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Now THAT'S a fault line. This is just to the right of one of the mine openings. FOV is about six feet.
I have always been fascinated by the shear line that shows so plainly on these hills to the east of Milpitas up toward Hayward. I've never seen any write-ups about this so obvious line.
I had to do extensive cropping, so, view large
Nearly got run over on Highway 166 as I pulled over to the spot where the San Andreas crosses the highway. Didn't have time to get a great pic, but you can see two examples of the fault here. Click the notes in the pic to see more.
Hollister, California
In the San Francisco Bay area there are three major faults, from west to east the San Andreas, the Hayward, and the Calaveras; all are part of the San Andreas fault system. All of these are "right-lateral strike-slip faults," which means that the motion is predominantly horizontal, with the land on the west side of the fault moving north.
South of the Bay Area the Hayward and Calaveras merge into the San Andreas. Hollister is located just north of where this happens, right on top of the southern end of the Calaveras fault.
What makes Hollister particularly interesting is that from San Juan Bautista to just north of Parkfield the faults in the San Andreas system are not "stuck": instead of moving only during major earthquakes, they continuously "creep." As a result of this creep, Hollister is being slowly ripped in two, for the most part along a remarkably narrow zone running right through the middle of town.
The rate of creep is inconsistent, and has been measured at between 6 and 15mm per year at various times throughout the 20th century. Underground pipes, road paving, curbing, and foundations all show signs of being gradually shifted apart. Notice how the distortion always bends structures to the right- no matter from which side of the fault the photo was taken. Also notice that the motion is horizontal: the ground is remaining level as it moves. Together, these two observations define right-lateral strike-slip motion.
(revised) Faulted slaty mudstones just above (on the hanging wall of) the Taconic Frontal Thrust (Logan's Line; not exposed), at the leading edge of the Giddings Brook Slice of the Taconic Allochthon, State Route 40 (just south of the Old Scotch Cemetery), south of the Argyle Village on the east side of the highway. Landing (in Landing et al, 2007) identified this outcrop as "Deepkill Formation" (Lower Ordovician = Poultney Formation of others), while Platt (1960, PhD Diss.) mapped this as Lower Cambrian West Castleton Formation (=Browns Pond Fm and Middle Granville Slate of Landing and others). UC Davis geology student for scale.
Landing (in Landing and Webster, 2018) has a new (Oct. 2018) field trip location at this roadcut (his Stop 11) with an interesting description of the lithologies here (Lower Ordovician Deepkill Formation), but not the structural elements: castleton.s3.amazonaws.com/files/pages/a5-final.pdf
This sandstone from the Black Hills of South Dakota, was faulted, and the layers offset, during compaction.
Ramp in minor thrust fault set in the limestone-dominant Bellevue Member of the McMillan Formation in the Upper Ordovician rocks of Kentucky.
Thrust faults are compressional features that push, or "thrust," older rocks over younger strata at a relatively low angle (this is a minor example only limited to one member within a formation).
This is a ramp where the fault "ramps" up, cutting through the strata at a low angle. When it reaches an area that is easier to propagate though (such as a small sequence dominated by softer shales) it will flatten out to an appropriately named "flat."
The Bellvue Member is comprised of thin to thick bedded gray fossiliferous limestones (the larger ledges in the picture above) and classified as storm deposits or tempestites.
In between are relatively thin gray, thinly bedded shales beneath the large limestone ledges.
Stratigraphy: Bellevue Member, McMillan Formation, Maysvillian Stage, Covington Subseries, Cincinnatian Series, Late (upper) Ordovician.
Locality: southwest side of road cut along U.S Highway 92, south of Lawrence Creek and Germantown Road, east-northeast of Maysville, north Mason County, Kentucky, USA (38°40'05.8" North latitude, 83°48'05.6" West longitude).
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Cincinnatian Lithostratigraphy:
Weir, G. W., W. L. Peterson, W. C. Swadley, and J. Pojeta. 1984. Lithostratigraphy of Upper Ordovician strata exposed in Kentucky. United States Geological Survey Professional Paper 1151-E:1–121.
Peck, J.H., 1966, Upper Ordovician formations in the Maysville area, Kentucky: USGS Bulletin 1244-B, 30 p.
Thrust Fault Tectonics:
Berberian Manuel, Master “blind” thrust faults hidden under the Zagros folds: active basement tectonics and surface morphotectonics. 1995. Tectonophysics
Volume 241, Issues 3–4, 30 January 1995, Pages 193-195, 197, 199-224
K. R. McClay, Thrust Tectonics, 1992, print book.
Turtleback Peak, Red Rock Canyon National Conservation Area, Las Vegas, Nevada, USA
The most significant geologic feature of Red Rock Canyon is the Keystone Thrust Fault. .... A thrust fault is a fracture in the earth's crust that is the result of compressional forces that drive one crustal plate over the top of another. This results in the oldest rocks on the bottom of the upper plate resting directly above the youngest rocks of the lower plate. At Red Rock Canyon, the gray carbonate rocks of the ancient ocean have been thrust over the tan and red sandstone in one of the most dramatic and easily identified thrust faults to be found. (http://www.redrockcanyonlv.org/GEOLOGY.htm)
Fault gouge from California, USA. (~8.75 centimeters across at its widest)
Fault gouge is a relatively soft, unlithified, mud-like to clayey material that occupies some fault zones or fills matrix between fault breccia clasts. It forms from extreme pulverization and grinding during multiple fault movements over geologic time.
This sample is from the San Andreas Fault Zone, which is a transform plate boundary between the Pacific Plate and the North American Plate. These two tectonic plates are sliding past each other. Despite the word "sliding", movement along the fault zone is usually in the form of sudden jolts - these movements send out shock waves, which cause earthquakes.
The San Andreas fault gouge specimen seen here is principally composed of montmorillonite (also known as smectite), a silicate mineral often called "swelling clay". When wet, it expands. When dry, it shrinks. Surface outcrops of this fault gouge display popcorn weathering, which forms by repeated wetting and drying of swelling clays. Minor amounts of kaolinite clay are also present.
Locality: San Andreas Fault Zone, Mecca Hills, Riverside County, southern California, USA
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