PALEOGENE BASEMENT THRUST REACTIVATION OF MESOZOIC NORMAL FAULTS: EVIDENCE FROM THE LOBO FORMATION, SOUTHWEST NEW MEXICO

Lobo Formation outcrops in the Florida Mountains and at Granite Hill in southern New Mexico provide a record of late Laramide (Paleogene) intermontane basin formation and basement-involved thrusting. The Lobo records thrust uplift of alkali-feldspar granite along inferred basement-involved reverse faults that in part reactivated middle Mesozoic normal faults. We define two members of the Lobo: (1) a lower Granite Hill Mbr, 57 m thick, composed of granite-cobble and -boulder conglomerate of uncertain age deposited in an alluvial-fan setting; (2) an upper Capitol Dome Mbr, 120 m thick, composed of fluvial, palustrine and lacustrine deposits of probable late Paleocene-early Eocene age. The contact between the members at Granite Hill is transitional. At Granite Hill, the lower member overlies late Cambrian (497±1 Ma, U-Pb zircon) granite; in the Florida Mountains, the lower member is absent, and the unfolded upper member locally overlies folded Paleozoic strata and Proterozoic to Cambrian basement, and elsewhere is overthust by basement. One km west of Granite Hill, lower Permian limestone at Eagle Nest is thrust over overturned Lower Cretaceous strata. Rare sedimentary clasts in the Lobo indicate that these nearby Paleozoic and Mesozoic strata were not significant sources for synorogenic conglomerate and alternatively suggest a basement buttress, not presently exposed, thrust over the sedimentary rocks at Eagle Nest as the source for lower-member granite clasts. We measured gravity along two transects with stations separated by ~80 m and a trend of ~065, approximately perpendicular to the strike of the Eagle Nest thrust. Gravity data indicate a multi-phase fault zone between Eagle Nest and Granite Hill dipping ~65° to the SW. Although the precise location of the source for basement clasts in the lower Lobo remains uncertain, granitic basement uplifts exposed nearby in the Florida Mountains, and our geophysical, geochronologic, stratigraphic and structural data indicate a geologic history wherein Paleocene(?)-Eocene Laramide thrusting reactivated Jurassic-Early Cretaceous normal faults and ceased in the middle Eocene. Laramide basement faults were subsequently reactivated in a normal sense during Basin and Range extension resulting in burial of the nearby granitic source for Lobo Formation sediment.