Abstract

The Goodsight-Cedar Hills volcano-tectonic depression in south-central New Mexico formed concurrently with and following deposition of about 550 km3 of Oligocene volcanic and volcaniclastic rocks. The depression is an asymmetric basin about 80 km long and 38 km wide. It is filled to a maximum depth of about 550 m by early rhyolitic ash-flow tuff, medial epiclastic strata derived partly from marginal raised rims (Bell Top Formation), and late basaltic andesite (Uvas Basalt). Much of the 295 km3 of effusive rock was erupted from vents located near the center of the depression and from a major subsidence fracture zone along the eastern margin. Subsidence of the depression floor was noncatastrophic and approximately kept pace with basin filling, except along the eastern margin.

Following eruption and broad, regional subsidence of the Uvas Basalt, the central floor of the depression was arched upward to form the Sierra de las Uvas dome and adjacent synclinal moat. The fault pattern of the dome and its association with known vents and the thickest part of the volcanic pile suggest that it formed by vertical movement of magma from an underlying chamber. Its development, following subsidence of the Uvas Basalt, suggests that it is essentially resurgent in origin. Although no postdoming volcanism is known, indirect evidence indicates that intrusion of silicic magma probably caused resurgence.

It is clear that the Goodsight-Cedar Hills depression is not a resurgent cauldron of the classic Valles or Toba types. Rather, it appears to be a structure transitional in character between a cauldron and a fault trough. Subsidence apparently followed volcanism. Resurgence followed eruption of basaltic andesite rather than the usual ash-flow sequence, and the 10- to 12-m.y. history of the Goodsight-Cedar Hills depression is 5 to 6 times as long as other well-known resurgent cauldron cycles. Locally, deposition in the depression continued without interruption into early late Tertiary time, when extensional faulting occurred.

The depression is elongated parallel to the late Tertiary Rio Grande Rift and may be a precursor of Basin and Range structure in the area. The late Tertiary fault pattern in the area indicates that earlier volcano-tectonic structures had important effects on the pattern of late Tertiary Basin and Range structures. Locally, late Tertiary faults were inherited from and duplicate the position of earlier subsidence faults or the synclinal moat. Elsewhere, the north-trending regional fault pattern was considerably modified by structural and litho-logic inhomogeneities of volcano-tectonic origin, particularly the Sierra de las Uvas dome and its buried intrusive masses.

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