overlain by Triassic and then Jurassic sedimentation (e.g. onlap structures, mesoscale paleorelief infilling). This scenario implies extremely low erosion rates to allow the preservation of a significant relief; such feature has already been reported

for other regions of Central Asia from thermochronological (AFT) and field data analyses (Jolivet et al., 2007). In parallel, Central Asia Mesozoic tectonic history is marked by the polyphased Cimmerian orogeny in Tibet area (e.g. Hendrix et al., 1992;Sobel and Dumitru, 1997;De Grave et al., 2007). Low amplitude, far-field effects can thus be recognized in the Mesozoic northern Tian Shan: one early collision stage -Qiantang/Kunlun blocks during Late Triassic/Middle Jurassic -would correspond to the drastic coarsening of the Lower Jurassic sedimentation. In turn, a later collision stage -Lhasa/Qiantang blocks during Late Jurassic/Early Cretaceous -well corresponds to the deposits of the thick and coarse breccias of the Kalaza Fm. at that time. In this case, a tectonic origin of the relief rejuvenation is attested by a major angular unconformity observed regionally.

As unambiguously demonstrated, the physiography of the modern Tian Shan dominantly results from the recent tectonic evolution of the area (e.g. Tapponnier and Molnar, 1977;Avouac et al., 1993;Métivier and Gaudemer, 1997;Charreau et al., 2009). However, this study put forward that the current Tian Shan topography can probably not be considered as the consequence of the Cenozoic intracontinental reactivation alone, but as a combination of the Cenozoic deformations superimposed on a reminiscent Mesozoic paleo-range. These new results question on the amplitude of the movements that could be estimated along the northern front structures during Cenozoic intracontinental reactivation.

Figure captions

Fig. 1

 1 Fig. 1 Simplified geological map of the northern Tian Shan from Wusu to Urumqi areas.

Fig. 2

 2 Fig. 2 Synthetic and representative stratigraphic log made in the Manasi He area, northern

Fig. 3

 3 Fig. 3 Landscape pictures of representative features of the Mesozoic sedimentary series (see

Fig. 4

 4 Fig. 4 Structural and sedimentological observations in the Houxia area. (a) Simplified

Fig. 7

 7 Fig. 7 Structural and sedimentological observations in the Wusu area. (a) Simplified

  7a for location). (c) Picture and corresponding field sketch interpretation of the unconformable contact between Jurassic layers and the Carboniferous basement (see Fig.7afor location).

Fig. 8

 8 Fig. 8 Sketch views of the paleogeographic evolution of the northern piedmont of Tian Shan

  Fig 2.

  Fig 4.

  Fig. 4b

  Fig. 6b

  Fig.3b

  Fig. 7c

  Fig. 7b

  within and silty mudstones (red-beds). At the scale of the northern Tian Shan piedmont, Lowermost Jurassic layers indicate a change in the sedimentation as they consist in thick and Page 11 of 50 For Review Only

Fig. 1

 1 Fig. 1 Simplified geological map of the northern Tian Shan from Wusu to Urumqi areas.

Fig. 2

 2 Fig. 2 Synthetic and representative stratigraphic log made in the Manasi He area, northern

Fig. 3

 3 Fig. 3 Landscape pictures of representative features of the Mesozoic sedimentary series (see

Fig. 4 Fig. 6

 46 Fig. 4 Structural and sedimentological observations in the Houxia area. (a) Simplified

Fig. 7

 7 Fig. 7 Structural and sedimentological observations in the Wusu area. (a) Simplified

Fig. 8

 8 Fig. 8 Sketch views of the paleogeographic evolution of the northern piedmont of Tian Shan

Fig. 1

 1 Fig. 1 Simplified geological map of the northern Tian Shan from Wusu to Urumqi areas.

Fig. 2

 2 Fig. 2 Synthetic and representative stratigraphic log made in the Manasi He area, northern

Fig. 3

 3 Fig. 3 Landscape pictures of representative features of the Mesozoic sedimentary series (see

Fig. 4

 4 Fig. 4 Structural and sedimentological observations in the Houxia area. (a) Simplified

  5b), (b) Picture and interpretation field sketch of a small valley, west of the Hutubi He (see location on Fig. 5a): meso-scale basement relief covered by Jurassic sandstones as evidenced by onlaps over a steep inherited ~150 m high basement paleorelief (scarp). Basement rocks are deformed with a large inclined Paleozoic fold.

Fig. 7

 7 Fig. 7 Structural and sedimentological observations in the Wusu area. (a) Simplified

  7a for location). (c) Picture and corresponding field sketch interpretation of the unconformable contact between Jurassic layers and the Carboniferous basement (see Fig.7afor location).

Fig. 8

 8 Fig. 8 Sketch views of the paleogeographic evolution of the northern piedmont of Tian Shan

Page 1 of 50 For Review Only

			Clays	s t l i S	Sands f m c Cngl
	5Km 6Km	Cretaceous	K1tg K2d		Donggou Tugulu	Fm. Gr.
	4Km		J3q J3k		Qigu Fm. Kalaza Fm.	Upper Jurassic
	3Km	Jurassic	J2x J2t		Xishan Toutunhe	yao Fm. Fm.	Middle Jurassic
	2Km		J1b J1s		Badaowan Sangong	Fm. he Fm.	Lower Jurassic
	1Km	Triassic	T3 T2			Xiaojiagou	Gr.
	0Km	Permian	Basement T1 Carboniferous		Upper	Cang fang	gou Fm.
			Main Unconformity

Acknowledgements

Field and laboratory work in the present study are financially supported by Chinese National 973 Program (2009CB825008) and Chinese National S&T Major Project 2011ZX05008. Prof. A.R. Caroll, Dr. J. Charreau, and an anonymous reviewer are warmly thanked for their constructive reviews. We are also grateful for the help by Prof. Carlo Doglioni (Editor) and Prof. Gerhard Wörner (Associate Editor). Discussions on seismic profiles" interpretation with Dr. Y. Branquet have also been greatly appreciated. The first author benefices a Ph.D scholarship from French Embassy in Beijing and IGGCAS. Acknowledgements Field and laboratory work in the present study are financially supported by Chinese National 973 Program (2009CB825008) and Chinese National S&T Major Project 2011ZX05008. Prof. A.R. Caroll, Dr. J. Charreau, and an anonymous reviewer are warmly thanked for their constructive reviews. We are also grateful for the help by Prof. Carlo Doglioni (Editor) and Prof. Gerhard Wörner (Associate Editor). Discussions on seismic profiles' interpretation with Dr. Y. Branquet have also been greatly appreciated. The first author benefices a Ph.D scholarship from French Embassy in Beijing and IGGCAS. Acknowledgements Field and laboratory work in the present study are financially supported by Chinese National 973 Program (2009CB825008) and Chinese National S&T Major Project 2011ZX05008. Prof. A.R. Caroll, Dr. J. Charreau, and an anonymous reviewer are warmly thanked for their constructive reviews. We are also grateful for the help by Prof. Carlo Doglioni (Editor) and Prof. Gerhard Wörner (Associate Editor). Discussions on seismic profiles' interpretation with Dr. Y. Branquet have also been greatly appreciated. The first author benefices a Ph.D scholarship from French Embassy in Beijing and IGGCAS.

coarse conglomeratic deposits, as exposed in the south of Qingshui He (Fig. 3a). Thickness of Lower Jurassic reaches 1000 m over the area extending from Manasi to Urumqi, and decreases westwardly and eastwardly. Sediments become progressively finer upward, grading into ~3 km thick series of Lower/Middle Jurassic gray sandstones, silty shales (Fig. 2).

Middle Jurassic sandstones and mudstones locally contain coal-rich layers consistent with a lacustrine depositional facies (Hendrix et al., 1992). Upper Jurassic comprises typical finegrained red beds where conglomerates remain scarce. The transition from Jurassic to Cretaceous series is marked by up to 800 m thick sedimentary breccias (Kalaza formation -J3k; Fig. 3b) made of coarse-grained angular clasts (Fig. 3c). It rapidly dies laterally and grades to sandstones within the Junggar basin itself (XBGMR, 1978;1993). The marked unconformity between the Kalaza formation and the underlying finer-grained Upper Jurassic red beds (Fig. 3b) suggests a short transport distance of material before deposition and nearby relief uplifting.

In this study, Mesozoic sediments have also been extensively observed within internal parts of the range, and more particularly in the eastern part of the studied area (Fig. 1).

Triassic to Jurassic sediments are encountered on top of the Paleozoic basement units, at rather high altitudes (2000 to 3000 m) in comparison with the mean altitude of the foreland fold-and-thrust belts (1000 to 1500 m high). In the Houxia valley for instance -65 km SSW of Urumqi -a Lower to Upper Jurassic continuous sedimentary series is preserved in a syncline structure (Fig. 4a). According to the geological map (Fig. 4a; XBGMR, 1977), Lower Jurassic formations -i.e. Shangonghe Fm. to the northwest and Lowermost Jurassic Badaowan Fm. to the east -directly overlie on top of Carboniferous basement units, without intercalated Triassic sediments. Besides, south and north verging thrusts developed along parts of the northern and southern limbs of the fold, respectively (Fig. 4a). A section made across the north limb of the syncline displays continuous Lower to Middle Jurassic series with discordance at the base).