BUILDING MOUNTAINS IN OLD CONTINENTAL CRUST: EXAMPLES FROM ARGENTINA AND IDAHO-MONTANA

Many orogenic belts are characterized by along-strike and foreland-ward changes in thrust geometry and structural style. Given that thrust systems often result from shortening of a pre-existing sedimentary basin, these changes can typically be related back to a prior basin-forming (or destroying) orogenic event. In the retroarc thrust belts of the North and South American Cordillera, two prominent orogen-scale changes are the focus of this study: the Wyoming salient of the northern Rocky Mountains and the Bolivian salient of the southern central Andes.

In both regions, many have hypothesized that the along-strike changes are linked to intrinsic properties: e.g., the distribution and thickness of pre-orogenic stratigraphy applied in the context of critical taper theory; others have implicated extrinsic drivers including major geodynamic changes at the plate interface, such as dip angle of the subducting oceanic plate. Recent field mapping, structural analysis, U-Pb geochronology, and (U-Th) thermochronometry from east-central Idaho and northern Argentina point toward an intrinsic control. The recesses marking the edges of both salients locate along thick, fine-grained clastic Proterozoic rift basins, precluding basin thickness as an origin for the change in structural style. The studied areas are also distant from proposed regions of shallow subduction, obviating plate dip as a major control. Instead, both regions are marked by pre-Cordilleran tectonism that prevented deposition or removed mechanically weak units that were elsewhere exploited as décollement horizons. In east-central Idaho, the Lemhi arch was a region of erosion/nondeposition during Laurentian rifting and precluded deposition of weak Cambrian shales exploited throughout the Wyoming salient. In northern Argentina, the Transpampean arch persisted following Ordovician tectonism, which precluded deposition of units exploited as detachment horizons within the Bolivian salient. In the absence of these décollement horizons, alternate favorably oriented weaknesses were instead exploited, such as the brittle-ductile transition or older rift-related detachments. In addition, pre-orogenic faults were also reactivated, resulting in a thick-skinned or hybrid structural style.