UBC Theses and Dissertations
Structural geology and processes of deformation in the mesozoic and cenozoic evolution of the Queen Charlotte Islands Lewis, Peter D.
The Queen Charlotte Islands, located within the Insular Belt off the central British Columbia coast, preserve a lithologically diverse group of volcanic, sedimentary, and intrusive rocks ranging in age from Late Paleozoic to Late Tertiary. This study analyzes the deformation history recorded in these rocks through regional and detailed structural analyses of five map areas comprising 20% of the islands' landmass. The structural model constrained by these studies involves at least four distinct deformation events. Earliest formed structures are Middle Jurassic northwest-trending, southwest-verging folds and contractional faults which accommodated approximately 50% regional structural shortening. During the late Middle Jurassic to Early Cretaceous, dip-slip motion along steep faults tectonically elevated northwest-trending fault blocks, exposing uplifted areas to preferential erosion of Middle Jurassic and older strata. In the Late Cretaceous to Early Tertiary, a second episode of northeast-southwest shortening added an additional 10% regional shortening, concentrated in high-strain zones in Cretaceous strata overlying re-activated block faults. Tertiary structural styles were dominated by strike-slip and extensional faulting, coinciding with the onset of syn-tectonic sedimentation in the adjacent Queen Charlotte Basin. Structural styles associated with these events reflect the variable mechanical properties of the deforming units. During periods of crustal shortening, most regional strain was accommodated by movement along steeply-dipping reverse faults in homogeneous, poorly-layered rocks, while well-bedded rocks shortened by mesoscopic to macroscopic folding and movement along shallowly-dipping thrust faults. Strike-slip and extensional faulting episodes were characterized by displacements along steeply-dipping fault surfaces in all units. Penetrative structural fabrics preserved in Mesozoic rocks include axial planar slaty cleavage, mylonitic foliation, and wall-rock foliations adjacent to major fault surfaces, and all can be tied to some aspect of the documented structural history. Spaced cleavage fabrics in Cretaceous mudstones are interpreted to have formed as dilatent fractures during sediment dewatering and lithification. Throughout deformation, strains were accommodated by semi-brittle processes of crystalline plasticity (dislocation glide and dislocation creep, twin gliding), pressure solution creep, and brittle failure (extensional and shear fracturing, microcracking, andcataclastic flow). Regional metamorphic conditions were sub-greenschist grade, except for locallized areas affected by pluton heating. This structural history includes substantial refinements of existing models for the structural evolution of the Queen Charlotte Islands, the most significant of which include: 1. ) the characterization of Mesozoic contractional and extensional deformation events, and the recognition of their significance to the present stratigraphic distribution and structural configuration, 2. ) the delineation of a concentrated strain zone in the central Queen Charlotte Islands, and the recognition that its formation was linked to Mesozoic contraction and extensional events, rather than Tertiary strike-slip faulting as previously interpreted, 3. ) the evolution of a general structural model for the Queen Charlotte Islands region in Tertiary time integrating structural, magmatic, sedimentological, and geophysical constraints.
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