THE MANY FACETS OF TRANSTENSION, FROM OBLIQUE RIFTING TO OBLIQUE OROGENIC COLLAPSE

Transtension occurs in tectonic settings ranging from oblique continental rifting to oblique orogenic collapse of thickened crust. Surface processes (erosion/sedimentation) and heat/mass transfer at depth strongly influence the structural and thermal evolution of these systems and determine their rheological transitions (brittle/ductile crust and lithospheric/asthenospheric mantle). In oblique rifts, the theory of transtensional strain, including strike-slip partitioned strain, predicts quite successfully the orientation and distribution of faults, the geometry of sedimentary basins, the rotation of crustal blocks, and the behavior of ductile crust. Mismatch between structures in the brittle and ductile crust can be related to the presence of detachments or attachments, depending on the nature of the brittle-ductile transition and the degree of mechanical coupling between these layers. Strain and kinematic models of attachment zones predict specific structural patterns with which natural structures are compared. Examples from the circum-Caribbean region and from intracontinental zones in North America and central Anatolia indicate that observed deformation patterns are consistent with a set of boundary conditions combining oblique divergence and the role of rheological layers in the lithosphere. Oblique collapse of thickened orogenic crust involves a coupled erosional/depositional system in the upper crust and a sharp rheological transition between the brittle crust and, typically, a partially molten layer beneath. Flow of the partially molten crust accommodates upper-crustal deformation (tilted blocks, en échelon basins) and controls the structural pattern of the viscous crust. Foliation and lineation trajectories in high-grade metamorphic rocks of North American and central Anatolian metamorphic core complexes indicate that flow is driven by oblique divergence, but also by pressure gradients related to uneven crustal thickness, density distribution, and local density inversion that drives diapiric flow in migmatite domes.