ISOCHRON STRUCTURE MAPS BRING NEW INSIGHTS TO LOW-TEMPERATURE THERMOCHRONOLOGY - COMBINING LOW-TEMPERATURE THERMOCHRONOLOGIC DATA WITH SURFACE AND SUBSURFACE DATA ACROSS COLORADO

Low-temperature thermochronologic (LTT) data are typically shown in 2-component plots of age vs. elevation or age vs. location. Isochron structure mapping allows all three components of age, elevation and location to be represented on a single display. Age is captured at 67 Ma coinciding with the shift from foreland basin subsidence to basement uplift during the Laramide orogeny in Colorado. To represent elevation, a normalized datum is calculated for each sample by projecting to 67 Ma using a burial rate of 1.75 m/Myr for samples with ages > 67 Ma, or an exhumation rate of 50 m/Myr for samples with ages of 40-67 Ma. These rates reflect reasonable estimates given the available data. Location is shown by contouring the normalized datums as a structure map. This crude use of constant gradients to normalize the data created a well-behaved surface with multiple points defining highs and lows. Complementing 2-component plots with isochron structure helps avoid some of their pitfalls, such as interpreting an age-elevation profile when sampling along a sloping isochron surface, or trying to make structural sense of LTT ages on a map where there is significant topographic relief.

The utility of this approach is best seen by melding and comparing the isochron structure with other surface and subsurface data in Colorado. Isochron highs correlate with structural highs, or areas of high heat flow such as Tertiary igneous centers and Proterozoic shear zones. In areas exhumed to basement, but not affected by high heat flow, two estimates are possible assuming a burial of 4000 m between the top Cretaceous and the 67 Ma isochron. Where the stratigraphy to the top Cretaceous is known from nearby outcrops and wells, the top Cretaceous and Great Unconformity elevations can be determined. Alternatively, given a LTT sample at the Great Unconformity and control for the older stratigraphy, the upper Cretaceous thickness can be calculated. Having a constraint for the position of the cover across areas exhumed to basement is better than guessing when constructing cross sections. Isochron structure can also be readily compared to other surfaces such as the Rocky Mountain Erosion Surface. Seeing these surfaces act in concert points to post-Laramide structuring. Conversely, seeing them trend in opposite directions further pinpoints loci of high heat flow.