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BAUER, Paul, New Mexico Bureau of Geology and Mineral Resources, Socorro, NM 87801, bauer@nmt.edu; RALSER, S., New Mexico Tech; ILG B., Glorieta, NM; KELLEY, S., NM Tech; MARCOLINE, J., Dixon, NM.

Detailed mapping in the S. Sangre de Cristo Mountains east of Santa Fe has delineated major fault systems characterized by complex geometries of N-striking, high-angle, anastomosing brittle fault systems. The Picuris-Pecos (P-P) fault extends 85 km south from Taos and projects northward to the Sangre de Cristo fault. The Borrego fault, to the west, juxtaposes disparate Proterozoic units. The major splays in these two fault systems contain breccia zones (tens of m wide), and fracture zones more than a km wide. Locally, horses of undeformed Paleozoic limestone "float" in fractured Proterozoic granites. Strike-slip slickenlines are predominant, but oblique- and dip-slip striae are common. Preliminary apatite fission-track (AFT) data suggest that the major fault strands have had different Cenozoic kinematic histories along strike; across the southern P-P and Borrego faults we infer Neogene vertical movement, whereas, farther north, no significant difference in post-Laramide cooling history across the P-P is detected.

The Apache Canyon (AC) fault near Canoncito, is a peculiar, moderately S-dipping, arcuate fault, with down-dip slickenlines, that links the P-P and Borrego faults. The AC fault juxtaposes highly fractured Proterozoic gneiss against moderately fractured Penn-Permian sedimentary strata along a zone of silicified breccia. Six km south of the AC fault, the P-P and Borrego faults appear to merge into a single fault with a combined fault separation (from Triassic strata) that is much less than that on either fault to the north (in Proterozoic and Penn rocks). AFT ages for 4 samples across the AC fault are similar (49-52 Ma), indicating little post-Laramide vertical offset. The AC fault is interpreted as a Laramide transtensional normal fault between the two strike-slip zones. The remarkable decrease in fault separation to the south reflects the reactivation history (e.g., older rocks display more net slip than younger rocks), and/or that strain was transferred to parallel faults to the west.

Geometric and kinematic data gathered from several hundred sq km of this faulted terrain are interpreted as evidence for a long history of fault activation and reactivation: 1) post-1.4 Ga, pre-Penn. strike-slip(?); 2) Miss. and Penn. dip-slip(?); 3) major Laramide right-slip, with overall geometry of a positive flower structure, with coeval strike-slip, oblique-slip, and dip-slip faulting; and 4) Neogene, rift-related normal faulting.

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