Tilted middle Tertiary ash-flow calderas and subjacent granitic

plutons, southern Stillwater Range, Nevada : Cross sections of

an Oligocene igneous center

By

David A. John

Introduction

Steeply tilted late Oligocene caldera system record a number of unusual features Extremely thick (>4-5.5 km) caldera related

deposits Lack of evidence of structural doming Preservation of compositional zoning Cogenetic plutonic rocks are exposed over

depth range of~10 km

Stillwater Caldera Complex

Job Canyon (29-28 Ma) Poco Canyon (25-23 Ma) Eleven Mile Canyon (25-23 Ma) Composite Freeman Creek pluton

Unusual features

Extreme thickness caldera related deposits (>4-5.5 km)

Unusual features continued

Preservation of compositional zoning

Caldera collapse

Miocene extension Basement rocks tilted 40°-90° Subvertical ring-fracture faults (>3-5 km)

depth Piston like blocks Lack of evidence of structural doming

Conclusions

Oligocene caldera complex in the southern Stillwater Range

Extremely thick caldera related deposits due to extreme subsidence

Cogenetic plutonic rocks over a depth range of ~10 km due to tilting during Miocene extension