Timing of Bedrock Exhumation Across the Columbia River Fault near Revelstoke, BC
Abstract
In the southeastern Canadian Cordillera, Paleocene-Eocene post-orogenic extension exhumed mid-lower crustal rocks along low-angle normal detachment faults. The Columbia River Fault (CRF) is one such detachment, bounding the eastern margin of the Monashee Complex. The CRF initiated ~56 Ma and cooling ages in the Monashee Complex record rapid exhumation from 25 km to ~2 km depth during Eocene time. In this study we present new apatite (U-Th)/He ages that indicate that the Monashee Complex underwent a later phase of rapid cooling during the late Oligocene-Miocene. We present 61 new AHe ages from 12 samples collected across the CRF north of Revelstoke, BC. Age-elevation relationships and inverse thermal history modelling show that the footwall of the CRF experienced a phase of rapid cooling (10°C/Myr) from 25-18 Ma. In comparison, the hanging wall experienced slower cooling rates (<5°C/Myr) during this time, and likely spent prolonged periods in the apatite He-partial retention zone during its protracted cooling. We propose that the CRF reactivated in the late Oligocene and rapidly exhumed footwall rocks another 2 km over ~7 Myr. Integration of existing thermochronology data allows three phases of bedrock exhumation in the Monashee Complex to be defined: 1) very rapid cooling (>30°C/Myr) from 56-40 Ma during detachment-driven exhumation of the core complex; 2) rapid cooling (10°C/Myr) from 25-18 Ma during CRF reactivation; and 3) erosion and incision of modern-day Columbia Mountains starting 5 Ma. Miocene reactivation of the CRF is coeval with late brittle normal faulting to the southeast of the CRF, which is suggested to have been driven by partitioning of displacement from the Rocky Mountain Trench along a basal detachment. The close timing and structural similarities of these events suggests that reactivation of the CRF from 25-18 Ma may have also been driven by the same forces.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMEP0290012D
- Keywords:
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- 1125 Chemical and biological geochronology;
- GEOCHRONOLOGY;
- 1130 Geomorphological geochronology;
- GEOCHRONOLOGY;
- 1140 Thermochronology;
- GEOCHRONOLOGY;
- 1150 Cosmogenic-nuclide exposure dating;
- GEOCHRONOLOGY