Structural and seismic hazard implications of geologic map and potential field geophysical observations at Mount Diablo, California
Abstract
Mount Diablo has been characterized as a large overturned anticline, but its actual structure is more complex. Offset geologic units and gravity/magnetic anomalies show the mountain is bisected by the reverse-oblique right-lateral Greenville-Diablo-Concord fault zone (GDCFZ) with 15-20 km of Pliocene and younger strike-slip offset. Thus the mountain is formed of two structural blocks with different stratigraphy and structural style. The western block is characterized by a thin (<1 km) Paleogene section and thick (>4 km) Neogene section, with a broad active fold and thrust belt formed in response both to a restraining bend in the GDCFZ and to regional fault-normal compression. The uplifted eastern block is characterized by a locally thick (~3 km) Paleogene section and a thin (<1 km) exposed Neogene section, with a Paleogene normal fault system forming a stepped graben tilted up by Neogene compression. The Neogene compression has also formed roughly fault-parallel folds, including the Mt Diablo anticline on the uplifted eastern block near the GDCFZ. At the peak of Mt Diablo, intense deformation in the restraining stepover has distorted the fold axes and maximized basement uplift, so that the basement rocks of the Franciscan Complex and Coast Range ophiolite are only exposed at the highest parts of the mountain. The elongate gravity high associated with the Mt Diablo anticline is offset 2-3 km to the east of the fold axis, reflecting either a fault detachment between the basement and overlying strata or the pre-folding basement geometry. Lack of appreciable offset on the gravity high precludes significant strike-slip offset on faults directly east of Mt Diablo. Some earlier workers have included the Mt Diablo anticline in a fold and thrust belt rooted in a blind Mt Diablo Thrust extending beneath the mountain and ~20 km to the east. This neglects the 15 km strike-slip offset on the GDCFZ, however, and there is significantly less compressive deformation to the east. The model proposed here is that the Mt Diablo Thrust roots into the GDCFZ at depth, and the fold and thrust belt lies entirely to the west, limiting the area of fault rupture (and maximum earthquake) on the Mt Diablo Thrust. The Late Cretaceous and Paleogene stratigraphy at Mt Diablo shows that there is now a mountain where once was the depocenter of the Great Valley forearc basin.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.T22D..02G
- Keywords:
-
- 4302 Geological;
- NATURAL HAZARDS;
- 8106 Continental margins: transform;
- TECTONOPHYSICS;
- 8170 Subduction zone processes;
- TECTONOPHYSICS;
- 8175 Tectonics and landscape evolution;
- TECTONOPHYSICS