CROSS FAULTS AND LATERAL HETEROGENEITY IN MOUNTAIN BELTS

Despite the general, lateral continuity of contractional structures in orogenic belts, there has been documentation of important orthogonal/cross structures as well as lateral heterogeneity in deformation style, igneous activity, metamorphic grade, geomorphology and seismic activity in several orogens. To assess the occurrence, causal mechanisms, and implications of these lateral heterogeneities, a selection of convergent orogens, with different tectonic settings and history are reviewed. Major factors driving the lateral heterogeneity and/or cross structures include the pre-existing deformational history of the cratonic blocks involved, lateral change in lithology of crustal rocks, variations in crustal/lithospheric rheologic properties, or changes in plate kinematics. The Appalachian orogenic front mimics the Iapetan rift margin. Pre-existing basement structures have control on pre- and syn- orogenic sedimentation, which subsequently impacts orogenic wedge evolution. A thicker sedimentary sequence generally evolves into a salient (as opposed to a recess), which is further enhanced by the presence of weak horizons as seen in the Zagros and the Cordillera. During orogenic contraction, inherited basement structures are preferentially reactivated based on their orientation. Several cross faults in the Himalayas spatially coincide with orogen-perpendicular, lower plate, basement structures. In a similar way, oceanic subducting plate physiography can also influence deformation in the overriding plate. Along-strike variations in subduction dynamics have been reflected in the Andean deformation. Orogenic extension in the Alps has been accompanied by a system of orogen-parallel strike slip faults and extensional cross faults. It is evident that lateral heterogeneities can form crucial control on the evolution of orogenic belts and can influence seismic rupture patterns, resource occurrence, and landslide-related natural hazards.