Dominique Chardon

Strain partitioning in an obliquely convergent orogen, plutonism, and synorogenic collapse: Coast Mountains Batholith, British Columbia, Canada

[1]xa0We describe the crustal structure of the Coast Mountains batholith between 54° and 55°N, within the Canadian Cordillera, with emphasis on emplacement of the 7 km thick Kasiks sill complex (KSC). Kinematic patterns that developed during emplacement of the KSC are the result of interactions between magma transport, magma accumulation and regional deformation. The sills were emplaced during NW directed normal shearing and flattening of country rocks that host the KSC. A ∼2 km thick shallowly NE dipping mylonite zone cuts the eastern side of the KSC. Kinematic indicators within the mylonite zone record top to the east normal displacements. Structural analysis shows that mylonite formation occurred during subvertical shortening and east-northeast, subhorizontal extension. U/Pb zircon age dates show that ENE directed normal shearing along the eastern side of the KSC and WNW directed normal shearing within the KSC occurred contemporaneously between ∼54 and 51 Ma, indicating strong strain partitioning between the mylonite and the KSC. This pattern of strain partitioning is interpreted to have been driven by return flow of melt-laden crust in response to tectonic denudation of the upper crust. Seismic profiling shows that many of these structures extend to mid and lower crustal depths. Comparison of our results with other regions within the Canadian Cordillera indicates that orogen-scale right-lateral strike-slip faults deformed synchronously with wide spread magmatism and formation of extensional gneiss domes. Thus the crustal structure of the Coast Mountains batholith was the result of early Tertiary batholith construction during dextral oblique convergence and synorogenic collapse.

Large‐scale transpressive shear zone patterns and displacements within magmatic arcs: The Coast Plutonic Complex, British Columbia

The Coast Plutonic Complex is the largest magmatic arc of the North American Cordillera, extending from northwestern Washington State to eastern Alaska. It forms the transition between two tectonic domains that are suspected to have undergone several phases of large (several thousands of kilometers) orogen-parallel displacement during the Mesozoic and early Cenozoic. A compilation of fabric data, published isotopic ages, and new structural observations shows that the western Coast Plutonic Complex was affected by subvertical, orogen-parallel, crustal-scale shear zones. These shear zones mainly reflect sinistral transpression and were sequentially active from ∼110 to 87 Ma during the intrusion of voluminous batholiths. Sinistral shearing was roughly coeval with the development of the thrust belts flanking the Coast Plutonic Complex (between ∼101 and ∼85 Ma), suggesting plate-scale transpression was a first-order process in the construction of the Coast Mountains orogen. These shear zones separate panels with distinct plutonic and cooling histories, suggesting the sinistral displacements between crustal blocks were large (greater than tens to hundreds of kilometers). This transpressive shear system likely reflects the Jurassic to early Late Cretaceous migration of outboard Cordilleran terranes to the south suggested by paleomagnetic evidence and plate reconstruction models. This example from the Coast orogen shows how transpression is partitioned between a thermally weakened magmatic arc and outwardly vergent fold-and-thrust belts. Our analysis further shows that the ∼2000-km-long Late Cretaceous to early Tertiary Coast shear zone has a minimum extent toward the south to at least 51°30′N.

KINEMATICS AND TECTONIC SIGNIFICANCE OF TRANSPRESSIVE STRUCTURES WITHIN THE COAST PLUTONIC COMPLEX, BRITISH COLUMBIA

Abstract Structural data from the Coast Plutonic Complex, near Prince Rupert, British Columbia, are consistent with a deformational history dominated by dextral transpression from Campanian to Paleocene time. Penetrative east-side-up, southwest-directed, ductile shearing produced moderately northeast-plunging overturned kilometer-scale isoclinal folds. These folds are dextrally sheared and refolded into kilometer-scale upright northwest-plunging folds and steeply dipping transposed foliations with moderate to shallow northwest-plunging lineations along their western side. An east-side-up component to the transcurrent shearing is kinematically compatible with east-side-up shearing found within the Great Tonalite Sill. Kinematic and geometric gradients and the spatial distribution of the finite stretching direction are interpreted to result from partitioning of transpression. The location of these structures and overprinting relationships suggest the Great Tonalite Sill intruded late-kinematically into a crustal-scale dextral transpressive shear zone. These results indicate this shear zone could form part of the Baja-B.C. fault system that would have accommodated large northward displacements of the terranes making up western British Columbia and southeast Alaska. This conclusion is based on: (1) It is favorably located to accommodate the proposed displacements; (2) Deformation occurred during the time period of proposed large displacement (83–59xa0Ma); (3) The 15-km thickness of the shear zone indicates it records large displacements.

Strain partitioning and batholith emplacement at the root of a transpressive magmatic arc

Abstract An integrative study of the high-pressure Ecstall batholith and its country rocks (Prince Rupert area, British Columbia) reveals the interplay of deformation and plutonism produced in the largest Cordilleran magmatic arc (i.e. the Coast Plutonic Complex) during mid-Cretaceous convergence between the Farallon oceanic plate and North America. The results emphasize the interference between three strain fields: (1) an early (>92xa0Ma) crustal wedge produced by orogen-perpendicular, SW-vergent thrusting, (2) an orogen-parallel sinistral strike-slip shear zone active until 87xa0Ma, and (3) the lateral expansion of the batholith (93–91xa0Ma) against the shear zone. The batholith expanded in a direction oblique with respect to the shear zone trend (+20°) by extruding country rocks against its head in a direction normal to its expansion direction during sinistral, strike-slip partitioned transpression. The batholiths emplacement combined far-field batholith boundary-normal translation, extrusion-related rotation and radial and concentric elongation in its structural aureole. This case study may typify the process by which a modern magmatic arc grows longitudinally at depth during transpression, the direction and sense of growth being determined by the direction of plate motion relative to the magmatic arc and the degree of strike-slip partitioning of transpression.

Stabilization of large drainage basins over geological time scales: Cenozoic West Africa, hot spot swell growth, and the Niger River

Reconstructing the evolving geometry of large river catchments over geological timescales is crucial to constraining yields to sedimentary basins. In the case of Africa, it should further help deciphering the response of large cratonic sediment routing systems to Cenozoic growth of the basin-and-swell topography of the continent. Mapping of dated and regionally correlated lateritic paleolandscape remnants complemented by onshore sedimentological archives allows the reconstruction of two physiographic configurations of West Africa in the Paleogene. Those reconstructions show that the geometry of the drainage stabilized by the Late Early Oligocene (29 Ma) and probably by the end of the Eocene (34 Ma), allowing to effectively link the inland morphoclimatic record to offshore sedimentation since that time, particularly in the case of the Niger catchment – delta system. Mid-Eocene paleogeography reveals the antiquity of the Senegambia catchment back to at least 45 Ma and suggests that a marginal upwarp forming a continental divide preexisted Early Oligocene connection of the Niger and Volta catchments to the Equatorial Atlantic Ocean. Such a drainage rearrangement was primarily enhanced by the topographic growth of the Hoggar hot spot swell and caused a major stratigraphic turnover along the Equatorial margin of West Africa.