DEFORMATION PATTERNS ACROSS THE LARAMIDE AND SIERRA PAMPEANAS THICK-SKINNED FORELAND SYSTEMS; RELATIONS TO PLATE DYNAMICS, LITHOSPHERIC STRESS TRANSMISSION, AND CRUSTAL ARCHITECTURE

Outstanding questions regarding the transmission of stresses from active plate boundaries into continental interiors have existed ever since linkages were first made between low-angle subduction and thick-skinned foreland systems. Our analyses of deformation fabrics and paleomagnetic directions across both ancient (Laramide) and modern (Sierras Pampeanas) thick-skinned foreland systems show consistent regional patterns of shortening that are spatially and temporally linked to convergent plate kinematics and dynamics, along with inherited crustal architecture, at the time of deformation initiation. Early layer-parallel shortening (LPS) fabrics across these systems, which include minor conjugate wedge and strike-slip faults, fracture sets, cleavage, and anisotropy of magnetic susceptibility (AMS) fabrics, record shortening directions that initiated in the sedimentary cover and were subsequently tilted and locally rotated about vertical axes during development of basement-cored uplifts. Stress inversion indicates low stress ratios in most areas, with higher values where multiple fold trends are developed. AMS Kmax lineations, defined by kinked, rotated, and intersecting phyllosilicate fabrics, formed during limited LPS in red beds. Early LPS directions in the Laramide of WY are oriented 050–070° for sites located along NW- to NNW-trending arches, and in the Sierra Pampeanas are oriented 080-100^o along N- to NW-trending arches. In both systems, LPS directions display deflections that partly correlate with regional structural grain of obliquely trending arches that formed along pre-existing basement weaknesses. Regionally, orientations of fully restored LPS fabrics across the forelands are consistent with an integrated tectonic system driven to a first-order by initiation of flat-slab subduction at the plate boundary. This geodynamic transformation likely resulted in removal of weaker mantle asthenosphere from the overriding plate along with increased inter-plate coupling with the down-going plate. This coupling allowed efficient transmission of stresses across the strong (old, cold, and possibly dry) continental lithospheric mantle and mafic lower crust, into the upper crust where strain was localized along relatively weak, inherited structural heterogeneities.