LARAMIDE-STYLE BACKTHRUSTS AND TRIANGLE ZONES IN THE U.S. ROCKIES AND SIERRAS PAMPEANAS, ARGENTINA

Backthrusts are antithetic faults associated with the up-dip portions of basement-rooted, Laramide thrust systems and provide important and necessary kinematic avenues for displacement transfer into high-level folds in sedimentary cover rocks. We define two general classes of backthrusts: 1) thin-skinned backthrusts at the up-dip termination of basement-rooted Laramide thrusts, often creating triangle zones and stacked duplex structures with multiple detachment horizons in the trishear zone; and 2) basement-rooted backthrusts in the hanging wall of major Laramide thrusts. A classic example of the first is in southwest Montana where late stage motion on the sub-Bridger thrust zone delaminated the Upper Cretaceous Livingston Group along the Billman Creek Formation, forming a major backthrust uplift and footwall triangle zone. The triangle zone crops out up-plunge along the northern third of the Sedan quadrangle (Skipp et al., 1999) and is one of the best documented examples of a backthrust/triangle zone associated with a Laramide uplift in the Rocky Mountains. Other examples occur throughout the Rockies based on surface geology and subsurface data, but only in areas where erosion has not obliterated the up-dip portions of Laramide thrusts. Modern analogs of backthrusts are documented in the active Laramide-style uplifts of the Sierras Pampeanas of Argentina, where relicts of a pre-Tertiary regional erosion surface reveal the uplift geometry of basement blocks. Despite most uplifts exhibiting a dominant bounding fault and strong asymmetry related to block tilting, antithetic faults are present as: a) backthrusts giving rise to symmetric block uplifts, as evidenced by flat-lying remnants of the erosion surface and/or sedimentary cover (Hualfin and El Gigante ranges); b) secondary faults disrupting the smooth backslope of asymmetric ranges, accommodate substantially less displacement during uplift than main bounding faults (Pocho and Velazco Oriental ranges). All these backthrusts are thought to be long-lived, inherited structures with several reactivations during different stress conditions since Late Paleozoic.