Franciscan Complex at Mount Diablo, California: A Progress Report
Abstract
Franciscan Complex rocks crop in the higher parts of Mount Diablo (MD), including the summit, as part of a ~10 km by 5 km tectonic window elongated ENW-WSW, 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, or 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 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 (tectonic mélange) relationships lacking in exotic blocks. The clastic sections include bedded turbidites and olistostromal (sedimentary mélange) horizons, and the latter include high-grade exotic blocks up to 120 m in long dimension, with amphibolite and eclogite assemblages overprinted by blueschist assemblages. Olistostromal horizons range in thickness up to about 150 m, but many are thin (<10 m). Nearly all clastic rocks have a cleavage or foliation, whereas basalts show undeformed pillow forms in most outcrops. Excluding the high-grade blocks, most Franciscan rocks display lawsonite-albite facies mineral assemblages. Foliation parallels lithologic contacts and lawsonite apparently grew during and after foliation development indicating development of the imbricates at depth along the subduction interface. 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 marks a 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.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.T23F0445W
- Keywords:
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- 4302 Geological;
- NATURAL HAZARDS;
- 8106 Continental margins: transform;
- TECTONOPHYSICS;
- 8170 Subduction zone processes;
- TECTONOPHYSICS;
- 8175 Tectonics and landscape evolution;
- TECTONOPHYSICS