High temperature orogen parallel extension during arc construction and exhumation

M.L. Crawford, Dept. of Geology, Bryn Mawr College, Bryn Mawr, PA 19010 and K.A. Klepeis, Dept. of Geology and Geophysics, University of Sydney, Sydney, NSW 2006, Australia

We present a model of arc emplacement and exhumation using new structural and kinematic data that emphasize the importance of horizontal arc parallel extension, sinistral transtension, and arc normal to oblique convergence in the Coast Mountains orogen, northern coastal British Columbia. The early phases of this ~65-51 Ma regional scale orogen parallel intra-arc extension occurred in the deep crust concurrent with arc-normal convergent deformation. This extension accompanied and outlasted the emplacement of the northern Coast Mountains batholith and culminated in rapid uplift and exhumation of the deep roots of the batholith prior to 50 Ma by continued orogen parallel extension, by near vertical displacements on shear zones on both the eastern and western side of the batholith, and by top northeast extension at upper crustal levels on the eastern side of the batholith. These orogen parallel and orogen normal displacements accompanied the transition from late Mesozoic plate convergence to Recent right lateral transform displacement along the northwestern margin of North America.

Our study area lies in the central part of the Coast orogen, east of Prince Rupert (~54oN latitude), between the Skeena River and Portland Inlet. It is east of the Coast shear zone that marks the western side of the batholith complex and that records a complex history of dominantly orogen normal to high angle crustal scale displacements (Klepeis et al., 1998). The area consists of upper amphibolite to granulite facies metamorphic rocks intruded by tabular gabbro, tonalite and granodiorite plutons that range from 100's of meters to 10 km in thickness, and are either (1) low angle generally north dipping bodies or (2) vertical.

In the southern part of the area a 7-8 km thick series of sheeted, tabular leucotonalite and granodiorite sills and gneiss dip gently to moderately northward in a direction oblique to the regional north-northwest trend of the Coast Plutonic belt. Screens of conformable migmatitic metasediment occur between the sills. In the migmatite and orthogneiss of the complex kinematic indicators (C' shear bands, S-C fabric, asymmetric tails on feldspar porphyroclasts) show a top north and northwest transport parallel to regional north and northwest plunging stretching lineations. Some of the sills share the country rock fabric, in others the fabric is dominantly magmatic. The observed structural relations suggest that emplacement of the sills accompanied the deformation that generated the extensional planar fabric in the country rocks. The tonalite sills and the foliation are reoriented by upright folds with gently north-plunging axes that parallel the north-plunging stretching lineation. This suggests that the top northwest extension that produced the pervasive lineation and the kinematic indicators was accompanied by east-west or northeast-southwest contraction. Superimposed on these fabric elements are hundreds of minor, oblique sinistral, north-down transtensional shear zones that truncate and transpose the regional N-dipping foliation as well as extensional (north side down) shear zones that dip gently to the north and contain strong northwest-plunging down-dip mineral lineations. These shear zones are identified as extensional by kinematic indicators including asymmetric boudinage, S-C fabrics, synthetic C' shear bands and asymmetric tails on mafic clasts. Felsic dikes were emplaced within many of the steep shear zones. The mostly solid state fabric of the shear zones locally is transgressed by migmatitic melt patches and dikes that both crosscut the shear zone foliation and contain, albeit weakly, the regional north-dipping foliation. These relationships suggest that synkinematic magma emplacement continued during the top northwest, arc-parallel extension recorded by these shear zones. A major sub-vertical, sinistral transtensional shear zone oriented obliquely to the overall north-northwest trend of the orogen, cuts across the complex. This large shear zone displays the same geometry, steep orientation, oblique transport direction and oblique sinistral (north-down) sense of motion as the majority of the minor shear zones. The trace of this shear zone is at least 12 km long but may be much longer. The large shear zone merges with a detachment zone at the top of the complex. The detachment zone is characterized by tabular lithologic units with planar fabric that uniformly dips gently to the north. No fabrics older than those ascribed to the detachment, such as characterize the rocks north and south of the detachment, can be observed. The detachment separates the complex of sheeted intrusive rocks and migmatitic gneiss described above from overlying amphibolitic and metasedimentary rocks in the northern part of the study area. Rocks above the detachment zone experienced a minimum of intrusive activity, little to no melting, and display a strong structural discordance with the detachment zone and with the fabric of the underlying complex. A large, gently northwest-plunging synform deforms the foliation of the upper plate. The synform may have formed as a result of down-dip drag induced by top northwest motion on the extensional detachment.

The Quottoon pluton, a long relatively narrow steeply dipping tabular body, lies along the western side of the sheeted complex and the overlying schist and gneiss of the northern part of the map area. It is one of a number of similar tabular plutons that lie between the high temperature metamorphic rocks of the Central Gneiss Complex and lower temperature but higher pressure rocks to the west. We interpret the shape of this pluton, its relations to the several generations of extensional shear zones, the steeply dipping magmatic fabrics, the remarkably concordant pluton margins, and the lack of evidence of tectonic strain until late in its crystallization history to suggest that space for the pluton was created during the orogen parallel extension recorded in the adjacent Central Gneiss Complex.

This documentation of arc-parallel extension and transtension yields new information on the way that deformation was accommodated across the Central Gneiss Complex. Structural evidence from the western side of the orogen, west of the study area, documents dominantly contractional deformation in the mid-Cretaceous oriented approximately EW to NE-SW normal to the orogen. After ~5 Ma extension along the orogen became the dominant deformation mechanism and accommodated significant magma emplacement of the Paleogene arc. The later stages of arc-parallel extension and transtension in the Central Gneiss Complex on the eastern side of the Quottoon pluton occurred at the same time as near vertical displacement within the Coast shear zone on the western side of that pluton. We suggest that these vertical displacements on the Coast shear zone combined with the tectonic unroofing of the batholith via arc-parallel extensional displacements led to rapid exhumation of the batholith. The late stages of the deep crustal, arc-parallel extension we describe on the western side of the Coast Mountains batholith was also approximately simultaneous with near vertical displacements and top northeast extension on upper crustal shear zones on the eastern side of the batholith. Together these processes facilitated the tectonic unroofing of the mid-crustal roots of the batholith prior to 50 Ma.