Cao, Hui (2011) The tectonics of the Arkansas river region, Colorado, USA. PhD thesis, James Cook University.

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Abstract

[Section A] Five lengthy periods involving multiple phases of cordierite and andalusite growth were revealed by detailed studies of foliation inflection/intersection axes (FIA) preserved in porphyroblasts in schists from the Arkansas River region in Colorado, USA. The regionally consistent character of the succession of five different FIA trends enabled the relative timing of each FIA with respect to the next to be determined. The FIA succession from first to last is: FIA 1 trending W-E, FIA 2 trending SW-NE, FIA 3 trending NNW-SSE, FIA 4 trending NW-SE and FIA 5 trending SW-NE. For four of the FIA sets, samples were found containing monazite grains preserved as inclusions. These were dated on an electron microprobe. The ages obtained concur exactly with the FIA succession, with FIA 1 at 1506 ± 15 Ma, FIA 2 at 1467 ± 23Ma, FIA 3 at 1425 ± 18 Ma, FIA 4 not dated and FIA 5 at 1366 ± 20 Ma. These ages are directly reflected in a succession of plutons in the surrounding region dated by other isotopic approaches, suggesting that deformation, metamorphism and pluton emplacement occurred together episodically, but effectively continuously, for some 140 million years.

[Section B] Whether isoclinal folds originate as antiforms or synforms in steeply-dipping multiply-deformed high metamorphic grade gneisses and schists is commonly difficult to ascertain because sub-vertical stretching and high strain rotates fold axes towards the vertical. A succession of five FIAs (foliation intersection/inflection axes preserved within porphyroblasts) trending SSW-NNE, SW-NE, W-E, NW-SE and NNW-SSE in cordierite schist units interlayered with gneisses in the Arkansas River region of South Central Colorado reveal the timing and origin this fold. The asymmetry of curvature of the extremities of foliations preserved as inclusion trails within porphyroblasts, from gentle to steep pitches in sections near orthogonal to the associated FIA trend, reveal that a steeply plunging isoclinal fold, which was originally interpreted to be a synform, formed as an antiform. It developed during the first stages of porphyroblast growth. This approach can be applied to all similar rocks and can be used to resolve stratigraphic successions in rocks containing no younging criteria.

[Section C] Fold axial plane traces and schistosity in high temperature-low pressure regime PreCambrian rocks in the Arkansas River region preserve the effects of a succession of 5 FIA trends (foliation intersection or inflection axes in porphyroblasts). Similar fold axial plane trace measurements in high temperature-medium pressure regime Palaeozoic rocks in Vermont preserve the effects of only the youngest two NNW-SSE and NNE-SSW trending FIAs of the 5 sets that developed in this region. The other three have no equivalent fold axial planes. This difference from shallow to deeper orogenic regimes reflects decreasing competency at greater pressure with collapse and unfolding of earlier formed folds. The greater overlying load of rocks has tended to flatten all but the very largest early-formed structures, preserving only those folds that were more recently developed.

[Section D] A similar succession of Foliation Inflection/Intersection Axis trends preserved within porphyroblasts (FIAs) is present in two areas separated by 200 km along the Rocky Mountains. PreCambrian rocks in Central Colorado and Northern New Mexico were affected by deformation and metamorphism from approximately 1506 to 1370 Ma. A succession of five FIAs trending W-E, SSW-NNE, NNW-SSE, NW-SE and WSW-ENE has been distinguished in Central Colorado that have been dated at 1506±15Ma, 1467±23Ma, 1425±18 Ma, not dated and 1366±20 Ma, respectively. To the south in Northern New Mexico, a succession of five FIAs trending SSW-NNE, WNW-ESE, NNW-SSE, NW-SE and WSW-ENE have been distinguished dated at 1482±48Ma, 1448±12Ma, 1422±35Ma, not dated and 1394±22Ma. The excellent correlation of the sequence of FIA trends and their ages between regions reveals a six fold FIA succession across the region with the first developed FIA not being present in Northern New Mexico and the third not being present in Central Colorado. Preferential partitioning of W-E trending deformation into the Central Colorado region around 1506±15Ma was followed by SSW-NNE trending deformation that affected both regions at 1470±20 Ma. However, preferential partitioning of WNW-ESE trending deformation into Northern New Mexico at 1448±12 Ma left Central Colorado unaffected. Both regions were then affected by the three remaining periods of orogenesis, the first trending NNW-SSE at 1424±15 Ma followed by one trending NW-SE that has not yet been dated, and then one trending WSW-ENE at 1390±19 Ma. This suggests that the Yavapai terrain was tectonized at approximately 1506 Ma, prior to amalgamation with the Mazatzal terrain around 1470 Ma. Subsequent orogenesis was initially partitioned preferentially into the Mazatzal terrain, but the following three periods of tectonism affected both terrains in a similar manner.

Item ID:	23850
Item Type:	Thesis (PhD)
Keywords:	antiform development; Arkansas River region; Arkansas River Valley; Central Colorado; chemical U-Th-Pb monazite dating; deformation; FIAs; fold axial planes; folding; foliation inflection / intersection axes; foliations; geochronology; geotechtonics; inclusion trails; monazite dating; orogenesis; orogenic belts; orogenic regimes; orogens; partitioning; porphyroblasts; radioactive dating; radiodating; radioisotopic dating; radiometric dating; rock deformation; schists; stratigraphy; tectonic terrains; tectonics; tectonism; uranium-thorium-lead dating
Related URLs:	http://eprints.jcu.edu.au/17210/ http://eprints.jcu.edu.au/8754/
Additional Information:	Publications arising from this thesis are available from the Related URLs field. The publications are: Cao, Hui (2010) Multiple successions of foliation development in shallow versus deep orogenic regimes and their relationship to regional folding. Proceedings of 10th International Multidisciplinary Scientific Geoconference SGEM 2010 10th International Multidisciplinary Scientific Geoconference. , 20-26 June 2010, Albena, Bulgaria. Cao, Hui (2009) Chemical U-Th-Pb monazite dating of deformations versus pluton emplacement and the proterozoic history of the Arkansas River region, Colorado, USA. Acta Geologica Sinica, 83 (5). pp. 917-926.
Date Deposited:	07 Nov 2012 04:02
FoR Codes:	04 EARTH SCIENCES > 0403 Geology > 040312 Structural Geology @ 33% 04 EARTH SCIENCES > 0403 Geology > 040313 Tectonics @ 33% 04 EARTH SCIENCES > 0403 Geology > 040303 Geochronology @ 34%
SEO Codes:	84 MINERAL RESOURCES (excl. Energy Resources) > 8401 Mineral Exploration > 840199 Mineral Exploration not elsewhere classified @ 100%
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