RESOLVING MECHANISMS OF MID-CRETACEOUS OROGENY AROUND CENTRAL ADMIRALTY-REVILLAGIGEDO BELT GRANITOIDS, WRANGELL, ALASKA

Petrologic and structural data in central southeastern Alaska provide constraints on the mechanism of crustal thickening associated with Mid-Cretaceous orogeny. The area comprises 99 to 88 Ma igneous rocks that intrude Gravina Belt carbonaceous metasediments with minor metavolcanic layers. On northwest Woronkofsky Island, these metamorphic rocks show E-dipping layering with up-facing indicators signifying that there are no overturned sections. Sections of homoclinal E-dipping strata along central Zimovia Strait are inferred thrust faults. To the southeast, deformation is characterized by map- to outcrop-scale isoclinal and asymmetric folds. Fold axes and mineral lineations mostly plunge moderately to the southeast. The structures suggest that these features developed in a contractional regime via thrust faulting and folding associated with underthrusting of the Alexander terrane and Gravina belt rocks beneath the Yukon Tanana terrane to the east. Tonalite and hornblendite sills, probably older than 95 Ma, are folded with the country rocks. Contact metamorphic aureoles associated with the Admiralty-Revillagigedo belt Woronkofsky plagioclase porphyritic tonalite (93±5Ma), weakly foliated central Wrangell tonalite (88±2 Ma), and equigranular hornblende-biotite foliated Bell Island tonalite (90±1 Ma) record a northwest to east and southeast increase in metamorphic grade, pluton emplacement level, and fabric development. Emplacement of these granitoids predated, accompanied, and outlasted the deformation associated with thrusting. The Woronkofsky and central Wrangell tonalites are discordant and were emplaced at shallow crustal levels as indicated by andalusite in contact aureoles. Intrusions along southern Zimovia Strait and to the east, including the Bell Island pluton, are concordant with the SE-dipping structural fabric. The widespread occurrence of kyanite and kyanite ± staurolite pseudomorphs after andalusite in the contact aureoles of the plutons in this eastern area documents a medium to high pressure regional metamorphic event synchronous with thrusting. Most of the data are consistent with thrusting and/or pure shear thickening as the loading mechanism in this part of the orogen; magma loading played a subordinate role.