Comment and Reply on “Extension across the Eocene Okanagan crustal shear in southern British Columbia”

Tempelman, Dirk; Parkinson, Dave

doi:10.1130/0091-7613(1987)15<188:caroea>2.0.co;2

Cited by 2 publications

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“…During this time (∼100 Ma), the allochthon is interpreted to have experienced protracted internal deformation and metamorphism [ Parrish , 1995]. Early Tertiary normal faulting and associated exhumation has dissected and exposed all levels of the allochthon [e.g., Read and Brown , 1981; Parrish et al , 1988; Tempelman‐Kluit and Parkinson , 1986; Johnson and Brown , 1996], as well as the cratonic basement within the Monashee complex [ Armstrong et al , 1991; Parkinson , 1991; Crowley , 1999] (Figure 1b).…”

Section: Geologic Setting: Overviewmentioning

confidence: 99%

Tectonic evolution of the Selkirk fan, southeastern Canadian Cordillera: A composite Middle Jurassic–Cretaceous orogenic structure

2008

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The eastward transition from penetrative ductile deformation, metamorphism, and plutonism in the hinterland of the southern Canadian Cordillera to the “thin‐skinned” deformation of the foreland represents a significant change in tectonic style and process. The transition is also marked by a zone of structural divergence across which SW vergent hinterland structures pass into NE vergent foreland structures. The Selkirk fan within the Selkirk Mountains of the southern Canadian Cordillera is considered a type locality for this zone. This structure trends SE–NW for more than 120 km in greenschist to upper amphibolite facies rocks that have at least three generations of superposed deformation. The kinematic development of the Selkirk fan has been controversial, but its age generally has been inferred to be Middle to Late Jurassic. However, new U‐Th‐Pb dates obtained by ID‐TIMS and SHRIMP indicate a more protracted history. The data demonstrate that a protofan developed in the Middle Jurassic (172–167 Ma) and that the west flank of the fan was exhumed to upper crustal levels at this time. Conversely, the eastern flank was progressively buried to >25 km depth and pervasively overprinted by Cretaceous deformation (104–84 Ma) and Cretaceous‐Paleocene metamorphism (144–56 Ma). A new tectonic model is proposed to reconcile the composite nature and protracted development (>100 Ma) of the Selkirk fan within an orogenic system that evolved in response to periods of terrane accretion on the western margin of the North American craton.

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Section: Geologic Setting: Overviewmentioning

confidence: 99%

Tectonic evolution of the Selkirk fan, southeastern Canadian Cordillera: A composite Middle Jurassic–Cretaceous orogenic structure

2008

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“…, 2008). Metamorphism and deformation occurred during Late Cretaceous to early Tertiary time (Brown & Read, 1983; Tempelman‐Kluit & Parkinson, 1986; Parrish et al. , 1988; Carlson & Moye, 1990).…”

Section: Geological Setting and Field Relationsmentioning

confidence: 99%

Metamorphic evolution of sapphirine- and orthoamphibole-cordierite-bearing gneiss, Okanogan dome, Washington, USA

Kruckenberg

2011

Journal of Metamorphic Geology

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Gneiss domes are commonly cored by quartzofeldspathic rocks that provide little information about the pressure-temperature-fluid history of the domes. Three northern Cordilleran migmatite domes (ThorOdin and Valhalla ⁄ Passmore, British Columbia, Canada; Okanogan, Washington, USA), however, contain Mg-Al-rich orthoamphibole-cordierite gneiss as layers and lenses that record metamorphic conditions and pressure-temperature (P-T) path information not preserved in the host migmatite. These Mg-Al-rich rocks are therefore a valuable archive of metamorphic conditions during dome evolution, although refractory rocks such as these commonly contain reaction textures that may complicate the calculation of metamorphic conditions. In the Okanogan dome, Mg-Al-rich layers are part of the Tunk Creek unit, which occurs at the periphery of an underlying migmatite domain. Bulk compositional layers (mm-to m-scale) consist of gedrite-dominated, hornblende-dominated and biotite-bearing layers that contain variable amounts of gedrite, hornblende, anorthite, cordierite, spinel, sapphirine, corundum, kyanite, biotite and ⁄ or staurolite. The presence of different compositional layers (some with reaction textures, some without) allows systematic analysis of metamorphic history by a combined petrographic and phase equilibrium analysis. Gedrite-dominated layers containing relict kyanite preserve evidence of the highest-P conditions; symplectitic and coronal reaction textures around kyanite indicate decompression at high temperature. Gedrite-dominated layers lacking these reaction textures contain layers of sapphirine and spinel in apparent textural equilibrium and record a later high-T-low-P part of the path. Phase equilibria (pseudosection) analysis for layers that lack reaction textures indicates metamorphic conditions of 720-750°C at a range of pressures (>8 to <4 kbar) following decompression. Elevated crustal temperatures and concordant structural fabrics in the Tunk Creek unit and underlying migmatite domain suggest that the calculated P-T conditions recorded in Tunk Creek rocks were coeval with anatexis, extension, and dome formation in Palaeocene-Eocene time. In contrast to orthoamphibolecordierite gneiss in the other Cordilleran domes, the Tunk Creek unit occurs as a discontinuous km-scale layer rather than as smaller (m-scale) pods, is more calcic, and lacks garnet. In addition, kyanite did not transform to sillimanite, and spinel commonly occurs as a blocky matrix phase in addition to vermicules in symplectite. These differences, along with the compositional layering, allow an analysis of bulk composition v. tectonic (P-T path) controls on mineral assemblages and textures. Pseudosection modelling of different layers in the Tunk Creek unit provides a basis for understanding the metamorphic history of these texturally complex, refractory rocks and their host gneiss domes, and other such rocks in similar tectonic settings.

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Comment and Reply on “Extension across the Eocene Okanagan crustal shear in southern British Columbia”

Cited by 2 publications

References 0 publications

Tectonic evolution of the Selkirk fan, southeastern Canadian Cordillera: A composite Middle Jurassic–Cretaceous orogenic structure

Tectonic evolution of the Selkirk fan, southeastern Canadian Cordillera: A composite Middle Jurassic–Cretaceous orogenic structure

Metamorphic evolution of sapphirine- and orthoamphibole-cordierite-bearing gneiss, Okanogan dome, Washington, USA

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