Oligocene laccoliths on the Colorado Plateau: A key to understanding Cenozoic rock cooling and erosion
Filling out the summit roster on Mount Ellen, the highest point in the Henry Mountains. Photo credit: Xavier Robert.
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The landscape evolution of the Colorado Plateau has fascinated geologists since the days of Powell, Gilbert, and Dutton. Today, low-temperature thermochronology is being applied across the region to constrain the timing and rate of late Cenozoic denudation and canyon cutting, although the thermal histories of the Plateau’s sedimentary rocks push the current limits of interpreting low-temperature thermochronology data -- hence the rekindled debate on the “age” of Grand Canyon. Using thermochronology in the these rocks close to the Oligocene laccoliths in the Henry, La Sal, and Abajo Mountains, we overcome this problem because the thermal histories are clearly punctuated ca. 27-25 Ma by localized magmatic heating, which erased the detrital memory of the apatite grains we date and clarifies the record of subsequent erosional cooling.
Published in Geology. |
Towards more robust interpretation of cooling ages in magmatic terranes
Because they can reliably yield abundant apatite and zircon, igneous rocks in active and ancient orogens are often targeted for low-temperature thermochronologic dating to constrain the timing, rates, and patterns of erosion. However, interpreting the thermal histories of regions currently or formerly magmatically active is complicated by (1) transient heating and cooling within and adjacent to plutons, (2) changes in regional geothermal gradients, (3) interaction between advective and conductive cooling, and (4) the potential for coupled magmatic and erosional exhumation. In these settings, when is it appropriate to interpret low-temperature cooling ages as reflecting the timing or rate of purely erosional exhumation?
The effects of U-Th-rich grain boundary phases on apatite helium ages
Apatite crystals from an Oligocene pluton in the Henry Mountains coated with Fe-rich grain boundary phases. Murray et al. (2014).
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We document a source of apatite He age variability amongst single apatite grains from the same sample: U-Th-Sm-bearing phases precipitated on apatite grain boundaries. These phases interfere with the key assumptions made when we apply the alpha-ejection correction to raw apatite He ages. We model these effects and demonstrate strategies for retrieving useful information from otherwise problematic apatite He datasets.
Published in Chemical Geology. |