USING SAND PROVENANCE TO CONSTRAIN THE TECTONIC HISTORY OF THE OUTER CALIFORNIA BORDERLAND

Detrital modes of forty sand/ash and sandstone/tuff samples from DSDP sites drilled during Leg 63 along the outer California Borderland reflect the dismemberment and overprinting of a continental forearc during triple-junction migration and plate margin reorganization. Variable sand provenance along this margin attests to its complex tectonic history.

Site 467 lies in the San Miguel Gap at the northwestern extreme of the outer Borderland. Samples from this site fall into three compositional groups: the oldest (~16.5-14 Ma) is quartzo-lithic with a mean composition of Q35F20L45, Qm60K11P29, and Lm17Lv64Ls19; the middle (~13-11 Ma) consists of extremely altered mafic (?) tuff; and the youngest (~5-2 Ma) is quartzo-feldspathic with a mean composition of Q29F47L24, Qm35K12P53, and Lm9Lv67Ls24. Local submarine geology suggests an eastern source for the quartzose sand. Compositional shifts at Site 467 may reflect rifting of the Transverse Ranges from the outer Borderland, then rotation of the Transverse Ranges, and subsequent northward translation of the outer Borderland.

Site 468 lies in an embayment along the Patton Escarpment, and Site 469 lies further west, on oceanic crust. Within the sand-bearing section at Site 469 (18-16 Ma), the oldest samples are volcaniclastic (e.g., Q2F9L89, Lm1Lv96Ls3), but there is an upsection increase in the amount of sedimentary and metamorphic lithic fragments (e.g., Q29F27L44, Lm8Lv19Ls73). The oldest sample (~17 Ma) examined from Site 468 is also slightly enriched in metamorphic and sedimentary fragments (Q19F27L54, Lm5Lv79Ls16). However, younger samples (16-2 Ma) at Site 468 are generally more volcaniclastic (mean=Q7F41L52, Qm12K17P71, and Lm1Lv98Ls1). We concur with shipboard scientists that shallow-water bioclasts and rounded glassy epiclasts found at both sites suggest that they were derived from emergent island(s), probably on the Patton escarpment. Sedimentary and metamorphic lithic types are consistent with an accretionary prism source. The sand detrital modes for Sites 468 and 469 record forearc uplift and magmatism likely associated with ridge subduction during the southward passage of the Rivera Triple Junction.