Eruption of the Oligocene Wah Wah Springs tuff and Development of the Indian Peak caldera, Great Basin, USA

The Indian Peak-Caliente caldera complex was a major focus of explosive silicic activity in the subduction-related Great Basin ignimbrite province during a middle Cenozoic ignimbrite flareup. The Indian Peak caldera is the largest in the complex; the topographic margin extends ~40 km north-south and, after correction for post-eruption east-west extension, the area of the structural caldera is about 1,000 km². It formed 30 Ma when an estimated 5,900 km³ of crystal-rich dacitic magma erupted to form the Wah Wah Springs tuff. A maximum of about 6 km of total subsidence occurred. The magma erupted from depths of about 4-10 km based on hornblende geobarometry and comparison with experimental data. The internal structure of the caldera has been exposed by basin and range faulting and tilting; geologic relations suggest the following sequence of events. 1. Surface tumescence as the shallow magma chamber grew. 2. Eruption was triggered by failure of the roof which sagged inward as the eruption progressed. 3. Subsidence continued by movement on a system of reverse and normal faults in the extending 11 km wide collar zone which developed between the inner reverse ring fault and the outward retreating topographic margin. Caldera wall-rock collapsed into the deepening depression; entrainment of rock fragments into the vesiculating magma created lithic-rich intracaldera tuff. 4. In the northern, inward-sloping collar zone, wall-collapse breccia accumulated to a depth of hundreds of meters on top of pre-collapse rocks. In the deeper levels of the sliding breccia, sufficient load existed to create intense cataclasis at the bases of sliding blocks. Some wall collapse breccia cascaded beyond the collar zone and formed lenses 10s of m thick in the accumulating intracaldera lithic tuff. 5. Boiling magma continued to erupt through the thickening pile of intracaldera tuff and wall-collapse breccia, consuming energy and constraining the lithic-rich ignimbrite within the topographic margin of the caldera. 6. As the erupting magma lost its explosiveness, it continued to rise intrusively along the northern inner reverse ring fault and initiated resurgent uplift of the collar zone and a reversal of displacement along the inner ring faults. 8. This magma solidified as porphyritic granodiorite that intruded intracaldera tuff. 9. Later, as much as 1 km of post-collapse, caldera-filling ignimbrites accumulated in the caldera moat between the topographic margin and resurgent intracaldera pile.