Geologic map of the Fort Rock Dome, Yavapai County, Arizona: Supplement 1 from "The geology and mechanics of formation of the Fort Rock dome, Yavapai County, Arizona" (Thesis)

Description

The Fort Rock dome, in Yavapai County, Arizona, is a recently discovered circular geological structure, two miles in diameter, which initially aroused interest because of its similarity to an impact crater. Features of the dome that are similar to features of an impact crater include a crater-like depression in its center underlain by brecciated Precambrian igneous and metamorphic rocks, upturned Tertiary volcanic and sedimentary rocks on its rim, radial and tangential faults, and an apron of breccia on its flanks showing a crude inversion of the pre-breccia stratigraphy. Studies by the writer have indicated that the structure is primarily a volcanic dome caused by an intrusion at depth. The crater on top is erosional in origin. Rocks exposed in the vicinity of the Fort Rock dome range in age from Precambrian to Quaternary. Precambrian rocks include layered metamorphic rocks, cataclasites, granite, granite pegmatite, and veined rocks. These rocks are part of a broad belt of Precambrian rocks exposed in the mountains of central Arizona along the southwestern margin of the Colorado Plateau. Paleozoic sedimentary rocks, seen northeast of the dome, range in age from Cambrian to Mississippian. Tertiary volcanic rocks overlie chiefly Precambrian rocks in a large area, about 1000 square miles in extent, west and south of the dome. Numerous volcanic centers are scattered throughout this area; the Fort Rock dome is one relatively small center. Quaternary deposits include alluvium and terrace deposits. The chief structures in the region are Basin and Range faults. Between the dome and Seligman, Arizona, to the northeast, north-south normal faults with displacements of hundreds of feet form low, linear ranges and broad valleys. In the Aquarius Mountains, west of the dome, several east-west normal faults offset rocks by hundreds of feet. On the eastern edge of Big Sandy Wash, west of the Aquarius Mountains, a north-south fault with a displacement of 1000 feet is responsible for a major escarpment in the Basin and Range Province that is continuous with the Grand Wash Cliffs to the north. Most Phanerozoic rocks in the region are relatively flat-lying. The Fort Rock dome is an anomalous area where Tertiary rocks are highly deformed. Precambrian rocks on the Fort Rock dome are similar to older Precambrian rocks at the Grand Canyon, Arizona. Layered metamorphic rocks on the dome include amphibolite, schist, phyllonite, and gneiss. These rocks belong to the epidote-amphibolite metamorphic facies. Cataclasite bodies, which are present in the layered metamorphic rocks, appear to be granite intrusions that are crushed and foliated. Granites and granite pegmatites intrude the layered metamorphic rocks and cataclasites. They are also crushed but are not foliated. The Precambrian rocks are fractured and veined in an east-west shear zone that crosses the southern half of the dome. Veined rocks are covered by Tertiary volcanic rocks on the rim of the crater. Veined rocks in a similar shear zone several miles north of the dome are covered by Tapeats Sandstone, of Cambrian age. Tertiary volcanic rocks lie on a mid-Tertiary erosion surface that covered most of northwestern Arizona and is responsible for much of the topography seen today in this area. Volcanic rocks in the vicinity of the Fort Rock dome comprise chiefly two formations, here given the names Crater Pasture Formation and Fort Rock Creek Rhyodacite; the Crater Pasture Formation is the older of the two. In addition to these two formations, several lava flows and intrusive bodies are present which are younger than the Fort Rock Creek Rhyodacite, including a unit herein referred to as the basalt of Buttox Hills. Other volcanic rocks in the region include the Peach Springs Tuff, of middle Miocene age, which overlies the Fort Rock Creek Rhyodacite ten miles west of the dome, and younger Mohon Mountain volcanics, south of Trout Creek. The Crater Pasture Formation includes a series of lava flows, agglomerates, tuffs, and associated intrusive and sedimentary rocks in the vicinity of the Fort Rock dome. The volcanic rocks range in composition from ultramafic to intermediate. In the vicinity of the dome, eleven chief subunits have been recognized. Reconnaissance mapping has indicated that volcanic rocks in an extensive area west and north of the dome are of similar age and composition and can also be included in this formation. The subunits recognized at the dome include, from oldest to youngest (where relative age is known), a limburgite flow, an olivine-trachyandesite volcanic breccia and flow, a hornblende-trachybasalt flow, a pyroxene-trachyandesite tuff and flow, three hornblende-trachyandesite flows, an olivine and hornblende trachyandesite tuff and conglomerate, a sedimentary breccia, an olivine-trachybasalt tuff and agglomerate, and an olivine-sanidine-trachybasalt flow. Most of these rocks were erupted from vents located on or near the periphery of the Fort Rock dome, and most are traceable for distances of a mile or less. The sedimentary breccia, which in this report is referred to as the sedimentary breccia of One O'Clock Wash, is a local deposit on the flanks of the dome that consists of clasts eroded from the dome. This unit was deposited during uplift of the dome. The youngest unit in the formation, an olivine-sanidine-trachybasalt flow, herein referred to as the flow of Fault Canyon, was erupted from a vent on the southwest flank of the dome. This flow apparently came from a magma beneath the dome, which is believed to have been responsible for the uplift. The Fort Rock Creek Rhyodacite includes a series of ash-flow tuffs and other massive tuffs, volcanic breccias, lava flows, and associated intrusive and sedimentary rocks in the Aquarius Mountains and vicinity. The composition of the volcanic rocks of this formation is chiefly rhyodacite. Subunits recognized are here given the names, in order from oldest to youngest, the sedimentary breccia of Noon Gorge, the Old Stage Road Member, the Three Sisters Butte Member, and the breccia and conglomerate of the Crossing. The sedimentary breccia of Noon Gorge is a local deposit on the flanks of the Fort Rock dome consisting chiefly of clasts eroded from the dome. The Old Stage Road Member is a unit chiefly of non-welded ash-flow tuff that extends throughout the Aquarius Mountains and into surrounding areas. The Three Sisters Butte Member is a unit of interbedded volcanic breccia and massive tuff which is found throughout the Aquarius Mountains. The breccia and conglomerate of the Crossing is chiefly a volcanic breccia; it occurs on the northeast flank of the Aquarius Mountains. The Old Stage Road Member, the Three Sisters Butte Member, and the breccia and conglomerate of the Crossing were erupted from a large rhyodacite center in the Aquarius Mountains located about 2-1/2 miles southwest of the Fort Rock dome. The basalt of Buttox Hills is a local flow that overlies ash-flow tuff on the southeast side of the dome. A dike of this unit intrudes rocks on the dome. Quaternary units include alluvium in washes, older alluvium on terraces, and colluvium. Terrace deposits are extensive east of the Fort Rock dome. They contain abundant clasts of Paleozoic sedimentary rocks that were derived from terrane to the north and east. Precambrian structures on the dome include folds of different amplitudes in the layered metamorphic rocks and an east-west shear zone in the southern half of the dome. The shear zone, which is characterized by fractured and veined Precambrian rocks, brings together in the dome differing rock assemblages and also rocks of slightly differing metamorphic grade. This shear zone appears to be part of a major discontinuity in Precambrian rocks that is traceable for 70 miles on an aeromagnetic map from near Ashfork, Arizona, to the western edge of the Aquarius Mountains. This discontinuity is associated with Tertiary normal faults and may be an ancient strike-slip fault zone in the Precambrian rocks. Tertiary structures on the dome include relatively minor faults that pre-date the uplift of the dome and the dome itself, with its associated folds and faults. The dome is a structural dome. Its central part is a low, quaquaversal arch, now deeply eroded. Its edge is, in most places, a sharp monocline. Structural relief on the dome is estimated to be about 900 feet. Dips on the steep-dipping limb of the monocline, where it is sharp, vary from about 40° to about 90°(?) and average about 66°. Most faults mapped on the dome are confined to the vicinity of the monocline, partly as a result of poor exposure and lack of structural control in the deeply eroded central part of the dome. These faults have chiefly radial and tangential strikes and steep dips, where attitudes can be determined. Normal, dip-slip movement appears to have been dominant, although radial faults may have had a strike-slip component of movement. Block rotations are observed along relatively long faults. Ages of faults, where they can be determined, range from the beginning of uplift, which was signaled by deposition of the sedimentary breccia of One O'Clock Wash, to after the emplacement of the ash-flow tuff of the Fort Rock Creek Rhyodacite. Major offsets, however, appear to be confined to the times of emplacement of the various units of sedimentary breccia on the flanks of the dome including the sedimentary breccia of One OClock Wash, the sedimentary breccia of Noon Gorge, and a basal sedimentary breccia in the Old Stage Road Member of the Fort Rock Creek Rhyodacite. In addition to mapped faults on the rim of the crater, abundant small faults, with displacements of inches to tens of feet, and lenses of unmineralized, open breccia were formed during doming. These are most easily seen in the Precambrian rocks in the interior of the crater. Units of the Crater Pasture Formation older than the Fault Canyon flow are strongly uplifted and deformed on the dome, whereas the Fault Canyon flow and units of the Fort Rock Creek Rhyodacite are uplifted and deformed by much smaller amounts. Major Tertiary structures near the dome include two west- to northwest-trending normal faults, traceable for several miles, on which movement occurred both during and after emplacement of the Fort Rock Creek Rhyodacite. Rocks on the southern and southwestern sides of these faults moved up during emplacement of the Fort Rock Creek Rhyodacite but moved down in subsequent times. Major events in the Precambrian geologic history at the Fort Rock dome include deposition of volcanic and sedimentary rocks, metamorphism, intrusion of granites, probably at about 1725 million years before the present, re-metamorphism accompanied by intrusion of granites and granite pegmatites, probably at about 1695 million years before the present, uplift, strike-slip(?) faulting, which probably occurred prior to about 1400 million years before the present, and erosion. Major pre-volcanic events in the Phanerozoic geologic history at the dome include deposition of sedimentary rocks in times ranging from the Cambrian to the Mississippian, uplift, and erosion, in mid-Tertiary times. The first major events in the Tertiary volcanic history at the dome include the eruption of various ultramafic to intermediate flows, tuffs, and volcanic breccias of the Crater Pasture Formation. Subsequently, during the intrusion of a large body of magma at some depth beneath the surface, overlying Precambrian rocks and volcanic rocks were domed. Erosion of the dome occurred and was coincident with eruption of volcanic rocks south of the dome. The Fault Canyon trachybasalt flow was erupted next on the southwest flank of the dome; it apparently came from the magma beneath the dome. Uplift of the dome subsequently diminished and some collapse may have occurred. Eruption of the Fort Rock Creek Rhyodacite began at a large center in the Aquarius Mountains southwest of the dome following extrusion of the Fault Canyon flow. Ash flows, ash and block falls(?), and volcanic mudflows, all of rhyodacite composition, were emplaced. Ash-flow tuff buried most of the terrane around the Fort Rock dome. Some faulting accompanied emplacement of the Fort Rock Creek Rhyodacite. The basalt of Buttox Hills was later erupted on the southeast side of the dome. Ten miles east of the dome, the Peach Springs Tuff, of middle Miocene Age, was emplaced on the western flank of the Aquarius Mountains. The Mohon Mountain volcanics, of Trout Creek, followed. Late geologic history in the region includes formation of Basin and Range fault in Miocene to Pliocene times and cutting of the present Colorado River drainage, beginning in early to middle Pliocene times. The deep canyons of Trout Creek and other creeks south and southwest of the dome presumably were formed during the latter period of erosion. At the dome, erosion led to the carving of a crater-like depression on the dome. This depression is presently surrounded by volcanic rocks in the steep, dipping limb of the monocline on the edge of the dome. These rocks form a circular in-facing scarp, which is the rim of the depression. A mathematical model of doming is developed in this report. The Precambrian rocks underlying the Fort Rock dome are treated as a layer of viscous fluid. Below this layer is an inviscid magma. Intrusion of the magma into the layer of viscous fluid is modeled by applying an axisymmetric pressure distribution on the bottom of the layer. After the pressure is applied, a dome is created on the surface. This dome has a smaller amplitude than the arch created on the bottom of the layer by the intrusion. By examining layers of different thickness, h, relative to the radius, a, of the applied pressure distribution, it appears that a dome like the Fort Rock dome cannot be created unless h/a is near 1. In addition, for a cooling time of around 10(4) years for the magma that caused uplift of the Fort Rock dome, an effective viscosity for the Precambrian rocks underlying the dome is calculated to be around 4 x 10(20) poises. Planes of maximum shear generated in the model are consistent in attitude and sense of displacement with most faults mapped on the Fort Rock dome. Discrepancies that are observed can be explained in several ways. The history of doming predicted by the model is consistent with the history of doming at the Fort Rock dome. It includes an initial stage of accelerating uplift followed by lesser uplift or collapse upon formation of a vent for the magma. Uplift ceases upon crystallization of the magma.

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