The Coast Belt Thrust System: Evidence of Late Cretaceous shortening in southwest British Columbia

Abstract

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...

Episodicity of Mesozoic terrane accretion along the Pacific margin of Gondwana: implications for superplume-plate interactions

“…Regional midCretaceous deformation is recognized in British Columbia (BC: Fig. 2), with, for example, ocean-vergent thrusting (Journeay & Friedman 1993) of upper Valanginian to middle Albian volcanic rocks. Thrusting was active from at least mid-Albian times to the emplacement of synkinematic plutons dated at ~97 Ma and ~96 Ma and deformed an accretionary complex that formed prior to 92 Ma (Chardon 2003).…”

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

“…The Methow terrane was imbricated structurally with the Bridge River and Cadwallader terranes along its western margin during Cenomanian time ( Figs. 1 and 4; Journeay and Friedman, 1993). Along the eastern margin of the Methow terrane, the Pasayten fault has a multistage deformational history that includes Late Cretaceous sinistral displacement, Late Cretaceous to Tertiary west-vergent contractional motion, and Tertiary extension (Hurlow, 1993).…”

Arc and Slab-Failure Magmatism in Cordilleran Batholiths II – The Cretaceous Peninsular Ranges Batholith of Southern and Baja California

“…A west-vergent fold-thrust belt, that developed around 100 Ma (Rubin et al 1990;Haeussler 1992), places highgrade rocks of the eastern belt over lower grade rocks of the Gravina-Dezadeash-Nutzotin belt to form a thrust stack that is of higher metamorphic grade upwards (Lynch 1992(Lynch , 1995Journeay and Friedman 1993;Crawford et al 2000;McClelland and Mattinson 2000). Although the deformation is about the same age, the polarity is the reverse of the more southerly batholiths.…”

“…Similarly, detailed field studies and geochronology within the Coast batholith of Canada and southeastern Alaska led to the realization that it too could be readily divided into two parts, there separated by the collapsed Nutzotin-Dezadeash-Gravina basin and related fold-thrust belt (Rubin et al 1990;Journeay and Friedman 1993;Gehrels et al 2009). And farther south, additional studies within the Peninsular Ranges batholith demonstrated that it could be readily divided into an older, more westerly, magnetite-bearing suite and a younger, ilmenite bearing, high Sr/Y suite to the east Silver et al 1979;Silver and Chappell 1988;Kimbrough et al 2001;Tulloch and Kimbrough 2003).…”