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Origin of Rocky Mountain-Colorado Plateau

For the sake of analysis, the structures studied (fault systems resulting from inversion processes)have been classified into two categories on the basis of their orientation(west to northwest and north to northeast). The direction of alignment is important in geologic analysis just like it is in geophysical surveys that could be targeting a specific strike during exploration. It is vital to note that the plateau of Colorado, Midcontinent, Rocky Mountains and generally the regions of cratonic platform in North America have their Phanerozoic deformation patterns and styles identically displayed.

The accuracy of the model of analysis predicts that Ancestral Rockies patterns and those of Laramide resulting from contraction processes are a replica of the trends drawn from extensional faults of the Proterozoic while on the other hand, it also suggests that during inversion, antecedent fault dips controlled the fold geometry resulting from fault propagation as it is evident from the regional variation in forced-fold vergence. Late Proterozoic rifts pronounced all over Rodinia is a critical indicator of inversion tectonics as the most probable occurrence in all cratonic platforms worldwide.

The structural relief of the fore mentioned features and their extent of exposure are noted to differ drastically but with a great similarity in their regional orientation and structural style (Marshak p. 151–154 and; McBride275–286). This is then classified as a process of intractonic deformation with a major focus on Rocky Mountains and the Colorado plateau. It is argued that during the Proterozoic, the assembly and breakup of supercontinents resulted into inversion processes forming extensional faults that are now reflected by both Laramide and Ancestral Rockies deformation.

(Marshak p. 151–154). The Rocky Mountain-Colorado Plateau province were majorly influenced by two Phanerozoic deformation events with the basins and basement-cored uplifts (both found in the region occupied by the present day Rocky Mountains and the Mid-continent) created by the late Paleozoic Ancestral Rockies events. (McBride p. 275–286) The concern basins and uplifts are a representative of responses to stress from collisional orogeny by the craton. (Kluth p. 10–15 ).

Faults: A Rift-Inversion Model The Rocky Mountains-Colorado Plateau province has a well defined geometry and an elaborate kinematics of Phanerozoic faulting. Despite this fact, the origin of these faults having been associated with the rupturing of the once stable crust remains a mystery. In an attempt to unravel this mystery, two theories have been developed; 1) They must have developed from compressional forces experienced during the Laramide and the Ancestral Rockies events

2) The origin of the feature is also suggested to have occurred prior to the Ancestral Rockies and Laramide events a process initiated by reactivation activities resulting from the movements (Davids, p. 215-233) Basing our argument on the second theory, it is further proposed that weak faults found on the cratonic platform in the Rocky mountain-Colorado Plateau province, also resemble those of the Mid-continent, and were formed by crustal rupturing during Proterozoic rifting. (Marshak p. 151–154 )

Thus, the Phanerozoic movement on faults in these features Reverse movement or the oblique-slip movement are generated from the reactivated faults resulting in the inversion of normal faults. During inversion, it is the normal faults that are targeted as the major centers of weaknesses that permit slip under low differential stress when the crust undergoes later shortening. (Cooper p. 375) The critical stress necessary to initiate sliding on the preexisting faults is significantly less than that needed to form new faults in compact rock.

This is partly attributed to the fact that frictional resistance is generally less than the shear rupture strength under the same pressure conditions and confinement. (Etheridge p. 179–194). It is also a fact that alteration in fault zones does produce weak rocks. (Holdsworth p. 73–78). The presence of preexisting fractures provides a reservoir for ground water and this may become over pressured during compression, thereby decreasing the effective normal stress holding opposing walls of the fault together. Works Cited:

Cooper, M. A. , and Williams, G. D. Inversion Tectonics: 1989, Geological Survey: London, Special Publication 44, Marshak, S. , and Paulsen, T. , “Mid-continent U. S. fault and fold zones”: A legacy of Proterozoic intracratonic extensional tectonism: Geology, 1996, v. 24 Davis, G. H,” Monocline fold pattern of the Colorado Plateau, in Matthews, V, III, ed, Laramide folding associated with basement block faulting in the western United States”: Geological Society of America Memoir , 1978 ,151, McBride, J. H. , and Nelson, W. J., Style and origin of mid-Carboniferous deformation in the Illinois Basin.

USA-Ancestral Rockies deformation tectonophysics: 1999, v. 305 Kluth, D. F. , and Coney, P. J. , Plate tectonics of the Ancestral Rocky Mountains: 1981, Geology, v. 9 Holdsworth. R. E, Butler, C. A, and Roberts, A. M, The recognition of reactivation during continental deformation: 1997, Geological Society (London) Journal, v. 154 Etheridge, M. A. On the reactivation of extensional fault systems: 1986, Royal Society of London Philosophical Transactions, ser. A, v. 317

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