Laetitia Le Pourhiet

Rifting through a stack of inhomogeneous thrusts (the dipping pie concept)

[1]xa0Orogenic building leaves a complex heritage consisting of a stack of nappes that may have contrasting lithologic structures resulting in heterogeneous mechanical behavior of the system during the postorogenic stages. While the thermal state of the region is reequilibrating, strong lateral variations of the depth to the brittle-ductile transition develop as a consequence of these preexisting heterogeneities. We use a thermomechanical model to quantify how an inherited weak nappe influences the development of fault patterns resulting from postorogenic extension. The competence contrast between the nappe and the rest of the upper crust as well as the strength of the crust itself are the principal variable parameters of our experiments. The results suggest that a dipping weak nappe introduces a lateral velocity discontinuity and serves as a localization factor for deformation. The presence of a preexisting nappe with a low competence contrast is sufficient to localize strain along the nappe leading to the formation of a flexural rolling hinge. In this case, the migration of the basin is slow, continuous, and limited by gravity driven processes that lead to the rise of hot (weak) material under the subsiding basin. In case of a high competence contrast, overall crustal strength is reduced by a “dipping pie” effect. Assuming overall high crustal strength, the presence of a contrasting nappe leads to a bimodal fault pattern governed by two types of faults: crustal-scale planar faults rooting on the brittle ductile transition of the crust and thin-skinned listric faults rooting on the nappe itself. This bimodality results in a jump-like migration of the basin downward along the dipping weak nappe. Applying this model to the case of the Gulf of Corinth (Greece) allows us to explain, in the case of assumed high competence contrast, the observed microseismicity patterns, the asymmetry of the Gulf, and the kinematics of fault migration within the basin.

Initial crustal thickness geometry controls on the extension in a back arc domain: Case of the Gulf of Corinth

[1]xa0Since 60 Myr, Peloponnesus and continental Greece have been affected by the Hellenidean compressional and the Aegean extensional phases. This complex evolution resulted in development of a strongly inhomogeneous crust in the Gulf of Corinth region. To study this area, we use a large strain thermomechanical numerical code PARAVOZ previously used for a number of similar problems such as rift evolution. Yet, instead of varying boundary and initial conditions applied to a plane-layered model, we use available geophysical constraints on the actual deep structure of the lithosphere to test its different possible initial structures. By varying the position of the initial crustal heterogeneity versus the position of the lithospheric slab, we are able to explain the origin of the internal structures and the kinematics of the Gulf of Corinth. The results suggest that the development of shear zones in the lower crust is favored by the gravitational collapse of the thicker part of the crust, whereas the geometry and the kinematics of these shear zones are controlled by the position of the edge of the slab. Asymmetry is seen in cases when a horizontal shift exists between the edge of the slab and the thicker part of the crust. Our model explains the differences between the northern shore and the southern shore as well as the east west variations observed in the Gulf of Corinth.

Lithospheric convective instability could induce creep along part of the San Andreas fault

Along the western border of the Sierra Nevada microplate, the San Andreas fault (California, United States) is comprised of three segments. Two (north and south segments) are locked and support large earthquakes (e.g., the M 7.7 1906 San Francisco and the M 7.8 1857 Fort Tejon earthquakes), while the central segment, from Parkfield to San Juan Bautista, is creeping. Based on mechanical models, we show that the late Pliocene–Quaternary convective removal (delamination) of the southern Sierra Nevada mantle lithosphere and associated uplift of the Sierra Nevada Mountains causes the Great Valley upper crust to deform by flexure and buckling. Additional three-dimensional flexural models imply that the local flexural bulge overlaps with the creeping segment of the fault system, while geological observations indicate that the local weakening of the San Andreas fault started at the same time that the Sierra Nevada started its recent phase of uplift. We argue that bending stresses promote lithostatic pore pressure to occur in the depth range of 7–15 km, causing the effective strength of the fault to vanish, and locally favoring creep. Our results suggest for the first time that earthquake cycles along a major plate boundary may be influenced by convective instabilities in the adjacent upper mantle.