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Cordilleran Section - 111th Annual Meeting (11–13 May 2015)

Paper No. 5
Presentation Time: 9:25 AM

PRECISE TEMPORAL CONSTRAINTS ON EOCENE STRIKE-SLIP FAULTING IN WASHINGTON AND ITS RELATIONSHIP TO TRIPLE-JUNCTION MIGRATION AND TERRANE TRANSPORT


EDDY, Michael P.1, BOWRING, Samuel A.1, UMHOEFER, Paul J.2 and MILLER, Robert B.3, (1)Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, (2)School of Earth Sciences & Environmental Sustainability, Northern Arizona University, 625 Knoles Drive, Box 4099, Flagstaff, AZ 86011, (3)Department of Geology, San José State University, One Washington Square, San Jose, CA 95192, mpeddy@mit.edu

Western Washington preserves the southern terminus of a Paleogene belt of forearc magmatism that is believed to represent a period of ridge-trench interaction. This interpretation is consistent with plate reconstructions that require subduction of the Kula-Farallon ridge beneath North America during this time. However, the geometry of this interaction remains uncertain due to disagreements over whether or not parts of this magmatic belt are far-traveled from their Paleogene position.

U-Pb zircon geochronology from Paleogene terrestrial sedimentary units in central and NW Washington provides temporal constraints on strike-slip faulting in this region, and demonstrates that right-lateral motion initiated on the Darrington-Devil’s Mountain, Straight Creek-Fraser, Leavenworth, Eagle Creek, and Entiat fault zones coeval with ca. 50 Ma ridge-trench interaction. Importantly, the Darrington-Devil’s Mountain and Straight Creek-Fraser fault zones displace sedimentary units that likely formed a regional depositional system from ca. 57-51 Ma and contain little evidence for syn-depositional strike-slip faulting. The dates also show that the Chumstick Basin formed at 49 Ma between the Leavenworth and Entiat fault zones. This basin records rapid depocenter migration, high sediment accumulation rates (> 5 m/kyr), and marginal coarse-grained facies that are indicative of its origin as a strike-slip basin. Displacements on the Entiat and Eagle Creek fault zones remain poorly constrained. However, right-lateral displacements are estimated between 110-150 km on the Straight Creek-Fraser fault zone bracketed between 51-35 Ma, ca. 110 km on the Darrington-Devil’s Mountain fault zone after 51 Ma, and 25-30 km on the Leavenworth fault zone from 48-45 Ma. We interpret the formation of large-displacement, right-lateral, strike-slip faults after 51 Ma to mark a fundamental change to transform motion or oblique subduction along the North American margin following the southward migration of a triple junction. This observation provides an important constraint on the nature of this triple-junction and may have implications for terrane transport during the Eocene.

Session No. 18

T16. Terrane Accretion and Translation in the Cordillera
Wednesday, 13 May 2015: 8:00 AM-12:00 PM
Room 116 (Fine Arts Building)
Geological Society of America Abstracts with Programs. Vol. 47, No. 4, p.51
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