U.S. Department of the Interior Saguaro National Park Service Saguaro National Park Geology of the Rincon Mountains General Setting of the The Rincon Mountains are one of many mass of bedrock referred to as a metamor- Rincon Mountains relatively small ranges that dot the south- phic core complex, one of several that western U.S. belonging to the Basin and extend from northern Mexico into south- Range Province. These ranges are the ern Canada. The highest peak in the result of block faulting which occurred Rincons is Mica Mountain. It is 8666’ in 10-25 million years ago (MYA) and are elevation and supports Ponderosa Pine separated by basins filled with thousands and spruce vegetation. Several trails lead of feet of alluvial sediment derived from to its summit, including one along Tanque the erosion of these mountains. The Verde Ridge, the portion of the Rincons Rincon Mountains are a highly eroded visible from park headquarters. The Building Blocks Before we begin to look at the origin of The theory of plate tectonics states that the the Rincon Mountains, it is important to crust is made up of many plates, some of look briefly at the rocks which make up which are thousands of miles in diameter the mountains and at the theory of plate and up to sixty miles thick. These plates tectonics, which holds the key to are in constant motion, breaking apart understanding the origin of the rocks and along the mid-ocean ridges, as molten structures which make up the Earth’s material (magma) wells up beneath them, crust. Rocks of the three major classes, and coming together along the margins of igneous, sedimentary, and metamorphic, certain continents. are found within the park. The igneous rocks include coarse grained intrusive Where these plates meet, one of three rocks, such as granite, and extrusive lava things may happen. If one plate, usually flows and intrusive basalt dikes which the oceanic plate, is denser than the others, cooled much more rapidly. Sedimentary it will descend (subduct) under the other rocks are formed from the consolidation plate and a trench will form at that point. of sediment derived from weathering and As the oceanic plate continues to descend erosion of preexisting rocks and deeper, the rocks become plastic and then deposited in layers by streams, wind or in molten, leading to the formation of a chain the shallow waters of the ocean. The most of volcanoes as the less dense magma rises common of these rocks include to the surface. Such is the case today as the sandstone, shale and limestone. Pacific Plate subducts under the North Metamorphic rocks form deep within the American Plate or Asian Plate. If neither earth when heat, pressure, and chemical plate is dense enough to subduct, the fluids alter preexisting rocks. These plates may collide and push up very high include slate, marble, gneiss, and schist. mountains such as the Himalayas, or slide Specific examples of these major rock past one another forming shear zones such types will be discussed in the following as the San Andreas Fault Complex. sections as they help to explain the geologic history of the Rincon Mountains. How it Came to Be* The oldest rocks found in the park are rocks. Also present are 1.4 billion year old dark gray metamorphic rocks, known as granites, which, although altered, form *Much of the information in this section comes from articles and field the Pinal Schist, which represent the much of the Tanque Verde Ridge. trip itineraries of Dr. George H. original crust in southern Arizona. These Davis. rocks are approximately 1.7 billion years There is little evidence of what happened old and belong to an era of geologic time over the next few million years as the known as the Precambrian. They formed region was subjected to extensive erosion. during a plate collision at that time which Approximately 600 MYA, at the beginning altered preexisting sediments and volcanic of the Paleozoic Era, gentle rises and falls rocks to schist and other metamorphic of the crust occurred. This led to the How it Came to Be encroachment of shallow seas and the Then, approximately 20-30 MYA, the Continued* deposition of sedimentary rocks, mostly Earth’s Crust in this region was stretched limestones, sandstones, and shales, and sheared in a northeast to southwest separated by extensive periods of erosion. direction. As arching continued, a huge The limestones are exposed in several slab of rock broke loose and slid to the places within the park, those near Lime northwest along a special type of fault Kiln Falls were at one time mined and known as a detachment fault. This used to make hydrated lime for mortar. movement took place over thousands of years shattering the rocks along the fault During the early part of the Mesozoic Era, zone. These shattered but cohesive rocks continual uplift of the region led to the are called cataclasites and are exposed in erosion of exposed rocks by streams. several places along the Cactus Forest These sediments were deposited as Loop Drive, but can best be seen exposed floodplains in the shallow waters of in the hillside as you look north from Lime intermontane basins. Falls. Above the fault, forming the upper plate are Paleozoic limestones, whereas Late in the Mesozoic Era, as the ancient below the cataclasite forming the lower Pacific Plate continued to descend plate, are stretched, highly altered granites, (subduct) under the North American which form a type of metamorphic rock Plate, extensive volcanic activity and called mylonite. These mylonites form the mountain building occurred in the area. impressive Tanque Verde Ridge seen to the This event is known as the Laramide east of the loop drive. Other highly Orogeny. Although not present in the deformed lower plate rocks can be seen as Rincon Mountains, these volcanic rocks strikingly banded rocks along the Catalina form a majority of the rocks which Highway to Mt. Lemmon. This compose the Tucson Mountains to the metamorphic rock is the Catalina Gneiss. west. During and following this volcanic period, masses of granite were implaced 5 to 8 miles below the surface and slowly Tucson Mountains cooled. Catalina Mountains The Last Stages Following the detachment of the upper At one time valley floors may have been as plate rocks, which incidentally is much as eight to ten thousand feet below responsible for the location of the Tucson the mountain crests, but today relief is Mountains (before detachment they were much reduced as alluvial (stream) deposits located further east of the present Rincon of gravel, sand and mud have filled the Mountains), stresses relaxed. The entire basins to their present levels. southwestern portion of the United States became stretched as the Pacific Plate What does the future hold for the region? began to pull away from and shear past Erosion will continue to reduce the relief the North American Plate, between 10-20 of the mountains, which may lead to MYA. The extension of this area renewed uplift of the mountain fault block produced block faulting, where many and potential future earthquakes. A major blocks separated from other blocks along earthquake has not occurred in the Tucson steep normal faults producing the basins region since the 1880’s, but could happen which today surround the Rincon at any time. However, such events will Mountains and other similar ranges in the most likely be few and far between over southwest. the next few thousand years! block falls, valley fills with sediments expansion EXPERIENCE YOUR AMERICA.