Constraints on Mesozoic Translation of Cordilleran Terranes from a Transect Across the Peninsular Ranges Batholith in Northern Baja CaliforniaSchmidt, K L (1), Paterson, S R (1), Johnson, S E (2),
(1) Earth Sciences SCI 117, University of Southern California, 3651 University Ave., Los Angeles, CA 90089-0740 United States;(2)Department of Geology and Geophysics, Macquarie University, Sydney, NSW 2109 Australia
Abstract:
It has been hypothesized that Mesozoic arc terranes now located in the Coast Ranges of British Columbia originated at the present latitude of Baja California. In the Sierra San Pedro Martir(SSPM) region of northern Baja California, remnants of a J(?)-K continental margin arc are preserved as the Peninsular Ranges batholith (PRb) and intervening country rocks consisting of Mesozoic arc volcano-sedimentary assemblages and Proterozoic(?)-Paleozoic passive margin sedimentary assemblages. A number of constraints tie this arc remnant across the Gulf of California to Sonora, Mexico, and restrict deformation that may have been involved in collision or translation of terranes both in time and location at this latitude.
As noted by Gastil (1993), a transition between Proterozoic(?)-Paleozoic miogeoclinal shallow- and deep-marine facies on the eastern side of the peninsula can be fit to the same feature in Sonora, Mexico by restoring Neogene deformation in the Gulf of California. This feature links the eastern side of the Baja peninsula with Sonora since Paleozoic time, and precludes any Mesozoic margin-parallel strike-slip faults that may have been involved with terrane translation in this region.
In the center of the peninsula, the transition between western and eastern zones of the PRb, which corresponds to plutons of oceanic and continental geochemical affinity respectively, occurs within a $\sim$20 km wide zone of strongly deformed plutons and gneissic host rocks. New age data and detailed studies of fabric relationships between plutons and deformed host rocks suggest this zone has a protracted history of complex, predominately contractional deformation that occurred synchronous with intrusion. Orthogneisses of unknown age host $\sim$133 and $\sim$118 Ma plutons that were deformed syn-post intrusion. The undeformed, zoned SSPM pluton, which is part of the regionally extensive La Posta type plutonic suite, precludes any major deformation through this zone after $\sim$97 Ma.
Gneisses of the central zone lie in fault contact with shallow-level arc volcanic rocks of the Early K Alisitos Fm to the west. This fault also has a complex history including mylonitic contractional deformation that occurred in conjunction with the central zone, and a younger, contractional, brittle overprint. Regionally, this contact is sinuous and no Mesozoic strike-slip features have been noted in it or in Alisitos rocks to the west.
Cordilleran terrane models that propose terrane amalgamation and/or translation along the North American margin at this latitude must comply with the following constraints: (1) any large-scale strike-slip fault must either be older than $\sim$133 Ma or lie west of the Alisitos assemblage; (2) terrane collision is restricted to $\sim$pre-97 Ma if its effects were recorded on the continental margin; and (3) arc magmatism was active within the time frame of $\sim$133 to $\sim$80 Ma.
