Mitchell G. Mihalynuk

Coherent French Range blueschist: Subduction to exhumation in <2.5 m.y.?

Coherent oceanic strata in the French Range belong to the exotic Cache Creek terrane of the Canadian Cordillera. They were metamorphosed to blueschist grade, tectonically extruded, eroded, and intruded by plutons—perhaps in <2.5 m.y. Sodic amphibole overprint chert as young as late Pliensbachian to Toarcian age (ca. 191 to ca. 177 Ma). Blueschist mineral assemblages defi ne the early metamorphic fabric along with phengite dated by 40 Ar/ 39 Ar as 173.7

Stratigraphic and isotopic link between the northern Stikine terrane and an ancient continental margin assemblage, Canadian Cordillera

Geologic and isotopic data strongly imply a Late Triassic depositional link between a juvenile volcanic arc (northern Stikine terrane) and an outboard ancient continental margin assemblage (Nisling terrane) in the Canadian Cordillera. Two sandstone samples and a schist clast from a conglomerate layer at the base of the Upper Triassic Stuhini Group (northern Stikine terrane) have Nd-depleted mantle model ages of 1400-1430 and 1600 Ma, respectively; other Stuhini Group rocks have model ages of 390, 660, and 690 Ma. Three samples of Nisling terrane schist and gneiss yield Nd model ages of 910, 1770, and 2450 Ma and highly radiogenic 87 Sr/ 86 Sr ratios. These isotopic data corroborate stratigraphic evidence that detritus at the base of northern Stikine was shed from the Nisling terrane and strengthen interpretations that these terranes became linked by Late Triassic time. Thus, Upper Triassic strata of the northern Stikine terrane may have accumulated on top of or adjacent to an exotic continental fragment, a rifted fragment of the North American margin, or the in situ North American margin.

First evidence for ultrahigh-pressure garnet peridotite in the North American Cordillera

Constraints on the thickness of mantle lithosphere involved in collisional orogenesis are fundamental for understanding the geodynamics of mountain building and the overall growth of continents by accretionary tectonics. Garnet peridotite and ultrahigh-pressure (UHP) crustal rocks provide such a constraint in many collisional orogens but have hitherto been unrecognized in western North America’s Cordillera. Here we show the first evidence for exhumation of UHP (.2.8 GPa) garnet peridotite and eclogite and for deposition of these rocks as detritus in an Early Jurassic forearc basin (Laberge Group, Yukon Territory and British Columbia). Our results suggest that collision in this part of the North American Cordillera must have been thick skinned, involving a Proterozoic continental mass with a lithosphere .100 km (and possibly to 150 km) thick. Our discovery also provides insight into the vigor of uplift and erosion of deep-seated rocks in a nascent continental arc.

Structure, Stratigraphy and Petroleum Resource Potential of the Central Whitehorse Trough, Northern Canadian Cordillera

Abstract The Whitehorse Trough is an early Mesozoic marine sedimentary basin, which extends from southern Yukon to Dease Lake in British Columbia. This paper outlines the stratigraphy and structure, and characterises the overall petroleum resource potential of the central Whitehorse Trough. Strata within the central Whitehorse Trough include carbonate rocks of the Upper Triassic Sinwa Formation, and interbedded sandstone, siltstone and argillite of the Lower Jurassic Inklin Formation. The central Whitehorse Trough is structurally dominated by a Middle Jurassic southwest-vergent fold and thrust belt. Programmed pyrolysis data indicate that potential source rocks in the Inklin Formation are gas-prone, and, along the northeastern flank of the central Whitehorse Trough, are within the oil and gas window. Variably tuffaceous feldspathic arenites in the Inklin Formation have fair to good reservoir characteristics, but are a minor component of the stratigraphy. Potential petroleum traps are provided by antiforms, thrust faults and stratigraphic pinch-outs. Three conceptual gas plays for the central Whitehorse Trough are proposed: (1) the Sinwa Structural Gas Play; (2) the Inklin Structural Gas Play; and (3) the Inklin Stratigraphic Gas Play. The Inklin Structural Gas Play presents the greatest potential in the central Whitehorse Trough because levels of organic maturation along the northeastern flank of the basin are favourable and structural traps are readily identifiable. This prospective belt of strata may extend to the northeast beneath the structurally overlying Cache Creek terrane.

Mantle and geological evidence for a Late Jurassic–Cretaceous suture spanning North America

Crustal blocks accreted to North America form two major belts that are separated by a tract of collapsed Jurassic–Cretaceous basins extending from Alaska to Mexico. Evidence of oceanic lithosphere that once underlay these basins is rare at Earth’s surface. Most of the lithosphere was subducted, which accounts for the general difficulty of reconstructing oceanic regions from surface evidence. However, this seafloor was not destroyed; it remains in the mantle beneath North America and is visible to seismic tomography, revealing configurations of arc-trench positions back to the breakup of Pangea. The double uncertainty of where trenches ran and how subducting lithosphere deformed while sinking in the mantle is surmountable, owing to the presence of a special-case slab geometry. Wall-like, linear slab belts exceeding 10,000 km in length appear to trace out intra-oceanic subduction zones that were stationary over tens of millions of years, and beneath which lithosphere sank almost vertically. This hypothesis sets up an absolute lower-mantle reference frame. Combined with a complete Atlantic spreading record that positions paleo–North America in this reference frame, the slab geometries permit detailed predictions of where and when ocean basins at the leading edge of westwarddrifting North America were subducted, how intra-oceanic subduction zones were overridden, and how their associated arcs and basement terranes were sutured to the continent. An unconventional paleogeography is predicted in which midto late Mesozoic arcs grew in a long-lived archipelago located 2000–4000 km west of Pangean North America (while also consistent with the conventional view of a continental arc in early Mesozoic times). The Farallon Ocean subducted beneath the outboard (western) edge of the archipelago, whereas North America converged on the archipelago by westward subduction of an intervening, major ocean, the Mezcalera-Angayucham Ocean. The most conspicuous geologic prediction is that of an oceanic suture that must run along the entire western margin of North America. It formed diachronously between ca. 155 Ma and ca. 50 Ma, analogous to diachro nous suturing of southwest Pacific arcs to the northward-migrating Australian continent today. We proceed to demonstrate that this suture prediction fits the spatio-temporal evidence for the collapse of at least 11 Middle Jurassic to Late Cretaceous basins wedged between the Intermontane and Insular-Guerrero superterranes, about half of which are known to contain mantle rocks. These relatively late suturing ages run counter to the Middle Jurassic or older timing required and asserted by the prevailing, Andean-analogue model for the North American Cordillera. We show that the arguments against late suturing are controvertible, and we present multiple lines of direct evidence for late suturing, consistent with geophysical observations. We refer to our close integration of surface and subsurface evidence from geology and geophysics as “tomotectonic analysis.” This type of analysis provides a stringent test for currently accepted tectonic models and offers a blueprint for similar, continental-scale investigations in other accretionary orogens.

Sedimentary record for exhumation of ultrahigh pressure (UHP) rocks in the northern Cordillera, British Columbia, Canada

We use field relationships, paleoflow indicators, petrography, and major and trace element mineral chemistry to examine the protolith and provenance of detrital mantle-derived ultrahigh pressure (UHP—2.8 GPa) minerals in immature clastic sediments of an Early Jurassic basin (Laberge Group) in northwestern British Columbia, Canada. Our results show that fresh mantle detritus in the Laberge Group was derived from mantle lithosphere that equilibrated at >2.8 GPa and temperatures of 850–1100 °C, exhumed in orogenic massifs and quickly deposited over a restricted time interval. Two models are proposed for the exhumation and denudation of the UHP rocks in either (1) an arc-continent collision between the Stikinia and Yukon Tanana terranes, or (2) an exposed forearc and accretionary melange in a convergent margin in the Cache Creek terrane. A collision between the Stikinia and Yukon Tanana terranes has not previously been documented, but this scenario is the most consistent with widespread evidence for rapid exposure of deep-seated rocks at the required time period for deposition of UHP detritus ca. 185 Ma.

Erratum to Geological symbol set for Manifold ® Geographic Information System

e front matter Crown Copyright r 2006 Published by geo.2006.10.002 inal article: 10.1016/j.cageo.2005.11.003 ing author. Tel.: +1250 9520431; 0381. ess: Mitch.Mihalynuk@gov.bc.ca uk). improved and expanded symbol set. New symbols can be obtained directly from: http://www.em. gov.bc.ca/Mining/GeolSurv/Publications/Geofiles/ Gf2006-6/toc.htm. Both old and new symbol sets are posted on the Computers and Geosciences website at: http:// www.iamg.org/CGEditor/index.htm.