geothermal resource potential of the safford-san simon basin, arizona

The upper basaltic andesite, overlying the rhyolite/latite flows in the
Gila Mountains, is dark gray and occurs in massive flows two to five meters
thick. These rocks are considered equivalent to the 20-25 m.y. Bearwallow .
Mountain basaltic andesites of Elston and others (1973).
Contemporaneous with
this basaltic andesite volcanism, a large rhyolite/latite dome was extruded
in Tollgate Canyon of the northern Peloncillo Mountains ,(D. Richter, pers.
comm.).
Thin unwelded tuffs from this center of silicic volcanism are
quarried for cinder.
Other centers of silicic volcanism are identified at Dry Mountain in the
Whitlock Mountains and in the Peloncillo Mountains northeast of San Simon.
No thick, extensive sequences of welded ash flow tuff are identified in the
northern and central areas of the Safford-San Simon Basin; therefore, no
large Tertiary, cauldron subsidence features are believed to be present.
Also, no thick sequences of clastic sediment are interbedded in the mid-Tertiary
volcanic sequence that crops out in the mountains bordering northern
and central portions of the Safford-San Simon basin.
On the whole, the volcanic
rocks in the Gila and Peloncillo mountains dip gently northeast and have
a total thickness between 0.6 and 1.2 km.
During and after the mid-Tertiary volcanism, the Pinaleno Mountains "metamorphic
core complex" probably evolved.
"Metamorphic core complexes" may
have formed as a result of regional strain, "thermal perturbation, and tectonic
denudation" (Davis and Coney, 1979).
Studies show that core complexes in
southern Arizona have a four-element morphology (Davis and Coney, 1979):
(1) unmetamorphosed cover rocks are deformed by listric normal faulting, overturned
folds, and bedding plane faults; (2)
a low-angle decollement zone of
erosionally resistant, and frequently chlorititized, mylonitic breccia, mylonite,
and microbreccia separates the cover rocks from the metamorphic carapace;
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