CRUSTAL EVOLUTION DURING THE LAST 105 MILLION YEARS, CENTRAL COAST OROGEN, BC AND SOUTHEASTERN ALASKA

The ~200 km wide central Coast Orogen lies between Prince Rupert, BC and Wrangell, AK. The salient tectonic feature of this NW-striking orogen is a shift from convergence and crustal thickening from 105 to 50 Ma to crustal thinning by orogen parallel extension followed by orogen-normal block tilting that continues today. Igneous activity accompanied deformation and influenced crustal evolution throughout the orogen during the entire 105 Ma time period. Across the orogen structures that document a convergent tectonic setting from 105 to ~58 Ma include kilometer-thick thrust slabs accompanied by regional scale recumbent folds, some of which developed into kilometer scale sheath folds. Many of these were refolded by upright folds with north-plunging fold hinges.

The Coast Shear Zone (CSZ) divides the orogen into two segments. On the west, the western metamorphic belt is an east-side-up tilted crustal block exposing regionally metamorphosed rocks that record pressures of 4 kbar or less in the west to a maximum of 9-10 kbar on the east. Regional metamorphic temperatures up to ~500 ºC document thermal gradients on the order of 15-16 degrees/km. In this belt, crustal thickening due to regional-scale thrusting of kilometer-thick slabs was accompanied by syntectonic emplacement of tabular plutons between 105 and 90 Ma. On the east, in the Central Gneiss Complex (CGC), regional crustal temperatures reached 750 ºC. Kilometer thick N to NE gently dipping tabular dioritic to tonalitic plutons and coeval tabular vertical bodies 100's of meters thick were syntectonically emplaced. Crustal melting at successively shallower crustal levels reflecting crustal heating coincided with the time of major igneous activity (65-53 Ma).

After 60 Ma, crustal thinning related to extensional shear zones and crustal-scale normal faults, including the CSZ and faults on the eastern side of the CGC, resulted in crustal uplift. Location of the normal faults that bracket the orogen may reflect ductility contrasts between the hot CGC and cooler rocks on either side. This uplift (52-50 Ma) was accompanied by decompression melting in the CGC and emplacement of large volumes of granite along its eastern side. After 50 Ma, extension across the belt resulted in bimodal basaltic and associated granitic magmatism and continued formation of tilted crustal blocks.