PUTTING IT ALL TOGETHER: EXHUMATION HISTORIES FROM A FORMAL COMBINATION OF HEAT FLOW AND A SUITE OF THERMOCHRONOMETERS

To determine the long-term thermal history of a region, researchers can draw on a wide range of techniques including a suite of thermochronometers and near-surface heat flow. Each thermochronometer is sensitive to and therefore records a different segment the temperature-time pathway experienced by a rock. Because the methods provide complementary information, an ideal analysis would combine a suite of thermal datasets. This approach is frequently employed in the thermochronology community, but the complementary data sets are often treated independently. Additionally, most traditional fission track modeling only explores time-temperature space and therefore requires assumptions about the paleo-geothermal gradient in order to make interpretations about exhumation history. By using vertical profiles of a suite of thermochronometers and heat flow measurements, we can solve for the evolution of the geothermal gradient over time along with the exhumation history. Here we present a methodology for formally integrating a suite of thermochronometers to constrain the thermal history of a region. We use a genetic algorithm to efficiently explore the time-exhumation history of a vertical sample profile (such as a borehole). We couple a thermal model of burial and exhumation with (U/Th)-He age modeling and fission track age and length modeling. As an example of the process, we present new analyses of previously published data from the SAFOD pilot hole, a 2 km-deep scientific borehole near Parkfield, California.

We recognize that there are limitations to a strict quantitative interpretation of thermochronologic models related to extrapolation of laboratory-derived diffusion parameters to geologic timescales. While these errors are systematic and might be correlated within a given thermochronologic system, we join several systems together. Formally integrating the data sets into a combined inversion balances the strengths and weaknesses of each thermochronologic system and allows us to quantify the uncertainty of the resulting geologic interpretation.