Description:
The Picacho Mountains are north-south trending mountain range completely surrounded by
Quaternary alluvium, and consists of Tertiary and older granitic and gneissic rocks (Figure 1). Picacho
Peak, located south of the south end of the range, is also surrounded by alluvium, and consists of Tertiary
andesitic volcanic rocks. Picacho Peak and the Picacho Mountains are separated by a gap of shallowly buried
bedrock through which pass Interstate 10, the Southern Pacific Railroad, and the Central Arizona Project
canal. The Picacho Mountains consists of a compositionally diverse suite of Tertiary, Cretaceous or
Proterozoic granitoids, heterogeneous to gneissic granite, muscovite granite, schist, and gneiss, much of
which has been affected by middle Tertiary mylonitic deformation and probably by late Cretaceous synplutonic
deformation [Rehrig, 1986]. Mylonitization is inferred to have accompanied normal faulting and
ascent of the bedrock from mid-crustal depths in the footwall of a moderate to low-angle normal fault
commonly known as a "detachment fault". Older gneissic fabrics may record Laramide mid-crustal deformation,
or relict Proterozoic fabric. The crystalline rocks of the Picacho Mountains are part of the footwall
of a south- to southwest-dipping detachment fault that is exposed in only one small area in the southern Picacho
Mountains. Picacho Peak, which consists almost entirely of northeast-dipping basaltic and andesite
volcanic rocks, is part of the hanging wall of the detachment fault.
Geologic map of the Picacho Mountains and Picacho Peak, Pinal County, southern Arizona - report and two map sheets, scale 1:24,000.