The appearance of highly reflective fault mirrors (FMs) (Figure 1) in carbonate rocks is a topic that is only recently receiving interest. Knowing the conditions that are required to produce these FMs is important as it can indicate how the fault has ruptured providing a mitigation tool for appropriate plans to be put in place for similar events in limestone dominated regions. This is geologically important, has economical significance and could save lives. Recognised FMs occur in carbonate rocks during presumed faulting which is a common occurrence in the earth’s upper crust (Barnhoon et al 2005), especially through Mediterranean countries such as Greece and Italy (Smith et al 2011). Fault mirrors have previously been studied at larger outcrop scale, but the nanoscale study has been much overlooked.
Figure 1. Highly reflective FM in Eocene
Limestone, Kfar Giladi, Israel (Siman-Tov et al)
The first major nanoscale study was undertaken using hand samples of carbonate FMs from three different well-preserved locations along the Dead Sea transform. They were compared with a non-mirror like fault surface from the Nahal Uziyahu Fault, Gulf of Eilat as a control. The surface topography of the samples were scanned down to the submicron scale using an optical profilometer as well as using atomic force microscopy (AFM), both which are non-contact instruments that are used to map surface profiles by using the reflections of a laser beam to accurately map the surface of a sample. The optical profilometer is quicker but does not have as high resolution as the AFM. Scanning electron microscopy (SEM) was used to image the surface structures and transmission electron microscopy was used to study a cross sectional area of the gathered samples (Siman-Tov et al 2013).
Structures and smoothness of the FMs vary depending on the scale (Siman-Tov). A ~10-2 m layer of 90-100% matrix called ultracataclasite is observed (Sibson 1977), and the surface has corrugations with wavelengths ranging from ~ 0.1 – 1.0 m in the larger outcrop scale overview of the surface. Between hand sample scale and ~1 μm, most the observed surfaces showed a few subparallel striations but were mostly smooth. Striations are common on slip surfaces (Siman-Tov). Roughness decreases with increasing slip, which suggests the area has been subjected to large slip amounts. However the occurrence of extensional fractures known as Riedel shears which branch off the striations suggest that the area was exposed to varying amount of slip. When analysed at in the nanoscale, carbonate grains with diameters of tens to hundreds of nanometers are visible (nanograins).
Marshak, S. (2009) Essentials of Geology, 3rd ed. New York: W.W. Norton & Company, ch. 11, p. 298-320.
The shelf-edge includes carbonate-to-clastic facies transition and tectonic uplift and erosion of the carbonates followed by deposition of the clastics. The Saint Peter Sandstone is a well-sorted, almost pure quartz arenite deposited during a major mid-Ordovician low stand. Clastics spread across an exposed carbonate platform by transportation. This is shown by the well-rounded, frosted texture of the quartz grains.
This sedimentary rock has hardened over the many years with sand shells, small pebbles, grains of sand and rocks of various sizes. In comparison to our 4.5 billion year old Earth, these sand shells might as well be brand new, when in reality they could be up to 1,000 years old. If the sandstone were to be replaced with calcite it would completely change the subclass of rock, it would then be chemical & organic limestone. The variation in sand stone is due to different rates of deposition and change in patterns of the sediment movement (Mc Knight, p. 384). These tightly compacted varying stones and shells will be weathered away by wind and waves over time and could eventually be reduced to a rock the size of your hand.
More recently than Zeck’s work, Cesare et al. (1997), only divided the xenoliths into two main types: garnet-biotite-sillimanite and spinel-cordierite xenoliths. The quartz-cordierite rocks, distinguished by Zeck (1970), were interpreted as the products of interaction between garnet-biotite-sillimanite xenoliths and blebs of mafic magma and are not recognised. The xenoliths were observed to contain widespread occurrences of rhyolitic glass as...
and Metamorphic rocks can be found. There are also a lot of crusted plates, and violent
Explain how the slab component is generated in island arc igneous rocks. Consider both the fore-arc and back-arc basin environments
Ralph Waldo Emerson once said, “we learn geology the morning after the earthquake.” Fortunately for those living along the San Andreas Fault line in California, there are people behind the scenes, from geologists to city and emergency planners, who have no intention of waiting that long.
Four tectonic environments in which magmatism occurs: destructive plate margins, constructive plate margins/divergent plate boundaries, oceanic intraplate and continental intraplate. Arcs are magmatic products of destructive plate margins (stern, 2001) which are referred to as sites where new continental crust is created and old oceanic crust is subducted back into the mantle (Hawkesworth , Hergt, McDermott, Ellam, 1991). An island arc is a form of an arc produced by the subduction of an oceanic plate beneath another oceanic plate (Hu, 2013). Island arcs are characterised by high large ion lithophile elements and low high field strength elements ((Hawkesworth , Hergt, McDermott, Ellam, 1991). New oceanic crust is created at the mid ocean ridges and by mass balance is then destroyed (recycled back into the mantle) at the subduction zones (Jarred, 2003). As it forms it undergoes hydrothermal alteration where the composition is changed from olivine, plagioclase and glass dominated to being dominated by clay minerals i.e. low grade metamorphism (green schist facies) (Jarred, 2003) where it becomes enriched in water, carbon dioxide and incompatible trace elements such as uranium and potassium. As the crust moves away from the ridge it accumulates sediments and becomes colder, older and dense enough to subduct. As the cold slab subducts into the hot mantle, it heats up. The slab goes to greater depths faster than it’s heated therefore pressure increases therefore hydrous minerals start to break down (Aizawa, Tatsumi, Yamada, 1999). The slab undergoes metamorphic reactions from hydrous green schist facies to amphibolite facies and finally to anhydrous eclogite facies at depths of less than 100 km (Ringwood, 1977). ...
Since California had previously been mentioned, it is a great place to begin explaining the complex geological impacts starting with the San Andreas Fault. The San Andreas is what geologists know as a strike-slip fault, meaning that when two pl...
Indicate the pressure sites in the earth's crust; the volcanic sites consistent with the pressure sites in the cortex where there are significant cracks and deep.
...se two tectonic plates generated intense friction and pressure that generated enough heat to melt rocks. The descent of denser oceanic Farallon Plate into the asthenosphere produced magma that was made from basalt or andesite. Which is dark colored rocks with little silica. The buoyant magma pushed through the silica-rich continental crust, partially melting the crust that it moved though, and becoming more granitic in composition. About 100 million years ago, the granitic magma pooled at depths of only 2 to 5 miles beneath the surface (United States of America). The granitic terrain that makes up the Sierra, was once thought to have only local variations but was produced from one large mass of rock. It has been discovered however that hundreds of intrusions caused the variations in the granite that is displayed in Yosemite and in the Sierra Nevada range (Huber).
Concretions are hard compact accumulations of mineral matter and are found inside sedimentary rocks. Some examples of this are Sandstone and in some weathered volcanic rock. Concretions come in many different shapes and the most common of the shapes is spherical or disk shaped. Concretions are the most varied-shaped rocks of the sedimentary world. The way concretions come to be is the mineral matter concentrates around the nucleus of a host rock. The nucleus is often organic such as a tooth or leaf or shell or fossil. As the mineral matter concentrates around the nucleus it forms harder zones known as nodules.
Feldspar and Olivine are two silicate minerals commonly found on the Earth’s surface, their chemical formulas are KAlSi3O8 – NaAlSi3O8 –CaAl2Si2O8 for feldspars and (Mg, Fe)2SiO4 for olivine. Olivine is a nesosilicate, whereas feldspars are a group of tectosilicates, both minerals are anhydrous, since they contain no water in their chemical structure. Feldspars hold differing quantities of different elements, such as potassium, calcium, and sodium; making them classifiable in terms of composition, for instance, NaAlSi3O8 is Albite, while CaAl2Si2O8 is referred to as Anorthite.
It is composed of two elements; oxygen and sulfur. This mineral can either be white in colour, gray, brown, orange, green, red, pink, yellow, beige, or it can be colourless, and the streak that it leaves is white. Based on the Moh’s Hardness scale, Gypsum falls somewhere around the 2 margin. Its lustre is close to that of glass (in properties and in appearance), and is considered vitreous. The cleavage—where it breaks along its line of weakness—is 1,1 – micaceous ; 2,2 and its fracture is uneven. Gypsum is slightly flexible, has low hardness, where it can be scratched by a fingernail, and is sectile (can be cut with a knife). Gypsum is a sedimentary rock, meaning it was formed by cementation at the surface of the Earth near bodies of water; it causes the mineral (gypsum) to settle within the
The field of geology has many different branches. Some of these areas have hardly anything in common. The one thing that they all include, though, is that each one concentrates on some part of the Earth, its makeup, or that of other planets. Mineralogy, the study of minerals above the Earth and in its crust, is different from Petrology, the st...