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Description
A prebatholithic boundary between a western domain of Jurassic-Cretaceous volcanic arc rocks and an eastern domain of continentally derived flysch trends northwest along the axis of the Peninsular Ranges. A study of the rocks within a 45 square kilometer area near Rancho El Rosarito, Baja California, Mexico (30° 30' N. Latitude), indicates that east over west ductile thrusting occurred along this boundary during the latest Albian. The main lithologic units in the Rancho El Rosarito area, from west to east, are the "western metavolcanic assemblage" (WMA), the "eastern metasedimentary sequence" (EMS), and the "Rancho El Rosarito gneiss" (RERG). Occurring west of the prebatholithic boundary, the WMA is comprised of Cretaceous age, rhyolitic to basaltic volcanic flows and tuffs. These rocks have been metamorphosed to greenschist facies, exhibit primary igneous textures, and are generally undeformed. Occurring east of the boundary, the EMS is predominantly comprised of clastic sedimentary rocks that have been variably metamorphosed from greenschist to amphibolite facies (metamorphic grade increases towards the east). In sharp contrast to the adjacent WHA, the rocks of the EMS are pervasively and multiply deformed L-S tectonites. Regional (main phase) deformation produced a northwest-trending, east-dipping, bedding-parallel foliation, a down-dip extensional mineral lineation, and isoclinal folds that are axial planar to the foliation and which have axes parallel to the lineation. Post main phase deformation produced, northwest-trending, steeply east-dipping, westward-vergent folds that occur in several domains near the prebatholithic boundary. Structurally overlying the EMS is the concordantly deformed RERG. It is dominated by pervasively foliated and lineated orthogneisses of granodioritic composition that have been dated by the U-Pb zircon method at 108 Ma. within the RERG, main phase deformation produced a composite S-C mylonitic fabric that consistently indicates an east-over-west sense of shear. The abrupt change in lithology and structural style at the prebatholithic boundary suggests that the WMA and the EMS are structurally juxtaposed. Down-dip lineations in the EMS and the RERG suggest that movements were predominantly dip-slip. The eastward increase in metamorphic grade, the west-verging folds, and the S-C fabric all indicate that the east side was thrust over the west side. U-Pb geochronology from the RERG and the nearby, undeformed, Sierra San Pedro Martir pluton constrain regional deformation to the latest Albian. All lines of evidence are compatible with west directed translation along the prebatholithic boundary during the medial Cretaceous. Deformation may have been the consequence of externally imposed forces related to the accretion of the Jurassic-Cretaceous arc to cratonal North America. Alternatively it may be a manifestation of internally imposed forces related to the dynamic evolution of the batholith.
