S.S. Egan

Modeling the tectonic development of the Tucano and Sergipe-Alagoas rift basins, Brazil

Abstract The Tucano and Sergipe-Alagoas basins of northeast Brazil form part of a regional extensional basin system that was operative during the Mesozoic breakup of South America and Africa with both basins developing on Precambrian crust. The synchronous development of the rift basins suggest that they are genetically linked in space and time. Whereas the offshore Sergipe-Alagoas basin is characterized by a substantial thickness of post-rift sediment, the Tucano basin either failed to develop or at least preserve a significant thickness of post-rift sediment. Observed primary porosities within both pre- and syn-rift Tucano basin sediment imply that significant post-rift sedimentation never occurred. Failure to develop significant post-rift subsidence has important thermal and mechanical implications for the reaction of the lithosphere to rifting and can be explained in terms of: (1) depth-dependent lithospheric extension in which intracrustal detachments allow the extension of the crust to be decoupled from the thinning of the lithospheric mantle, (2) small rates of extension that allow the lithosphere to cool during rifting, and/or (3) lithospheric rifting during which the flexural strength of the lithosphere remains high. With respect to points (2) and (3), forward modeling demonstrates that finite rifting rates over a 20–25 m.y. period are insufficient to cool the lithosphere to the point where post-rift subsidence fails to develop. An interesting complication arises when the flexural strength of the lithosphere remains large during rifting: it tends to suppress the vertical motions of the lithosphere, such as those engendered by the cooling of the lithosphere following rifting, thereby reducing significantly the amplitude of the post-rift subsidence. Thus, the lack of post-rift sedimentation within a basin does not necessarily imply that extension has been limited to the crust. From our kinematic modeling of the Tucano basin, the observed negative free-air and Bouguer gravity anomalies (− 120 mGals) suggest that the flexural strength of the lithosphere has remained sufficiently large to maintain the load of the sediment. We can model successfully the observed Tucano basin architecture and gravity anomaly irrespective of whether we assume depth-dependent or depth-independent lithospheric extension primarily because the flexural strength of the lithosphere tends to “buffer” or suppress the amplitude of the post-rift subsidence. In contrast, the adjacent Sergipe-Alagoas basin is associated with low-amplitude gravity anomalies which may reflect a resetting of flexural strength during rifting. Prior to rifting, however, there was no appreciable elevation difference between the Tucano and Sergipe-Alagoas regions (as might be induced by a hot-spot for example) implying that the flexural strength of the lithosphere was similar. The total thickness of the syn- and post-rift sediments within the Sergipe-Alagoas basin is about the same as the thickness of rift-phase sediments in the Tucano basin. Thus, the amount of crustal extension responsible for each basin was similar. However, since the Sergipe-Alagoas basin contains 4–5 km of post-rift sediments, lithospheric mantle thinning in this region must have been significantly greater than the crustal extension to overcome the effects of “flexural bufferring”. The need for lithospheric mantle thinning to be greater that crustal extension in the Sergipe-Alagoas region was also a result obtained by modeling the development of the ocean/continent boundary between Brazil and Africa. From our coupled kinematic and rheological model of lithospheric extension, we predict that the ocean/continent boundary should form preferentially at the location of greatest crustal and lithospheric mantle thinning, that is, within the region of maximum depth-independent lithospheric extension. The ocean/continent boundary formed east of the Sergipe-Alagoas basin despite the fact that the Tucano basin represents the region of greatest crustal thinning. If extension had been uniform with depth beneath these basins, the ocean/continent boundary should have formed in the Tucano basin. As it did not, some form of intracrustal detachment appears to have been operative to reduce lithospheric mantle thinning beneath the Tucano basin while enhancing it beneath the offshore regions of the Sergipe-Alagoas basin.

The geological and geodynamic evolution of the eastern Black Sea basin: insights from 2-D and 3-D tectonic modelling

Abstract Subsidence mechanisms that may have controlled the evolution of the eastern Black Sea have been studied and simulated using a numerical model that integrates structural, thermal, isostatic and surface processes in both two- (2-D) and three-dimensions (3-D). The model enables the forward modelling of extensional basin evolution followed by deformation due to subsequent extensional and compressional events. Seismic data show that the eastern Black Sea has evolved via a sequence of interrelated tectonic events that began with early Tertiary rifting followed by several phases of compression, mainly confined to the edges of the basin. A large magnitude (approximately 12 km) of regional subsidence also occurred in the central basin throughout the Tertiary. Models that simulate the magnitude of observed fault controlled extension (β=1.13) do not reproduce the total depth of the basin. Similarly, the modelling of compressional deformation around the edges of the basin does little to enhance subsidence in the central basin. A modelling approach that quantifies lithosphere extension according to the amount of observed crustal thinning and thickening across the basin provides the closest match to overall subsidence. The modelling also shows that deep crustal and mantle–lithosphere processes can significantly influence the rate and magnitude of syn- to post-rift subsidence and shows that such mechanisms may have played an important role in forming the anomalously thin syn-rift and thick Miocene–Quaternary sequences observed in the basin. It is also suggested that extension of a 40–45 km thick pre-rift crust is required to generate the observed magnitude of total subsidence when considering a realistic bathymetry.

Computer modelling and visualisation of the structural deformation caused by movement along geological faults

Abstract The computer modelling and visualisation of deformation caused by movement along faults has enhanced our understanding of the evolution of fault-related structures in the geological record. In particular, the development of computer software to carry out structural restoration and section balancing has provided earth scientists with an effective tool for validating structural interpretations constructed from geological and geophysical data. This paper describes both two- and three-dimensional geometric methods for modelling hanging wall deformation in response to fault movement. Equations are presented for the definition of two-dimensional fault geometries and for the determination of hanging wall geometry following movement over these faults. The Chevron and inclined shear constructions and fault-bend fold theory are described in a format to enable easy conversion into computer algorithms. The modelling of fault movement in three-dimensions is also considered in the context of the Chevron construction. Schematic models are presented which show hanging wall deformation caused by extensional, compressional and, most importantly, strike-slip movement over a complex fault surface. In addition, a new geometric technique for the restoration of deformed hanging wall surfaces is described. This technique has been called flexural flattening and involves flattening a surface represented as a mesh of triangles, back to horizontal. It has the advantages of maintaining the area of the surface before and after restoration and is relatively simple to apply in comparison to three-dimensional implementations of existing geometric methods.

The flexural isostatic response of the lithosphere to extensional tectonics

Abstract The tectonic loading resulting from the extension of the lithosphere is examined. Loads resulting from crustal thinning, thermal perturbations and basin infill are recognised and the resulting flexural isostatic deflection of the lithosphere is modelled in response to each of them in the presence of a listric fault. During rifting regional uplift or rebound occurs as a result of both crustal thinning due to normal faulting (simple shear) and the elevation of the geothermal gradient, while subsidence is generated by the downward acting loads created by both basin infill and thinning of the lower crust by stretching (pure shear). Following rifting further flexural subsidence occurs in response to the cooling of the temperature field and associated sedimentary infill. One of the major tectonic effects, resulting dominantly from crustal thinning by normal faulting, is the generation of footwall uplift. The magnitude of this uplift, along with basin depth and underlying crustal structure, are strongly determined by the flexural rigidity of the lithosphere, the amount of extension and the dip of the major basement fault along which extension is taking place. Depending upon the values of these parameters, model calculations predict between a few tens of metres and 1.5 km of footwall uplift. These values correlate closely with the magnitude of footwall uplift estimated from examples within the geological record. The erosion of this rift induced topography is also proposed as a possible driving mechanism for the generation of major basin unconformities.

Subsidence and uplift mechanisms within the South Caspian Basin: insights from the onshore and offshore Azerbaijan region

Abstract A combination of fieldwork, basin analysis and modelling techniques has been used to try and understand the role, as well as the timing, of the subsidence–uplift mechanisms that have affected the Azerbaijan region of the South Caspian Basin (SCB) from Mesozoic to Recent. Key outcrops have been studied in the eastern Greater Caucasus, and the region has been divided into several major tectonic zones that are diagnostic of different former sedimentary realms representing a complete traverse from a passive margin setting to slope and distal basin environments. Subsequent deformation has caused folds and thrusts that generally trend from NW–SE to WNW–ESE. Offshore data has been analysed to provide insights into the regional structural and stratigraphic evolution of the SCB to the east of Azerbaijan. Several structural trends and subsidence patterns have been identified within the study area. In addition, burial history modelling suggests that there are at least three main components of subsidence, including a relatively short-lived basin-wide event at 6 Ma that is characterized by a rapid increase in the rate of subsidence. Numerical modelling that includes structural, thermal, isostatic and surface processes has been applied to the SCB. Models that reconcile the observed amount of fault-controlled deformation with the magnitude of overall thinning of the crust generate a comparable amount of subsidence to that observed in the basin. In addition, model results support the tectonic scenario that SCB crust has a density that is compatible with an oceanic composition and is being under-thrust beneath the central Caspian region.

Numerical modelling of lithosphere shortening: Application to the Laramide erogenic province, western USA

Abstract A two-dimensional model of lithosphere shortening is presented which quantifies crustal thickening, temperature perturbations and flexural isostatic components. The model assumes that the upper brittle layer of the crust deforms by thrusting, while the lower ductile lithosphere accommodates compressional deformation by a pure shear or “squashing” mechanism. Model results suggest that basement uplift, foreland basin development and underlying crustal structure are controlled by the amount and configuration of the compressional deformation in the upper and lower lithosphere, the perturbation of the temperature field, erosion and the flexural rigidity of the lithosphere. The model is applicable to regions of thick-skinned thrust tectonics and is applied to the Laramide orogenic province in the western USA, Model simulations of structural cross-sections across the Laramide province show basement uplifts and adjacent foreland basins that are comparable in magnitude with those suggested by geological data. The model has also been used to provide insights into the tectonic evolution of the Laramide province and an attempt has been made to determine the flexural rigidity of the lithosphere at the time of Laramide deformation as well as quantify both the effects of the post-shortening re-equilibration of the temperature field and thrust-uplift erosion. The amount of crustal thickening and post-shortening cooling of the geotherm predicted by the two-dimensional model of lithosphere shortening are used in strength calculations to determine the relative strength or weakness of the lithosphere. The results suggest that the lithosphere is relatively strong immediately following shortening, due to the cooling of the geotherm. Following shortening, however, gradual weakening occurs as the temperature field returns back to its unperturbed state in the presence of an enhanced crustal thickness. The results are compatible with the evolution of the Laramide province, which has experienced a change in tectonic regime to one of extension over the last 20–30 Ma.

Flexural modelling of continental lithosphere deformation: a comparison of 2D and 3D techniques

Abstract The flexural isostatic response of the lithosphere in response to loading caused by continental tectonics has been modelled in 3D. The modelling approach used has been to determine hanging wall deformation following movement over a pre-defined fault surface. In addition, the lower crust is assumed to deform by a pure shear mechanism. The changes in crustal thickness resulting from these structural processes impose loads upon the lithosphere, which responds by isostatic adjustment. Algorithms have been developed to quantify the flexural isostatic response to these loads in 3D. These deflections are then superimposed upon the results from the structural modelling to generate isostatically compensated hanging wall, footwall and fault surfaces. Schematic models are presented for extensional, compressional and strike-slip deformation. Model results are dependent upon the interaction between fault geometry, displacement along the fault, which can be varied along strike, and the methodology used to quantify the flexural response of the lithosphere to loading. Emphasis has been placed upon contrasting models, which include a structural component only with those that incorporate both structural and flexural isostatic processes. Following extension, structural processes generate a relatively deep half-graben with no deformation at the basin edges. Isostatic compensation modifies this structure to produce a relatively shallow, but variable, basin depth with uplift (typically between 1 and 2 km) experienced at the basin edges. Compressional models have been generated which show the formation of large uplift structures, which are modified by isostatic compensation so that they are considerably reduced in magnitude. A regional depression (i.e. foreland basin) is also generated adjacent to the remaining uplift. Both 2D and 3D implementations of flexural isostasy have been investigated to provide insights into the validity of results provided by commonly applied 2D methods. A major advantage arising from 3D tectonic modelling is the ability to investigate the effects of oblique or entirely strike-slip components of fault movement. Strike-slip deformation has been modelled in the context of a single fault surface, which varies along strike, to show the development of pressure ridge and pull-apart basin structures. The isostatic compensation of these structures shows complex patterns of uplift and subsidence due to the interference of negative and positive loading and associated flexural deflections.

A kinematic modelling approach to lithosphere deformation and basin formation: application to the Black Sea

Abstract A kinematic model of lithosphere deformation has been developed that integrates the following components: structural deformation of the crust and mantle lithosphere; thermal conditioning, perturbation and subsequent re-equilibration of the lithosphere temperature field; flexural isostatic adjustments; and surface processes, including both lateral and temporal variations in basin fill and bathymetry. This approach enables the forward modelling of extensional basin evolution in two and three dimensions followed by deformation due to subsequent extensional and compressional (i.e. inversion) events. The model has been applied to the Black Sea, which is one of the deepest basins in the world and yet it is poorly understood in terms of the mechanisms that have controlled its evolution. Although it is widely accepted that this basin was initiated by Mesozoic back-arc extension related to the subduction of the Tethys Plate to the south, most of the subsidence observed today occurred within the Palaeogene and Neogene (i.e. within the framework of the Alpine–Himalayan orogenic belt). The modelling approach described above has been used to test possible geological and geodynamic mechanisms that have controlled the subsidence history of the Black Sea. In particular, the investigation has focused on trying to explain the anomalously thick post-rift subsidence that occurred in the basin. Models assuming uniform lithosphere extension do not generate the observed thickness of sediment infill in the basin. Similarly, modelling of the compressional deformation around the edges of the basin structure does little to explain the large magnitude of subsidence within the centre of the basin. Model results show that the observed basin depths can be attained only when the total magnitude of deformation is constrained from crustal thickness changes rather than by fault displacement measurements.

The Obduction of the Northern Oman Ophiolite — Crustal Loading and Flexure

The unique opportunity to study a 20 km-deep section of oceanic lithosphere in the Oman Mountains has attracted a wealth of expertise and generated a great deal of geological data. Lippard et al. (1986) offered a solution to the problem of obduction at the former passive margin of Eastern Arabia which incorporated all age data known at that time and accounted for the broad spectrum of geological evidence presented in the mountains.

GERYA, T. 2010. Introduction to Numerical Geodynamic Modelling. xvii + 345. Cambridge University Press. : Price £40.00, US

In terms of popularity, geodynamics is a relatively new area of the Earth Sciences that is concerned with understanding the mostly large-scale processes occurring within the Earths interior, such as heat generation and transfer, isostasy, mantle convection, faulting, etc. This new textbook by Taras Gerya attempts to provide ‘a practical, hands-on introduction to numerical geodynamic modelling for inexperienced people’. This point is emphasised within the introduction section to the book, which prepares the reader for the emphasis on numerical modelling that underpins a large part of geodynamics. In particular, the author presents his seven ‘Golden Rules’ that readers are advised …