Paper No. 7-2
Presentation Time: 9:00 AM-6:00 PM
FRANCISCAN COMPLEX AND BOUNDING FAULTS, MOUNT DIABLO, CALIFORNIA: IMBRICATED OCEAN PLATE STRATIGRAPHY WITH COAST RANGE FAULT FOLDED IN SOUTH-VERGENT OVERTURNED ANTICLINE
For decades without peer Elizabeth Miller has demonstrated the value of detailed geologic mapping as the foundation for regional structural-tectonic analysis. In this spirit, I show that remapping of Franciscan Complex geology of Mount Diablo, California may give insight into subduction-accretion and exhumation processes. The Franciscan forms an eye-shaped exposure, elongated E-W, and bounded to the north and south by rocks of the Coast Range ophiolite (CRO) and Great Valley Group (GVG), respectively, that lack burial metamorphism. The bounding faults, that may be termed the Coast Range fault (CRF), strike E-W to NE and dip northward at low average angles. Along the southern fault, the GVG and directly overlying Franciscan are overturned. The Franciscan comprises a stack of ocean plate stratigraphy (OPS) comprising pillow basalt overlain successively by chert and clastic sedimentary rocks. This stack has been imbricated into duplexes at km to <1 m scales, with local development of block-in-matrix relationships lacking in exotic blocks. The clastic sections include bedded turbidites and olistostromal horizons, and the latter include high-grade exotic blocks up to 100 m size, with amphibolite and eclogite assemblages overprinted by blueschist assemblages. Nearly all clastic rocks have a cleavage or foliation, whereas basalts show undeformed pillow forms in most outcrops. Excluding the high-grade blocks, most of the Franciscan displays blueschist (jd + gln + lws) or lawsonite-albite mineral assemblages. Along the northern and southern margin of Franciscan exposures, a clastic-dominated unit locally crops out (thickness ≤100 m) that lacks penetrative fabric and shows sparse pumpellyite growth; one outcrop along the contact with CRO serpentinite has abundant prehnite. The contact between this unit and the "core" Franciscan rocks truncates the internal contacts within the latter and shows an apparent normal fault relationship (downward increase in metamorphic pressure) similar to the CRF, which also truncates internal Franciscan contacts. These relationships suggest accretion and imbrication of the Franciscan, followed by exhumation accommodated by normal faulting along the CRF, followed by the folding of the CRF into a south-vergent overturned anticline.