Paleomagnetism of the Quottoon plutonic complex in the Coast Mountains of British Columbia and southeastern Alaska: evidence for tilting during uplift

The Quottoon plutonic complex (part of the great tonalite sill) was emplace d into eastern parts of the Coast shear zone along the west flank of the Co ast Mountains. U-Pb crystallization ages range from 72.3 to 55.5 Ma. A regi onal compilation of K-Ar hornblende dates from the tonalite sill indicates a west to east decrease from similar to 60 Ma to similar to 54 Ma across th e intrusive suite. Paleomagnetic samples were collected at 160 sites (great er than or equal to 8 samples per site) along six transects across the Quot toon complex between the Skeena River and Willard Inlet. Directions of char acteristic remanent magnetization (ChRM) were successfully determined by pr incipal component analysis of thermal demagnetization data for about half o f the samples collected. Site-mean ChRM directions were determined for 56 s ites from the Skeena River, Quottoon Inlet, Steamer Passage, and Wales Isla nd transects, using criteria of greater than or equal to 4 sample ChRM dire ctions per site and 95% confidence limit (alpha (95)) less than or equal to 25 degrees. For Filmore Island and Willard Inlet, few sites provided adequ ate determinations of site-mean ChRM directions, but many sample ChRM direc tions were well determined and consistent. The mean direction from the Skee na River - Quottoon Inlet collection is nearly concordant with the expected Eocene direction. Mean directions from other transects have clockwise-rota ted declinations and inclinations that are shallower than the expected dire ction by up to 23 degrees. Consistent with expectations for east-side-up ti lting of crustal panels, sites from the western part of each transect have mean directions farther from the Eocene expected direction than do sites fr om the eastern part of the transect. The magnitude of east-side-up tilt var ies along strike: the Skeena River to Quottoon Inlet segment experienced li ttle or no tilt; Steamer Passage south of Portland Inlet records 15 degrees tilt; north of Portland Inlet, tilts are 40 degrees for Wales Island and 3 5 degrees for Filmore Island; farther north, tilt decreases to 12 degrees f or Willard Inlet. Tilting was driven by Eocene extension of the Coast Mount ains, with tilted crustal panels bounded by northwest-striking, east-side-d own normal faults and northeast-striking transfer faults or shear zones. Th e ACCRETE seismic transect imaged east-dipping reflectors down to similar t o 15 km depth, which likely correspond to the east-side-down normal faults. The most significant of the northeast-striking transfer structures must un derlie Portland Inlet. Based on a tilting domino model, about 30% extension is required to produce the 40 degrees tilt of crustal blocks at Wales and Filmore islands.