Abstract

The major structural features of the northeast part of the Rio Puerco fault zone are (1) north- to northwest-trending folds, (2) northeast-trending normal faults, and (3) north-trending normal faults. These structures formed in response to an older north-oriented shear couple and a younger east-west-oriented tension. Laramide orogenic movements of late Paleocene through Eocene age resulted from a north-trending, right-shift force couple related to northeast shift of the Colorado Plateau. The folds and the northeast-trending normal faults formed in response to the right-shift couple, and the faults are interpreted as tensional fractures that developed at 45° to the trend of the couple. Miocene to Holocene tectonism is dominated by north-trending, east-dipping normal faults related to the development of the Rio Grande rift. Many southeast-dipping normal faults of Laramide origin were rejuvenated during Miocene and younger rifting.

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