Abstract

Pre-caldera and caldera-related igneous rocks near Questa, New Mexico, provide a well-constrained example of upper-crustal extension over a magma chamber. Extensional strain exceeding 200% occurred over many square kilometers in a region now underlain by a batholith. The most intense deformation followed emplacement of intrusions related to formation of Questa caldera at 25.7 Ma and preceded the cooling of 24.6 Ma plutons. Detailed mapping of the southern part of Questa caldera indicates the sequence (1) extension concurrent with plutonism, resulting in an overlying carapace of highly tilted blocks bounded by low-angle faults; (2) renewed intrusion into overlying units, with the orientation of pluton tops, dikes, and molybdenum-bearing alteration zones controlled by previously formed low-angle faults; (3) reactivation or continued motion of low-angle normal faults, possibly aided by hydrothermal fluids; and (4) high-angle normal faults that formed after the magmatic system waned. Early high-angle faults remained active, again possibly aided by fluids, during their rotation to low angles.

At upper-crustal levels, the magma chamber is believed to have provided a zone of ductility into which overlying brittle normal structures were rooted. At mid-levels in the crust, regional magmatism thermally softened the crust and allowed it to deform in a ductile manner. Regional extension appears to have been focused in a thermally and mechanically weakened area during the period of maximum plutonic activity. The features at Questa provide insights into processes that may have occurred in similar systems in the Basin and Range province and elsewhere.

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