Sascha Brune
email: brune@gfz-potsdam.de

Julia Autin

The rift to break-up evolution of the Gulf of Aden: Insights from 3D numerical lithospheric-scale modelling

The Gulf of Aden displays an ideal setting to study oblique rift processes since numerous structural data are available onshore and offshore, down to the ocean -continent transition (OCT). We investigate key observations by means of 3D numerical thermo-mechanical experiments on lithospheric scale. The Gulf formed with a rift-normal of -15° azimuth and under the influence of a supposedly 25° trending far field extension. By reproducing oblique rifting with 40°, we study the evolution of the Gulf of Aden in terms of crustal fault geometries and stress patterns from rift initiation to break-up. Our model suggests that intermediate faults dominate during the initial rift phase, followed by rift-parallel normal faulting at the rift flanks and strike-slip faults in the central part of the rift system. Upon break-up, displacement-orthogonal as well as intermediate faults occur. We compare our results to previous analogue experiments of oblique rifting on lithospheric scale as well as to the structural evolution of the Gulf of Aden. The basic evolution is in accordance with the development of fault patterns in the analogue model and allow to propose further interpretation of the distal margin evolution of the Gulf of Aden. To large extent, this study supports previous hypotheses about the processes taking place during oblique rifting, however, it proposes different deformation

Introduction

Oblique rifts are evolving through a complex pattern of deformation, which changes through time. By nature, they cannot be assimilated to 2D structures since they display strong lateral variations. Therefore, they need to be studied in three dimensions and their understanding still depends on the ability to reproduce 3D processes.

The obliquity of the Gulf could be linked to the interaction between the laterallyevolving subduction of the Tethyan Ocean toward the north and the Afar hot spot in the south-west [START_REF] Bellahsen | Why did Arabia separate from Africa? Insights from 3-D laboratory experiments[END_REF]. Field and seismic studies were conducted onshore and offshore Oman [START_REF] Fournier | Oblique rifting and segmentation of the NE Gulf of Aden passive margin[END_REF]d'Acremont et al., 2005;[START_REF] Bellahsen | Fault reactivation and rift localization: Northeastern Gulf of Aden margin[END_REF] and in Yemen [START_REF] Huchon | Rotation of the syn-rift stress field of the northern Gulf of Aden margin, Yemen[END_REF], and allow to recognise three fault populations (extension-normal, rift-parallel, intermediate). They correspond to several directions of extension in Oman [START_REF] Lepvrier | Cenozoic extension in coastal Dhofar (southern Oman)/: implications on the oblique rifting of the Gulf of Aden[END_REF][START_REF] Fournier | Oblique rifting and segmentation of the NE Gulf of Aden passive margin[END_REF][START_REF] Bellahsen | Fault reactivation and rift localization: Northeastern Gulf of Aden margin[END_REF], in Yemen (Huchon et al., 1991;[START_REF] Huchon | Rotation of the syn-rift stress field of the northern Gulf of Aden margin, Yemen[END_REF] and on Socotra Island [START_REF] Fournier | Segmentation and along-strike asymmetry of the passive margin in Socotra, eastern Gulf of Aden: Are they controlled by detachment faults[END_REF]. Offshore, near the Ocean-Continent Transition (OCT), both the faults and basins mainly strike perpendicular to the Gulf opening (d'Acremont et al., 2005). The OCT most likely exhibits exhumed serpentinized mantle (d'Acremont et al., 2006;Leroy et al., 2010c;[START_REF] Leroy | From rifting to oceanic spreading in the Gulf of Aden: a synthesis[END_REF]. The Gulf of Aden display also a high segmentation with first-order and second-order segmentations [START_REF] Leroy | From rifting to spreading in the eastern Gulf of Aden: a geophysical survey of a young oceanic basin from margin to margin[END_REF]d'Acremont et al., 2005). Due to these structural data sets that were collected both onshore and offshore, the Gulf of Aden is an ideal area to study rifting with moderate obliquity.

Fault patterns of oblique rifts have been investigated during the last decades using analogue models on two different levels of complexity: (i) Crustal scale models simplify the rift system to a deforming crust influenced by a basal zone of extension that involves an oblique velocity discontinuity [START_REF] Withjack | Deformation produced by oblique rifting[END_REF][START_REF] Tron | Experiments on oblique rifting in brittle •] ductile systems[END_REF][START_REF] Mcclay | Analogue modelling of orthogonal and oblique rifting[END_REF][START_REF] Clifton | Influence of rift obliquity on fault •] population systematics: Results of experimental clay models[END_REF][START_REF] Mart | Analogue experiments of propagation of oblique rifts[END_REF][START_REF] Corti | Centrifuge models simulating magma emplacement during oblique rifting[END_REF][START_REF] Corti | Analogue modelling of continental extension: A review focused on the relations between the patterns of deformation and the presence of magma[END_REF][START_REF] Corti | Centrifuge modelling of the influence of crustal fabrics on the development of transfer zones: Insights into the mechanics of continental rifting architecture[END_REF][START_REF] Sokoutis | Modelling the extension of heterogeneous hot lithosphere[END_REF]. The advantage of this setup is that crustal strain patterns can be studied independently of mantle deformation, but this also limits the applicability to the first rift stage where isostatic balancing with the mantle and lithospheric necking can be neglected. Furthermore, the role of the basal discontinuity is overestimated intrinsically. (ii) Analogue experiments on lithospheric scale have been conducted recently and did successfully reproduce lithospheric thinning and its effect on crustal fault patterns [START_REF] Sokoutis | Modelling the extension of heterogeneous hot lithosphere[END_REF][START_REF] Agostini | Evolution, pattern and partitioning of deformation during oblique continental rifting: Inferences from lithospheric •] scale centrifuge models[END_REF], Autin et al. 2010).

However, thermal effects or rheological changes occurring during rifting are not modelled in these experiments and their absence remains a significant limitation of such analogue models. They also do not show the progression from oblique rift initiation to plate rupture.

Within the recent decade, numerical models became more and more important in understanding lithospheric deformation. In contrast to analogue models, state-of-the-art geodynamic codes are capable of computing realistic temperature dependent viscosity as well as complex elasto-visco-plastic rheologies. Despite these advantages, numerical models of oblique rifting intrinsically require computationally expensive calculations in three dimensions which is why few numerical models have been published so far: Van Wijk (2005) pioneered this topic by investigating rift evolution using a relatively coarse lithospheric-scale model. She showed that individual rift segments will cross an inherited weak zone that is oblique to the direction of extension. [START_REF] Allken | Three-dimensional numerical modeling of upper crustal extensional systems[END_REF][START_REF] Allken | Factors controlling the mode of rift interaction in brittle-ductile coupled systems: A 3D numerical study[END_REF] studied the influence of an offset between two rift segments on the structures of an extensional system. While using a high resolution of 1.3 km, they restricted their models to crustal scale. Brune et al. (2012a) showed by means of a simple analytical model that oblique rifting is energetically preferred over rift-perpendicular extension, which they corroborated by means of lithospheric-scale numerical experiments. This model has been extended in order to investigate the influence of plume-related lithosphere erosion on the dynamics of continental break-up (Brune et al. 2012b).

In this paper, we show that lithospheric-scale numerical experiments are capable to reproduce extensional structures from initial rifting to break-up. We thereby apply elastovisco-plastic rheology with laboratory-based flow laws for temperature/pressuredependent viscosity. We investigate the fault geometries during of oblique rifting based on strain-rate and plastic strain patterns. Moreover, we exploit the fact that numerical models provide direct access to the stress tensor at any numerical element, which allows to evaluate fault patterns on a sub-shear zone level. We explicitly compare our experiments to previous analogue modelling results and relate them to present structural knowledge of the Gulf of Aden.

Model description

We consider a rectangular Earth segment that measures 249 km times 249 km horizontally and 120 km vertically (Fig. 1a). We thereby use 275560 cubic elements with a length of 3 km. The modelled Earth segment consists of a 20 km thick upper crustal layer with a wet quartzite rheology [START_REF] Gleason | A flow law for dislocation creep of quartz aggregates determined with the molten-salt cell[END_REF], a lower crustal layer of 15 km thickness of granulite properties [START_REF] Wilks | Rheology of some continental lower crustal rocks[END_REF], and a 45 km thick layer of strong mantle material with dry olivine rheology [START_REF] Hirth | Rheology of the upper mantle and the mantle wedge: A view from the experimentalists, in Inside the Subduction Factory[END_REF]. We introduce a chemical asthenosphere by applying the flow law of wet (i.e. 500 ppm H/Si) olivine below 90 km depth [START_REF] Hirth | Rheology of the upper mantle and the mantle wedge: A view from the experimentalists, in Inside the Subduction Factory[END_REF]. All rheological parameters are listed in Table 1.

We impose a full extension velocity of 10 mm/yr through velocity boundary conditions at the model sides facing in x-direction so that they move symmetrically with 5 mm/yr. This velocity is equivalent to 10 km/My which results in a maximum extension of 200 km after 20 My model time. We refer to the angle between boundary velocity and the boundary itself as the angle of obliquity α. In this study, we use α=40° which represents the obliquity encountered in the Gulf of Aden. The front and back sides of the model are connected via periodic boundary conditions which effectively realises an infinitely long rift zone.

We apply the implicit, finite element code SLIM3D (Semi-Lagrangian Implicit Model for 3 Dimensions) to solve the thermo-mechanically coupled conservation equations of momentum, energy and mass in three dimensions. Detailed description of the numerical methods can be found in [START_REF] Popov | SLIM3D: A tool for three-dimensional thermo mechanical modeling of lithospheric deformation with elasto-visco-plastic rheology[END_REF] and Brune et al. (2012a).

In many cases oblique rifting arises because inherited lithospheric weak zones like sutures are reactivated with an oblique extensional component [START_REF] Ziegler | Dynamic processes controlling evolution of rifted basins[END_REF]. We introduce a weak zone by implementing a small linear temperature heterogeneity in the centre of the prospective rift (Fig. 1b). In doing so we anticipate a small amount of lithospheric necking that focuses the extensional deformation into the desired rift axis. This is one possible way of rift initialization. Alternative means are mechanical anisotropy [START_REF] Tommasi | Continental rifting parallel to ancient collisional belts: An effect of the mechanical anisotropy of the lithospheric mantle[END_REF], implementation of a weak plastic seed [START_REF] Huismans | Symmetric and asymmetric lithospheric extension: Relative effects of frictional-plastic and viscous strain softening[END_REF], or crustal thickening [START_REF] Van Wijk | Role of weak zone orientation in continental lithosphere extension[END_REF]. Note that after a small amount of extension, all of these techniques will result in lithospheric necking comparable to our initial condition.

Three weakening mechanisms are reproduced in the model. (i) Friction softening is introduced using a strain-dependent effective friction coefficient that decreases linearly from 0.6 to 0.06 for plastic strains between 0 and 1 while it remains constant at 0.06 for plastic strains larger than 1. (ii) Shear heating results in increased temperature proportional to stress times strain rate. (iii) Stress softening and strain rate softening are intrinsic to dislocation creep and reduce the local viscosity.

In most geodynamic codes (including SLIM3D), fault structures are represented by finite width shear bands that localize within a couple of elements. Here we utilize a technique that allows to use the principal stress components and their orientation in order to extend the geodynamic interpretation of our models (Brune et al., to be submitted): At each surface element we therefore evaluate the scalar Regime Stress Ratio (RSR) that indicates extension (RSR=0.5), strike-slip motion (RSR=1.5), and compression (RSR=2.5) on a continuous scale [START_REF] Simpson | Quantifying Anderson's fault types[END_REF]. Once the stress regime is computed for each surface element, we assume Andersonian faulting to infer the optimally oriented fault direction:

For extensional and compressive stress regimes in isotropic and homogeneous materials, faults emerge with azimuths orthogonal to σ 3 . Strike-slip faults occur at ±30° from σ 1 .

For reason of symmetry it is not possible to differentiate between sinistral and dextral strike-slip faulting based on stress tensor information only which is why we account for both conjugate fault populations. In the azimuth diagram, however, they are scaled with a factor of 0.5 so that the overall number of evaluated elements is not affected. Azimuth is measured as the clockwise angle from northward direction. This method delivers a preferred fault mechanism and orientation for any given stress tensor, even if no deformation takes place inside the element. Taking this into account, we restrict our stress analysis to a zone of tectonic activity where the strain rate is larger than 10 -15 s -1 .

The number of elements within the active region varies with time and is indicated in the azimuth diagrams of Fig 3c in the upper left corner (#Elements).

Model results

The model resolution of 3 km inhibits the formation of faults in a strict sense, since faults localize in nature on a much smaller scale. Instead, we observe finite-width shear zones with a typical width of few elements (Fig. 2a). The spontaneous formation of shear zones takes place within the first computational time steps. Fault spacing is directly related to the brittle-ductile transition depth in the upper crust (e.g. [START_REF] Vendeville | Physical models of extensional tectonics at various scales[END_REF].

Due to strain softening, shear zones become weaker with accumulated deformation. Thus, the individual small-scale shear zones compete and their number reduces with time.

The largest amount of deformation is taken up by shear zone parts atop the lithospheric necking peak. During the first 6 My large conjugate normal faults develop within the shear zones that cut through the whole crust so that surface spacing is controlled by the Moho depth (Fig 2e). Since the rift process decreases crustal thickness, the distance of the conjugate faults at the surface is reduced with time until it vanishes and break -up takes place at 14 My. This model does not account for petrophysical formation of oceanic crust so that continental break-up is complete when the crust is broken and asthenospheric material reaches the surface.

Three fault azimuths will play a fundamental role during the discussion of the model, i.e. rift-parallel (75°), displacement-orthogonal (115°), and intermediate (95°). These directions result from a 25°-oriented direction of extension so that the global orientation of the least principal stress σ 3 is 5°. The evolution of the numerical model can be divided in three main phases. Note, however, that the transitions between phases are not abrupt but take place over one or two Note that the model capabilities are limited in several aspects. Most importantly, magma migration and dike formation that tend to decrease lithospheric strength perpendicular to the direction of extension are not accounted for. Moreover, the limits of computational power restrict our model resolution to 3 km which is still far from resolving individual faults. Nevertheless, the presented model is one of the first that reproduces lithosphericscale rift evolution from initial deformation until break-up.

Numerical model vs. Gulf of Aden analogue model

Before we compare the numerical model to the Gulf of Aden, we will explore differences and similarities to the previously conducted lithospheric-scale analogue model of Autin et al. (2010). Both experiments feature an obliquity of 40°. As the numerical model uses a prescribed weakness in the lithosphere, we compare it to an analogue model which also contains a pre-existing lithospheric weakness (Autin et al., 2010, model B).

The analogue model is constructed in order to reproduce oblique rifting by the way of shifted lateral discontinuities (see Autin et al., 2010 for details). Moreover, an oblique weakness trends parallel to the direction of obliquity imposed by the lateral velocity discontinuities, and joins them. It displays a four-layer type lithosphere strength profile (Fig. 1c) modelled using granular materials and silicone. This modelled lithosphere overlies a low viscosity, higher density glucose syrup that mimics the asthenosphere.

Lateral dimensions of the setup (56 cm times 30 cm in the laboratory) scale to 750 km times 400 km in reality. Another fundamental similarity between both models is the fault repartition in the rift.

In both the numerical and the analogue models, rift-parallel faults are always located along the rift borders. Rift borders is where the overall oblique thinning of the lithosphere creates the strongest thickness variations, inducing density variations which are thought to enhance the buoyancy forces, perpendicular to the oblique rift (Bellahsen et al., this volume). On the other hand, displacement-normal faults are always created in the rift centre, particularly in later stages of deformation, when no more stresses perpendicular to the rift occur in the rift centre.

Numerical model vs. Gulf of Aden natural rift

Final deformation pattern

The overall deformation pattern of the Gulf shows an en-echelon disposition of the synrift faults and grabens, on both side of the oblique OCT (Fig. 5a). When the Gulf is closed to the OCT (Fig 5b), the tertiary main depocentres show en-echelon sigmoid grabens. This pattern is similar to the plastic strain pattern observed in the numerical model (Fig. 2b). The modelled basin topography is also comparable with en-echelon sigmoid basins progressively linked together and finally separated when the final localization occurs at 10-11 My (Fig 2d). Nevertheless, this first order deformation occurs at larger scale (ca. 100 km) than in the numerical model (40-50 km) and is partly controlled by the mesozoic inheritance [START_REF] Ellis | A tectono-stratigraphic framework for Yemen and its implications for hydrocarbon potential[END_REF][START_REF] Granath | The Nogal Rift of northern Somalia: Gulf of Aden. Reactivation of a Mesozoic Rift[END_REF][START_REF] Leroy | From rifting to oceanic spreading in the Gulf of Aden: a synthesis[END_REF]Autin et al., this volume). This wavelength difference is certainly due to the initial pattern of the inherited, widely spaced, mesozoic basins, which have focused the deformation during their reactivation, preventing the appearance of a more distributed pattern as in the numerical model. Although, the main tertiary depocentres focus in these inherited basins, deformation is also observed outside of them as for example in the Ashawq graben (Fig 5c). Thus, it appears that the distributed deformation pattern of the model can be recognized in the Gulf of Aden but is locally controlled and enhanced by the inheritance overprinting the general pattern.

As proposed above, the sigmoid shape in the model can be due to successive rotations during ongoing extension of long-lived shear zones (Fig. 4). In the Gulf of Aden, the inherited mesozoic basins have a sigmoid shape and thus could have experienced such rotations. Novel analogue models that reproduce this inheritance during oblique rifting show indeed that the inherited structure ends with a sigmoid shape (Autin et al., this volume).

Chronology and localisation of the fault populations

Another point of comparison is the distribution of the fault populations in the rift and their chronology. As describe in Bellahsen et al. (this volume) The internal parts are mainly composed of intermediate (rather in the OCT) and extension-orthogonal faults (ridge trend). If we consider that the deformation localizes progressively towards the rift centre, then the most proximal structures are older than the distal ones. This evolution is in accordance with the 3 step evolution of the numerical model. Indeed, figure 3b and3c show that (i) the intermediate faults are created first and will then be located in the external parts of the rifts; (ii) rift-parallel faults form later and are localised where the thinning is the strongest (and thus the slope the steepest), i.e. at 100 km and 150 km along the x-axis at 6 My or 110 km and 140 km at 10 My (Fig. 2e);

(iii) intermediate and extension-orthogonal appear in the rift centre during the final evolution of the model.

Processes at work

We propose that the numerical model of oblique rifting captures the main evolution of the deformation through the Gulf of Aden and in particular in the Western part, where a localising effect of the high thermal regime could be assimilated to the initial weakness introduced in the models. In this frame, the deformation processes occurring during oblique rifting proposed in previous works are partly supported by our model.

For the Phase 1 (1 to 5 My), the stress-inferred azimuth diagrams show that the normal faults have first an "intermediate" direction which results from the combination of the far field stresses and the local stresses induced by the weakness zone as proposed by [START_REF] Withjack | Deformation produced by oblique rifting[END_REF].

During Phase 2 (6 to 13 My), the progressive development of rift-parallel faults seemingly indicates that deformation localises along the oblique trend. This localisation could be linked to enhanced buoyancy forces that induce a rift-orthogonal extension (Bellahsen et al., this volume). Buoyancy forces arise from density variations in the lithosphere (Artyushkov, 1973;Fleitout and Froidevaux, 1982) and thus are perpendicular to the major lithosphere thinning. They are thought to be driving forces for rift localization even in orthogonal settings (Huismans et al., 2001;Davis and Kusznir, 2002;Burov, 2007). This hypothesis is supported by the localisation of rift-parallel faults in the rift borders above the maximal thinning area (Fig. 2e) and the emergence of rift shoulders (Fig 2c). Moreover, this deformation is correlated with a strong localisation of the deformation, which would confirm the localising effect of buoyancy forces, that are thought to allow the distal margin formation.

Deformation during the third step (from 14 My on) is localised in the rift centre. The intermediate and extension-orthogonal faults indicate that the far-field extension dominates. The rift centre is far enough from the thinning zones (rift borders) so that the newly formed faults develop mainly in response to the far field stresses, as proposed in Autin et al. (2010). Nevertheless, the presence of intermediate faults suggests that a local stress field is still active. The reason is probably that plate cooling takes place para llel to the former rift zone. The ongoing evolution of the model shows that intermediate fault proportion tends to decrease with time compared to the extension-orthogonal faults, suggesting that the far-field is more and more dominant.

The numerical model also coincides with a general conceptual model of passive margins. Indeed, the strain rate shows clearly the transition from the distributed deformation (stretching mode) to a more localizing rift pattern from 6 to 9 My (thinning mode) and the final narrow oblique localization at 10 My and progressive exhumation of the deep layers before the lithospheric break-up at 14 My (exhumation mode), correlating with the general margin formation modes described by [START_REF] Lavier | A mechanism to thin the continental lithosphere at magma-poor margins[END_REF] and [START_REF] Péron-Pinvidic | The final rifting evolution at deep magma-poor passive margins from Iberia-Newfoundland: a new point of view[END_REF]. Exhumation of lower crust and mantle material under a poor magmatic regime suggests the formation of magma-poor rifted margins in the numerical model. Although, geophysical data allow to propose the presence of exhumed serpentinised mantle in the Eastern Gulf of Aden (d'Acremont et al. 2006;Leroy et al. 2010a and b), such dataset is not available in the Western Gulf of Aden.

Nevertheless, in the eastern part of the Western Gulf, margins are thought to be magmapoor as no magmatic structures as seaward dipping reflectors were recognised (e.g. [START_REF] Bosworth | The Red Sea and Gulf of Aden Basins[END_REF]. If so, our numerical model would suggest that such exhumation will take place when the deformation pattern is dominated by a combination of numerous intermediate faults and only few extension-orthogonal faults, i.e. at around 13 My.

Conclusion

We identify a characteristic evolution of fault patterns: At first, faults develop with orientations that are intermediate between the rift-direction and the displacement-normal. [START_REF] Gleason | A flow law for dislocation creep of quartz aggregates determined with the molten-salt cell[END_REF], lower crust: Pikwitonian granulite [START_REF] Wilks | Rheology of some continental lower crustal rocks[END_REF], lithospheric mantle: dry olivine [START_REF] Hirth | Rheology of the upper mantle and the mantle wedge: A view from the experimentalists, in Inside the Subduction Factory[END_REF], asthenospheric mantle: wet olivine, i.e. 500 ppm H/Si [START_REF] Hirth | Rheology of the upper mantle and the mantle wedge: A view from the experimentalists, in Inside the Subduction Factory[END_REF]. Peierls creep parameters for mantle: (Kameyama et al. 1999). *the friction coefficient decreases linearly by 90 % of the initial value when plastic strain reaches 1, and remains constant for larger strains.

  My. Figures showing the evolutionin steps of 1 My can be found in the supplementary materials. Phase 1. (1-5 My): At 1 My, the strain rate pattern of Fig 2a shows small-scale shear zones that strike at an angle of 95° azimuth that is intermediate between the extension -orthogonal direction and the rift orientation. Within few million years, they develop into an en-echelon system with a wavelength of several tens of kilometres. The stress-inferred fault mechanism (Fig. 3a), is of normal type everywhere and shows intermediate fault orientation (Fig 3b,c). Phase 2. (6-13): Deformation of the en-echelon shear zones strongly localises towards the lithospheric necking region. (Fig 2a). The normal fault azimuth map (Fig 3b) shows rift-parallel and intermediate normal faulting at the rift flanks. In the rift centre, however, extension-orthogonal faults occur together with strike-slip faults that delimit individual shear zones (Fig 3a). The azimuth diagram (Fig. 3c) shows a shift from intermediate to rift-parallel directions (the distribution starts to be asymmetric). At 10 My, rift-parallel faults are dominant at the rift borders (Fig. 3b and 3c) and a strong localization of the deformation occurs again. Phase 3. (14 My and after): Incipient break-up links up the individual shear zones. Only intermediate and extension-orthogonal faults develop during break-up of the lithosphere. The final strain distribution (Fig 2b at 14 My) shows sigmoid deformation patterns. The sigmoidal shape can be explained by successive rift localisation and the longevity of individual shear zones: After formation of the initial en-echelon pattern, the central portion of each shear zone gets stretched parallel in direction of extension which appears as a counter-clockwise rotation (Fig 4). Since deformation localizes towards the rift centre, the area where rotation occurs narrows with time. Thus, shear zones of the proximal margin experience less rotation while distal margin shear zones are deformed until they are nearly parallel to the direction of extension.

  Autin et al (2010), as well, observed three main fault populations: rift-parallel (75°), intermediate faults (95°), and displacement-normal (115°). The main results ofAutin et al. (2010) was the recognition of 3 main steps of development: (i) The fault populations, especially during the early stages of deformation, are composed of faults that in strike, are largely intermediate between rift-parallel and perpendicular to displacement. This fault population is characteristic of oblique rifts as observed in previous studies . (ii) In later stages, faults parallel to the rift become numerous.Autin et al. (2010) propose that buoyancy forces related to thickness variations in the lithosphere during rift localization play a significant role and control the initiation of rift-parallel faults (see also Bellahsen et al., this volume). (iii) During the final stages of extension, the small-scale deformation pattern is composed of displacement-normal faults in the deepest parts of the rift. The three stages are very similar to the ones observed in the numerical models. Nevertheless, several differences exist: The numerical model indicates that during the beginning of Phase 2, rift-parallel and extension-orthogonal faults develop simultaneously. Contrarily to analogue models, Phase 3 of the numerical model displays extension-orthogonal as well as intermediate faults (Fig. 3c). In the numerical model, this stage is controlled by the ascent of the hot asthenosphere and subsequent plate cooling, which cannot be reproduced in analogue models. These processes induce a strong localization of the deformation, where oblique weakening combined with the far -field stress could lead to the intermediate fault development. The azimuth diagram (Fig 3c) shows clearly extension-orthogonal faults at 14 My, when the breakup occurs. It is slightly different from the analogue model, which suggests that they appear at earlier stages, during hyper-extended domain formation. It is noteworthy that clockwise rotations of the structural pattern start at 7 My in the numerical model. Rotations are observed at the boundaries of the analogue models, which are either clockwise or counter-clockwise depending on the rift border where it occurs. They are thought to be responsible of the initiation of second order transfer zones in oblique rifts.

  , the Western Gulf of Aden displays a general fault organisation: (i) The external parts of the rift show intermediate and extension-orthogonal faults; (ii) The steep slopes focus rift-parallel faults and; (iii)

  Then rift-parallel normal faults occur at the rift flanks simultaneously with strike-slip faults and extension-orthogonal faults in the central part of the rift system. Finally displacement-orthogonal as well as intermediate orientation dominate during break-up. This evolution is in accordance with the emergence of the fault pattern in lithospheric analogue models and allows to propose further interpretation of the distal margin evolution of the Gulf of Aden. The comparison with the natural oblique rift confirms that this evolution is highly probable. The final pattern of the deformation, the distribution of the fault populations in the rift and their probable chronology are all compatible with the deformation steps. As already proposed in other studies, we correlate these steps to the following deformation processes: (i) Interaction of the far-field stress and the local stresses induced by the weakness zone, which form intermediate faults. (ii) Buoyancy forces induced stress field that strongly localises the deformation and creates rift-parallel faults. (iii) A progressive return to far-field stress conditions as the thinning of the rift centre becomes important, creating intermediate and displacement-orthogonal faults. Our model suggests that the exhumation of lower crustal and mantle material could take place when intermediate faults are most likely to dominate the deformation pattern.

Figure captions Figure 1

 1 Figure captions

Figure 2

 2 Figure 2 Model evolution at 1 My, 6 My, 10 My, and 14 My (i.e. 10 km, 60 km, 100 km, and 140 km extension, respectively). (a) Initially, shear zones are parallel to the expected intermediate azimuth of 95°. Localisation occurs towards the rift centre. (b) The central shear zone part rotates successively to form sigmoidal deformation patterns. (c) Topography shows rift shoulder uplift due to hot asthenospheric upwelling at 6 My followed by subsidence due to lithospheric cooling. (d) Basin geometries is strongly affected by shear zones. (e) Mid-model cross section shows successive localisation towards the rift centre. Black lines indicate boundaries between material layers. Note that figures showing the whole evolution in steps of 1 My can be found in the supplementary materials.

Figure 3

 3 Figure 3 Stress-inferred fault evolution at 1 My, 6 My, 10 My, and 14 My. (a) Normal faulting is the dominant mechanism except for a temporary strike-slip region in the rift center. (b,c) Normal fault orientations are intermediate at the beginning, rotate towards rift -parallel at the rift flanks until 10 My where after they show intermediate and extension-normal orientation at break-up. Note that figures showing the whole evolution in steps of 1 My can be found in the supplementary materials.

Figure 4

 4 Figure 4 Illustration how long-lived shear zones generate sigmoidal strain patterns (Compare to Fig 2b). Note that the azimuth of final shear zone pattern depends on the distance from the continent-ocean boundary.

Figure 5 (

 5 Figure 5 (a) Structural map of the Gulf of Aden with the main tertiary depocentres as well as the

  Figure 1

Table caption Table 1 .

 caption1 Model parameters. Dislocation creep parameters for upper crust: wet quartzite

Table 1 Click here to download Table: Table1.pdf Outside Rift Inside Rift Upper crust: Wet Quartz Lower crust: Mafic Granulite Lithospheric mantle: Dry Olivine

 1

Acknowledgments

Sascha Brune is funded by SAMPLE (South Atlantic Margin Processes and Links with onshore Evolution), Priority Program 1375 of the German Research Foundation (DFG).


Markus J Loegering

Z Anka

Julia Autin

Rolando Di Primio

Denis Marchal

Jorge F Rodriguez

D Franke

Eduardo Vallejo

R Di Primio

Tectonic evolution of the Colorado Basin, offshore Argentina, inferred from seismo-stratigraphy and depositional rates analysis

Keywords: Colorado Basin, Passive margin, Offshore Argentina, Depocenter migration, Sedimentation rates, South Atlantic

come   L'archive ouverte pluridisciplinaire

Introduction

The Argentine Colorado Basin is one of the major sedimentary basins in the South Atlantic. It is located on the eastern South-American passive continental margin (Fig. 1), where a complex set of sedimentary basins exist in the coastal and inner shelf area.

Passive continental margins are dynamic environments characterised by shallow shelf areas containing thick sedimentary packages derived from onshore erosion. They are therefore an ideal recorder of sedimentary depositional evolution and of the geological development in general. Although passive margin settings are considered to be the result of the transition from a rift phase to drift phase, most passive margins have undergone a complex deformation history. Such deformations are described as a result of salt mobility, margin instability, post breakup flank uplift, and localized compressional deformation (e.g. Brown et al., 2004;Fort et al., 2004;Hudec andJackson, 2002, 2004;Rowan et al., 2004). Despite the widespread occurrence of strata distribution in most passive margins, few examples exist where there is a good preservation of the entire depositional sequence and its geometry.

The Colorado basin records the deposition of the entire syn-rift and post-rift successions, with a clear tectono-stratigraphic history owing to the apparent absence of salt deposition as well as minimal uplift and erosion. Therefore it is ideally suited to reconstruct the depositional geometry evolution throughout the entire development of a passive margin.

Geological and tectonic setting

The Argentine passive margin is mainly of the volcanic rifted type resulting from the breakup of Gondwana and the northward-propagating opening of the South Atlantic [START_REF] Gladczenko | South Atlantic volcanic margins[END_REF][START_REF] Hinz | The Argentine continental margin north of 48°S: sedimentary successions, volcanic activity during break-up[END_REF][START_REF] Rabinowitz | The Mesozoic South Atlantic Ocean and evolution of its continental margins[END_REF][START_REF] Sibuet | Early evolution of the South Atlantic ocean: Role of the rifting episode[END_REF][START_REF] Uchupi | The tectonic style of the Atlantic Mesozoic rift system[END_REF][START_REF] Uliana | Permian to Late Cenozoic evolution of Patagonia, main tectonic events, magmatic activity, and depositionaltrends[END_REF]Uliana et al., 1989;[START_REF] Franke | Margin segmentation and volcano-tectonic architecture along the volcanic margin off Argentina/Uruguay,South Atlantic[END_REF][START_REF] Ghiglione | Structure and tectonic history of the foreland basins of southernmost South America[END_REF]. The Colorado Basin records the development of the Lower Cretaceous to present-day volcanic-rifted passive margin of Argentina. After a Permian pre-rift period, the formation of the Colorado Basin is related to the break-up of Gondwana, starting with the rift systems from Triassic/Jurassic (?) to Early Cretaceous, followed by a drift phase and thermal subsidence period through the Cretaceous and ultimately reaching the present-day passive margin configuration of the basin.

The number of extensional phases affecting the continental shelf before the final Late Jurassic -Early Cretaceous phase of the Atlantic extension is still under discussion. [START_REF] Keeley | Basin evolution and prospectivity of the Argentine continental margin[END_REF] suggested two phases of active rifting in the Late Triassic-Early Jurassic and in the Mid-Jurassic. The continental extension may have begun in isolated centres in South America during the Late Triassic/Early Jurassic and almost all of south and west Gondwana was affected by magmatism resulting in a high heat flow [START_REF] Macdonald | Mesozoic break-up of SW Gondwana: implications for regional hydrocarbon potential of the southern South Atlantic[END_REF]. Early Jurassic intraplate extension reactivated pre-existing depocenters and originated new ones such as the San Jorge Basin (Fitzgerald et al., 1990). This early extensional phase may be reflected by the lower syn-rift succession in the North Malvinas Graben [START_REF] Richards | Geology of the North Falkland Basin[END_REF]. The major Late Jurassic -Early Cretaceous crustal stretching episode is found throughout the sedimentary basins of the Argentine continental shelf [START_REF] Keeley | Basin evolution and prospectivity of the Argentine continental margin[END_REF].

One of the early rift phases (either Late Triassic-Early Jurassic, Middle Jurassic or Late Jurassic-Early Cretaceous) was likely oblique and resulted in basins oriented at high angle with respect to the trend of the continent-ocean boundary [START_REF] Franke | Crustal structure across the Colorado Basin, offshore Argentina[END_REF][START_REF] Macdonald | Mesozoic break-up of SW Gondwana: implications for regional hydrocarbon potential of the southern South Atlantic[END_REF]. The final opening took place in the Early Cretaceous with proposed ages ranging from 127.7 to 135.5 Ma [START_REF] Lawver | A tight fit -Early Mesozoic Gondwana, a plate reconstruction perspective[END_REF]Nürnberg and Müller, 1991;[START_REF] Rabinowitz | The Mesozoic South Atlantic Ocean and evolution of its continental margins[END_REF]. Therefore, sediments within the basin represent the entire sedimentary record from the Late Jurassic rifting of Gondwana (and possibly pre-rift sediments) through the Cretaceous rift and drift episodes until the present day (Fig. 2).

The basement is made up of Precambrian igneous and metamorphic rocks. A blanket of glacial poorly or non-sorted conglomerate or breccia deposited in Late Carboniferous-Early Permian time over most of Gondwana [START_REF] Macdonald | Mesozoic break-up of SW Gondwana: implications for regional hydrocarbon potential of the southern South Atlantic[END_REF] is preserved in the area of the Colorado Basin [START_REF] Keeley | Basin evolution and prospectivity of the Argentine continental margin[END_REF]. This was covered by dark, organic-rich shales deposited in a major fresh or brackish transgression at the end of the glaciation before a more arid continental environment established in the Triassic [START_REF] Macdonald | Mesozoic break-up of SW Gondwana: implications for regional hydrocarbon potential of the southern South Atlantic[END_REF]. The Permo-Carboniferous sedimentary rocks are exposed as a succession of diamictites, sandstones and dark shales in the La Ventana Hills (Sierras Australes) (Fig. 1b) [START_REF] Lopez Gamundi | Basin fill evolution and paleotectonic patterns along the Samfrau geosyncline: the Sauce Grande basin-Ventana foldbelt (Argentina) and Karoo basin-Cape foldbelt (South Africa) revisited[END_REF]. The succession shows consistent affinity to the Great Karoo Basin of Southern Africa [START_REF] Keidel | La geologia de las sierras de la provincia de Buenos Aires y sus relaciones con las montañas de Sudàfrica y los Andes[END_REF][START_REF] Keeley | Basin evolution and prospectivity of the Argentine continental margin[END_REF]Lopez-Gamundi and Rosello, 1997).

The syn-rift stratigraphy of the Colorado Basin is speculative as no wells penetrate these sequences. However, sandstones of Early Cretaceous age were drilled in smaller flanking fault blocks that may be equivalents of the depositional infill of the east-west to NW-SE trending grabens and half grabens (Figs. 2 & 3) [START_REF] Bushnell | Petroleum system analysis of the eastern Colorado Basin, offshore northern Argentina[END_REF]. Strong and continuous reflectors may additionally indicate lacustrine syn-rift shales of (?)Jurassic and Early Cretaceous age (Fig. 2). In the Cruz del Sur well a mix of sandstones with andesites and tuffs was drilled in the syn-rift interval [START_REF] Bushnell | Petroleum system analysis of the eastern Colorado Basin, offshore northern Argentina[END_REF]. The Cretaceous Colorado and Fortin Fms. (Albian to Campanian) are dominated by coarse continental to shallow marine sandstones and conglomerates [START_REF] Bushnell | Petroleum system analysis of the eastern Colorado Basin, offshore northern Argentina[END_REF] but as their lithological content is highly variable the distinction between these two formations is difficult [START_REF] Fryklund | Cuenca del Colorado[END_REF]; thus they are jointly referred to the Colorado Fm. in this study. The successions above the Cretaceous sandstones were deposited in a more marine environment and consist mainly of bathyal siltstones and claystones of Upper Maastrichtian to Miocene age [START_REF] Bushnell | Petroleum system analysis of the eastern Colorado Basin, offshore northern Argentina[END_REF]. Palaeocene basalts were encountered within the Pedro Luro Fm. [START_REF] Lesta | Actualization de la information estratigrafica en la Cuenca del Colorado[END_REF] at the southern centre part of the basin, evidenced by drilled wells. The Tertiary Facies distribution shows a transition from fluvial-deltaic and shallow marine environment to a deeper marine environment towards the shelf edge [START_REF] Fryklund | Cuenca del Colorado[END_REF][START_REF] Bushnell | Petroleum system analysis of the eastern Colorado Basin, offshore northern Argentina[END_REF].

Data set and methodology

The study area covers approximately 95.000 km 2 . Using a dense 2D seismic reflection dataset (about 30.000 km) and data from 8 exploration wells (Fig. 1b) provided by Petrobras Argentina and the Federal Institute of Georesources Germany (BGR), we reconstructed the geological evolution of the Colorado Basin. The 2D seismic lines consisted of processed standard industrial reflection seismic lines (post-stack time-migrated). The grid spacing varies from 2.5 to 35 km and the seismic signal reached down to 12.000 ms in two-way-travel time (twt). In addition to the seismic data, some stratigraphic well tops and lithological/biostratigraphic data, as well as checkshots were also provided by Petrobras.

Stacking velocities used in time depth conversion were provided by the BGR. Interpretation and time-depth conversion were performed with Petrel TM Interpretation Software (Schlumberger) version 2009.

Seismic-to-well log correlations and integration with biostratigraphic data were carried out in order to provide a chrono-stratigraphic framework for the interpreted surfaces, as well as a detailed seismo-stratigraphic interpretation and tectono-stratigraphic analysis of the basin.

Following seismic interpretation of the main reflectors in the study area, a velocity model to convert twt interpreted horizons into depth was defined. The time-depth relation from checkshot data of the individual wells (Fig. 4a) and the stacking velocities from the BGRlines were used to calculate the interval velocities for the defined seismic units (Fig. 4b).

Stacking velocities were determined from analysis of normal move-out measurements from common depth point gather of seismic data. The interval velocity, typically P-wave velocity, of a specific layer of rock was calculated from the change in stacking velocity between seismic events on a common midpoint gather. Using the interval velocity from 6 wells and from the Dix-conversion of stacking velocity of 3 seismic lines, a final velocity model was elaborated, allowing the conversion from the interpreted twt seismic horizons into truevertical depth (tvd). By validating the well tops to the depth-converted seismic dataset a uncertainty of ±5% can be estimated. Finally, depth and thickness maps were created from the interpreted sections. Using these maps the volume of each seismic unit was derived and sediment accumulation as well as sedimentation rates were calculated. In addition to the seismic horizons, the major bounding fault systems were mapped on the 2D seismic sections.

Results

Seismo-stratigraphic units

As a result of seismic-to-well log correlations and the integration of biostratigraphic well tops the major syn-and post-rift seismic sequences and their bounding surfaces, such as unconformities and regional seismic markers, were identified and mapped. Furthermore as a result of seismo-stratigraphic interpretation and tectono-stratigraphic analysis the top pre-rift and the break-up unconformity were identified and mapped throughout the basin. The main seismic units identified in the Colorado Basin are shown in Figures 2 and3. Overlying the pre-rift sequences the Cretaceous succession was subdivided into four seismic units. The synrift sediments (?Jurassic/Early Cretaceous) beneath the break-up unconformity were deposited in graben and half-graben systems within the main depocenters of the basin. The post-rift sediments starting with the Sag II and Sag I units (Barremian to Aptian), as well as the Colorado Fm. (Albian to Maastrichtian) were deposited during the basin sag-phase and thicken above the depocenters of the basin. Four Tertiary units were distinguished: the Pedro Luro Fm. (Paleocene), the Elvira Fm. (Eocene), the Caotico informal unit (Oligocene), and the Baranca Final Fm. (Miocene). These sequences are present on the continental shelf and slope region with water depths ranging from 50 to 2250 m (Fig. 1b).

Main structures

Several structural elements have been analysed by mapping the major bounding faults that can be traced through the Colorado Basin. These faults include:

(1) Rift-related faults resulting in graben and half-graben systems beneath the break-up unconformity. These extensional faults are rooted in the deep pre-rift successions and generally terminate within the syn-rift successions, although some extent into the Lower Cretaceous (Fig. 3 &5). The normal faults control the geometry of the main syn-rift depocenters (Fig. 5)

(2) Seismic data shows that the basin was subject to an extensional episode post-dating the formation of the South Atlantic, reactivating the rift-related normal faults during the Cretaceous. These reactivated structures were later capped by Tertiary sediments (i.e. Baranca Final Fm.) (Fig. 6). The reactivated structural elements are commonly found in conjunction with and above the rift faults and do not show any evidence of structural inversion or compression.

(3) The Upper Cretaceous and Lower Tertiary sequences are dissected by a second generation of post-rift normal faults affecting the depositional units of the Colorado and Pedro Luro Fms.

(Fig. 7). A semi vertically-faulted interval with dense fault spacing has been identified within these formations in the distal slope area of the basin. These faults are likely to have been generated by slope gravitational tectonics.. We built up four regional profiles accross the Colorado Basin (Fig. 8). They depict clearly present-day basin structure and the main seismo-stratigraphic units identified, including the locations of main depocenters and the relationship between the rift faults and the syn-rift and post-rift units. The NE-SW Profile 1 at the slope area of the basin shows inferred pockmarks, submarine channels, and thick, up to 7800 m accumulations of syn-rift and post-rift sediments up to the top of the Colorado Fm. The main depocenter is dissected by the rift-controlled faults and the Cretaceous extensional-reactivation of these faults is illustrated above the smaller depocenter located in the south-west area. Profile 2 shows a cross-section in N-S direction across the central part of the basin, depicting the onlaps of the Cretaceous and Tertiary successions onto the pre-rift structures. In this profile rift-controlled faults show no signs of reactivation, but result in a high amount of syn-rift grabens and half-grabens, which also controlled the location of the overlying sag-phase depocenters. Profile 3, across the eastern part of the basin, shows a series of grabens and half-grabens, controlled by rift-related faults on both sides. The grabens are filled with thick successions of syn-rift and post-rift sediments and to the south-west the rift faults have been reactivated. Profile 4 displays the entire basin in E-W direction, including the slope, the Continental-Oceanic Boundary (COB), and the oceanic domain. The slope area is characterised by relatively high sedimentation rates and the development of basinward prograding sedimentary wedges. The COB is confirmed by seaward-dipping-reflectors (SDR's), also described by [START_REF] Hinz | The Argentine continental margin north of 48°S: sedimentary successions, volcanic activity during break-up[END_REF] and [START_REF] Franke | Margin segmentation and volcano-tectonic architecture along the volcanic margin off Argentina/Uruguay,South Atlantic[END_REF]. The profiles show thick successions of syn-rift sediments and the increasing thickness of the section between the break-up unconformity and the Colorado unconformity towards the centre. The main depocenters are illustrated by the development of syn-rift sediments and the deposition of the basin's sag phases, where the extensional/rift-related faults occur at the flanks of these depocenters. All profiles show relatively thin sedimentary layers of the Tertiary Pedro Luro, Elvira and Caotico units, however the Upper Tertiary Baranca Final Fm. is a thick sedimentary package with a continuous high sediment supply until present day.

Surface-and isopach maps

The construction of isochronal (twt) maps provided a 3D spatial visualisation of the stratigraphic relationship and the structures of the identified sequences. The depth-converted surface (Fig. 9-i), and isopach (Fig. 10a-h) maps display the evolution of the sedimentary basin infill through time. We observe a clear change in configuration from the break-up unconformity (127.7 to 135.5 Ma) to present-day. In its present-day configuration, the basin is shallower (infill of up to 2000 m) in its flanks, and deepens towards the centre (16,000 m of sediment infill / Fig. 9a). The syn-rift depocenters are comparatively limited in size and contain up to 5500 m of sediments (Fig. 10a). Within the syn-rift sediments, the presence Jurassic and Early Cretaceous organic-rich source rock intervals have been suggested (Fig. 2). The basin sag phase is represented by the sequences deposited between the break-up unconformity (Barremian/Fig. 9b) and the Colorado unconformity (Campanian/Fig. 9e).

These Cretaceous successions (Sag II, Sag I, and Colorado Fm.) contain a maximum of 6500 m sediments (Fig. 10b, 10c, and10e) and onlap on the pre-rift structures in the southern and northern part of the basin (Fig. 9d and9f). The depth maps of the Sag I and the Colorado Fm.

(Figs. 9d and9e) show that the basin deepens and sediments prograde towards the east. The deposition of the seismic unit Sag I coincides with the possible occurrence of potential Aptian source rocks in the basin (Fig. 2). The continued subsidence and resulting accommodation space has been filled by Tertiary units (Pedro Luro, Elvira, Caotico, and Baranca), with up to 4000 m of sediments deposited until the end of the Miocene (Fig. 10e, 10f, 10g, and10h). The Pedro Luro Fm. marks the Cretaceous/Tertiary boundary in the sedimentary succession (Fig. 9f). The Pedro Luro, Elvira and Caotico units onlap onto the pre-rift structures towards the north and south (Fig. 10e and10f, and10g). The Baranca Final Fm. (Miocene) and the Belen Fm. (Pleistocene) concludes the history of basin filling (Fig. 10h).

Depocenter migration and stratigraphic evolution

As mentioned, the pre-rift successions are dissected by numerous extensional faults, resulting in the formation of graben and half-graben systems in the basin, controlling the location of the syn-rift depocenters (Fig. 5). The pre-rift deposits appear as mainly parallel seismic reflectors, both at the base of the graben structures and at the horsts (Figure 11). It is suggested from well data, that this succession represents Late Carboniferous-Early Permian conglomerate or breccia and finer lithologies [START_REF] Macdonald | Mesozoic break-up of SW Gondwana: implications for regional hydrocarbon potential of the southern South Atlantic[END_REF][START_REF] Keeley | Basin evolution and prospectivity of the Argentine continental margin[END_REF]. The pre-rift succession displays a central EW-elongated graben which influences thicknesses in the overlying sequences up to the Miocene. To the East, the E-W Colorado Basin turns NW-SE, becoming perpendicular to the present-day continental margin (oriented NE-SW).

Three major syn-rift depocenters were identified in the basin with a maximum thickness of 5500 m within one of the main depocenter (Fig. 10a). Beneath the break-up unconformity, (Fig. 7) at the slope of the basin, resulting from gravitational tectonics and slumping episodes.

In summary, from the Aptian to Maastrichtian, shelf progradation dominated whilst during the Paleocene to Oligocene, aggradation prevailed. During the Eocene sediment deposition decreased, related to limited available accommodation space (Table 1). From the Oligocene to the Miocene sediment deposition increased due to higher available accommodation space and relatively high sedimentation rates continued to the present day with the development of basinward prograding sedimentary wedges.

Sedimentation rates

, The sedimentation rates for each unit were derived by calculating volumetric accumulation rates over time and averaging these over the area of deposition (Table 1). The sedimentation rates are independent measures of sedimentation in meters per million years (Ma). Assuming an accumulation time span of 25 My for the syn-rift successions, sedimentation rates of 118 m/My are estimated (Table 1, Fig. 13). The accumulations in this period most likely represent a dominantly continental environment with siliciclastic lacustrine sediment infill and volcanics. By Late Cretaceous post-rift thermal subsidence dominated the basin development.

During the early phase of subsidence, high sedimentation rates of up to 122 m/My occurred in the basins sag-phase, which was mainly restricted to the structural geometry of the rift basin.

During the deposition of the Colorado Fm. (Albian to Maastrichtian), with coarse continental to shallow marine sandstones and conglomerates, sedimentation rates decrease dramatically to 30 m/My. During this period (44 Ma) the bulk of the sediment were deposited by progradation to the east of the basin, whilst throughout the Late Cretaceous sedimentation took place in elongated depocenters with oceanward progradation. A second phase was related to thermal subsidence along the trend of the South Atlantic margin throughout the Tertiary (Fig. 13). During this time, the basin developed under truly marine conditions and sedimentation rates are relatively high (47 m/Ma) in the Paleocene, when the section becomes marine for the first time. The sediments consist mainly of bathyal siltstones and claystones.

This major Paleocene marine transgression was followed by a period of uplift of South America that triggered a regression and low sedimentation rates (12 m/Ma) during Elvira Fm's deposition (Eocene), which is a condensed section characterised by shales deposition .

The Late Paleogene-Neogene facies distribution shows a transition from shallow marine and fluvial-deltaic environment to a deeper marine environment towards the shelf edge and relatively high sedimentation rates (54 m/Ma) are calculated during Oligocene time and remained high (40 m/Ma) until the end of Miocene. These relatively high sedimentation rates continued until the present day with the development of large basinward-prograding sedimentary wedges. This increase in sedimentation rates correlates with the global climatic transition from greenhouse to icehouse conditions (Fig. 13), which is thought to have influenced the enhancement of continental erosion (Seranne 1999, Anka and Seranne 2004).

In addition, subsidence was still ongoing creating the necessary accommodation space during this time and both aggradations within the central part of the basin, as well as progradation towards the east took place.

Discussion

Basin evolution and comparison with conjugate margin

This This study shows that the Colorado Basin initially evolved in an NW-SE trending direction during pre-rift/early syn-rift and changed to a E-W trending direction during the Sag II phase. The direction of extension changed again to NE-SW towards the East during early basin formation, and shifted to a NW-SE direction of extension during the Late Cretaceous along the Atlantic margin, corresponding to the opening direction of the South Atlantic. [START_REF] Franke | Crustal structure across the Colorado Basin, offshore Argentina[END_REF] conclude that the basin is mainly an extensional basin and not an intracontinental sag basin, which coincides with our observations. The strong obliquity of the basin with respect to the South Atlantic opening direction (NE-SW) could be explained by assuming t that the Colorado Basin represents a failed rift which developed in early stages of the South Atlantic formation, whereby the two other arms of the tripartite system have formed the main Atlantic spreading centre. However, this study confirms that the origin of the NW-SE oblique rift most probably resulted from extensional stress, either acting through or interacting with the prevailing Palaeozoic basement fabric, which coincides with the observations from [START_REF] Franke | Crustal structure across the Colorado Basin, offshore Argentina[END_REF]. Hence, we suggest that the geometry and structure of the inherited part of the Colorado Basin represents an earlier rift phase pre-dating the main break-up of the South Atlantic, which could be related to pull-apart basins formation. Such pull-apart basins have been proposed in the South Atlantic offshore and onshore Argentina [START_REF] Franke | Crustal structure across the Colorado Basin, offshore Argentina[END_REF][START_REF] Tankard | Structural and Tectonic Controls of Basin Evolution in Southwestern Gondwana During the Phenerozoic[END_REF]Uliana et al., 1989;[START_REF] Uliana | Permian to Late Cenozoic evolution of Patagonia, main tectonic events, magmatic activity, and depositionaltrends[END_REF], as well as in the foreland basins of southernmost South America [START_REF] Ghiglione | Structure and tectonic history of the foreland basins of southernmost South America[END_REF]Uliana, et al 1989). The Colorado Basin still may be a failed rift; however, we propose that pre-rift inheritance (Fig. 11) was the main control for the direction of the NW-SE segment of the basin (Fig. 5, 6 and Fig. 8/Profile 1). We do not have a conclusive timing regarding the initiation of the basin, but the large dimensions of the syn-rift graben structure -extending up to 30 km in width-(Fig. 10a, suggest that rifting took place earlier than the final rift phase (156 Ma) that resulted in the break-up of the South Atlantic. The well penetration of basalts within the Paleocene (Pedro Luro Fm.) does not support the existence of another late rifting phase as these mapped volcanic areas do not correspond to any depocenter.

The evolution of a sedimentary basin depends on the interplay of various factors: the thermal subsidence history, compaction, tectonic uplift, and climatic conditions that affect the style and magnitude of erosion and sedimentation. The interpretation of more than 30,000 km of seismic reflection data of the Colorado Basin has led to a better understanding of the subsidence history during the early rift/drift stages. In the basin tectonic subsidence started with the initial rifting around 156 Ma and it was relatively rapid for approximately 50Ma, before it decreased and continued with a more or less constant rate until today. The Colorado Fm. is interpreted to reflect sedimentation during the thermal subsidence stage, which affected the Argentine continental margin during the first part of the drift phase. Seismic data and well data give no evidence for erosion of more than a few tens of meters after the break-up unconformity.

Although the duration of extensional phases preceding the final break-up of the South Atlantic is unclear, most authors suggest an extensional period of about 25 Ma (Uliana et al., 1989;[START_REF] Harry | Basaltic volcanism, mantle plumes and the mechanics of rifting: the Parana flood basalt province of South America[END_REF]. Assuming an accumulation time span of 25 Ma for the syn-rift successions, sedimentation rates of 118 m/Ma are estimated (Table 1, Fig. 13). Such high sedimentation rates of syn-rift deposits have so far not been described on the eastern Argentine margin, nor in the conjugated Orange Basin in South Africa. By late Cretaceous, the Gondwana break-up had stopped in almost all areas and post-rift thermotectonic subsidence and minimal faulting dominated the basin development from there on [START_REF] Vayssaire | Petroleum systems of Colorado and Malvinas basins, deep water Argentina, South Atlantic petroleum systems[END_REF]. This study confirms that the basin underwent two phases of thermal subsidence resulting in distinct variations of the post-rift successions, which also correspond to the observations made by different authors [START_REF] Galeazzi | Structural and stratigraphic evolution of the western Malvinas Basin[END_REF][START_REF] Hinz | The Argentine continental margin north of 48°S: sedimentary successions, volcanic activity during break-up[END_REF][START_REF] Uchupi | The tectonic style of the Atlantic Mesozoic rift system[END_REF][START_REF] Sibuet | Early evolution of the South Atlantic ocean: Role of the rifting episode[END_REF] (Fig. 13). During the early phase of subsidence (the sag phase) high sedimentation rates (122 m/Ma) are observed (Sag I and Sag II units; Fig. 13,Table 1).

Whereby the first phase was mainly restricted to the structural geometry of the rift basin.

During the deposition of the Colorado Fm. (Albian to Maastrichtian) sedimentation rates decrease dramatically to 30 m/Ma, such local sedimentation rates may not represent the entire system. However, these relatively low sedimentation rates can be explained by low accommodation space above the structural geometry of the rift basin. Throughout the Late Cretaceous (Albian to Maastrichtian) sedimentation took place in elongated depocenters, with the bulk of the sediment being deposited by sediment progradation to the east (oceanward) of the basin. This study shows that thick succession of Cretaceous and Tertiary sediments were deposited at various points along the shelf edge, which also correspond to observations made along the Argentine shelf edge in general [START_REF] Urien | The basins of southeastern South America (southern Brazil, Uruguay, and eastern Argentina), including the Malvinas Plateau and southern South Atlantic paleogeographic evolution[END_REF]. The second phase was related to thermal subsidence along the trend of the South Atlantic margin during the Tertiary (Fig. 13). During this time, the basin developed under marine conditions and sedimentation rates are relatively high (47 m/Ma) in the Paleocene. Relatively low sedimentation rates (12 m/Ma) occurred during the Eocene (Elvira Fm.) with the deposition of shales. This might be explained by the major marine transgression during the Paleocene, which was followed by a period of uplift of South America during the Eocene [START_REF] Urien | The basins of southeastern South America (southern Brazil, Uruguay, and eastern Argentina), including the Malvinas Plateau and southern South Atlantic paleogeographic evolution[END_REF] that triggered a regression of large magnitude resulting in the low sedimentation rates during the deposition of the Elvira Fm. (Eocene). This drop in sedimentation rates might also be the origin of the condensed section of the Elvira Formation. The Late Paleogene/Neogene facies distribution shows a transition from shallow marine and fluvial-deltaic environment to a deeper marine environment towards the shelf edge and relatively high sedimentation rates (54 m/Ma) are calculated during Oligocene time and remained high (40 m/Ma) until the end of Miocene.

This increase in sedimentation rates corresponds to the global climatic transition from greenhouse to icehouse environment (Fig. 13), which had a strong effect on the glaciation of the Andes and resulting glacial erosion. In addition, thermal subsidence was still ongoing, creating the necessary accommodation space during this time and both aggradation within the central part of the basin, as well as progradation towards the east took place until present day. The higher sedimentation rates in the Colorado Basin during the Oligocene and Miocene are in contrast to the low sedimentation rates registered in the southern Orange Basin, in the conjugate margin, during the same period of time, which might be explained by substantial sediment bypass the middle shelf area , although may also be explained by comparably smaller thermal subsidence in the southern Orange Basin (Kuhlmann et al., 2010). [START_REF] Vayssaire | Petroleum systems of Colorado and Malvinas basins, deep water Argentina, South Atlantic petroleum systems[END_REF] state that a second extensional episode occurred in the Colorado Basin between 95-80 Ma. This study confirms the reactivation of the main faults during this time interval as seen by the reactivated rift-controlled faults (Fig. 6 &8). The Upper Cretaceous and Lower Tertiary sequences are dissected by a second generation of post-rift normal faults affecting the depositional units of the Colorado and Pedro Luro Fms, during which the highly vertical faulted interval at the distal slope area was developed (Fig. 7). The migration of depocenters during Mid Cretaceous from the eastern centre to the slope of the basin is related to the progradation towards the shelf break. The progradation took place with minor slumping episodes indicating slope instability associated with gravitational tectonics at the distal slope.

These events/structures were later sealed by the Pedro Luro Fm. (Paleocene). From the Upper Cretaceous onwards the continental margin is affected by slope instabilities as seen in the high vertical faulted interval with seaward-dipping normal faults (Fig. 7). The main Cretaceous/Tertiary phases of slope instabilities affected the units of the Colorado and the Pedro Luro Fms. Cretaceous slumps are observed in the eastern part of the Colorado basin which had relatively high sedimentation rates during that time and shows interplay of progradation and slumping (Fig. 12b). It appears that the Upper Tertiary Caotico Unit (Oligocene) and Baranca Final Fm. (Miocene) are relatively undisturbed.

On the conjugate Orange Basin, Kuhlmann et al., (2010) stated that the seaward dipping extensional faults only developed above dipping decollement surfaces of the Upper Cretaceous at the shelf break, similar to the location of the observed Cretaceous slumps in this study (Fig. 7). Another controlling factor for the occurrence and style of the slumping is the variation in sedimentation rates (Fig. 13). However, paleogeographic conditions during the slumping episodes were different during both time intervals. While Late Cretaceous slumping took place in a relatively narrow proto-South Atlantic, with low discharge rates of the Colorado River [START_REF] Bushnell | Petroleum system analysis of the eastern Colorado Basin, offshore northern Argentina[END_REF][START_REF] Galeazzi | Structural and stratigraphic evolution of the western Malvinas Basin[END_REF][START_REF] Hinz | The Argentine continental margin north of 48°S: sedimentary successions, volcanic activity during break-up[END_REF] and decreasing crustal subsidence in the elongated depocenters, the Tertiary mass movements occurred during a time of higher sediment supply and thermal subsidence. Several triggering mechanisms have been postulated as the cause of slope instability and sediment collapse.

These include rapid sedimentation rates and resulting high internal pore fluid pressures, as well as down-slope undercutting [START_REF] Emery | Continental margin off western Africa: Cape St. Francis (South Africa) to Walvis ridge (Southwest Africa)[END_REF][START_REF] Dingle | Mesozoic and Tertiary Geology of southern Africa[END_REF]. This study shows that in the Colorado Basin the slumping episodes are mostly related to the progradation of the sediments, the sediment build up at the slope, and further development of seaward dipping normal faults.

Conclusions

The evolution of the Colorado Basin has been interpreted through the detailed analysis of the seismo-stratigraphic sequences identified in the basin and the estimation of the sedmentation Table 1. Estimation of total deposited area and volume as well as accumulation and sedimentation rates for the units identified in the Colorado Basin.

  the syn-rift sediments are predominantly present in NW-SE trending depocenters. The breakup unconformity appears as an erosional surface along the shelf and forms the base of the post-rift sediments in the basin. The pre-rift unconformity is characterized by erosional truncation of the pre-rift strata in the grabens and the syn-rift sequence (or sequences) shows onlap onto the pre-rift unconformity. Above the break-up unconformity the post-rift sediments starting with the thickness of the Sag II phase and the main depocenters are located in the centre of the basin trending in an E-W direction (Fig. 10b). During the Sag I phase deposition of these successions was confined to the centre-west and to the south-east parts of the basin (Fig 10c), maintaining the NW-SE and E-W trend. The thickness map of the Colorado Fm. (Albian -Maastrichtian) displays a thickening of the sediments in the eastern centre part of the basin and its depocenter turns NW-SE towards the East (Fig. 10d). From this time onward, a NE-SW trending shelf break developed with progradation and concurrent activity of mass movement of the sediments of the Colorado Fm. (Fig. 10d, 10e and Fig. 12b). Seismic data shows that the basin was subject to an extensional episode postdating the formation of the South Atlantic, reactivating the rift-related normal faults during the Cretaceous. The post-rift sequences of the Cretaceous deposits vary from less than 500 m to 6500 m and from the Aptian to Maastrichtian shelf progradation to the east dominated. The Colorado Fm. (Albian-Maastrichtian) is characterized by fairly parallel bedding and close to the continental slope its top is marked by an erosional unconformity. The top-Maastrichtian erosional unconformity was transgressed by the Paleogene (Paleocene-Eocene) sequences. The Tertiary units, starting with the Pedro Luro Fm. (Paleocene), are up to 4000 m thick and during the Paleocene to Oligocene aggradation prevailed in the centre of the basin. The Tertiary successions are characterized by parallel reflectors, indicating that they were deposited during a tectonically quiescent period. There is no evidence for erosions larger than a few tens of meters after the break-up unconformity. Throughout the deposition of the Pedro Luro Fm. (Paleocene) the depocenter is concentrated at the slope of the basin (Fig. 10e), probably as a result of progradation towards the shelf edge. During the deposition of the Elvira Fm. (Eocene) the depocenters migrated back towards the centre and western area of the basin (Fig. 10f) where aggradation became dominant and sediment thickness decreased. The Late Tertiary depocenter is mainly concentrated in the eastern part of the basin as documented by the deposition of the Caotico and Baranca/Belen units (Fig. 10g and 10 h). The Tertiary successions are more or less parallel layered in the centre part of the basin suggesting sediment aggradation during this time. Whilst from the Oligocene to the Miocene sediment deposition increased and relatively high sedimentation rates continued to the present day with the development of basinward prograding sedimentary wedges. The Cretaceous and Tertiary units onlap onto the pre-rift structures in the north and south of the basin (Fig. 12a). The Upper Cretaceous and Lower Tertiary sequences are dissected by a second generation of post-rift normal faults affecting the depositional units of the Colorado and Pedro Luro Fms.

  rates through time. The syn-rift and post-rift (thermal sag phase) evolution of the Colorado Basin and its geometry is mainly controlled by rift bounding faults, which resulted in the formation of graben and half-graben systems where the main depocenters developed. Three main syn-rift depocenters have been identified, whereby the thickest one contains up to 5.5 km of sediments. Three faulting events are recognized in the basin: (1) The rift-related faults resulting in graben and half-graben systems, (2) A second extensional episode, which reactivated the rifting normal faults during the Upper Cretaceous and (3) A generation of post-rift normal faults, which affected the Upper Cretaceous and Lower Tertiary sequences (Colorado and Pedro Luro Fms.) in the slope. Our results confirm that the Colorado Basin is a typical extensional basin which formed in close connection with the development of the South Atlantic. The origin of the NW-SE oblique rift most probably resulted from extensional stresses, either acting through or interacting with the prevailing Palaeozoic basement fabric. Hence, we propose that the geometry and structure of the inherited part of the Colorado Basin represents an earlier rift phase pre-dating the main break-up of the South Atlantic. The thermal subsidence in the basin started with the initial rifting around 156 Ma ago and it was relatively rapid for approximately 50Ma, before it decreased and continued with a more or less constant rate until today. The Colorado Fm. is interpreted to reflect sedimentation during the thermal subsidence stage, which affected the Argentine continental margin during the drift phase. With an accumulation time span of 25 Ma for the syn-rift succession, which correspond to the duration of extensional phases preceding the final break-up of the South Atlantic, estimated sedimentation rates are of 118 m/Ma. By Late Cretaceous, the Gondwana break-up had stopped in almost all areas and post-rift thermal subsidence dominated the basin development. During the early phase of subsidence (Early Cretaceous), high sedimentation rates of up to 122 m/Ma occurred in the basins sag-phase, which was mainly restricted to the structural geometry of the rift basin. Low sedimentation rates occurred throughout the Late Cretaceous and sedimentation took place in elongated depocenters with oceanward progradation. A second phase of thermal subsidence along the trend of the South Atlantic margin during the Tertiary was concomitant with a drastic increase in sediment supply during the Oligocene and Miocene, resulting in the development of large basinward prograding sedimentary wedges. These high sedimentation rates are associated to continental dynamics, thermotectonic subsidence, and the change from greenhouse to icehouse conditions. From the Cretaceous onwards the continental margin is affected by slumping episodes. The main Cretaceous/Tertiary phases of mass movements affected the Colorado Fm. (Campanian) and the Pedro Luro Fm. (Paleocene). The slumping episodes are related to the progradation of the sediments and the sediment build-up at the slope as well as to the development of the seaward dipping extensional faults.

Figure 1 .

 1 Figure captions:

Figure 2 .

 2 Figure2. Chronostratigraphic and sequence stratigraphic chart for the Colorado Basin including seismic horizons mapped within this study. In addition, the different elements of potential petroleum systems are specified (Modified after[START_REF] Vayssaire | Petroleum systems of Colorado and Malvinas basins, deep water Argentina, South Atlantic petroleum systems[END_REF].

Figure 3 .

 3 Figure 3. Major seismic units identified in the Colorado Basin. These sequences were mapped through the basin.

Figure 4 .

 4 Figure 4. (a) Depth to TWT plot showing the velocities of used wells. (b) Spatial distribution of velocities, exemplary of top pre-rift, including locations of used well velocities (white dots) and the dotted blue line represent available stacking velocities in the study area. This unit that has the best data coverage shows a distinct increase towards the main depocenter location.

Figure 5 .

 5 Figure 5. Rift-related faults along the margin of the Colorado Basin. These extensional faults are rooted in the deep basement, including pre-rift sediments, and generally terminate within the syn-rift successions, although some may extent within the Lower Cretaceous.

Figure 6 .

 6 Figure 6. Basement-controlled faults beneath the break-up unconformity and evidences of a second extensional episode, which reactivated the rifting normal faults (white arrows).

Figure 7 .

 7 Figure 7. A highly-faulted interval within the Colorado (Campanian) and Pedro Luro (Paleocene) Fms. at the distal slope region of the basin. High-amplitude reflectors within the upper part of the Colorado Fm., between the Tertiary/Cretaceous bounding faults, suggest gas accumulation in these levels.

Figure 8 .

 8 Figure 8. Regional TWT profiles showing the main seismo-stratigraphic units and structures identified in the Colorado Basin, including the locations of main depocentres and how the basement faulting affects the upper sequences. Profile 4 shows the entire basin in E-W direction, including the slope and the location of the Continental-Ocean-Boundary (COB). See locations in Figure1b.

Figure

  Figure 9a-i. Depth-converted maps depicting the structure of the main syn-rift (Permian/Jurassic/Early Cretaceous) and post-rift (Cretaceous/Tertiary) units identified in the basin.

Figure

  Figure 10a-h. Isopach maps depicting the main depocenters of the main identified units in the basin.

Figure 11 .

 11 Figure 11. Seismic parallel reflectors at the base of the graben structures characterize the prerift deposits. The syn-rift sequences show onlap onto the top of the pre-rift unconformity.

Figure 12 .

 12 Figure 12. (a) Seismic section in south to north direction, showing the onlaps of Cretaceous and Tertiary units onto the pre-rift structures in the northern part of the basin. (b) Seismic section at the slope of the basin, showing progradation and Cretaceous slumps.

Figure 13 .

 13 Figure 13. Comparison of accumulation and sedimentation rates for the Colorado Basin at the Argentine margin, western South Atlantic and the southern Orange Basin at the western South African margin, eastern South Atlantic, together with the tectonic development of the eastern margin and global climate proxies. The blue arrows indicate main phases of erosion as observed in the study area and the green arrows indicate phases of fault activity. SR = source rock.

  Figure 1 Click here to download high resolution image

  Figure9a-i

  Fig. 9b

  Fig. 9i

  Figure10a-h

  Fig. 10b

  Fig. 10d

  Fig. 10f

  Fig. 10h

  Figure 11 Click here to download high resolution image

Table 1 Click here to download high resolution image

 1

Acknowledgements

This study was funded by the German Science Foundation (DFG) priority program "SAMPLE" Phase I. Z. Anka's position is funded by a Helmholtz-University Young Investigators Group grant from the Helmholtz Association of German Research Centres. We gratefully acknowledge Petrobras Argentina (PESA) and the German Federal Institute for Geoscience and Natural Resources (BGR) for supplying data and allowing publication of these results. Prof. Eduardo Rosello and an anonymous reviewer are thanked for their insightful comments and suggestions which helped to improve earlier versions of this manuscript.

Depth (m)

Top Sag II (Lower Cretaceous)