DETRITAL ZIRCON GEOCHRONOLOGY OF THE WESTERN BAJA TERRANE: CONSTRAINTS ON THE TIMING OF MESOZOIC SUBDUCTION AND ACCRETION IN BAJA CALIFORNIA

Blueschist facies subduction assemblages of the Western Baja terrane (WBt) crop out on several islands west of the Baja California Peninsula. These metasediments and metabasites represent the exhumed products of Mesozoic tectonic underplating beneath western North America. Constraints on the timing of deposition, underthrusting, and unroofing are sparse, rendering the tectonic significance of the WBt uncertain. In this study we present new U-Pb detrital zircon geochronologic ages for the WBt, thereby placing new constraints on the detrital sources and the maximum depositional ages of the WBt. Thirteen samples were collected from Isla Cedros, Islas San Benitos, and Isla Santa Margarita, producing a total of 1023 concordant analyses. The WBt yields two distinct age peaks: at ca. 100 Ma and ca. 140-190 Ma, with subsidiary peaks at ca. 1.2, 1.5, and 1.7 Ga. The most important provenance implication for this study is that the WBt is ultimately a domestic product of Laurentia and is not an exotic terrane. The WBt has two primary lithological subunits: a predominantly metasedimentary unit and a predominantly metabasaltic unit. WBt subunits yield maximum depositional ages (MDAs) of 85-98 Ma and 93-105 Ma, respectively. Further constraints are placed on the timing of the WBt through comparisons with temporally overlapping forearc assemblages on mainland Baja California. Forearc assemblages yield MDAs of ca. 65-92 Ma, suggesting sedimentation was continuous through the Late Cretaceous. These relations require the WBt to have been isolated from <85 Ma sedimentation and the most likely mechanism responsible for shielding the WBt from post-85 Ma detritus was its sequestration in the subduction zone. Analysis of the WBt implies accretion in Baja California occurred for ~15 Ma, beginning ca. 100 Ma and ending ca. 85 Ma. The timing of WBt accretion and impingement of a large igneous province with the margin of southwestern North America (75-95 Ma) overlap significantly, suggesting that accretion was driven by slab flattening.