Jon Spencer, Senior Geologist with AZGS, gave a tour de force presentation at the monthly meeting of the Arizona Geological Society here in Tucson last night, offering a tectonic reconstruction of Southern Arizona through the late Cenozoic, to propose a model for the origin of the regions huge porphyry copper deposits.
His talk, "Restoration of tectonic extension in the greater Tucson area and implications for original distribution of porphyry copper deposits," integrated results from decades of geologic mapping and subsurface geophysics to estimate amounts and direction of faulting, primarily extensional, during the Late Cenozoic.
Restoring porphyry copper deposits along the displacements resulted in bunching them into two long ovals [above, right], trending WSW-ENE, generally parallel to Proterozoic structural fabric. Jon suggested there may be a causal relationship.
Some of the questions addressed age dates and Eric Seedorf noted that deposits such as Safford and San Manuel, shown by Jon as possibly part of the same tectonic feature, are about 12 million years apart in age.