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PARTIONING OF DEXTRAL SLIP IN AN INCIPIENT TRANSVERSE SHEAR ZONE OF NEOGENE AGE, NORTHWESTERN ALBUQUERQUE BASIN, RIO GRANDE RIFT, NEW MEXICO

Richard M. Chamberlin

Geological Society of America, Abstracts with Programs, v31, no7, pA-113, 1999

ABSTRACT--Recent geologic mapping near the northwest margin of the Albuquerque Basin (Loma Creston quadrangle) has revealed a subtle ENE-trending zone of distributed dextral shear between Bodega Butte and Mesita Cocida. The Bodega-Cocida transverse zone is expressed as a 1.5 km-wide belt of deflections, splays and terminations within the dominant northerly-striking system of high-angle normal faults that controlled Neogene basin subsidence. Restored cross sections near Loma Creston indicate about 10% westerly directed extension.

Gently tilted basin fill strata consist of fluvial siliciclastics and alluvial volcaniclastics generally shed from the northwestern basin margin; sedimentary facies are locally intercalated with basaltic to rhyolitic lavas and tuffs erupted from the southern periphery of the Jemez volcanic field. Published radiometric dates of correlative volcanic units range from about 10.4 to 2.5 Ma. Footwalls of longitudinal rift faults, dominantly downthrown to the east, are locally well defined by north-trending ridges of hydrothermally silicified sandstones and conglomerates. Alteration patterns suggest two episodes of southerly fault-guided hydrothermal flow spatially linked to rhyolitic eruptive centers dated at about 10 and 7 Ma.

Sliplines (striations and secondary "Riedel" shears) are often well preserved and exposed on silicified faces of the Neogene rift faults. A limited number of fault slip observations (n=12) suggests the following pattern. The predominant east-down high-angle normal faults, which exhibit throws of 100-500 m, are essentially dip-slip with a very minor component of dextral slip (striae pitch 78-87 south). Within the Bodega-Cocida transverse zone, NNE-striking high-angle normal faults of moderate throw (10-60 m) show moderate to strong dextral oblique slip (striae pitch 28-52 south on east-down faults and 11-63 north on west-down faults). Two NNE-striking sub-vertical dextral-slip faults (striae pitch at 2-8 ) are well exposed in an incipient antithetic accommodation zone where opposing dextral oblique faults locally overlap on the south flank of Mesita Cocida. Regional patterns suggest the Bodega-Cocida transverse zone may represent deflections of north and south propagating rift faults by a preexisting basement shear zone. (Poster is available for viewing at the Bureau of Geology in Socorro)