USING LUMINESCENCE SIGNALS FROM BEDROCK FELDSPARS TO QUANTIFY RAPID COOLING IN THE SAN BERNARDINO MOUNTAINS, SOUTHERN CALIFORNIA

Luminescence signals from bedrock feldspars contain information about a sample's recent thermal history. For example, we show that the burial temperature of drill core samples controls which luminescence traps can accumulate electrons and which will remain empty. We illustrate this effect for burial temperatures ranging from -4.1 to 60.2°C.

Because the time required for these luminescence traps to fill is on the order of 10^5 - 10^6 years, samples in regions of active tectonic uplift may be in thermal disequilibrium. We demonstrate how luminescence signals from the rapidly-cooling San Bernardino Mountains in Southern California can be used to estimate the recent (past 1.2 Ma) cooling paths of bedrock samples. Our results reveal an elevation-dependent trend, with higher-elevation samples reaching surficial temperatures at about 1 Ma and lower-elevation samples cooling rapidly (~300°C/Ma) to surficial temperatures beginning at 0.1 Ma. The onset of lower-elevation cooling may be coincident with the shutdown of the Mill Creek fault and subsequent fluvial downcutting.

In summary, we demonstrate that at sufficiently high cooling rates (though perhaps as low as 10°C/Ma according to numerical simulations), luminescence signals from feldspars contain a record of continuous thermal history for bedrock at surficial and near-surface temperatures.