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Geologic Framework (continued) 2

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Whenever two crustal plates collide extensive zones of rock breakage (faulting) are formed as the two plates jostle each other. In the overriding plate, faults form parallel to the converging margin because the two plates usually approach each other obliquely. The ground breaks roughly parallel to the Earth's surface and the resulting faults are called lateral and wrench faults. However, faults also form perpendicular to the converging plate margin, i.e., transverse to the volcanic arc. These faults can have a variety of movement directions including both horizontal and vertical. In the Jurassic arc of south-central Arizona, the arc-parallel faults trended northwest to southeast and primarily had a left-lateral sense of movement. This means that looking across the fault, the opposite fault block appears to have moved relatively to the left. These were regionally extensive fracture zones, several of which can still be readily recognized on the landscape of today. The fracture zones also tended to be pathways for hot fluids associated with the magmatism, and thus ore deposits may be focused within these fracture zones. However, the Jurassic age ore deposits tend to be small and were economically worked for only small quantities of precious metals.

The end of the Jurassic Period and beginning of the Cretaceous Period, 144 million years ago, was a time of waning volcanic activity in south-central Arizona. The area was primarily the site of deposition of quartz-rich sediments associated with only small amounts of volcanic rock. However, between 90 and 80 million years ago (Late Cretaceous Period) magmatism began again and continued up to 52 million years ago, within the Tertiary Era. Rocks formed during this time span are said to be of Laramide age. The magmatic activity was very important with respect to present-day environmental geology because it was accompanied by the development of large, economic ore deposits.

The Cretaceous and Tertiary volcanism and plutonism occurred further inboard on the North American continent than the Jurassic age magmatism. The rocks produced were generally more siliceous and the resulting volcanism was highly explosive leading to the formation of very large circular depressions (calderas) associated with the volcanoes. Lateral faulting accompanied the volcanic activity, but unlike the Jurassic faulting, the sense of movement was now right-lateral, i.e., when looking across the fault line the opposite fault block appears to have moved relatively to the right. Many of these faults did not represent new crustal breaks but reactivation of the earlier Jurassic faults.

The regional faulting was a key ingredient in determining the localities of magmatism and the very large ore deposits associated with the magmatism. These ore deposits result from the cooling of metal-enriched ground waters that circulated through the shallow continental crust. Initially these fluids were heated during the phases of magmatism and produced very large areas of altered rock associated with the ore deposits. South-central Arizona is especially famous for its large "porphyry-type" copper deposits, several of which are currently being mined. The region also has numerous lead-zinc-copper-silver ore deposits found in sedimentary carbonate rocks.

Following the end of Late Cretaceous to Early Tertiary magmatism and mineralization the region become geologically quiet again until approximately 35 million years ago when magmatism once more reappeared. More importantly, however, the system of faults changed from those previously observed. Superimposed on the older northwest trending fault systems were north to northeast trending faults. Movement of large blocks of the crust along these fault systems resulted in the formation of large basins, including the Tucson Basin. The sedimentary rocks that have accumulated in the Tucson Basin up to the present time form the main ground water reservoir in the Santa Cruz River drainage network.

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