The Coast Belt Thrust System (CBTS) is the leading edge of a west vergent contractional belt that formed along the inboard margin of the Insular superterrane in late Cretaceous time. The foreland of this contractional belt comprises a system of east dipping frontal thrusts that imbricate Jurassic and Early Cretaceous supracrustal arc sequences and related plutonic suites of the Westem Coast Belt (WCB). These faults root eastward into an imbricate zone of folded thrusts and out-of-sequence reverse faults across which the metamorphic gradient has been stmcturally inverted. To the east and stmcturally above the imbricate zone is a folded stack of high-pressure thrust sheets comprising metamorphosed island arc and oceanic rocks of the Central Coast Belt (CCB). These rocks represent the exhumed crustal root of the CBTS. Metamorphosed volcanic and volcaniclastic rocks that occur in thrust sheets of the imbricate zone yield U-Pb zircon dates which support a correlation with lower-plate Jurassic and Early Cretaceous arc sequences of the WCB. Structural and geochronologic data indicate a two-stage history of Late Cretaceous shortening. During the early stage of shortening, supracrustal arc and related basin sequences along the inboard margin of the Insular superterrane were stmcturally imbricated and accreted to the toe (footwall) of a westward prograding accretionary complex made up of previously assembled terranes of the CCB. These early-stage structures are represented by folded thrusts in the imbricate and hinterland zones of the CBTS. The timing of this earlystage thrusting is bracketed by the emplacement of synorogenic plutonic suites, which yield U-Pb zircon dates of 97 + 1 Ma and 96 +6/-3 Ma. Late-stage shortening involved both thin-skinned thrusting and folding of Jura-Cretaceous arc sequences in the WCB foreland and out-ofsequence thrusting of higher-grade thrust sheets in the imbricate and hinterland zones of the CBTS. In the foreland this episode of deformation is bracketed by late-and postkinematic plutons, which yield U-Pb zircon dates of 94 + 2 Ma and 91 +4/-3 Ma, respectively. In the imbricate zone, this episode of deformation is bracketed by the emplacement of synkinemafic and postkinematic plutons, which yield U-Pb zircon dates of 96 +6/-3 Ma and 94 +6/-5 Ma, respectively. The development of thick-skinned out-of-Copyfight 1993 by the American Geophysical Union. Paper number 92TC02773. 0278-7407/93/92TC-02773510.00 sequence thrusts in the imbricate and hinterland zones of the CBTS signals a northeastward migration of the deformation front in Late Cretaceous time. This shift in the locus of thrusting resulted in southwestward telescoping and structural inversion of the metamorphic hinterland and may account for the complex stacking order and thermal history of the Southern Coast Belt. INTRODUCTION The Coast Belt of southern British Columbia straddles the boundary between the Insular and Intermontane superterranes (Figure 1). It comprises an array of faultbounded island arc and oceanic terranes that were ac...

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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.