Lithoprobe crustal reflection structure of the Southern Canadian Cordillera 2: Coast mountains transect

Abstract

The Lithoprobe seismic reflection transect across the southern Coast Mountains of the Canadian Cordillera images fundamental crustal structures presumably related to collision of the Intermontane and Insular composite terranes, and deep levels in the upper plate of the offshore Cascadia subduction belt. The eastern part of the Coast Mountains are characterized by east dipping upper crustal reflectors that project to exposed faults and east dipping lower crustal reflectors; they are truncated by subhorizontal to west dipping middle and upper crustal reflectors. These geometric relationships are interpreted to have formed during an early phase of primarily west directed contraction that created the east dipping structures of the upper and lower crust, and a later phase of east directed shortening caused by wedging of the Intermontane belt into the lower and middle crust of the tectonic stack. Subsequently, the Coast belt may have been displaced eastward on contractional faults that ascend from the lower crust beneath the Intermontane belt and surface in the Omineca and Foreland belts. Extensional faults bounding the east flank of the Coast Mountains and west flank of the central Nicola horst in the Intermontane belt flatten into the middle and lower crust of the intervening region and geometrically outline crustal boudinage. Within the western Coast Mountains, east dipping reflections spanning the middle crust to upper mantle are traced updip to Vancouver Island and the underlying Cascadia subduction zone. The C reflector on Vancouver Island is believed to separate Wrangellia from underlying accreted terranes and is correlated to the mainland where it forms the upper boundary of a reflective lower crustal wedge that flattens into the Moho. If the Moho is not a young feature, then some accreted material appears to have wedged into the continental framework above the crust‐mantle boundary, possibly causing shortening in the overlying crust and creating midcrustal ramps observed on the reflection data. The structurally lower E reflections, interpreted as shear zones, originate at the subduction contact offshore and project landward into sub‐Moho reflections within the upper plate on the Mainland. The region between the E reflector and the descending oceanic plate is interpreted to be subducted lower continental crust and mantle.

Deep seismic reflection constraints on Paleogene crustal extension in the south-central Intermontane belt, British Columbia

“…Comparable large-scale antiforms have been identified in a deep seismic profile across the northern part of Stikinia where no major crustal extension is identified (Cook et al 2004). An antiform that flattens with depth, soling into the reflective lower crust, forms the core of the Nicola horst in the southern Canadian Cordillera (Varsek et al 1993), and this geometry is similar to that observed on line 13; however, we do not observe isolated strong reflectors within the lower crust that are as laterally continuous as those seen beneath, and adjacent to, the Nicola horst.…”

“…The seismic reflection lines of the Southern Cordillera (SC) transect, which were acquired in the late 1980's, were the first seismic transect across the Cordillera, and revealed the complex history of the orogen and its subsequent modification by extension (Cook et al 1992;Varsek et al 1993). The seismic lines of the Slave -Northern Cordillera Lithosphere Evolution (SNORCLE) transect were acquired ϳ10 years later, and, with the further development of seismic acquisition technology during that period, provided significantly improved reflection data from the deeper crust and uppermost mantle (Cook et al 2004;Evanchick et al 2005).…”

Crustal velocity structure in the southern Coast Belt, British Columbia

“…McLean, in preparation), high heat flow (Lewis et al 1988(Lewis et al , 1992, low Bouguer gravity (Riddihough and Seeman 1982), and high electrical conductivity (Gough and Majorowicz 1992;Jones et al 1992). As well, Lithoprobe seismic reflection data show intracrustal reflections which have been interpreted in terms of structural features (Monger and Journeay 1992;Varsek et al 1993); line 10 offers a means of extending these results beyond 2D interpretations. Can.…”

Lithofacies control in detrital zircon provenance studies: Insights from the Cretaceous Methow basin, southern Canadian Cordillera

“…Seismic reflection data indicate that the Pasayten fault dips eastward, juxtaposing Cretaceous volcano-plutonic rocks of Quesnellia over sedimentary successions of the Methow/ Methow-Tyaughton basin (Figs. 1, 4;Varsek et al, 1993).…”