LATE CENOZOIC EXTENSIONAL TECTONICS OF THE CAÑON EL PARRAL BASIN, SIERRA SAN FELIPE, NE BAJA CALIFORNIA

The Gulf Extensional province (GEP) of NE Baja California has been extending since Late Miocene time. The GEP has recorded the process of rifting from ENE distributed extension to dextral shear, clockwise-block rotation, and rapid extension in pull-apart basins in the present day Gulf of California. Strata in the Cañon El Parral (CEP) area, 35 km SSW of San Felipe, record extensional tectonics related to opening of the gulf. Early Miocene eolian sandstones (Tss0) and conglomerates containing exotic-clasts (Tcg0) are intercalated with olivine basalts (Tb1) and overlain by the tuff of San Felipe (Tmr1, 12.6 Ma, Stock et al., 1999 J.Volc.& Geoth. Res.) Unconformably overlying Tmr1 are tuffaceous sandstones (Tss1) and higher 6.3 Ma ignimbrite deposits (Tmr3 and Tmr4) followed by Plio-Pleistocene conglomerates (TQcg1). Regionally, most extension post-dates Tmr1 and most NE-striking sinistral/normal faults probably developed in Pliocene time (Lewis and Stock, 1998, J. Struc. Geol.) However, thickness changes, paleoflow indicators, and grain size changes in Tss1 indicate deposition synchronous with one large ENE-striking fault in CEP. Tss1, Tmr3, Tmr4, and TQcg1 also record waning activity on N- to NNE striking, W-dipping faults that accommodated E-W to ESE-WNW extension. To the SE, sinistral/normal faults accommodated Mio-Pliocene clockwise block rotations (Lewis and Stock, 1998). In the study area, a gently NE-plunging syncline-anticline pair is defined by Tmr1 and overlying pre-6 Ma tuffaceous sandstones. This fold pair probably reflects a zone of distributed sinistral shear, ~2.5 km wide, between clockwise-rotating blocks. Sparse kinematic data from faults cutting TQcg1 in the CEP area could suggest ENE-striking sinistral faults were active into the Plio-Pleistocene. The absence of Holocene faults implies that the period of clockwise block rotation has ended. Present extension is isolated along the active Sierra San Pedro Mártir fault and in active gulf spreading centers.