Theoretical studies of the geodynamics of accretion boundaries in the plate tectonics

Various aspects of the physical processes occurring at the accretion plate boundary in plate tectonics have been investigated. Regional stresses have been investigated, arising from lateral density contrasts in the ocean lithosphere. Elastic, visco-elastic and elastic/visco-elastic models predict regional stresses in the ocean basin of the order of 0.25 kb. Investigation of the thermal stresses created in the oceanic lithosphere as a consequence of the cooling of the ocean lithosphere as it moves away from the ridge axis, shows that tensional stresses occur in the upper lithosphere and compressional stresses in the lower lithosphere. An elastic/viscous model of the lithosphere predicts deviatoric stresses of the order of 3 kb. in the upper crust. The temperature distribution beneath the ocean ridge with magma solidifying to form crustal layer 3 has been investigated. Numerical models show that the width of the magma chamber and the thickness of the dyke complex depends on half spreading rate. If there is significant crystal settling, the width of the chamber is predicted to be considerably reduced. A critical half spreading rate of 0.45 cm/yr is predicted, below which the intruded material solidifies instantaneously. Computations support Cann's petrological model. Investigation of the magnitude of the stresses caused by the buoyancy of a magma chamber in the lower crust at the ridge axis suggest that the magma chamber is unable to cause crustal fracture and is, alone, a dynamically stable structure. The additional stresses due to the upthrust of molten upper mantle material is required to cause crustal fracture and a zone of fracture of less than 5 km wide is predicted. The stress field created in the oceanic lithosphere by a mantle plume has been calculated analytically. Estimates of the plume dimensions and velocity suggested by Morgan are predicted to be just sufficient to cause fracture of the lithosphere above the plume axis

Pre‐breakup extension in the northern North Sea defined by complex strain partitioning and heterogeneous extension rates

The early stages of continental rifting are accommodated by the growth of upper‐crustal normal fault systems that are distributed relatively evenly across the rift width. Numerous fault systems define fault arrays , the kinematics of which are poorly understood due to a lack of regional studies drawing on high‐quality subsurface data. Here we investigate the long‐term (~150 Myr) growth of a rift‐related fault array in the East Shetland Basin, northern North Sea, using a regionally extensive subsurface dataset comprising 2D and 3D seismic reflection surveys and 107 boreholes. We show that rift‐related strain during the pre‐Triassic‐to‐Middle Triassic was originally distributed across several sub‐basins. The Middle‐to‐Late Triassic saw a decrease in extension rate (~14 m/Myr) as strain localized in the western part of the basin. Early Jurassic strain initially migrated eastwards, before becoming more diffuse during the main, Middle‐to‐Late Jurassic rift phase. The highest extension rates (~89 m/Myr) corresponded with the main rift event in the East Shetland Basin, before focusing of strain within the rift axis and ultimate abandonment of the East Shetland Basin in the Early Cretaceous. We also demonstrate marked spatial variations in timing and magnitude of slip along‐strike of major fault systems during this protracted rift event. Our results imply that strain migration patterns and extension rates during the initial, pre‐breakup phase of continental rifting may be more complex than previously thought; this reflects temporal and spatial changes in both thermal and mechanical properties of the lithosphere, in addition to varying extension rates

Formation and deformation of hyperextended rift systems: Insights from rift domain mapping in the Bay of Biscay-Pyrenees

International audienceThe Bay of Biscay and the Pyrenees correspond to a Lower Cretaceous rift system including both oceanic and hyperextended rift domains. The transition from preserved oceanic and rift domains in the West to their complete inversion in the East enables us to study the progressive reactivation of a hyperextended rift system. We use seismic interpretation, gravity inversion, and field mapping to identify and map former rift domains and their subsequent reactivation. We propose a new map and sections across the system illustrating the progressive integration of the rift domains into the orogen. This study aims to provide insights on the formation of hyperextended rift systems and discuss their role during reactivation. Two spatially and temporally distinct rift systems can be distinguished: the Bay of Biscay-Parentis and the Pyrenean-Basque-Cantabrian rifts. While the offshore Bay of Biscay represent a former mature oceanic domain, the fossil remnants of hyperextended domains preserved onshore in the Pyrenean-Cantabrian orogen record distributed extensional deformation partitioned between strongly segmented rift basins. Reactivation initiated in the exhumed mantle domain before it affected the hyperthinned domain. Both domains accommodated most of the shortening. The final architecture of the orogen is acquired once the conjugate necking domains became involved in collisional processes. The complex 3-D architecture of the initial rift system may partly explain the heterogeneous reactivation of the overall system. These results have important implications for the formation and reactivation of hyperextended rift systems and for the restoration of the Bay of Biscay and Pyrenean domain

A flexural isostatic model of lithosphere shortening and foreland basin formation: Application to the Eastern Cordillera and Subandean belt of NW Argentina

A numerical model of lithosphere shortening has been developed to combine flexural isostatic and structural balancing. This allows us to quantitatively reconcile lithosphere shortening, thrust sheet emplacement, and foreland basin formation. Lithosphere shortening is accommodated by thrusting in the upper crust and by distributed deformation in the lower crust and the lithospheric mantle. This leads to loading by thrust sheet emplacement, crustal thickening, and thermal perturbation. Additional loading and unloading are generated by sedimentation and erosion, respectively. The model is kinematically controlled, each fault having its position, geometry, and movement prescribed. The model is applied to the Subandean late Oligocene to Recent foreland fold and thrust belt of NW Argentina at 22°15'S. The Subandean belt of NW Argentina contains three major stratigraphic units, the Lower, Middle, and Upper Terciario Subandino Formations. By modelling these formations we attempt to determine whether the Terciario Subandino Group can be explained as a foredeep fill in response to thrust sheet loading, and, if so, what is the required effective elastic thickness. A successful model must predict not only foreland stratigraphy but also exhumation, topography, and crustal thickness. We see a change in the wavelength of sedimentary deposition in the Subandean basin through time that can be accounted for by a change in flexural rigidity. Accommodation space for the Lower Terciario Subandino Formation can be generated using an elastic thickness of 70 km. For the Middle/Upper Terciario an elastic thickness of 15 km produces the correct basin geometries. Shortening estimates of 110 km have proved sufficient to generate the observed foreland stratigraphy, Bouguer anomaly, and topography across the Subandean belt and Eastern Cordillera. However, an additional 140 km of shortening is required to generate the observed Bouguer anomaly and topography across the western Andes. This discrepancy could be due to earlier shortening in the western Andes or to crustal thickening by magma addition

Characterizing and identifying structural domains at rifted continental margins : application to the Bay of Biscay margins and its Western Pyrenean fossil remnants. In : Gibson, G. M., Roure, F. & Manatschal, G. (eds). Sedimentary Basins and Crustal Processes at Continental Margins: From Modern Hyper-extended Margins to Deformed Ancient Analogues.

International audienc

Characterizing and identifying structural domains at rifted continental margins: application to the Bay of Biscay margins and its Western Pyrenean fossil remnants: Structural domains at rifted margins

International audienceWe use the Bay of Biscay and Western Pyrenees as a natural laboratory to develop and apply an approach to characterize and identify distinctive rifted margin domains in offshore and onshore settings. The Bay of Biscay and Western Pyrenees offer access to seismically imaged, drilled and exposed parts of one and the same hyperextended rift system. Offshore, we use gravity inversion and flexural backstripping techniques combined with seismic interpretation to provide estimates of accommodation space, crustal thickness and lithosphere thinning. Onshore, we focus on key outcrops of the former rift domain to describe the nature of sediment and basement rocks, and of their interface. This qualitative and quantitative characterization provides diagnostic elements for the identification of five distinct structural domains at magma-poor rifted margins and their fossil analogues (proximal, necking, hyperthinned, exhumed mantle and oceanic domains). This new approach can be used to reconcile offshore and onshore observations, and to aid interpretation when only local observations are available. Onshore remnants can be placed in an offshore rifted-margin context, enabling the prediction of first-order crustal architecture. For the interpretation of offshore seismic reflection sections, geological insights into rift structures and basement nature can be suggested based on onshore analogies