The Mexican Subduction ZoneToday, June 23 2020, there was a large magnitude earthquake near Oaxaca,

The Mexican Subduction ZoneToday, June 23 2020, there was a large magnitude earthquake near Oaxaca, Mexico, so today’s a good day for an overview of the geology of this part of Mexico. Mexico is a very active location tectonically, as seen in this map from a recent publication summarizing the geology of that area. It features a large volcanic arc, it is crossed by a number of smaller faults associated with the volcanic zones, and along its southern coastline, there are 2 oceanic plates that are gradually subducting beneath the continent.The Cocos and Rivera plates are small remnants of what was once a much larger oceanic plate called the Farallon plate. This large plate subducted beneath much of North America during the Jurassic to Cretaceous periods, creating large volcanic arcs along the western side of the continent including the Sierra Madre Occidental range in Mexico. But, about 28 million years ago, North America subducted that plate to pieces – the continent reached the spreading center between the Farallon plate and the Pacific Plate, breaking the Farallon plate into pieces and creating the modern Pacific-North American plate boundary.After that large plate was broken into two pieces, it also fractured into smaller bits based on transform faults – the locations that divide oceanic plates into segments. One of those older divisions, an old transform fault, separates the Cocos plate from the Rivera plate.After the breakup of the Farallon plate, the boundary between the plates re-organized. Part of the boundary between the Pacific and North American plates re-formed inland of the coast, causing part of Mexico to rift away and forming the modern day Gulf of California. The tiny plates that were subducting beneath Mexico each began to move in their own directions, causing the position of the volcanic arc in Mexico to shift inland.Eventually, a portion of the Cocos plate did something weird – it stopped going down. Instead, much of the Cocos plate became flat and slid north, reaching modern day Mexico City before it finally detaches and begins to sink. You can see that in the contours on this map – the Cocos plate goes down to some depth, then it bends and flattens and depth doesn’t increase until it reaches Mexico City. Flat plates in subduction zones are possible, they tend to occur when subducting plates are very young and where there is no old plate attached to it that is pulling downward. In Mexico, it is likely that the old Farallon plate that subducted into the mantle actually tore away from the Cocos plate about 10-15 million years ago, and that was part of why the Cocos plate stopped sinking.The earthquake today was at the southern tip of this map. It was on the subduction zone, where the Cocos plate sinks beneath the North American plate. To the north of the location of this earthquake, seismic surveys of the mantle have also suggested that the Cocos plate is ripping into pieces – another tear in the slab has been suggested to the north near Veracruz.This complex geology has built a set of mountains and volcanoes throughout central Mexico. This map also marks the location of the volcanic front in Mexico, including the large stratovolcanoes, and it shows the speeds the plates are moving at as they head into the trench. The subducting plate near Mexico City was the location of a large earthquake in 2017 associated with the bending of that plate. There are also other active faults running through the mountains throughout this area – a remnant of eons of processes along the western boundary of North America.-JBBImage source: https://www.sciencedirect.com/science/article/abs/pii/S0012825216304639Today’s earthquake: https://earthquake.usgs.gov/earthquakes/eventpage/us6000ah9t/executive -- source link
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