U–Pb Dating of Cave Spar: A New Shallow Crust Landscape Evolution Tool


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November 2017


In carbonate terranes, rocks types that provide apatite are not available to effectively use apatite fission track (AFT) or (U/Th)‐He chronometry (AHe). Here we suggest that calcite cave spar can be an effective chronometer and complimentary to AFT and AHe thermochronometers in carbonate regions such as our study area, the Guadalupe Mountains of southeastern New Mexico, and west Texas. Our measured depth of cave spar deposition is 500 ± 250 m beneath the regional water table, formed at temperatures of 40° to 80°C, indicating that these caves and their spar crystals form near the supercritical CO2‐subcritical CO2 boundary where we interpret the origin of both the caves and spar to occur. This depth‐temperature relationship suggests a higher than normal geotherm, likely associated with regional magmatic activity. As a case study we examined the timing of uplift of the Guadalupe Mountains previously attributed to the compressional Laramide orogeny (ca. 90 to 50 Ma), later extensional tectonics associated with Basin and Range (ca. 36 to 28 Ma) or the opening of the Rio Grande Rift (ca. 20 Ma to Present). We show that most of the spar origin is coeval with the ignimbrite flare‐up between 36 and 28 Ma. Our results constrain the initiation of Guadalupe Mountains block uplift, relative to the surrounding terrain, to between 27 and 16 Ma and reconstruct the evolution of a low‐lying regional landscape prior to block uplift from 185 to 28 Ma, in support of models that attribute regional surface uplift to extensional tectonics and associated volcanism.


U-Pb DATING, Cave Spar, Carbonate Terranes, Apatite Fission Track (AFT), (U/Th)‐He Chronometry (Ahe)

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Tectonics, Vol. 37, no. 1 (2017-11-20).