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The Laramide Zuni uplift, Southeastern Colorado Plateau: a microcosm of eurasian-style indentation-extrusion tectonics?


New Mexico Geological Society, Guidebook 40, p 81-90, 1989

ABSTRACT - Published accounts of Eurasian tectonics and indentation-extrusion experiments on plasticine support the concept that India has acted as a rigid indenter and driven 2000 km into "plastic" Eurasia thereby causing: (1) vertical extrusion of the Himalayas immediately in front of the indenter, (2) cradling and northward translation of the unstable wedge-shaped Tibetan Plateau in front of the Himalayan uplift, now accreted to the indenter and (3) lateral extrusion of Indochina, followed by China, eastward off the right face the Tibetan wedge, which now forms a streamlined secondary face accreted to the Indian indenter. The NW-trending Zuni uplift (60 x 100 km) lies within the Colorado Plateau, adjacent to the N face of a NNE-trending gravity high, which extends from El Morro, NM to the plateau margin near Morenci, AZ, about 250 km to the SSW. Limited evidence indicates that the El Morro gravity high represents a Precambrian mafic igneous belt (rift?); and that it was truncated by a NW-trending strike-slip shear zone during the late Paleozoic Ouachita-ancestral Rocky Mountain orogeny. We hypothesize that during the late Laramide (Eocene), the El Morro gravity high acted as a rigid crustal beam (indenter); it was detached at a mid-crustal ductile zone and driven northward by horizontal stresses generated at the edge of the plateau. The N face of the El Morro indenter functioned as a large hydraulic ram; it created hydrostatic pressures in excess of the lithostatic pressures, and thereby caused about 2400 m of vertical extrusion and uplift centered about the Mt. Sedgwick block complex ¾ the Precambrian core of the Zuni uplift. High-angle reverse faults that bound this core complex appear to grade laterally into left- and right-lateral strike-slip faults toward the corners of the main block. Extruded blocks preferentially slipped left (NW) across the indenter face, locally pulling apart and creating a cup in the N face of the uplift. Inward compression, caused by the cup, created the wedge-shaped Bluewater Lake block, now spearheading the streamlined indenter. The sinuous Nutria monocline was then formed at the leading edge of tapered blocks, uplifted and extruded to the west off the spearhead. At the same time, en echelon (left stepping) monoclines of the Grants-Fernandez system formed in front of NNE-extruded blocks on the right flank of the spearhead. Late-stage strike-slip faults outlined the contracted flanks of the the indenter as it weakened by strain heating(?). Near Fence Lake, along the west flank of the El Morro gravity high, as much as 5 km of left-lateral slip is inferred on a concealed fault that juxtaposes marine against nonmarine Cretaceous strata. We suggest these traits may qualify the Zuni uplift as a microcosm of Eurasian indentation-extrusion tectonics.