UTIG Seminar Series: Sarah Penniston-Dorland, University of Maryland
||February 12, 2016 at 10:30 am
||February 12, 2016 at 11:30 am|
| ||Location:||PRC, 10100 Burnet Road, Bldg 196, Rm 1.603, Austin, TX 78758|
| ||Contact:||Laura Wallace, email@example.com, 512-471-0324|
| ||URL:||Event Link|
"Strength and Heat in the Subduction Channel: Evidence from Metamorphic Rocks and Geodynamic Models"
The thermal structure and flow of material within subduction zones are closely linked and are important for our understanding of seismicity within subduction zones and for the generation of arc magmas. This is a talk in two parts investigating evidence from metamorphic rocks for the thermal structure and degree of material flow within subduction zones. Evidence from natural rocks is compared to that generated from computational geodynamic models.
Part 1. Thermal structure: The maximum-pressure P-T conditions (Pmax-T) and prograde P-T paths of exhumed subduction-related metamorphic rocks are compared to predictions of P-T conditions from computational thermal models of subduction systems. While the range of proposed models encompasses most estimated Pmax-T conditions, models predict temperatures that are on average colder than those recorded by exhumed rocks. In general, discrepancies are greatest for Pmax< 2 GPa where only a few of the highest-T modeled paths overlap typical petrologic observations and model averages are 100-300 °C colder than average conditions recorded by rocks. Prograde P-T paths similarly indicate warmer subduction than typical models. Our compilation and comparison suggest that exhumed high-P rocks provide a more accurate constraint on P-T conditions within subduction zones, and that those conditions may closely represent the subduction geotherm. While exhumation processes in subduction zones require closer petrologic scrutiny, the next generation of models should more comprehensively incorporate all sources of heat. Subduction-zone thermal structures from currently available models do not match the rock record, and this mismatch has wide-reaching implications for our understanding of global geochemical cycles, the petrologic structure of subduction zones, and fluid-rock interactions and seismicity within subduction zones.
Part 2. The Catalina Schist contains a spectacular, km-scale amphibolite facies mélange zone, thought to be part of a Cretaceous convergent margin plate interface. In this setting, mafic and ultramafic blocks ranging from cms up to 100s of m in diameter are surrounded by finer-grained matrix. All blocks throughout the mélange contain assemblages consistent with upper amphibolite-facies conditions, suggesting a relatively restricted range of depths and temperatures over which the mélange formed. This apparent uniformity contrasts with other mélanges, such as the Franciscan Complex, where rocks with highly variable peak metamorphic grade suggest extensive mixing of materials along the subduction interface. This mixing has been ascribed to flow of material within relatively low viscosity matrix. The Zr content of rutiles in samples from the amphibolite facies of the Catalina Schist were measured to determine peak metamorphic temperatures, identify whether these temperatures were different among blocks (within measurement error), and whether the spatial distribution of temperatures throughout the mélange was systematic or random. Resolvably different Zr contents are found among the blocks, corresponding to different peak metamorphic temperatures of 650 to 730°C at an assumed pressure of 1 GPa. No systematic distribution of temperatures was found, however. Therefore material flow within the Catalina Schist mélange was likely chaotic, but appears to have occurred on a relatively restricted scale.