STRUCTURAL DEFORMATION OF THE BACHELOR CALDERA, CENTRAL SAN JUAN MOUNTAINS, COLORADO

The Bachelor (27.4 Ma), San Luis (27-26.8 Ma), and Creede (26 Ma) calderas partially overlap each other and are nested within the large La Garita(27.6 Ma) caldera, forming the central caldera cluster of the San Juan volcanic field. The resurgent dome of the Bachelor caldera, in the middle of the cluster, contains a complex array of fractures that record episodic fault movement in the area related to structural history. Resurgent doming of the Bachelor caldera along NNW-SSE normal faults developed three vertical microjoint sets reflecting stress-release episodes in three different directions. These microjoint sets are most widespread within the lower, densely-welded zones of the intracaldera Carpenter Ridge Tuff. Relatively low-angle (<70°), south-dipping faults along the northern margin of the Creede caldera developed during to scalloping associated with the Creede caldera collapse. On the eastern margin, scalloping by the Creede caldera is responsible for NE-SW synthetic and antithetic fractures in intracaldera Carpenter Ridge Tuff that dip 70°. Finally, a second, late period of normal displacement along NNW-SSE Bachelor caldera crestal graben faults formed the Creede Graben. Creede Graben faults developed parallel synthetic and antithetic faults and their conjugates with dips no greater than 70°. To the east, Rio Grande Graben fault activation during the same period along a WNW-ESE trend formed antithetic fractures that dip anywhere from 60° to 70° N.