In-sequence Mid-crustal Flow Zones in the Core of the Southeastern Canadian Cordillera Linked to Rocky Mountain Thrust Tectonics
In the core zone of the southeastern Canadian Cordillera, a suprastructure with low-grade Jurassic metamorphism lies above an infrastructure with Early Cretaceous and Late Cretaceous to Paleogene high- grade metamorphism and melting. This infrastructure has an eastern subsurface tip line because in the external part of the orogen, the Cretaceous to Paleogene thin-skinned thrust belt does not have an infrastructure, and for much of its length, the tip line has not been exhumed. Models proposed in the last two decades suggest indirect linkages between infrastructural flow in the core and thrusting in the Rockies; however, we propose direct linkages. The core zone is dominated by zones of shear and flow that are several kilometers thick and deform North American basement. We documented that these zones are linked directly to particular Rocky Mountain thrust fault systems that lie above uninvolved basement. The thin-skinned tectonic style of the Rockies is approximately balanced in the west by eastward flow of basement sheets. Within the broad internal zone of the thrust belt there is a westward gradation from simple thrust sheets, to sheets with local flow zones, to sheets with significant flow, and ultimately, to infrastructural flow. Geochronology tied to cross-cutting plutons, mineral assemblages and deformation fabrics provides a temporal framework for the mapped structures. The shear and flow zones were activated "in-sequence" with the deepest being the youngest, and this is correlative with the "in-sequence" evolution of the thrust belt. The Early Cretaceous (ca. 125 to 115 Ma) Bearfoot-Ptarmigan décollement carried the Malton Complex basement sheets, and is linked to Main Range thrusts near Jasper. The Late Creatceous-Paleocene (ca. 90 to 60 Ma) Gwillim Creek shear zone carried the basement of Quesnel terrane, and perhaps North American basement down-dip to the west, and the shear zone is linked to the Lewis Thrust. In the Monashee complex, the northerly Frenchman Cap basement passes southward under the basement-cored Thor-Odin dome. We propose that the Late Cretaceous to Eocene (ca. 75 to 55 Ma) Monashee décollement, that bounds the upper margins of Frenchman Cap dome, passes under the basement cored Thor-Odin dome and that the Monashee décollement is linked to the McConnell thrust. The geometry that we propose for the Monashee décollement places the décollement at the base of a 10 to 15 km thick zone with substantial eastward "transport" (Couette) flow. The channels of the various recently proposed channel flow models lie within this zone; however, the stratigraphic and structural coherence documented within this zone limits any possible channel (Poiseuille) flow to 10 to 20 km. Such limited flow would not alter the kinematic framework that we suggest. We propose that, in the core zone, an Eocene (ca. 55 to 50 Ma) basal décollement lies at depth under all the basement sheets, including the Frenchman Cap dome, and that it is linked to the basal décollement of the Foothills thrusts.
Late Superstructure-Style Folding of the Monashee complex Infrastructure
In the classic infrastructure-superstructure association upright folds of the superstructure become progressively more recumbent downwards into the infrastructure zone, accompanied by an increase in metamorphism, commonly from greenschist- to amphibolite-facies conditions. Rock units are continuous through the transition zone and, in general, the earliest folds of the infrastructure are inherited from upright superstructure folds. With progressive deformation and changing boundary conditions, the position of the infrastructure-superstructure transition zone can migrate through the crust. Thus rocks previously deformed in the infrastructure zone might migrate back into the superstructure and undergo further deformation there. An example of this situation is observed in the Thor-Odin region of the Monashee complex in the Canadian Cordillera of southeastern British Columbia. Here, a tectonically exhumed section of the middle crust exposes a transposition foliation and attendant structures characteristic of an infrastructure zone that was developed and progressively modified by top-to-NNE penetrative non-coaxial shear between ca. 56 and 51 Ma. Evidence for an earlier superstructure stage of folding and fabric development occurs in the form of an early generation of transposed regional-scale periodic folds with abundant parasitic folds on either limb. Post-51 Ma crustal shortening and upright folding at shallow structural levels in Thor-Odin buckled the transposition foliation into a regional antiform in a style consistent with superstructure deformation. A suite of subvertical and N-striking ca. 52 to 48 Ma lamprophyre dykes cross-cut this late upright antiformal structure and experienced only a greenschist grade metamorphic overprint. At the same time, the deepest exposed regions were still deforming via top-to-NE shear contemporaneously with intrusion of a suite of mafic dykes interpreted to be co-magmatic with the lamprophyres. Melting was still ongoing at this structural level. These observations are consistent with a downward migration of the superstructure-infrastructure transition zone during the late ductile overprint of the Monashee complex. This transition zone migration can also explain the general pattern of downward younging of peak metamorphic ages throughout the Cordilleran orogenic hinterland. The structural evolution of the Monashee complex outlined above can be related to contemporaneous plate-scale tectonic events occurring along the active western margin of North America.
Thor-Odin Dome: Stacking, Burial, Heating and Flow of Crystalline Sheets in the Core Zone of the Southern Canadian Cordillera
The geometry and Late Cretaceous to Early Eocene thermotectonic history of the Thor-Odin dome, in the core zone of the Cordilleran orogen in southern BC, can be explained by a model of crustal shortening by progressive stacking, prograde metamorphism and penetrative ductile deformation of basement-cored crystalline sheets, in response to the westward under thrusting of the North American craton. Rocks that are now exposed in the Thor-Odin dome were buried to ca. 30 km in the Late Cretaceous (ca. 90 to 60 Ma) by the emplacement of a crystalline nappe, now exposed in Valhalla complex and in the rocks that structurally overly Thor-Odin dome, all carried by the Gwillim Creek shear zone. Rocks of the footwall of the Gwillim Creek shear zone, and in the base of the sheet it carried, underwent subsequent thermal relaxation, high-grade metamorphism and anatexis, thermal weakening and polyphase penetrative deformation (e.g. transposition foliation, and isoclinal, intrafolial and tight folds) with northeast directed flow. Penetrative high strain was localized in the basal 5 to 7 km (now Thor-Odin dome) during the Late Cretaceous transport of the sheet up a crustal-scale ramp, the Monashee ramp imaged in the Lithprobe seismic reflection profiles. The upper part of this sheet, that now structurally overlies Thor-Odin dome, started to cool and was structurally deactivated in the Paleocene to Early Eocene (U-Pb age of ca. 58 Ma cross-cutting pegmatite), whereas rocks of the dome itself continued to deform in the Early Eocene (U-Pb age of ca. 56 Ma. syntectonic leucosome) during decompression melting and extensional exhumation. The southwest dipping geometry of a ca.15 km thick panel of rocks on the southern flank of the dome was acquired prior to Eocene extensional denudation, on the basis of data from 40Ar/39Ar transects across the area, consistent with the ramp model. The adjacent basement-cored Thor-Odin and Frenchman cap domes, have historically been considered as having a similar geology (e.g. comparable supracrustal rocks and overlying basement rocks) and a shared Cordilleran tectonic history. However, on the basis of significant differences in lithology, structural style, and timing of deformation and metamorphism of the two domes we interpret Thor-Odin dome as a more outboard lithotectonic domain that formed part of a ca. 30 km thick sheet that overrode Frenchman Cap dome during the last stages of Cordilleran orogenesis. Rocks of Thor-Odin dome were completely transposed and deformed in the Early Eocene, whereas those of Frenchman Cap dome preserve a strain gradient, representing the base of penetrative ductile Cordilleran deformation, and the lower limit of the Cordilleran thermal overprint imposed on the Precambrian history of the craton. In our model, rocks of Thor-Odin dome are part of the sheet of metasedimentary rocks (e.g. Selkirk Allochthon), that structurally overrode rocks of Frenchman Cap; however, at the latitude of Thor-Odin, this sheet contains basement. Frenchman Cap gneisses, and its upper bounding shear zone, the Monashee décollement, project beneath Thor-Odin. The systematic stacking of internally deforming sheets in the core of the orogen was concomitant with and fed into thrust fault systems to the east in the Foreland fold and thrust belt.
Metamorphic evolution in the western Thor-Odin dome, Monashee Mountains, Canadian Cordillera
New data are presented for rocks from the Thor-Odin dome, to reconstruct the tectono-metamorphic evolution of high temperature metapelites and amphibolite boudins from a 5 km long E-W transect from the core of the dome to the Greenbush Lake normal shear-band zone, which rims the western margin of the dome. This work is aimed at describing mineral assemblages associated to the transposition foliation (ST), the dominant mesoscopic fabric in the Thor-Odin dome. Among the five lithostratigraphic complexes of the Canadian Cordillera, the Omineca belt consists of Precambrian intrusive and metamorphic rocks of the North American craton, exhumed as a consequence of the collisional accretion of the allocthonous terranes. This complex is the deepest metamorphic core of the Canadian Cordillera and includes the Monashee complex, in South East British Columbia. The southernmost part of the Monashee complex is the Thor-Odin dome, which consists of a Proterozoic basement sequence (migmatitic orthogneisses and paragneisses, with amphibolites) and of a Proterozic to Palaeozoic meta- sedimentary sequence (quartzites, schists, marble, calcsilicates, gneisses, and amphibolites). ST characterises both sequences. The ST of the Thor-Odin dome is a product of progressive deformation involving repeated cycles of perturbation by folding of ST, followed by fold tightening to transpose folded surfaces back into ST. The foliation contains a gently southwest-plunging Ky-bearing lineation. A regional westerly dipping normal shear zone, extending down to the Moho, reactivates the ST at the western margin of the dome (Thor-Odin detachment) and is associated with extensional tectonics; a westerly plunging Sil-bearing lineation develops during top-to-the-W shear. This lineation overprints the Ky-bearing lineation. A prominent micro-fracturing developed perpendicular to the lineation in the shear zone, during a later extension related to the N-S trending Victor Creek Fault. In Grt-free and Grt bearing amphibolite boudins Amp, Pl, Qtz and minor Bt are parallel to the dominant foliation. In Grt bearing amphibolites Oamp is interpreted as syn-kinematic. Coarse-grained Grt is in contact with Pl, Cpx and Amp, and locally has an internal foliation at a high angle to the matrix foliation. Grt is rimmed by symplectites of Amp and Pl. Chl and green Amp partially replace Oamp, Amp and Grt. In Grt-free amphibolites Amp parallel to the dominant foliation shows lower Ti content and higher Al content than the rim of new symplectitic Amp. The latest Amp, which grew in fractures, shows the lowest content in Ti and Al. In metapelites Sil, Bt, Kfs and Qtz are parallel to the ST foliation. Grt is rimmed by symplectites of Bt, Pl and Qtz. Ky crystals, enclosing Bt and partially replaced by Sil, are wrapped by the ST foliation and show inclusion trails at high angle to ST. Bt parallel to ST and enclosed in Ky shows a higher Ti content than the Bt in the symplectites. Wm and Chl grew in fractures intersecting ST -parallel Sil laths and between Grt and Bt. In Ky-free metapelites Bt, Sil, Pl, Kfs and Qtz have a SPO defining ST. Grt forms coarse-grained crystals, which are wrapped by ST, partially replaced by Bt, Pl and Qtz, and shows core-rim zoning; rare Wm grew at the Sil rim. In the polydeformed metamorphic rocks outcropping from Blanket Mt. to the Greenbush Lake shear-band zone, P-T estimates have been inferred by using mineral assemblages coeval with superposed fabrics in rocks of different bulk composition.
Structure and Thermochronology Studies on the Southwestern Flank of the Thor-Odin Dome: Implications for Tectonic Models and Lithoprobe Seismic Reflection Data, Southern BC
Cariboo Alp, a well-exposed km-thick belt of highly strained Kfs-sil migmatitic paragneiss on the SW flank of Thor-Odin dome, has been viewed as an important boundary between the basement-cored migmatitic gneiss dome and the overlying ∼7 km thick panel of high-grade supracrustal rocks. Interpretations include: i) a NE-directed ductile thrust duplex within the Monashee décollement; ii) part of a mid-crustal channel; and/or, iii) an Eocene extensional shear zone. Establishing the nature of this boundary is a test of possible models for the tectonic evolution of Thor-Odin dome and its role in the Late Cretaceous to Eocene construction of the Cordilleran orogen. Prominent structures at Cariboo Alp include a moderately S-SW dipping composite transposition foliation with intrafolial rootless folds, concordant leucosome, granitoid lenses and boudinaged layers, NE verging folds and W-SW plunging mineral and stretching lineations. NE-directed kinematic indicators such as shear bands, S-C fabrics and fold vergence indicate progressive northeastward flow. Deformation at Cariboo Alp was ongoing at ca. 62 Ma, during thermal-peak Kfs-sil-melt metamorphism, and had ended by ca. 58 Ma, based on U-Pb zircon ages of deformed and crosscutting pegmatites; whereas penetrative ductile flow continued in the rocks of the underlying dome until ca. 54 Ma, based on the U-Pb age of syntectonic leucosome. At Cariboo Alp, late structures indicating extension (e.g. muscovite and chlorite in pull- aparts, chlorite slickenlines on fractures and rare decimeter-scale steep brittle normal faults) are of minor extent and importance, and together with the thermochronology history, rule out the model of a SW dipping extensional shear zone at Cariboo Alp. Biotite and hornblende crystals from representative paragneiss, and biotite from pegmatites, from ∼60 samples collected throughout an ∼12 km thick structural section that transects Thor-Odin dome, Cariboo Alp and the overlying structural panel were dated using 40Ar/39Ar thermochronology. Throughout the tilted section and irrespective of structural level, robust plateau hornblende cooling dates are ca. 56-53 Ma, biotite cooling dates from paragniess are ca. 53-51 Ma, and biotite cooling dates from pegmatites are ca. 52-50 Ma. These data indicate that the tilting of the structural section predated high temperature exhumation, and that there is no significant Eocene displacement across the high-strain zone at Cariboo Alp. This rules out Eocene or younger extensional faults and/or channel boundaries at Cariboo Alp. Rather, the geology is consistent with that of a highly strained crystalline sheet that was deforming internally as it overrode a basement ramp, the Monashee ramp imaged in Lithoprobe seismic reflection profiles 6, 7 and 8. Cariboo Alp and structurally overlying rocks were structurally deactivated by ca. 58 Ma whereas rocks of the Thor-Odin dome underwent continued NE flow accompanied by decompression melting to as young as ca. 54 Ma. Exhumation and cooling occurred during extension on regional east- and west-dipping Eocene extensional ductile to brittle fault systems, the Columbia River and Okanagan Valley- Eagle River fault systems.
Microstructure, 40Ar/39Ar Chronology, Structure and Tectonics in Rodna Mountains, Eastern Carpathians, Romania.
The Rodna Mountains, a horst in the Eastern Carpathian internides, affords a window through postectonic cover into the Europe-derived Tisza-Dacia terrane. Mylonitic schists, which form a subhorizontal horizon defining a detachment between blocks of probable Variscan metamorphic heritage, display penetrative linear structures (fold axes, mineral orientation, stretched aggregates) trending NW-SE, parallel the trend of the Eastern Carpathians. The schists contain a variably preserved older microstructure formed under lower amphibolite facies conditions and characterized by straight or gently curved grain boundaries separating relatively large quartz grains with poorly developed substructure and decussate white mica texture. In a younger greenschist facies event, the quartz microstructure is partially to wholly modified, with formatiom of serrate mutual grain boundaries, fine new grains and subgrains, and asymmetric grain elongation; the quartz reworking is accompanied by a variety of intracrystalline deformation in white mica. The microstructural reworking appears to be mostly coplanar with the older fabric and is regionally widespread, indicating that the schistosity and other fabric elements measured in outcrop (e.g. lineations) predominantly reflect this younger event. 40Ar/39Ar total fusion ages were measured from single grains and grain aggregates of white mica in ten schist samples. Cooling ages cluster strongly in the 90-100 Ma range demonstrating an Upper Cretaceous age for the reworking of the mylonitic schist horizon. A second less developed cluster of ages centres on the Triassic-Permian boundary, with many grains grouped at ca. 300 Ma. This pattern most likely represents resetting of cooling ages related to Variscan metamorphism and suggests Alpine events older than 90-100 Ma did not occur. In agreement with others, we interpret the structure of Rodna Mountains schists to be related to the collision of terranes, offshore of their present position, and closely related to the nearly coeval and regionally widespread extensional Gosau-type basins.
New Insights in the Southern Part of the Variscan French Massif Central by Gravity Iversion, AMS and Geochronological Investigations on Syntectonic Granitic Plutons
Many natural examples suggest that granitic plutons are associated with faults that might have been used as channels for the magma ascent. Furthermore, the kinematics of these faults has a strong influence on the final architecture of these syntectonic plutons: bulk shape, deformation style, pluton internal fabrics. The Variscan French Massif Central (FMC) presents a suitable context to analyze these interactions between magmatic and tectonics processes. In the southern MCF, the Glénat, Omps and Boisset plutons crop out on both sides of the NNE-SSW trending crustal-scale Sillon Houiller Fault (SHF). A multidisciplinary study on these three plutons has been undertaken in order to elucidate the relationships between faulting and magmatic processes during the Late Carboniferous time. According to the Anisotropy of Magnetic Susceptibility (AMS) measurements and structural observations, the plutons acquired their final structure during the magma crystallization and record a NW-SE maximum stretching direction. In the Boisset pluton, post-solidus fabrics predominate, particularly a NNW-SSE trending mineral lineation is widespread. The NW-SE magnetic lineation pattern observed in the Glénat and Omps plutons agrees with the Middle to Late Carboniferous syn- to late orogenic extensional regime observed in other parts of the FMC. The 3D geological modelling refined by 3D gravity inversion refutes any evidence of rooting of the granites along the SHF. Therefore, despite the apparent cartographic relationship between the SHF and the three plutons, our study does not support a genetic link between fault and plutons. Our study also questions the existence of the SHF in this part of the Massif Central at the time of pluton emplacement, i.e. around 330-325 Ma. In agreement with the previous conclusions, we may conclude that during the Late Carboniferous time, in the Variscan FMC, the pluton emplacement is controlled by regional tectonics, i.e. by a generalized belt-parallel NW-SE extension, rather than by internal dynamics of the magma chamber or by local structures. Keywords: granite fabrics, monazite chemical U-Th-Pb dating, Anisotropy of Magnetic Susceptibility (AMS), gravity investigations and inversion, 3D modelling, French Variscan Belt, Sillon Houiller Fault.
Rapid subduction erosion linked to crustal failure across the forearc-arc region: a model for southern Mexico
Many ocean-continent subduction zones are characterized by a net loss of upper plate forearc material due to tectonic subduction erosion. Forearc material removed in this fashion represents an important component of the crustal material subducted into the mantle. It is commonly assumed that subduction erosion is a steady- state process that results in trench advance rates of less than 5 mm/yr. However in certain localities, e.g. Mexico, the geological record suggests that trench advance may once have proceeded much more rapidly. Results from finite element numerical modelling experiments suggest that convergence across an actively subducting region is accommodated across the weakest section of lithosphere at any given time. Typically, the weakest section is the boundary between oceanic and continental lithosphere. Accommodation of convergence may switch to a different thrust zone if a new zone of weakness becomes available, such as the melt-weakened forearc. In this case, upper plate material positioned between the old and new subduction thrust faults is subsequently subducted. In the models, the rate of trench advance equals the rate of plate convergence. In the Xolapa-South Guerrero region of Mexico, the forearc related to Oligocene subduction at the Acapulco trench is missing. The removal likely occurred between cessation of Eocene-Miocene arc magmatism in the southern Mexico Magmatic Arc at ca. 25 Ma and the start of arc magmatism in the central Mexico Trans-Mexican Volcanic Belt at ca. 22 Ma. Trench advance during this time may have exceeded 50 mm / yr. We suggest these rapid trench advance rates may be explained by the subduction of the forearc, as shown by the numerical experiments, and propose several ways this hypothesis might be tested against the geological record.