Southeastern Canadian Cordillera Research Articles

The volcanic architecture and tectono-magmatic framework of the Mount Grace carbonatites, southeastern Canadian Cordillera

The volcanic architecture and tectono-magmatic framework of the Mount Grace carbonatites, southeastern Canadian Cordillera

Regional-scale resistivity structure of the middle and lower crust and uppermost mantle beneath the southeastern Canadian Cordillera and insights into its causes

SUMMARYSubduction zones are recognized as an important class of plate boundaries and are the location of a number of important geological processes. They are also important because of the mineral and geothermal energy resources formed by plate convergence. While subduction zones around the world have a number of common features, there are also significant differences among them. The Cascadia subduction zone in southern British Columbia is characterized by a relatively hot subducting plate, and a broad backarc region that is believed to exhibit a shallow, convecting asthenosphere. The magnetotelluric (MT) method is a useful tool to study subduction zones and backarc regions because measurements of subsurface resistivity are sensitive to the presence of fluids. A number of previous MT studies have taken place in this region, but they were limited to a 2-D approach to data analysis. As the MT method has developed, it has become clear that there is a significant advantage to using a 3-D approach to data analysis. This paper presents the first regional-scale 3-D resistivity model of the southern Canadian Cordillera and provides new insights into the lithospheric structure and the distribution of fluids. The southeastern Canadian Cordillera has high heat flow and numerous thermal springs, the locations of which are often controlled by faults. However, the deeper thermal structure and origin of the fluids are poorly understood. To develop an improved understanding of the structure of this area, MT data measured at 331 locations were used to create a 3-D model of subsurface electrical resistivity. This study is primarily focused on the Omineca and Foreland morphogeological belts in southeastern British Columbia, which are separated by the southern Rocky Mountain Trench. The resistivity model is presented to a depth of 100 km and a number of conductive features are observed in the crust and uppermost mantle of the southeastern Cordillera. The locations of these conductors broadly matched previously reported conductors, but the 3-D inversion revealed new details of their geometry. The previously reported Canadian Cordilleran Regional conductor was modelled as a number of discrete conductors in the depth range 15–55 km beneath the Omineca belt. Temperatures approximately in the range 400–700 °C are expected at depths of 15–26 km and saline aqueous fluids are likely the cause of the low resistivity. Temperatures approximately in the range 700–1300 °C are expected at depths of 26–55 km and small volumes of partial melt may explain the low resistivity. The Southern Alberta–British Columbia conductor, Red Deer conductor and Loverna conductor were imaged as a single connected conductor, whose low resistivity is likely caused by sulphide mineralization. A group of conductors was imaged near the southern Rocky Mountain Trench in the depth range 10–70 km and their low resistivity is likely caused by interconnected saline fluids and possibly interconnected graphite films. To understand if the distribution of thermal springs was correlated with the 3-D resistivity model, a statistical study was undertaken. This showed no clear correlation between crustal conductance and the distribution of thermal springs.

Prolonged post-orogenic extension in the southeastern Canadian Cordillera: Miocene reactivation of the Columbia River Fault

Post-orogenic extension in the southeastern Canadian Cordillera manifested in the form of metamorphic core complexes exhumed during the Paleocene–Eocene, but new thermochronology data suggest this interval may be longer. The Columbia River Fault is a brittle-ductile normal fault bounding the eastern margin of the Monashee Complex, southeastern British Columbia. Vertical motions along this fault are investigated here using apatite fission-track and (U-Th)/He thermochronology and samples from two elevation profiles. Apatite fission track ages range from 29.3 to 17.7 Ma, and apatite (U-Th)/He ages range from 26.6 to 7.0 Ma. Inverse thermal history modeling of these profiles suggests the footwall underwent rapid cooling between 18 and 16 Ma, whereas the hanging wall experienced more protracted cooling since 40 Ma, with two small cooling pulses at 20 and 10 Ma. These new data suggest the Columbia River Fault was reactivated in the early Miocene and continued to exhume the Monashee Complex an additional ∼2 km. Combining all thermochronology data from the region suggests three phases of accelerated cooling in the Monashee Complex; 1) Eocene (56–40 Ma); 2) Miocene (20–16 Ma); and 3) Pliocene to recent. Thermal history modeling of thermochronometric data reported from elevation profiles from across the wider region of the southern Canadian Cordillera find early Miocene cooling phases contemporaneous with that seen along the Columbia River Fault. This widespread exhumation coincides with early Miocene plate tectonic reorganization of the Pacific-North American margin which resulted in increased transtension in northwestern North America. We propose that the early Miocene reactivation of the Columbia River Fault is a splay fault associated with dextral strike slip faulting along the Rocky Mountain Trench, driven by an increase in transtensional stresses along the Pacific-North American plate boundary. Building on earlier models, pre-existing gravitational instabilities in the southeastern Canadian Cordillera aided in lateral extension of the upper crust along unstable inherited structures throughout the Shuswap Metamorphic Core Complex

Modern dextral strain controls active hydrothermal systems in the southeastern Canadian Cordillera

Thermal springs in the southeastern Canadian Cordillera occur in association with several major fault zones, which may permit deep circulation of fluid through fractured reservoirs to depths greater than 2 km. Both the current stress field and the most recent kinematics of these faults likely play a significant role in localizing hydrothermal upwellings but are poorly resolved. In this paper, we present new data from structural mapping along the Columbia River, Slocan Lake, Purcell Trench, Southern Rocky Mountain Trench, and Redwall faults. Fault plane and slickenfiber orientations measured in the field indicate a previously unidentified, post-Eocene phase of dextral strike-slip kinematics on these faults, which have historically been mapped primarily as Eocene extensional structures. The NE−SW maximum principal stress required for these kinematics shares a similar orientation to the present-day stress field derived from crustal earthquake focal mechanisms, which suggests dextral slip may be a recent and ongoing development that continues at the present time. On a regional scale, there is a positive correlation between the locations of springs and regions with elevated seismicity. At the local scale, geothermal upwellings may be controlled by local zones of enhanced permeability including fault intersections and strain-transfer zones. The identification of favorable structural settings may enable discovery of hidden (a.k.a. blind) geothermal energy resources.

Re-evaluating monazite as a record of metamorphic reactions

This study presents a re-examination of historical specimens (DG136 and DG167) from the Monashee complex in the southeastern Canadian Cordillera that are critical to the current understanding of rare earth element (REE) distribution between garnet and monazite (and other accessory minerals) during metamorphism. Nine-hundred and fifty-one new monazite petrochronology spot analyses on 29 different grains across two specimens outline detailed (re)crystallization histories. Trace element data collected from the same ablated volume, interpreted in the context of new phase equilibria modelling that includes monazite, xenotime and apatite, link ages to specific portions of the pressure–temperature (P-T) paths followed by the specimens. These linkages are further informed by garnet Lu-Hf geochronology and xenotime petrochronology. The clockwise P-T paths indicate prograde metamorphism was ongoing by ca. 80 Ma in both specimens. The structurally deeper specimen, DG136, records peak P-T conditions of ~755–770 ℃ and 8.8–10.4 kbar, interpreted to coincide with (re-)crystallization of low Y monazite at ∼75–70 Ma. Near-rim garnet isopleths from DG167 cross in the observed peak assemblage field at ∼680 °C and 9.3 kbar. These conditions are interpreted to correspond with low Y monazite (re-)crystallisation at ∼65 Ma. Both specimens record decompression along their retrograde path coincident with high Y 70–55 Ma and 65–55 Ma monazite populations in DG136 and DG167, respectively. These findings broadly agree with those initially reported ∼20 years ago and confirm early interpretations using trace elements in monazite as generally reliable markers of metamorphic reactions. Modern phase equilibria modelling and in situ petrochronological analysis, however, provide additional insight into monazite behaviour during anatexis and the effects of potential trace element buffering by REE-bearing phases such as apatite.

Timing, rates and geomorphic controls on holocene loess and aeolian sand deposition using multiple chronometric methods, southeastern Cordillera, Alberta, Canada

Glacier-fed river systems represent potentially significant sediment sources for aeolian processes owing to sediment production, flow seasonality, and winds within mountain settings. We investigate the timing and rates of Holocene loess and aeolian sand deposition in three major river valleys (Bow, North Saskatchewan and Athabasca) of the southeastern Canadian Cordillera through aeolian stratigraphy and dating of sedimentary materials. Source sediments are glacially-derived, seasonally-exposed alluvial deposits. Variation in river morphology promotes source-sediment deposition influenced by bedrock configuration, tributary stream deposition and alluvial fans. Two main settings for aeolian deposition are source-proximal valley-bottom sites and more distal lower-valley sites in sub-alpine and montane forest ecozones. Dating is well-constrained by radiocarbon ages and tephrochronology. Quartz optical dating is complicated, likely owing to minimal recycling of grains and quartz provenance. Mass accumulation rates (MARs) indicate that aeolian deposition was initiated soon after deglaciation and has occurred nearly continuously for at least 10000 years. Loess MARs ranged between 100 and 2500 g·m−2·a−1 during the Holocene. Rates in the last 3000 years are generally two to three times higher than those observed for earlier periods, and are highest in most recent times. We attribute Holocene accumulation at these sites in general to fluvial source sediment availability and the efficacy of forested valleys as sediment traps. The late Holocene increase in MARs is primarily attributed to increased glacial silt and alluvial sediment supply during neoglacial fluctuations including the Little Ice Age.

Nailed to the craton: Stratigraphic continuity across the southeastern Canadian Cordillera with tectonic implications for ribbon continent models

Abstract Cambrian and Upper Devonian to Mississippian strata can be confidently traced westward, without strike-slip offset, from the autochthonous section above North American basement into the southeastern Canadian Cordillera, and are thus “nailed” to the craton. These strata are in turn stratigraphically pinned to older (Mesoproterozoic Belt-Purcell Supergroup, Neoproterozoic Windermere Supergroup, and Ediacaran), intermediate-aged (Ordovician–Silurian), and younger (Permian to Middle Jurassic) strata found only in the mountains, thus linking them to the adjacent autochthonous craton. The overlapping distribution of linking successions, regionally traceable unique stratigraphic horizons in the Belt-Purcell and Windermere Supergroups, and across-strike stratigraphic features show that the entire Cariboo, northern Selkirk, Purcell, and Rocky Mountains are directly tied to the adjacent North American craton without discernible strike-slip or oblique displacement, or substantial purely convergent plate-scale (>400 km) horizontal displacement. They link the entire width of the Belt-Purcell and Windermere basins in the southeastern Canadian Cordillera to the adjacent craton and show that any proposed Cretaceous ribbon continent suture, with its thousands of kilometers of proposed displacement, cannot run through the southeastern Canadian Cordillera.

Three modes of isograd formation in the northern Monashee Complex of the Canadian Cordillera

AbstractThis contribution re-examines textural relationships in metapelitic schist from five metamorphic zones separated by four isograds along the west flank of the Frenchman Cap dome in the southeastern Canadian Cordillera. There is a muscovite-out isograd formed by muscovite-dehydration melting in the kyanite-field. Two others, including one coinciding with the muscovite-out isograd, mark the appearance of sillimanite, which was produced along with biotite by garnet breakdown in the presence of melt. Two others mark the disappearance of kyanite: one formed during retrogression, the other coinciding with a shear zone (the Monashee décollement). Only the muscovite-out isograd, therefore, corresponds with the classical definition of an intersection of the topography with a prograde, isogradic-surface. This contribution exemplifies the need to use a descriptive definition of isograd as the trace of a surface across which a specific change in metamorphic mineralogy takes place. It is further proposed to specify their origin whenever possible with terms such as prograde-, retrograde- and structural-isograds. This revised terminology for isograds will improve our understanding of metamorphic terrains by helping to localize cryptic shear zones, gain a better understanding of the retrograde path and put firmer constraints on viable tectonic models.

The effects of lateral density gradients, slopes and buoyancy on channel flow: 1D analytical solutions and applications to the SE Canadian Cordillera

We present 1D analytical solutions for channel flow in orogens driven by various types of pressure gradients. Our calculations demonstrate that lateral density gradients in the upper crust, such as would occur across a suture zone separating arc rocks from pericratonic sediments provide a driving force for Poiseuille flow as large as topographic gradients observed in modern mountain belts. For cases for which the gradients are external (topographic and lateral density gradients) and internal (e.g. partial melting of channel material) to the channel, inclination decreases and increases the Poiseuille component of the average flow-velocity within the channel by the cosine and sine of the slope, respectively. The magnitude of the pressure gradient consequent upon the buoyancy generated by partial melting of metapelites in a channel with a 30° slope, such as would occur above an underthrusting basement ramp, is similar to that of topographic or lateral density gradients. Channel flow up a ramp could thus constitute an important exhumation mechanism in large hot orogens. Our calculations indicate that mid-crustal channel flow was a highly likely process in the Late Cretaceous-Paleocene setting of the southeastern Canadian Cordillera. The flow was first driven by the lateral density contrast between pericratonic sediments and the arc-related Intermontane terrane, then by combined effect of topographic gradient and melt-induced buoyancy of the Lower Selkirk Allochthon (part of the Shuswap Complex). Flow up the underthrusting basement ramp resulted in exhumation from mid- to upper-crustal levels. Channel flow then migrated downward to involve basement and overlying cover sequence rocks. Our results indicate that syn-convergent channel flow was a viable and very likely process in the southeastern Canadian Cordillera.

Prograde and near-peak zircon growth in a migmatitic pelitic schist of the southeastern Canadian Cordillera

U–Pb dates and trace elements acquired simultaneously on zircon from a migmatitic pelitic schist from the southeastern Canadian Cordillera are linked with results of phase equilibria modeling. The first of three phases of zircon growth occurred between 77 and 72Ma along a prograde path in the kyanite+melt field between the wet-melting and muscovite-dehydration melting reactions (675–775°C). This protracted phase of prograde growth is characterized by a shallow positive slope in heavy rare earth elements (HREE). The second phase occurred at c. 69Ma, after significant garnet growth by biotite dehydration-melting at 775–875°C, which produced zircon depleted in HREE. The last phase occurred at c. 63Ma on the retrograde path as garnet and rutile were consumed at >780°C, as indicated by enrichments in HREE and Nb, but prior to feldspar crystallization in the melt, as suggested by the moderate Eu anomaly. This path is consistent with rutile and monazite petrogenesis, documented herein and in a previous study, respectively. In contrast with the high temperatures at which zircon grew (675–875°C), Ti-in-zircon thermometry yields unrealistically low estimates (470–739°C) with 80% below the wet solidus; there are no uncertainties in the buffering assemblage, as rutile and quartz were present throughout, and no effect of pressure because a pressure-dependent calibration was used. This study demonstrates that zircon growth along a prograde path is possible and warrants further investigation on factors controlling the Ti content of zircon thermometry in metamorphic rocks.

Magnetic models of the Moyie anticline, Purcell anticlinorium, southeastern Canadian Cordillera

Two-dimensional magnetic models of the Moyie anticline contribute to an independent project targeting development of a regional three-dimensional (3D) model of much of the Purcell anticlinorium by compensating for shortcomings in reflection seismic signatures and distributions of drill holes and surface structural data. Modelling has focused on prominent anomalies related to relatively strong magnetizations within the Creston Formation and Moyie sills. Models of the western flank of the anticline indicate the Creston Formation forms the bulk of a westward-thickening, wedge-shaped sedimentary package whose west-dipping layers are truncated by a steeply west-dipping Moyie fault, truncating also Moyie sills on the opposite side. Near-surface dips of the fault range from 54° to 87°. Stratigraphic comparisons across the fault indicate minimum reverse dip displacements ranging from 5.6 to 8.5 km. The eastern flank of the Moyie anticline is modelled consistently as a monoclinal, eastward-dipping stratigraphic package comprising mainly Lower Aldridge Formation (with intruded Moyie sills) through Kitchener Formation, in general accord with geological models. One magnetic model, however, shows stratigraphy disrupted by shallow thrust faults, above which a series of near-vertical blocks is modelled. The Yahk River syncline is modelled as a symmetrical fold having an axial depth of 4.2 km to the top of the Upper Aldridge Formation. Collectively, the models outline a broad domal pattern of Moyie sills in the core of the anticline to 12 km depth, supporting a proposal that a gravity high correlating with the Moyie anticline relates to high-density Moyie sills at relatively shallow depths, rather than deeper high-density basement.

Pre- to post-Cordilleran transposition history of Joss Mountain: Insights into the exhumation of the Shuswap complex, southeastern Canadian Cordillera

We present new multiscale structural, mineral chemical, and U-Pb isotope dilution–thermal ionization mass spectrometry (ID-TIMS) data in order to unravel part of the tectono-metamorphic evolution of the Shuswap complex in the southern Canadian Cordillera. We reconstructed the pressure-temperature-deformation-time ( P - T - d - t ) history of the Joss Mountain domain within the Shuswap complex. The west-dipping Greenbush shear zone separates the Joss Mountain domain from the structurally lower Thor-Odin culmination to the east, the southern culmination of the Monashee complex, and one of the structurally deepest parts of the Shuswap complex. At Joss Mountain, the protolith of an orthogneiss crystallized at ca. 360 Ma which is consistent with Late Devonian arc magmatism along the western paleomargin of North America. Joss Mountain metasedimentary rocks and orthogneiss were transposed at ∼21–29 km depth over a period of at least 20 m.y., and possibly more than 38 m.y., during Late Cretaceous to Paleocene mature stages of Cordilleran continental collision. This mature collision took place while slow detachment of the subducted oceanic lithosphere occurred and thermal conditions were approaching those of a crust undergoing postorogenic thermal relaxation. Transposition at Joss Mountain ended earlier and exhumation started earlier than in the Monashee complex. Exhumation occurred under conditions of near-isothermal decompression and geothermal gradients consistent with lithospheric thinning. Earlier and slower exhumation of the Joss Mountain domain than of the adjacent northwestern Thor-Odin culmination may have resulted from normal movement along the Greenbush shear zone contributing to the exhumation of the Shuswap complex.

Middle Jurassic to earliest Cretaceous mid-crustal tectono-metamorphism in the northern Canadian Cordillera: Recording foreland-directed migration of an orogenic front

In situ sensitive high-resolution ion microprobe monazite geochronology and garnet isopleth thermobarometry reveal a previously unrecognized Middle Jurassic to earliest Cretaceous mid-crustal tectono-metamorphic event in the eastern part of the Yukon-Tanana terrane (Finlayson Lake district, southeast Yukon) in the northern Canadian Cordillera. Intersection of garnet end-member compositional isopleths applied to single-stage, growth-zoned garnet records progressive garnet growth from 550 °C and 6.1–6.6 kbar to 600 °C and 7.5 kbar. Monazite textures, chemical zoning, and in situ U-Pb ages record a single protracted episode of monazite growth from ca. 169 to 142 Ma coeval with the development of transposition fabrics and the late stages of garnet growth. This event post-dates widespread Early Jurassic exhumation of Yukon-Tanana terrane rocks west of the Tintina fault in west-central Yukon, which were previously ductily deformed and metamorphosed in the Permo-Triassic. The lack of evidence for Permo-Triassic ductile deformation and high-grade metamorphism within the Finlayson Lake district, and its position east of the Permian arc center and west of Permian blueschists and eclogites, suggests this eastern part of the terrane occupied the cool forearc at this time. These data indicate younger, more protracted mid-crustal orogenesis in the northern Cordillera than was previously recognized, with deformation and metamorphism migrating toward the foreland and downwards in the Middle Jurassic to Early Cretaceous, in part contemporaneous with and analogous to that in the southeastern Canadian Cordillera.

A window into the Early to mid-Cretaceous infrastructure of the Yukon-Tanana terrane recorded in multi-stage garnet of west-central Yukon, Canada

Amphibolite facies metasedimentary schists within the Yukon-Tanana terrane in the northern Canadian Cordillera reveal a two-stage, polymetamorphic garnet growth history. In situ U-Th-Pb Sensitive High Resolution Ion Microprobe dating of monazite provide timing constraints for the late stages of garnet growth, deformation and subsequent decompression. Distinct textural and chemical growth zoning domains, separated by a large chemical discontinuity, reveal two stages of garnet growth characterized in part by: (i) a syn-kinematic, inclusion-rich stage-1 garnet core; and (ii) an inclusion-poor, stage-2 garnet rim that crystallized with syn- to post-kinematic staurolite and kyanite. Phase equilibria modelling of garnet molar and compositional isopleths suggest stage-1 garnet growth initiated at ~600 °C, 8 kbar along a clockwise P–T path. Growth of the compositionally distinct, grossular-rich, pyrope-poor inner portion of the stage-2 overgrowth is interpreted to have initiated at higher pressure and/or lower temperature than the stage-1 core along a separate P–T loop, culminating at peak P–T conditions of ~650–680 °C and 9 kbar. Stage-2 metamorphism and the waning development of a composite transposition foliation (ST) are dated at c. 118 Ma from monazite aligned parallel to ST, and inclusions in syn- to post-ST staurolite and kyanite. Slightly younger ages (c. 112 Ma) are obtained from Y-rich monazite that occurs within resorbed areas of both stage-1 and stage-2 garnet, together with retrograde staurolite and plagioclase. The younger ages obtained from these texturally and chemically distinct grains are interpreted, with the aid of phase equilibria calculations, to date the growth of monazite from the breakdown of garnet during decompression at c. 112 Ma. Evidence for continued near-isothermal decompression is provided by the presence of retrograde sillimanite, and cordierite after staurolite, which indicates decompression below ~4–5 kbar prior to cooling below ~550 °C. As most other parts of the Yukon-Tanana terrane were exhumed to upper crustal levels in the Early Jurassic, these data suggest this domain represents a tectonic window revealing a much younger, high-grade tectono-metamorphic core (infrastructure) within the northern Cordilleran orogen. This window may be akin to extensional core complexes identified in east-central Alaska and in the southeastern Canadian Cordillera.

Barrovian metamorphism in the central Kootenay Arc, British Columbia: petrology and isograd geometry

A narrow, partly fault-bounded belt of Barrovian amphibolite facies rocks transects the central Kootenay Arc in the internal zone of the southeastern Canadian Cordillera. The following zones of increasing metamorphic grade are recognised in metapelites: chlorite/biotite, garnet, staurolite, kyanite, sillimanite, and sillimanite + K-feldspar. The garnet and higher-grade zones outline two joined domains: a N- to NNE-trending curved belt, >100 km long and 5–20 km wide, that is approximately parallel to the strike of stratigraphic units; and a SSE-trending belt, >70 km long and 10–15 km wide, that transects strike. Isograds in the N–NNE-trending belt outline an elongate bull’s-eye pattern with highest-grade rocks in the centre, coincident with the position of a structural culmination. Isogradic surfaces have an elongate domal form centred around this culmination. Rocks in the kyanite and sillimanite zones were metamorphosed at ~25 km (∼7 kbar (1 kbar = 100 MPa)) and >650 °C during Early Cretaceous crustal thickening; rocks in the garnet zone experienced peak temperature conditions (∼500 °C) at lower pressure, suggesting the piezothermic array for the belt has a positive slope. The western flank of the N- to NNE-trending belt is cut by the west-dipping Gallagher fault zone (GFZ), whereas the eastern boundary of the SSE-trending fork is marked by the Purcell Trench fault (PTF). These Palaeocene–Eocene normal faults truncate Early Cretaceous isogradic surfaces and juxtapose regions with contrasting structural and metamorphic histories. Low-grade rocks in the hanging wall of the GFZ underwent peak regional metamorphism during the Early–Middle Jurassic, prior to intrusion of the 159–173 Ma Nelson batholith at a depth of ∼12–14 km. With the exception of a zone along the southern “tail” of the Nelson batholith, rocks in the hanging wall of the GFZ were not affected by Cretaceous metamorphism or penetrative deformation. Rocks in the amphibolite-facies belt yield Palaeocene–Eocene K–Ar and 40Ar/39Ar mica cooling ages, whereas K–Ar biotite ages in the hanging wall of the GFZ record Jurassic – Early Cretaceous cooling, and those in the hanging wall of the PTF are mid-Cretaceous. Although the GFZ and PTF accommodated differential exhumation during the Palaeogene, significant relief on isogradic surfaces was established prior to normal faulting, during Early Cretaceous metamorphism and deformation.

Strong lateral strength contrasts in the mantle lithosphere of continents: A case study from the hot SW Canadian Cordillera

Abstract This study aims at quantifying the 3-D variability in lithosphere strength of the south-eastern Canadian Cordillera and adjacent craton to the east. Strength is calculated in a forward manner, starting from rheological laws of brittle and ductile deformation. The work flow calculates a temperature model based on a multi-layer compositional model and subsequently estimates the strength distribution from both the compositional and temperature models. The temperature modeling involves numerical inversion and cubic spline algorithms which enable to impose boundary conditions that account better for strong lateral variations in lithosphere thickness and lateral heat flow. This addresses the lithosphere structure between the Canadian Cordillera and craton. The Canadian Cordillera is marked by a hot and thin lithosphere of ~ 60 km thickness that stands in contrast to a cold and thick craton of ~ 170 km thickness to the east, which consequently results in pronounced changes in bulk lithosphere strength. The high surface heat flow of the Cordillera interior and its contrast with the Foreland Basin can be reproduced by temperature models that combine elevated mantle heat flow due to the thin lithosphere and higher crustal heat production from magmatic intrusions. A series of rheological models, which examines the role of different temperature input models, composition and strain rate, shows that the first-order strength pattern is persistent. For the hot Canadian Cordillera, strength resides for > 80% in the upper crust and with integrated lithosphere strength of 2.0–3.5⋅106 MPa⋅m. For the cold craton, the upper mantle provides > 75% of the integrated strength with values of 4.0–8.0⋅106 MPa⋅m for the crust and 20–65⋅106 MPa⋅m for the lithosphere. Effective elastic thickness is estimated between 5 and 15 km for the Cordillera and 40–80 km for the craton. This illustrates the Cordillera to craton transition as a prominent rheological feature for upper mantle flow dynamics and spatial seismicity trends.

Deep transverse basement structural control of mineral systems in the southeastern Canadian Cordillera1Geological Survey of Canada Contribution 20110294.

Intermittent reactivation of deep Paleoproterozoic- and Archean-aged transverse structures in the North American cratonic basement beneath the southeastern Canadian Cordillera exerted an important influence on the subsequent sedimentary, intrusive, deformation, and mineralization history. Two regional crossbelt transverse structural corridors recognized by recurrent episodic changes in sediment thickness and facies, and anomalous transverse structural trends, occur within the thrust and fold belt. Palinspastic restoration of the exposed transported structural corridors showed that the northern and southern structural corridors are cover expressions of the Red Deer zone and Vulcan Low structures in the cratonic basement, respectively. Transverse basement-controlled structural corridors enhanced the mineral potential and mineralization systems along them by localization of sedimentary facies favourable for mineralization, hydrothermal flow, intrusions, and favourable structures for mineralization. Greatest enhancement of hydrothermal flow and mineralization occurred near the intersection of basin-parallel and transverse structures. Greater opportunity for mineralization and intrusion occurred not only along the exposed corridors, but also in the west, above their undeformed “root” in the basement. The processes that intensified mineralization and localized intrusions along transverse basement-controlled structural corridors in the southeastern Canadian Cordillera should have occurred in thrust and fold belts elsewhere. The niobium-bearing Aley carbonatite complex in the eastern Cordillera of northeastern British Columbia may be an example of this.

Deformational and metamorphic history by multi-scale analysis at Joss Mountain, Shuswap complex, south-eastern Canadian Cordillera.

Deformational and metamorphic history by multi-scale analysis at Joss Mountain, Shuswap complex, south-eastern Canadian Cordillera.

Cretaceous to Eocene evolution of the southeastern Canadian Cordillera: Continuity of Rocky Mountain thrust systems with zones of “in-sequence” mid-crustal flow

The ∼400 km wide, east-verging, retrowedge side of the southeastern Canadian Cordillera was predominantly formed in a tectonic setting of oblique plate convergence during the Cretaceous to Eocene. This paper documents the internal geometrical development of the retrowedge. In the External zone, the Rocky Mountains and Foothills are characterized by three major east-verging, Late Cretaceous to Eocene, thin-skinned, piggyback thrust and fold systems. They root westward into a basal décollement and accommodated ∼180 km of shortening. The Western Internal zone is characterized by tracts of metamorphic rocks and metamorphic core complexes (e.g. Kettle, Okanagan, Priest River and Valhalla), some of which are basement-cored domes (e.g. Frenchman Cap, Thor-Odin, and Spokane). They have a downward-younging progression of Late Cretaceous to Eocene metamorphism and deformation in infrastructural flow zones characterized by transposition foliation, migmatites, flow folds and 1–7 km thick shear zones. In the Eastern Internal zone, a relict ∼100–200 km wide Early Cretaceous orogen, that predated emplacement of ca. 100 Ma plutons, is nested between the External and Western Internal zones. The geology and architecture of the Internal and External zones can be explained by progressive development of major Late Cretaceous to Eocene shear zone systems in the Internal zone that can be directly linked with coeval thrust and fold systems in the External zone. The linkage was via Late Cretaceous activation and Late Cretaceous to Early Eocene reactivation of the 150–200 km-wide central portion of the Rocky Mountain basal décollement that lies beneath and translated the intervening Early Cretaceous orogen. During the latest stages of shortening, in the Early Eocene, extensional shear zone systems in the Internal zone, localized on tectonothermal culminations, were concomitant with shortening in the External zone. Motion of deep-seated Early Eocene décollements beneath some of these culminations may have contributed to their doming. Crustal shortening ended at ca. 52 Ma due to a change in tectonic setting to that of a transtensional tectonic regime, coinciding with the end of thrusting in the External thrust belt and with crustal-scale extension in the Western Internal zone.

Testing modes of exhumation in collisional orogens: Synconvergent channel flow in the southeastern Canadian Cordillera

This study investigates exhumation processes in collisional orogens. A critical test between three modes of exhumation is presented based on a review of quantitative numerical and analogue modeling studies. The test is applied to the large tract of migmatites surrounding the Monashee Complex in the southeastern Canadian Cordillera. It reveals that the extensive and multidisciplinary database of this region is entirely compatible with the synconvergent channel-flow mode but not with critical wedge and gravitational collapse modes. We propose that a partially molten channel decoupled from its lid and base and started to flow in the middle crust toward the foreland at 100–90 Ma. A steady-state channel-flow system was established for the following ∼30 m.y., during which rocks at the front of the channel were exhumed to upper-crustal levels as they flowed above an underthrusting basement ramp. Flow was accommodated by oppositely verging shear zones bounding the channel, by internal ductile deformation, and by shortening in the foreland belt. The locus of flow migrated downward to exhume rocks of the Monashee Complex between 60 and 50 Ma by a similar process. The southeastern Canadian Cordillera thus constitutes an excellent natural analogue for the channel-flow model. In contrast to the commonly held view of large-magnitude extension and core complex formation, the role of extension was limited to the final ∼10–15 km of exhumation after 50 Ma.