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 Western 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 structurally inverted. To the east and structurally 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 structurally 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 early‐stage 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‐of‐sequence 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 synkinematic 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‐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.