#normal fault

[cordially-stupid]

Hello fellow geology enthusiasts! I've finished my structural class and have now put some of my homework on my redbubble! link is to the socks version >:3 Enjoy!

[earthstory]

The simplest kind of fault When rocks tear and break they can do so in a variety of ways, depending on whether they are being pulled apart (aka extension to geologists), pushed together (compression) or slid alongside each other (strike slip, like the San Andreas) by the tectonic forces affecting the area. This example is a normal fault from Iran, produced by pull apart forces, at least on a local level. The layers pick out very well the block of rock that has dropped between two others, and the two fault lines bordering it. The tectonics in Iran are complex, the main forces are compressive as Arabia separates from Africa and is in a slow motion collision that is closing the Persian Gulf. As the rock is pushed out of the way and uplifted to form mountains such as the Zagros range some regions are twisting and buckling in a rotatory motion, leading to local extensive forces and normal faulting. Loz Image credit: https://twitter.com/HaakonFossen

[weirdponytail]

Making little things for my Field Methods exam tomorrow and enjoyed this one. 

[therealuniverse]

The Straight Wall This shot compiled from images taken by NASA’s Lunar Reconnaissance Orbiter shows a large, partially destroyed crater known as the Ancient Thebit crater (the well-defined crater at the right is known as Thebit). At the center of the large crater is a famous spot known to lunar telescope observers known as Rupes Recta, which translates from Latin as the “Straight Cliff”. It is also more commonly known as the “Straight Wall”.

Rupes Recta is a 110-kilometer-long, steep fault scarp in the center of the old, degraded crater. It was originally noted by 17th century astronomer Christian Huygens, and modern day amateur astronomers can spot it in part thanks to the shadows it casts. As the sun rises on the lunar day, it casts a broad shadow out over the basin that can be readily seen as a straight, dark line. Rupes Recta is a normal fault. The crust on one side has dropped down by up to 400 meters, leaving a cliff with a slope of 85º. A study a few years ago characterized the geologic setting for this fault and first, by mapping which surfaces had been disturbed by the fault and counting the number of craters on those surfaces, they estimated the fault formed about 3.2 billion years ago. Most of the Moon’s surface today is in compression; the moon is actually shrinking ever so slightly as it cools, putting pressure on the outermost layer and forming thrust faults. This line is one of only a handful of normal faults on the Moon as they are produced during extension. To the west of Rupes Recta there is also a sinuous rille that runs nearly parallel to the fault scarp. Rilles are features thought to form through volcanic processes, either with molten rock flowing along or beneath the surface. The presence of a parallel sinuous rille caused the authors studying this feature to suggest that both were formed at about the same time when an extensional stress regime dominated this crater. The stresses in this crater allowed cracks to open and molten rock to flow upwards, in one case creating a volcanic feature and in another case creating a fault that breached the surface as a rigid feature, seen today as the Straight Wall. -JBB Image credit: https://en.wikipedia.org/wiki/Rupes_Recta#/media/File:Ancient_Thebit_(LRO).png Reference: https://sp.lyellcollection.org/content/401/1/377

[geologicaltravels]

2000: Fault scarp in the hills above Lastros, eastern Crete. Better photos can be found here. A paper, which is free to download, on the Lastros-Safka graben of which this fault is a part, is here.  

[relatetorocks]

[dinogami]

Cinder cone volcano (~32,500 years old) at the north end of Snow Canyon State Park, north of St. George, Utah. The volcano sits on Lower Jurassic (~180 million year old) Navajo Sandstone, which comprises the red and buff outcrops behind the volcano. This is one of two closely spaced cinder cones that produced the Santa Clara aa basalt flows. Several cinder cones, ranging in age from 2.1 million to 32,500 years old, dot the landscape from Grand Canyon through western Utah, created by decompression melting as this area, at the eastern margin of the Basin and Range Province of North America, is being pulled slowly apart.

[earthstory]

Piecing together an outcrop

This is a wall of sedimentary rocks exposed in Utah that has had a geologist take a pen to it. The digitally sketched lines mark specific beds that you can track across the outcrop as a method of reconstructing the motion on the faults.

Take a look at the rocks above and below the red line. Everywhere you see it, the red line marks the top of a gray/white colored sandstone unit. The purple line just above that marks a coal bed that sits atop a sequence of more fine-grained mudstones.

That sequence of rocks repeats at different elevations throughout this outcrop with faults in-between. Some of the layers have been tilted by the faults, some have not. The faults found here dip both ways, a commonly seen feature of faults produced by the fact that rocks often break at different angles.

These rocks have been extended, pulled apart by tectonic forces that created normal faults at this spot. See the fault that curves towards flat at the bottom of the outcrop? That’s a common shape for normal faults, they bend horizontal with depth in a geometry we call “Listric faulting”.

The state of Utah has been pervasively pulled apart over the past 30 million years as part of the Basin and Range extension (http://tmblr.co/Zyv2Js1JkP4qs ).

-JBB

Image credit: https://flic.kr/p/dfhDtC