Sierra Madre Occidental - Geology - Geological History

Geological History

The basement is thought to be Older North American basement, especially in the North, and a number of different accreted terranes, which are mostly covered by Cenozoic Volcanism. The northern extent of the Sierra Madre is the Cortes Terrane, although the northern extreme of this contains a small portion of the Carborca Terrane. Some of the northern end may also be underlain by a Precambrian basement placed around the same time as the Grenville Orogeny. Some of this area has a series of Paleozoic sedimentary sequences from a deep-marine environment underlying it, which considered by some to be part of the Cotes Terrane and by others to be transported. In some areas distinct sedimentary rocks can be identified by shallow-water Silurian and Mississippian sedimentary rocks underlying deep-water Pennsylvanian and Permian, which are divided by Mississippian rhyolite. The Permian clastics contain some detrital rock that sit alongside low-grade metamorphosed sedimentary and volcanic rocks with serpentinite.

South of the Cortes and Carborca Terranes is the San José de Garcia Terrane, which is a combination of Cretaceous arc volcanics and volcaniclastics, which may be thrusting over the Cortes terrane. The southern part of the range is made up of the Guerrero Composite Terrane, a large body, which is actually a complex of five different subterranes, although not all of these are within the main body of the Sierra Madre. However, the terrane is mostly covered by volcanics and sedimentary deposits, so it is only visible where erosion has revealed it. The main subterranes of the Guerrero Composite Terrane that are within the Sierra Madre are the Tahue and Zihuatanejo terranes. Dividing the Guerrero Terrane from the rest of the Sierra Madre terranes is a boundary that is thought to represent the Early Cretaceous Arperos Basin, a marine basin which separated the island arc that came to form the Guerrero terrane from the accretions that came to form the Sierra Madre terrane. It contains a lower formation made up of pillow basalts with pillow breccias, tuffs, and shales underlying a group of pelagic limestones, oozes, and turbidites.

Covering the southern basement are sedimentary and metamorphised sedimentary rocks containing argillite, limestone, volcanic schist, slate, and greywacke which were exposed in the canyon of the Santiago River. Near these sediments late Oligocene to early Miocene granite and granodiorite intrusive bodies occur. A possible cause of this is that these bodies were roof pendants which were uplifted by plutons.

Towards the end of the Cretaceous, the Laramide orogeny increased the activity of magmatism in the area, forming the first major igneous series in the area. The igneous series are made up of formations of plutonic and volcanic rock, which would later be exposed. Interbedded with these rocks are sedimentary deposit rocks. In the center of the range, some of these rock have been deformed by tectonic forcing that occurred at the same time. The southern part of the range contains none of the volcanism that is apparent in the northern range. These formations ended in the Paleocene. Eocene volcanism formed a series of andesitic and rhyolitic formations in the area, with spatial and temporal variations throughout. Most of the gold and silver deposits are also in these rocks.

In the Oligocene ash flows became the predominate deposit of the area, with interbedded lava flows between. These ash flows began the second series of high magmatism formations. The ignimbrite flare up formed a series of ignimbrite formations layered one atop another, that are sometimes broken by lava flows. The ignimbrite formations in this area cover the largest area of any known series, with ten calderas identified in the province. Three of these calderas are placed in Copper Canyon. The lava has formed a series of mafic rock that form the Southern Cordilleran Basaltic Andesite Suite. The tuffs are above a thick formation of lava rock. Around five or six units have been identified, mostly around the Copper Canyon area. These tuffs have actually allowed wide ranging correlation with formations in other geographical areas, for example Death Valley.

During the Miocene basin and range style faulting took place, as well as the placement of some small alkali basalts, forming a plateau. At the same time spreading began, eventually creating the Gulf of California. In the westernmost slopes mafic dikes formed. These events have also been linked to the subduction of the Farallon Plate.

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