Paper No. 29-12
Presentation Time: 8:30 AM-5:00 PM
DISPLACEMENT HISTORY OF THE PASAYTEN FAULT ZONE IN THE NORTH CASCADES, WASHINGTON AND BRITISH COLUMBIA
The Pasayten fault is a Cretaceous tectonic boundary between the Intermontane superterrane and the Coast belt that extends 250 km from north-central Washington into British Columbia (BC). It lies near the paleomagnetically defined boundary of Baja BC. Previous workers postulated that the Pasayten fault experienced left-lateral strike-slip and reverse slip, whereas major faults to the west record dextral slip; others speculated that the Pasayten fault is a dextral strike-slip fault. The fault separates the Jurassic-Cretaceous Methow basin on the southwest from Middle to Late Jurassic and mid-Cretaceous rocks of the Eagle Plutonic Complex (EPC) and Upper Triassic amphibolite-facies rocks of the Nicola Group of the Quesnel terrane to the northeast. The displacement history of the Pasayten fault was evaluated by detailed mapping of two poorly-studied areas along the fault zone. In the northern segment in BC, foliations in the EPC and beds in the Methow basin strike NW and dip, on average, 65-70° SW. Lineations plunge 25° NW. Protomylonites and mylonites in the EPC occur up to 1 km from the fault. Preliminary field and microstructural observations demonstrate oblique sinistral and reverse slip suggesting that the fault placed Methow basin strata over the EPC. The southern segment is directly SE of the previously mapped termination of the fault where the trondhjemitic Okanogan Range batholith intrudes undated amphibolite-facies Leecher Metamorphics, which may correlate with the Nicola Group. The batholith and Leecher Metamorphics are thrust over the southern Methow basin along the Red Shirt and Methow River thrusts. Field observations suggest that slip on the Pasayten fault was transferred to the west to the Methow River and Red Shirt thrusts. Foliations strike mostly N and NW and dip average, 62° E. Lineations plunge 45° SE. Fabric is well-developed, but not mylonitic. Lineations change to shallower angles ~4.5 km E of the Red Shirt thrust. Many foliations strike E and NE, markedly discordant to the fault. The observations from the study areas provide evidence for inversion of the Methow basin and sinistral shear on the Pasayten fault in the north, but no major structure lies along strike SE of the Pasayten fault.