Deformation of continental crust along a transform boundary, Coast Mountains, British Columbia

New structural, paleomagnetic, and apatite (U-Th)/He results from the continental margin inboard of the Queen Charlotte fault (~54°N) delineate patterns of brittle faulting linked to transform development since ~50 Ma. In the core of the orogen, ~250 km from the transform, north striking, dip-slip brittle faults and vertical axis rotation of large crustal domains occurred after ~50 Ma and before intrusion of mafic dikes at 20 Ma. By 20 Ma, dextral faulting was active in the core of the orogen, but extension had migrated toward the transform, continuing there until <9 Ma. Local tilting in the core of the orogen is associated with glacially driven, post-4 Ma exhumation. Integration with previous results shows that post-50 Ma dextral and normal faulting affected a region ~250 km inboard of the transform and ~300 km along strike. Initially widespread, the zone of active extension narrowed and migrated toward the transform ~25 Ma after initiation of the transform, while dextral faulting continued throughout the region. Differential amounts of post-50 Ma extension created oroclines at the southern and northern boundaries of the deformed region. This region approximately corresponds to continental crust that was highly extended just prior to transform initiation. Variation in Neogene crustal tilts weakens interpretations relying on uniform tilting to explain anomalous paleomagnetic inclinations of mid-Cretaceous plutons. Similarities to the Gulf of California suggest that development of a transform in continental crust is aided by previous crustal extension and that initially widespread extension narrows and moves toward the transform as the margin develops

Submillimetre CO observations of the Cepheus A outflow

The authors have carried out spectral mapping of the Cep A region in the J = 3-2 transition of CO. The high velocity wings symptomatic of outflowing gas are evident over a region ~6×6 arc min in extent (~1.3×1.3 pc). There appear to be 2 outflows, as found also for HCO+ by Loren et al. (1985). The CO J = 3-2 red wing intensity is enhanced relative both to lower transition CO data and to HCO+ J = 1-0, particularly to the south and east of Cep A, a result which suggests the presence of hot, optically thin gas in the flow. No such significant enhancement is seen, however, in the blue wing. The data are discussed in terms of a 2 component outflow in which the HCO+ emission originates mainly from dense clumps in the flow while a less dense (but >4×104cm-3) interclump medium is responsible for most of the 12CO emission

Identifying spatial variations in glacial catchment erosion with detrital thermochronology

Understanding the spatial distribution of glacial catchment erosion during glaciation has previously proven difficult due to limited access to the glacier bed. Recent advances in detrital thermochronology provide a new technique to quantify the source elevation of sediment. This approach utilizes the tendency of thermochronometer cooling ages to increase with elevation and provides a sediment tracer for the elevation of erosion. We apply this technique to the Tiedeman Glacier in the heavily glaciated Mount Waddington region, British Columbia. A total of 106 detrital apatite (U‐Th)/He (AHe) and 100 apatite fission track (AFT) single‐grain ages was presented from the modern outwash of the Tiedemann Glacier with catchment elevations between 530 and 3960 m. These data are combined with nine AHe and nine AFT bedrock ages collected from a ~2400 m vertical transect to test the hypotheses that erosion is uniformly or nonuniformly distributed in the catchment. A Monte Carlo sampling model and Kuiper statistical test are used to quantify the elevation range where outwash sediment is sourced. Model results from the AHe data suggest nearly uniform erosion in the catchment, with a preference for sediment being sourced from ~2900 to 2700 m elevation. Ages indicated that the largest source of sediment is near the present‐day ELA. These results demonstrate the utility of AHe detrital thermochronology (and to a lesser degree AFT data) to quantify the distribution of erosion by individual geomorphic processes, as well as some of the limitations of the technique.Key PointsDetrital thermochronometers record spatial pattern of erosionNearly uniform erosion under the present‐day glacierThe largest observed source area of erosion is near the ELAPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/112265/1/jgrf20399-sup-0001-supinfo.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/112265/2/jgrf20399.pd

Effect of Alpine glaciation on thermochronometer age‐elevation profiles

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94703/1/grl22545.pd

Elemental and Isotopic Evidence for Granitoid Genesis From Deep-Seated Sources in the Coast Mountains Batholith, British Columbia

Major element, trace element and Nd–Sr isotopic data are presented for 82 plutonic rocks from the southern Coast Mountains Batholith (CMB) in British Columbia, Canada, ranging in emplacement age from 210 to 50 Ma. The rocks are part of a large composite magmatic arc batholith, which the major element data show to be of calc-alkaline affinity. The majority of CMB samples lack the depletion in Eu that would be consistent with equilibration of magmas and plagioclase-bearing crystalline residues or fractionates, suggesting that equilibration took place deeper than the pressure limit of plagioclase stability at 35–40 km depth. The CMB samples show a wide variation in the slope of normalized rare earth element (REE) patterns, with chondrite-normalized La/Yb ratios above 10 being mostly confined to periods of high magmatic flux in the arc at 160–140, 120–80, and 60–50 Ma. The clearest relationships between major and trace elements are negative correlations between SiO2 and each of Sc, Y, and the heavier REE Gd to Lu. Nd and Sr isotopes mostly document juvenile origins for the granitoids, but show variations to higher 87Sr/86Sr and lower εNd during high-flux periods. The results are interpreted to indicate a deep origin for most CMB magmas, below ∼40 km where mafic to intermediate rock assemblages previously added to the arc crust by mantle melting were transformed to an (amphibole-bearing)-eclogite facies cumulate or restite, such that melting residues consisted mainly of two pyroxenes, garnet and variable proportions of amphibole. Thickened orogenic crust, for which there is clear geological evidence during the period 100–80 Ma, promoted this process. During high-flux periods, larger amounts of older rocks, mostly mafic rocks and some metasediments added to the base of the arc during orogenic shortening, became involved in magma genesis