Abstract: The Western Transverse Ranges Microplate as a Native Terrane
Michael D. Campbell, Walter E. Reed
Paleocurrent measurements from the entire Cretaceous section of the western Transverse Ranges microplate (WTRM) yield a northerly flow direction. Point count data indicate a mixed provenance for both conglomerates and associated sandstones. The dominant provenance was mixed magmatic arc/recycled orogen, and dissected/transitional arc terranes. Petrographic, quantitative SEM, and microprobe analyses also indicate the presence of diagnostic Franciscan mineralogy in these sediments, including glaucophane, riebeckite, lawsonite, and serpentine, suggesting derivation from a subduction complex. Olistoclasts of chert, jadeitic graywacke, serpentine, and blueschist are found intermixed within the arc-derived sediments. Olistoclasts range in size from sub-millimeter to centimeter scale, and ol stoliths range up to 150 m. Well preserved internal bedding in some of the olistoliths suggests emplacement by landsliding indicating very short transport distance. This Franciscan material represents the oldest melange-derived material reported from this part of California, and documents uplift and erosion of the subduction complex earlier than has previously been suggested.
These data are consistent with deposition in a Cretaceous fore-arc basin located west or south of the San Diego area. The allochthonous WTRM of southern California can be reconstructed to an originally north-south oriented fore-arc basin. After deposition of the Sespe Formation (22 Ma ±) the microplate was slivered by strike-slip faults and rotated clockwise approximately 90°. Subsequent to slivering and rotation, the block again accreted against the continental margin. Our reconstruction suggests that depositional and structural trends for Eocene and Cretaceous sediments is likely to be different from that observed in Miocene Monterey pay zones in the Santa Barbara channel region. If our reconstruction is correct, exploration strategy for Eocene and Cretaceous petroleum in he southern California Bight should take this tectonic model into account.
AAPG Search and Discovery Article #90981©1994 AAPG Pacific Section Meeting, Ventura, California, April 27-29, 1994