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Unroofing the Klamaths; blame it on Siletzia?

Rachel Piotraschke, Susan M. Cashman, Kevin P. Furlong, Peter J. J. Kamp, Martin Danisik and Ganqing Xu
Unroofing the Klamaths; blame it on Siletzia?
Lithosphere (May 2015) 7 (4): 427-440


The Klamath Mountains province of northwestern California-southwestern Oregon is an anomalous element in the Cascadia margin; these mountains have the highest average topography, the oldest rocks, and the only identified example of late Cenozoic detachment faulting in the coastal mountains of the Cascadia forearc. Low-temperature thermochronology (apatite fission-track, apatite [U-Th]/He) analyses from the central and southern Klamath Mountains province record two distinct exhumation events-a Cretaceous-Paleocene regional cooling and a southward-migrating locus of rapid cooling/exhumation in the Middle Tertiary. This younger event is localized within the geographic extent of the La Grange fault. We infer that this pattern reflects two distinct processes of exhumation: regional surface erosion (older) and migrating localized tectonic exhumation (younger). At the southern limit of this region of rapid cooling, slickenside striations on the exposed La Grange fault surface record southward displacement of the upper plate along a shallowly dipping ( approximately 20 degrees ) detachment surface. Thermochronologic data constrain average dip of the fault to a few degrees, upper-plate thickness to < approximately 6-8 km, and fault slip rate to <2 mm/yr for a duration of 30 m.y. (ca. 45 Ma to 15 Ma). The fault dip is unusually low compared to that of typical detachment faults; the duration of this extensional event is unusually long compared to other detachment faults; the north-south (margin-parallel) slip direction is roughly perpendicular to that of other Klamath Mountains province faults; and the Eocene to early Miocene timing of extensional faulting does not correlate with recognized tectonic events in northern California. Mid-Tertiary tectonic events in the Oregon Coast Ranges provide a context for understanding the unusual mid-Tertiary tectonism in the Klamath Mountains province. Immediately north of the Klamath Mountains province, early Eocene accretion of a large early Cenozoic igneous province, the Siletz terrane, initiated a westward jump of active subduction. Accretion was followed by late Eocene margin-parallel extension in the Oregon Coast Ranges, recorded by formation of a regional dike swarm. Both the timing of tectonic exhumation and the direction of extension on the La Grange detachment fault suggest that mid-Tertiary tectonism in the southern Klamath Mountains province was likely driven by plate tectonics associated with the accretion of Siletzia and the reestablishment of subduction outboard of the accreted terrane.

ISSN: 1941-8264
EISSN: 1947-4253
Serial Title: Lithosphere
Serial Volume: 7
Serial Issue: 4
Title: Unroofing the Klamaths; blame it on Siletzia?
Affiliation: Pennsylvania State University, Department of Geosciences, University Park, PA, United States
Pages: 427-440
Published: 20150513
Text Language: English
Publisher: Geological Society of America, Boulder, CO, United States
References: 82
Accession Number: 2015-049885
Categories: Structural geology
Document Type: Serial
Bibliographic Level: Analytic
Annotation: Includes appendix
Illustration Description: illus. incl. 3 tables, geol. sketch maps
N41°00'00" - N43°00'00", W124°30'00" - W123°00'00"
Secondary Affiliation: Humboldt State University, USA, United StatesUniversity of Waikato, NZL, New Zealand
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 201523
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