Your Fault of the WeekNASA has again provided us with an image that should find itself in all future
Your Fault of the WeekNASA has again provided us with an image that should find itself in all future structural geology texts: this one of the Piqiang Fault within the Tien Shan mountains of China. Structural geologists in the crowd, this is for you…For those in the “geologic know,” it’s easily recognizable as a strike-slip fault (where two rock masses are sliding against each other along a ~vertically oriented fault plane). The sense of movement on this fault is termed “left-lateral” because whichever side of the fault you’re standing on, the other side appears to be moving to the left.The San Andreas fault of California (http://tinyurl.com/phnga2f) is also a strike-slip fault, but of a completely different sort. The San Andreas marks the movement of two tectonic plates sliding against each other.The Piquiang Fault has formed as a result of two tectonic plates barging into each other head on so to speak, the Indian plate moving north against the Eurasian plate. During this motion, in addition to forming the Himalayas, rock strata become horizontally detached, and are “thrust” atop each other: not surprisingly, faults of this nature are call “thrust faults.” But – all the motions within a thrusting stack of rock do not occur at the same rate of speed. Rocks in the center of the thrust move faster than those that are sort of dragging along the margins. When this happens, the entire pile can be split, or torn apart, by series of vertical fault planes to allow for this differential speed. Each of these faults have strike-slip motion and are called, again not surprisingly, “tear faults.” The Piquiang Fault is a tear fault cutting a thrust stack. That it is left-lateral implies that it is on the left side of the thrust stack since the central parts of the thrust unit would be moving relatively faster than the external parts.Whew. Technical? Sorry, but sometimes the structural geologist in us simply breaks out…Annie RImage from NASA Earth Observatory: http://earthobservatory.nasa.gov/IOTD/view.php?id=82853&src=fbMore reading:http://www.mining-journal.com/supplements/mj-kyrgyz-republic-supplement-0313/an-extremely-complex-geology?SQ_DESIGN_NAME=print_friendlyhttp://maps.unomaha.edu/Maher/geo330/sandbox/tina3.html -- source link
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