HOT OROGENIC HINTERLAND CRUST DECOUPLED FROM BASAL TRACTIONS: A COMPARISON OF FLAT SLAB AND UNDERTHRUSTING EXAMPLES IN THE NORTH AMERICAN LARAMIDE, SOUTHEAST CHINA, AND SOUTHERN TIBET

During plate convergence, shallow subduction or underthrusting of the lower-plate lithosphere beneath an overriding plate often results in far-field intraplate deformation. Examples include the Late Cretaceous–Paleogene North American Laramide or the Cenozoic Himalayan-Tibetan orogen and related Tibetan plateau. Perplexingly, during this flat-slab or underthrusting process, wide expanses of crust between the plate boundary and intraplate orogen did not experience strong synchronous deformation. These apparently undeformed crustal regions may reflect (1) a strong, rigid plate, (2) increased gravitational potential energy (GPE) to resist shortening and uplift, or (3) localized decoupling of the upper-plate lithosphere from any basal tractions. Here we review the geology of three orogens that formed due to flat slab subduction or underthrusting: the Cenozoic Himalayan-Tibetan, the Mesozoic southeast China, and the Late Cretaceous–Paleogene Laramide orogens. These orogens all involved intraplate deformation >1000-km from the plate boundary and large regions of negligible crustal shortening between the plate-boundary and intra-plate thrust belts. A subhorizontal slab is thought to refrigerate the upper plate, but our observations show that the studied hinterland regions exhibited hot geotherms evidenced by peak thermometry transects, thermochronology, modern heatflow, and 3D thermal modeling. The ubiquity of this hinterland heating may reflect underthrusting-related crustal melting. Whatever the cause, a hot and weak hinterland is inconsistent with it persisting as an undeformed rigid block. Although the Tibet and Laramide cases may have been modulated by elevated GPE inhibiting further crustal thickening, the SE China hinterland involved tectonic quiescence and shallow marine sedimentation. This suggests that hinterland quiescence is not uniquely due to thickened crust and GPE. Comparison of these intracontinental orogens allows us to advance a general model, where hot orogenic hinterlands with a weak, mobile lower crust allow decoupling from underlying basal tractions exerted from flat-slab or underthrusting events. This hypothesis suggests that basal tractions locally drive intraplate orogens, at least partially controlled by the strength of the upper-plate lithosphere.