Sill Injection, Sinistral Transtension and the Formation of a Crustal Scale Gneiss Dome in the Core of the Central Gneiss Complex, British Columbia
Andronicos, C L (1), Hollister, L S (1), Chardon, D H (1)
(1) Department of Geosciences, Princeton University, Princeton, NJ 08544 United States
Abstract:
The Eocene Kasiks sill complex is a 7-km thick tonalite-diorite batholith exposed over a ~250 square km area. It played a key thermal and mechanical role in the doming and extension ofthis portion of the Central Gneiss Complex. The sill complex dips northward into the line of the Accrete seismic section along the Alaska-B.C. border and forms the root of a crustal scale northwest-plunging gneiss dome. The gneiss dome is asymmetrical with a steeply dipping west side and a more shallowly dipping east side.Foliations on the top of the dome are shallowly north dipping and lineations there indicate east-west stretching (see Davidson et al. this session). The eastern limb of the gneiss dome and the eastern side of the sill complex are deformed by mylonites with top-to-the east kinematic indicators. The overall geometry of the sill complex is that of a north-northwest striking negative flower structure, with sills on the east and west sides of the complex dipping away from a central shear zone that lies beneath and parallel to the axis of the dome. Lineations throughout the sill complex and adjacent country rocks are shallowly to moderately north-northwest plunging. The center of the sill complex is a domain of north-northwest striking steeply dipping foliations recording constrictive strains and has kinematic indicators consistent with sinistral strike slipshearing. The eastern side of the sill complex was deformed by top-to-the north shearing, and the western side records top-to-the south shearing. Boudinage, isoclinal folds and shallow foliations indicate subvertical flattening during shearing on both sides of the sill complex. The geometric and kinematic pattern of the sill complex and dome are consistent with magma intrusion and dome formation occurring synchronously. Melt was transported parallel to the axis of the dome during intense north-northwest stretching. Flattened country rock screens and shallowly dipping sills suggest magma pressure was greater than lithostatic pressure. We interpret these features to indicate that sill emplacement was facilitated by a combination of sill inflation and flattening of adjacent country rocks during sinistral transtension. Thus, dome formation occurred due to the inflation of the underlying sill complex in concert with down to the east shearing along the eastern limb of the dome. Strain was partitioned between upper levels of the dome that underwent east-west stretching and lower levels that underwent north-northwest stretching. Zones of high crustal reflectivity and high velocity imaged on the Accrete seismic line below Portland Inlet are likely to be the lower crustal continuation of the sill complex (see Morozov et al., this session).
