Low‐temperature thermochronologic record of Eocene migmatite dome emplacement and late Cenozoic landscape development, Shuswap core complex, British Columbia

Abstract

Exhumed mid-to-lower crustal rocks offer an opportunity to determine the mechanisms, conditions, timing, and consequences of the ascent of hot rocks from deep to shallow crustal levels. We used results of low-T thermochronology (zircon and apatite (U-Th)/He, apatite fission track) to document the very shallow emplacement (<2 km) of high-grade metamorphic rocks and to determine the timing and rates of Cenozoic cooling, exhumation, and subsequent incision of the Thor-Odin migmatite dome of the Shuswap metamorphic core complex, British Columbia (Canada). Samples collected at high elevation in the dome (>1800 m) have preserved Eocene fission-track ages and evidence of rapid cooling (≥60°C/Myr). This Eocene cooling event corresponds to rapid exhumation by upward flow of partially molten crust and final exhumation by detachment faulting. Samples collected below 1800 m in elevation display a wide range of apatite fission track ages (43-15 Ma) and track length distributions that reflect prolonged residence in the apatite partial annealing zone. These age-elevation relations imply that the dome rocks reached the near surface (<2 km) during initial upward flow and tectonic exhumation in the Eocene and that little erosion of the Eocene surface has occurred since that time. Thermal modeling of the lowest elevation samples (≤~600 m) and intrasample apatite (U-Th)/He age variations reveal enhanced erosion and relief production at the onset of continental glaciations at~3 Ma. Our work illustrates the dynamic links between deep and shallow crustal processes and the evolution of topography in a deeply incised hot orogen.

Orientation, composition, and entrapment conditions of fluid inclusions in the footwall of the northern Snake Range detachment, Nevada

“…The inferred P-T-t-d path is in good agreement with rapid exhumation of hot mid-to lower crustal rocks that can drive fluid circulation and heat advection into the upper crust (e.g. Morrison and Anderson, 1998;Person et al, 2007;Gottardi et al, 2011;Whitney et al, 2013;Siebenaller et al, 2013;Toraman et al, 2014;Quilichini et al, 2015;Platt et al, 2015). The data show that both crystal plastic and brittle deformation mechanisms affected the footwall rocks at the time of FI trapping.…”

Eocene extension and meteoric fluid flow in the Wildhorse detachment, Pioneer metamorphic core complex, Idaho

“…The northernmost metamorphic core complexes include the Shuswap, Valhalla, Okanogan, Kettle, and Priest River metamorphic core complexes, characterized by voluminous migmatite domes that represent former partially molten crust. The northern metamorphic core complexes preserve Paleocene-Eocene crystallization of partially molten crust (U-Pb zircon and monazite ages 55-50 Ma) that immediately followed exhumation-related cooling as documented by 40 Ar/ 39 Ar mica (49-47 Ma) and apatite fission-track ages (Ewing, 1980;Vanderhaeghe et al, 1999;Foster et al, 2001Foster et al, , 2007Teyssier et al, 2005;Hinchey et al, 2006;Mulch et al, 2006Gordon et al, 2008;Kruckenberg et al, 2008;Toraman et al, 2014;Quilichini et al, 2015), recording fast cooling and exhumation to within 1-2 km of the Eocene land surface only a few million years after high-temperature crystallization (Toraman et al, 2014).…”

Effects of Paleogene faults on the reconstruction of the metamorphic history of the northwestern Thor-Odin culmination of the Monashee complex, southeastern British Columbia

“…45 Ma (Lorencak et al, 2001). However, new analyses by Toraman et al (2014) resulted in additional apatite fission-track ages as young as 19.5 ± 2.6 Ma on the west side and 14.0 ± 4.0 Ma on the east side of the Victor Creek fault, which are within error of each other. Hornblende, muscovite, biotite, and K-feldspar 40 Ar/ 39 Ar ages do not show any specific differences or trends between the west and east sides of the Victor Creek fault (Johnson, 1994;Spark, 2001;Vanderhaeghe et al, 2003).…”

“…While that interpretation is valid for their observations, and could explain part of our observations in this contribution, it does not explain differences between sample 70 and samples 54 and 30, as they are all east of the Greenbush shear zone. In addition to the variation in ages of metamorphism across the Victor Creek fault, structures (Kruse and Williams, 2005) and detrital zircon populations (Kuiper et al, 2014), and to some extent apatite fission-track ages (Lorencak et al, 2001;Toraman et al, 2014), differ across the fault (see earlier herein).…”

“…The geological significance of the Victor Creek fault has been alluded to earlier by Lorencak et al (2001), Kuiper et al (2014), and (Toraman et al, 2014). Kuiper et al (2014) noted that detrital zircon in Neoproterozoic and younger metasedimentary units west of the Victor Creek fault (samples 30 and 54; same as used in this study) yielded fewer Archean and Paleoproterozoic ages and more Mesoproterozoic ages than those east of the Victor Creek fault (samples 70, 92, and 97; same as used in this study), possibly because they were deposited in different regions and later juxtaposed by the fault.…”