Structural Analysis and Thermochronological Constraints on Tertiary Extensional Faulting in the Klamath Mountain Province
Abstract
The Klamath Mountain Province (KMP), situated at the southern end of the Cascadia forearc is often considered a rigid crustal block in Tertiary tectonic models. Contrary to this idea, extensional faulting has been identified within the southern KMP in Tertiary time (Schweickert and Irwin, 1989), constraining N-S extension of up to 60 km between 50-15Ma by the La Grange fault, a low angle normal detachment fault (Cashman and Elder, 2002; Piotraschke et al., 2015). Localized high relief combined with limited and highly localized preservation of Tertiary sedimentary rocks further support the reconsideration of the KMP as a rigid block. In this study, we categorize all major Tertiary faults in the KMP and describe them based on their strike, trace length and offsets.
In our structural analysis, we define faults with >20 km of mapped trace as major structures. Eight major structures have an average strike of N45E with apparent horizontal displacements ranging from 10-60 km. Good exposures of faulted Tertiary sedimentary units that onlap the Mt Ashland pluton crop out along the eastern margin of the KMP. The Siskiyou Summit Fault (SSF), is of particular interest as it is a normal fault that displaces Cretaceous-Tertiary sedimentary rocks and bounds the northern edge of a major graben (Bestland, 1987). The SSF strikes N65E, dips 65 degrees SE and has a trace of >24 km, displacing the Cretaceous Hornbrook Formation by 11.3 km of right-lateral separation, with a normal separation of ~ 6 km. Offset of the Hornbrook Formation limits earliest fault activity to the late Cretaceous. Lack of offset within the upper Colestin Formation, which overlies offset units, indicates fault activity likely ceased by 27 Ma (Vance, 1984; Bestland, 1987). To quantify the vertical fault slip occurring in early Tertiary time we use low temperature thermochronology on bedrock samples from the Ashland pluton, located in the footwall of the SSF. As fault-driven exhumation likely accounts for a significant portion of the Ashland pluton's exhumation history, cooling ages may help place further constraints on the magnitude and extent of extension in the KMP during Cenozoic time.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.T41I0253T
- Keywords:
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- 7230 Seismicity and tectonics;
- SEISMOLOGY;
- 8158 Plate motions: present and recent;
- TECTONOPHYSICS;
- 8170 Subduction zone processes;
- TECTONOPHYSICS;
- 8488 Volcanic hazards and risks;
- VOLCANOLOGY