4D analogue modelling of transtensional pull-apart basins

Pull-apart basins are depressions that form as the result of crustal extension along strike-slip systems where the sense of fault stepping or bending coincides with that of fault slip. They are common features of strike-slip systems. We perform a number of numerical thermomechanical experiments to explore how the rheology of the lithosphere influences basin evolution and lithospheric structure beneath the basin. Our modeling shows that basin subsidence results from the competition of extension of the brittle part of the lithosphere, which leads to its subsidence, and of the compensating flow of the deeper ductile part of the lithosphere, which pushes the extended brittle block upwards. The result of this competition is the subsidence rate. Strain partitioning beneath the basin and crustal structures is controlled by (i) the thickness of the brittle layer and basin width, (ii) the magnitude of strike-slip displacement, (iii) the rate of frictional softening of the crust, and (iv) the viscosity of the ductile part of the lithosphere. The thickness of the brittle layer and the viscosity of the underlying ductile part of the lithosphere in turn depend on temperature, composition and material softening. We interpret the modeling results, deducing simple analytical expressions based on the "brittle brick stretching" (BBS) approach, which despite its simplicity describes the structure and evolution of pull-apart basins reasonably well. We also demonstrate that the structure and evolution of the Dead Sea Basin, located at a left step of the Dead Sea Transform in the Middle East, is consistent with a BBS type of deformation with only a minor contribution from compensational flow in the ductile part of the lithosphere. Finally, we show that the formation of a deep narrow pull-apart basin in relatively cold lithosphere, as in the Dead Sea Basin, requires very low friction at major faults (lower than 0.1-0.2). If this condition is not satisfied, strike-slip deformation does not localise and deep basins do not form.

Journal of Geology, 1983

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Geophysical Journal International, 2010

We address the mechanism of sedimentary basin formation along strike-slip fault systems with 3-D numerical simulations based on a continuum damage rheology model. The formation of these basins is usually explained by a pull-apart mechanism that predicts a rhomb-shaped basin geometry bounded by two longitudinal strike-slip faults and two transverse listric faults. Significant ductile deformation of the lower crust and upper mantle associated with basin growth requires normal or elevated heat flux. The Dead Sea continental transform is associated with some of the larger and unusually deep basins, among which the southern Dead Sea is the deepest. The heat flow in the Dead Sea basin is anomalously low and it is associated with deep seismicity. Moreover, the basin is bounded by deep transverse normal faults rather than the listric faults required by the pull-apart model. Hence, the formation of the basin cannot be explained by the existing pull-apart model. Ben-Avraham and Schubert proposed an alternative conceptual model for the formation of the deepest basin at the southern Dead Sea. They suggested that an isolated block of lithosphere has dropped into the mantle. We simulate the formation of this and other deep basins along the Dead Sea fault and demonstrate that the 'drop down' mechanism of the Dead Sea basin formation suggested by Ben-Avraham & Schubert is possible. Density heterogeneities formed in the crust or upper mantle during a previous stage of regional magmatism, drop into the upper mantle when strike-slip faults are created and detach them from the surrounding lithosphere. The simulations indicate that the resulting basin is rhomb-shaped and that with time it grows by the addition of distinct segments to its edges. The proposed mechanism could account for the formation and evolution of large sedimentary basins along other strike-slip fault systems, such as the San Andreas fault and other continental transform faults.

2010

The Vienna Basin is considered a classical thin-skinned pull-apart with a rather peculiar basement structure. Deformation and basin evolution are believed to be limited to the upper crust above the inclined Alpine-Carpathian floor thrust. Nine experiments were accomplished to estimate the effects of this special geometry and the resulting structures were compared with the Vienna basin. The key parameters for the models were inferred from a 3D Gocad model, which was compiled from seismic, wells and geological cross sections. The experiments were scaled 1:100.000 and built of quartz sand. An average depth of 6 km was calculated for the basal detachment; distances between the bounding strike-slip faults of 40 km and an overall length of the natural basin of 200 km were estimated. The following parameters were changed through the experimental process: syntectonic sedimentation; the stepover angle between strike-slip faults; moving of one or both fault blocks; inclination of the basal de...

Journal of Structural Geology, 1999

Seventeen small-scale experiments were performed to study the deformation induced in brittle±ductile models by a releasing stepover between two transform faults, and by intersection between transform and divergent plate boundaries. In both cases, faulting depends on the rheological layering (presence and strength of a ductile layer at depth) and the width of the basal stepover. Successive types of pull-apart basins are observed in the ®rst set of experiments, which are compared with natural examples. Firstly, a lazy Z-shaped basin appears, which is bounded by Y faults above transform boundaries and R faults propagating from each corner of the stepover. Then, R H faults replace R faults, leading to a rhomb-shaped graben. At every stage, the basin length-to-width ratio remains between 2.2 and 3.8, suggesting that scale independence of pull-apart basins is related to the geometrical shape of bounding faults. In a second set of experiments, deformation at the end of a transform boundary is characterized by a horsetail splay bounding a divergent basin. Within the horsetail splay, block tilting and block rotation about vertical axes lead to a surface slope perpendicular to the slope of the divergent basin, a feature that can be compared with marginal ridges at transform margins. #

Mechanism of Sedimentary Basin Formation - Multidisciplinary Approach on Active Plate Margins, 2013

Geological Society, London, Special Publications, 1996

The lithospheric driving forces which cause intraplate basin deformation are relatively constant over large areas. Consequently, lateral variations in deformation and stress and strain concentrations seem to be primarily caused by (pre-existing) heterogenities in the rheological signature of the continental lithosphere underlying the sedimentary basins. In this paper, we explore the weak character of upper crustal faults and their control on basin shape for a number of case studies on intraplate Phanerozoic basin settings, using a 2D finite element and a 3D flexure model. Of key importance is the integration of seismic data and field observations with the tectonic modelling, allowing the investigation of deformation processes and their expressions on different scales, operating on different levels of the lithosphere. Finite element models for sub-basin scales, incorporating weak upper crustal faults, predict strong control of these weak zones on local stress distributions and subsequent deformation, in agreement with observed deformation patterns. Slip along upper crustal faults control stress distribution and subsequent faulting in overlying sedimentary rocks. The effect of weak upper crustal fault movements on basin-wide (regional) upper crustal flexure is looked at in two case studies on: (1) extensional tectonics in the Lake Tanganyika Rift Zone (East Africa); and (2) compressional tectonics in the Central System and Tajo Basin (Central Spain). Both settings indicate that basement warpings are controlled by large amounts of slip along so-called weak crustal-scale border faults, which are mostly planar. Adopting border fault displacements in the 3D flexure model, the results indicate low effective elastic thickness (EET) values in a range of 3-7 km for induced basement deflection patterns in accordance with observations. The low EET values most likely reflect a (partly) decoupling of upper crustal and subcrustal deformation, facilitated by the weak lower crust, and in agreement with standard rheological assumptions for Phanerozoic lithosphere. In contrast, the inferred weakness of upper crustal faults relative to surrounding rock is not evident from uniform rheological assumptions. However, observations of reactivations of faults which are not preferably aligned with the stress field, and reactivations of basin deformation on long timescales are in support of this feature.

Tectonophysics, 2006

Tectonics, 2008

1] Contrary to other examples, like Death Valley, California, and the Sea of Marmara, Turkey, the Dead Sea-type pull-apart basins form within a narrow transform corridor between strike-slip faults that are less than 10 km apart, much smaller than the crustal thickness of 35 km. In this paper we investigate the role of fault zone width versus thickness and rheology on the mechanics of pull-apart basins through a series of laboratory experiments. Results show that pull-apart basins that develop above a small step over (i.e., smaller than the thickness of the brittle layer") are narrow and elongated parallel to the overall motion. This is enhanced by increased decoupling along a basal ductile layer. The experiment with the highest degree of mechanical decoupling shows a striking resemblance to the Dead Sea Basin (DSB). Comparison with modeling results suggests that the DSB's flat basin floor is bordered over its full length by strikeslip faults that control the basin geometry and temporal and spatial basin migration. This is in strong contrast to Death Valley-type pull-apart basins that are highly oblique to the transform direction with transverse normal faults dominating over longitudinal strike-slip faults. Results imply that lithosphere rheology and the ratio of basin width to crustal thickness are controlling factors in the mechanics of pull-apart basin formation within transform corridors like the Dead Sea Fault. Citation: Smit, J., J.-P. Brun, S. Cloetingh, and Z. Ben-Avraham (2008), Pull-apart basin formation and development in narrow transform zones with application to the

2004