Earth Sciences

Field investigations, including detailed geological and geomorphological mapping have been coupled with stratigraphic and structural studies of the Blais Creek Deep-seated Gravitational Slope Deformations (DsGSD), Monashee Mountains, British Columbia (BC). To reconstruct the DsGSD evolutionary stages and to evaluate its controlling factors, a complex methodology has been applied, integrating orthophotos, stereo models and 3D models of the DsGSD with field and literature data concerning tectonic and glacial history of the Seymour Valley. General geomechanical properties of the deforming rock mass has been then evaluated for using in numerical models of the failure mechanism at Blais Creek and to define a broad geomechanical characterization of different portions of the DsGSD.

Field investigations, including detailed geological and geomorphological mapping have been coupled with stratigraphic and structural studies of the Blais Creek Deep-seated Gravitational Slope Deformations (DsGSD), Monashee Mountains, British Columbia (BC). To reconstruct the DsGSD evolutionary stages and to evaluate its controlling factors, a complex methodology has been applied, integrating orthophotos, stereo models and 3D models of the DsGSD with field and literature data concerning tectonic and glacial history of the Seymour Valley. General geomechanical properties of the deforming rock mass has been then evaluated for using in numerical models of the failure mechanism at Blais Creek and to define a broad geomechanical characterization of different portions of the DsGSD. The combination between the aerial and terrestrial photogrammetry was appropriate in terms of the quality of the information obtained more than the quantitative information. Several Ground Control Points (GCPs) and Tie Points (TPs) were selected from the original DEM received by the BC Government. The use of a multitemporal aerial triangulation gave the possibility to minimize the error relative to every single block of images. Couples of oriented photos were used to create stereoscopic models. Multitemporal variations of the Blais Creek slope were observed and compared to the actual situation of the slope. The use of terrestrial photogrammetry through Adamtech software confirmed some of the qualitative data obtained from aerial interpretation and from field survey. The limited use of terrestrial photogrammetry was due to the impossibility of orienting the 3D terrestrial models. Anyway these models were also useful to confirm one of the possible mechanisms used to describe the evolution of Blais Creek. Geomechanical analysis was performed through field work and laboratory tests to characterize the entire slope and to produce some of the values useful for a possible numerical analysis of Blais Creek. It showed interesting differences in geomechanical properties between the calc-silicate and quartzite/gneiss. The kinematic analysis showed very the different instability areas along the slope, even if variations in landforms and rock masses volume weren't widespread along Blais Creek slope during the time span covered by aerial photographs . Indeed, the multitemporal analysis outlined very active instability along the large upper trench and the lateral active slopes of Blais Creek. Even without significant level of risks in the area, considering the remote area involved in this instability, some relevant hazards could occur, related to the possible collapse of SE side of Blais Creek DsGSD. Regarding the long term evolution of the DsGSD, the extensive network of linear features at Blais Creek is of a large deforming rock mass. Movement probably began with the retreat of valley glaciers during deglaciation when the oversteepened valley sides were debuttressed. By these evidences it is possible to theorize that the post-glacial retreat of the rock face and removal of the ice buttress from both the Seymour and the Blais Creeek Valleys lowered the factor of stability of the mass as a whole, allowing a deep-seated shear surface to develop gradually over time by progressive creep.

Analogue and computational models principally provide our present understanding of the mechanisms of prograde metamorphism and orogenesis, yet, due to the difficulty of linking prograde age information with the pressure-temperature (P-T) evolution of a rock, these models for the most part remain untested. Here, we describe an approach that allows multiple prograde pressure-temperature-time (P-T-t) points to be generated on single samples providing a means to construct detailed P-T-t loops, enabling the duration of metamorphic events, the timing of burial and the rate of heating experienced by individual samples to be determined. We use this combination approach to generate multiple prograde P-Tt points for three samples from the Himalayan and Canadian Cordillera and show for the first time that the duration of the metamorphic events are similar in each area and prograde heating rates vary from 2.4F1.2 8C/Ma to 5.0F2.0 8C/Ma. The P-T-t paths generated this way agree well with the established geological history of the studied areas and allow additional constraints to be placed on the mechanisms of orogenesis. D

Chemical mapping and in situ U-Th-Pb analyses reveal a link between age domains and zones of relative yttrium (Y) depletion or enrichment within monazite crystals and are correlated with metamorphic reactions involving garnet. Conventional, small-fraction isotope dilution thermal ionization mass spectrometry (ID-TIMS) and sensitive high-resolution ion microprobe (SHRIMP) techniques were utilized to measure U-Th-Pb isotopic compositions in metamorphic monazite from pelitic rocks of the southern Canadian Cordillera. Monazite ID-TIMS data from individual samples commonly demonstrate a 2 to 25 Ma range in U-Pb ages. This is difficult to reconcile using conventional regression techniques due to complexities such as excess 206 Pb or bulk mixing of discrete age domains. Backscattered electron (BSE) imaging and X-ray elemental mapping for Y, Th, and U revealed complex internal zonation within many of the monazite crystals and served as a guide for detailed, in situ (~30 Am) isotopic analysis using the SHRIMP. The Y maps generally provided the clearest indication of growth or recrystallization domains and were critical for targeting SHRIMP analysis because these relationships were not always clear in BSE, U, and Th images. Moreover, the Y domains consistently correlated with distinct age domains, with up to three or more in some crystals. These data clearly illustrate the cause of age dispersion within the analyzed monazite grains and demonstrate the significance of multiple age domains in metamorphic monazite that may be irreconcilable or misinterpreted when using conventional dating techniques such as ID-TIMS.

New U-Pb zircon dates from gneissic xenoliths in an Early Cambrian lamprophyre point to late Mesoproterozoic metamorphism and magmatism in the crust beneath northern Yukon. The data indicate a previously unrecognized thermal event in Yukon and extend the recognition of widely spaced 1.3–1.0 Ga igneous, metamorphic and tectonic events along the western margin of Laurentia. Together, these events demonstrate that the north-western margin of Laurentia was thermally active during a period of traditionally inferred tectonic quiescence. The tectonic setting, intensity and general nature of this late Mesoproterozoic tectonothermal activity are poorly constrained, but their recognition is key to expanding our understanding of the Proterozoic geology of North America. The recognition of this event places new constraints on the palaeotectonic reconstructions of the supercontinent Rodinia.