Thesis (M.S.)--Brigham Young University. Dept. of Geology, 2005. / Includes bibliographical references (p. 61-64).

The Eureka Lilly fault zone provides an impermeable barrier for groundwater flow in the East Tintic mining district. The fault zone separates two distinct groundwaters that have different temperatures, compositions, and potentiometric surfaces. The damage zone of the fault is an extensive network of interconnected open fractures and fault intersections that provide conduits for groundwater flow in otherwise impermeable units. The fault core breccia has been re-cemented and mineralized, which eliminates porosity in the rock by creating a thick impermeable zone, which has compartmentalized groundwaters across the fault zone. The compartmentalization of groundwater shows that fault zone variability (from strain partitioning and multiple deformation episodes) make traditional basin flow concepts inaccurate and difficult to apply in this area.

Voluminous Eocene to Oligocene intermediate to silicic volcanic rocks related to subduction erupted throughout the Great Basin and were supplanted by bimodal eruptions of basalt and rhyolite related to extension in the Miocene. Locally, in the northern East Tintic Mountains of central Utah, this important transition is marked by a distinctive package of mafic alkaline magmas that reveal important details about the nature of this fundamental change. A late Oligocene anorthoclase-bearing shoshonite lava in the Boulter Peak quadrangle contains megacrysts of anorthoclase, with phenocrysts of olivine, clinopyroxene, magnesiohastingsite, magnetite, and apatite. The anorthoclase grains occur as glomerocrysts with irregular, resorbed edges, indicating they are not in equilibrium with the mafic phenocrysts in the shoshonite. They are interpreted to be xenocrysts incorporated into an ascending mafic magma that came into contact with a partially crystallized syenite. The mafic magma involved was probably derived by partial melting of the lithospheric mantle based on its high Mg/Fe ratios, magnesian phenocrysts, high water content, and high ratios of lithophile to high field strength elements. The syenite body likely crystallized from a highly differentiated melt. The 40Ar/39Ar age of the shoshonite is 25.35±0.04 Ma, and appears to represent the transition from subduction before the onset of extension (Christiansen et al., 2007). Other Oligocene mafic units in the area may represent different variations of the mafic alkaline endmember for the mixing process. The Gardison Ridge dike, a potassic alkaline basalt with an 40Ar/39Ar age of 26.3±0.3 Ma, contains olivine and clinopyroxene phenocrysts that are compositionally very similar to those found in the shoshonite. Other mafic dikes have even higher alkalis. All of these dikes have similar trace element patterns, with negative Nb and positive Pb anomalies, and high Ba and K concentrations. The minette of Black Rock Canyon (28.45±0.13 Ma) also contains high alkalis, particularly K, and its trace element pattern shows positive Ba and negative Nb anomalies. The clinopyroxene phenocrysts in the minette are also very similar to those found in the other alkaline rocks. The high water contents of these units are evidenced by amphibole in the shoshonite, phlogopite in the minette, and the lack of plagioclase phenocrysts in the basaltic dikes. The ages, mineral assemblages, and chemical compositions show that these late Oligocene alkaline magmas formed after a shallowly subducting oceanic slab peeled away from the overlying continental lithosphere and rolled back. Hot asthenosphere flowed in to replace the subducting plate and caused partial melting of the variably metasomatized lithospheric mantle. These alkaline magmas include the shoshonite, mafic alkaline dikes, and minette of Boulter Peak; they mark the transition from older subduction-related magmatism to Miocene magmatism caused by lithospheric extension.

The mineral resources of the Tintic Mining District are influenced by three major events in its geologic history; the Mesozoic Sevier Orogeny, Paleogene volcanism and Late Neogene Basin and Range extension. In this paper a detailed analysis of each these geologic events is presented to help us understand the structural host, mineralization and exhumation of the Tintic Mining District ore. A kinematic analysis of the faults was completed to determine the origin of NE-trending faults, Sevier Orogeny or Basin and Range extension, in the northern part of the East Tintic Mountains in Allens Ranch 7.5' quadrangle, near the eastern margin of the Great Basin of central Utah. The structural history of the NE-trending faults found in the quadrangle was reconstructed to determine stress directions and fault kinematics. Maximum paleostress direction for the East Tintic fold and thrust system is between 80º–100º with fold axes oriented at ~350º. For example, the Gardison Ridge and Tintic Prince faults are NE-trending right-lateral transverse faults that formed at ~30º to paleostress directions similar to those of the Sevier Orogeny. The dominant NE-trending faults in the region are likely due to (1) differential shortening during progressive orocline development, (2) the pre-deformational Pennsylvanian-Permian Oquirrh basin geometry, and (3) the influence of the Leamington transverse zones of the Provo salient. Conversely, mixed paleostress directions for the north-trending Tintic Davis Canyon fault show it is a Basin and Range extension-related normal fault that may have originated as a Sevier related fault. Other N-trending faults within the quadrangle are only related to Basin and Range extension. However, large offset, range-bounding faults are buried by valley fill throughout the quadrangle and no young fault scarps are identified cutting Lake Bonneville deposits. An Oligocene to Miocene suite of extrusive volcanic units in the quadrangle correlates well with those of the East Tintic and Soldiers Pass volcanic fields. The Paleogene volcanic section is dominated by a suite of high-K calc-alkaline extrusive rocks (35 to 32 Ma). This intermediate to silicic sequence was followed by eruption of the mildly alkaline Mosida Basalt during the Miocene (19.5 Ma) marking the transition from subduction-related intermediate and silicic volcanism to extension-related mafic volcanism in the eastern Great Basin.

Geologic structures within the Birdseye 7.5 minute quadrangle Utah County, Utah have been related by previous workers to both the Jurassic Arapien Shale diapirism and to the mid-Cenozoic extensional collapse of the Charleston-Nebo Thrust. Whichever model proves valid, it will have implications for oil exploration and interpretation of the subsurface geologic structure in the region. A detailed map of the quadrangle was constructed to better constrain which mechanism was responsible for the deformation. Exposures of Arapien Shale near, and within the Birdseye quadrangle show no evidence of diapiric movement. Arapien involvement in the deformation of Tertiary rocks in the center of the quadrangle is therefore unlikely. Changes in the pattern of sedimentation of Eocene age rocks suggest a change in tectonics during this time. Restoration of the Eocene strata shows that the most plausible mechanism for this deformation is extension along reactivated thrusts in the Arapien Shale, Thaynes Formation, and Woodside Shale, related to Basin and Range extension. The Moroni Formation, a prominent Tertiary volcanic unit present throughout the Birdseye quadrangle, has been used to justify Eocene extension. Deformation with the formation was found to be present only along the Thistle Canyon normal fault, constraining movement along the fault to the Eocene and later. Dip and facies relationships present within the formation mainly are a result of paleotopography rather than extension. Several distinctive units were mapped within the formation, including lahar and fluvial deposits, as well as two different ash-flow tuffs. A depletion in nickel and chromium, an unusually ferroan composition, and distinctive Fe/Ti ratios suggest that the volcaniclastic rocks of the Moroni Formation are similar to volcanic rocks in the Slate Jack Canyon and Goshen quadrangles which lie about 35 km to the west. This implies that the ignimbrites and volcanic clasts in the Moroni Formation were sourced from the East Tintic volcanic center. It further implies that any mid-Tertiary extension between the East Tintic center and the Birdseye quadrangle did not create barriers to sedimentation and was limited in extent.