Distribution of faults in a transition zone: Bimodal faulting in the Pit River region, Shasta County, California
Abstract
Northern California marks a zone of transition between oblique subduction in Cascadia, dextral transtension in Walker Lane, and north-south compression of the Klamath Mountains. Because of its unique location, the region between Mt. Shasta and Lassen Peak provides insight into the distribution of deformation in regions of transitional tectonic regimes. In particular, the Pit River region provides several excellent exposures of faults in a diatomite quarry and in larger regional structures. We present information on the distribution, amount of slip, and orientation of local faults, and demonstrate how these data reflect the interaction of multiple regional stress fields. We have measured and compiled the orientations of many small faults to evaluate the distribution of deformation in a complex zone of oblique extension and compression. A ~0.5 km2 diatomite quarry near the Pit River and Lake Britton exposes hundreds of faults with small amounts of displacement. Two main faulting patterns emerge: 1) high angle NW/SE-striking faults characterized by normal, oblique normal, or strike slip kinematic indicators; and 2) lower angle E/W-striking faults with evidence of reverse to oblique reverse motion. We find that the regional landscape reflects a dominant mode of faulting that is NW/SE-striking normal, oblique normal, or strike slip; the Hat Creek and Rocky Ledge faults, each with tens of meters of oblique normal offset, exemplify this. Observations of numerous smaller faults in the diatomite quarry also show a dominant pattern of NW/SE-striking faults. E/W-striking compressional structures are present, but are less abundant. Faults of differing orientations occur together in the quarry and occasionally cross cut one another. Many faults cross but do not offset each other, indicating that they formed simultaneously. Where cross-cutting faults do exhibit offset, the NW/SE-striking faults offset E/W-striking faults, which suggests that NW/SE oriented faults have been longer lived or have had more recent activity. This relationship indicates that while the two main faulting patterns did coexist for a period of time, NW/SE-striking faults continued to deform after E/W features shut off.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.T33F2717A
- Keywords:
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- 8010 STRUCTURAL GEOLOGY / Fractures and faults;
- 8015 STRUCTURAL GEOLOGY / Local crustal structure;
- 8107 TECTONOPHYSICS / Continental neotectonics;
- 8118 TECTONOPHYSICS / Dynamics and mechanics of faulting