Sedimentary strata and layers in metamorphic and igneous rocks with different co
ID: 105262 • Letter: S
Question
Sedimentary strata and layers in metamorphic and igneous rocks with different compositions can have differing degrees of stiffness (competence) or fluidity (incompetence). Subject to extension, [] parts can be pulled apart into what are called [] while the relatively [] parts flow to fill the otherwise gaps between. Orogenesis due to crustal compression produces folds and reverse faults. The geometry of folds is described by: the inclination of the axial plain that passes though the [] (hinge line) of each flexed layer in the fold. the dip of the layers called the limbs of the fold. The limbs arc symmetrical in their spread to either side of the axial plain. With a vertical axial plane, the fold is symmetrical, and with a decreasing axial-plane dip varies through asymmetrical, overturned to []. The limb spread can be open (undulating) to closed (isometric). the fold-axis plunge (compass bearing and angular inclination in the same direction measured down from the horizontal). The folds can be describes as open (limbs are wide in their spread) to closed (limbs are parallel). Continental passive margins accumulate sedimentary strata with an overall wedge shape. Under regional compression these weakly lithified and water-logged sedimentary strata and any included volcanics deform under conditions of low grade metamophism that produces [] and metavolcanics. Tectonic structures that develop are thrust faults that slice the whole into fault-separated layers that override each other and which due to frictional drag and compression, have folds within that involve buckling and bending of competent layers and passive folds in incompetant layers. As compressional orogenesis thickens the continental crust, the lower part of the resulting mountain belt is deeply buried and there high-grade regional metamorphism produces [] metamorphic rocks such as schists. In these faults are few and deformation is elasticoviscous. Where temperatures here become elevated by the intrusion of magmas, the country rock is ductile and deformation is by passive folding where layers in gneisses are simply passive markers and do not influence the style of the folds. Continental crust that has not undergone orogeny for a long time has a thickness of about 40 km. The continental crust during orogenesis is greatly thickened. The base of orogenically thickened continental rock can be to a depth of 70 km. Following orogenesis, the region involved undergoes slow uplift due to the erosion of the upper part and [] rise of the mountain root. The last orogeny to build the Appalachian fold-mountain belt ended 250 million years ago. Since then erosion has exposed the cores of folds that formed at a depth of 35 km. Continent-wide, vertical isostatic adjustments and flexings of the otherwise stable crust (craton) is called epeirogeny. The present day relief of the Appalachians is the result of [] erosion (crosional etching) during the last 20 million years of a broadly uparched erosion surface called the Schooley Peneplain (that today has zero elevation at the coast and rises gradually westward to 3500 feet at its crest). Ridges in the present day scenery are underlain by erosionally resistant quartz [] while the valleys have been scooped-out of mechanically weak shale and chemically soluble limestone. axis boudins competent differential foliated incompetent isostatic metasediments recumbent sandstoneExplanation / Answer
1. Competent
2. Boudins
3. Incompetent
4. axis
5. recumbent
6. Metasediments
7 foliated
8. isostatic
9 differential
10. sandstone