2003 Seattle Annual Meeting (November 2–5, 2003)

Paper No. 11
Presentation Time: 3:30 PM


ALSLEBEN, Helge1, WETMORE, Paul H.1, SCHMIDT, Keegan L.2, GEHRELS, George E.3 and PATERSON, Scott R.1, (1)Department of Earth Sciences, Univ of Southern California, 3651 Trousdale Parkway, Los Angeles, CA 90089-0740, (2)Division of Natural Science, Lewis - Clark State College, Lewis - Clark State College, Division of Natural Science, 500 8th Ave, Lewiston, ID 83501, (3)Department of Geosciences, Univ of Arizona, Box 210077, Tucson, AZ 85721, alsleben@earth.usc.edu

The Alisitos arc, located in the western zone of the Jura-Cretaceous Peninsular Ranges batholith and extending from the ancestral Agua Blanca fault (aABF) south to at least the 28th parallel, collided with North America in the mid-Cretaceous. The aABF and Main Martir thrust (MMT) define northern and eastern boundaries of the arc, respectively. The northern end of the arc is characterized by subhorizontal tight to isoclinal folds and reverse faults that define a NW-trending, SW-vergent fold-thrust belt parallel to the aABF, which shows a component of sinistral transpression related to oblique convergence against a NW-trending continental margin. To the south, in the Sierra San Pedro Martir, where convergence was orthogonal, structures dominantly show east over west shear sense. Overall contraction may be greater than to the north as indicated by a significant metamorphic gradient across the MMT and steeply plunging fold hinges that rotated towards parallelism with the transport direction. Farther south, in the Canyon Calamajue, arc deformation is confined to a <10 km wide zone. Here, tight, shallowly plunging folds and S to SW-vergent brittle-ductile thrust faults define a fold-thrust belt that displays a weak W to E metamorphic gradient and arc strata may have accommodated less shortening than to the north.

Accretion-related deformation in the Alisitos arc is constrained to between 115 and 103 Ma. A broad zone of deformation with a strong strain gradient towards the suture zone is evident from ~115 to 105 Ma, as deformed, 115 Ma volcanogenic strata are intruded by 108 to 105 Ma plutons that deflect and truncate regional structures. Subsequent shear became more localized as both the aABF and MMT truncate and/or deform only the margins of plutons as young as 103 Ma.

Arc accretion requires closure of an inter-arc basin. This basin, preserved along the northeastern and eastern sides of the Alisitos arc, is represented by a mid-Cretaceous sequence of limestone, volcanogenic sandstone, and shale. A preliminary detrital zircon study of these strata tracks the approach of the Alisitos arc towards the North American continental margin. The oldest analyzed samples (~115 Ma) yield dominantly arc-derived zircons, whereas samples of younger strata (~105-103 Ma) yield progressively larger proportions of zircons with Precambrian ages.