Accessing The Tectonics Of Arkansas River Region, Colorad, USA Using Fia Measurement

Five lengthy periods involving multiple phases of cordierite and andalusite growth wererevealed by detailed studies of foliation inflection/intersection axes (FIA) preserved inporphyroblasts in schists from the Arkansas River region in Colorado, USA. The regionallyconsistent character of the succession of five different FIA trends enabled the relative timingof each FIA with respect to the next to be determined. The F1A succession from first to last is:FIA 1 trending W-E, FIA 2 trending SW-NE, FIA 3 trending NNW-SSE, FIA 4 trendingNW-SE and FIA 5 trending SW-NE. For four of the FIA sets, samples were found containingmonazite grains preserved as inclusions. These were dated on an electron microprobe. Theages obtained concur exactly with the FIA succession, with FIA 1 at 1506±15 Ma, FIA 2 at1467±23Ma, FIA3 at 1425±18 Ma, FIA4 not dated and FIA5 at 1366±20 Ma. These agesare directly reflected in a succession of plutons in the surrounding region dated by otherisotopic approaches, suggesting that deformation, metamorphism and pluton emplacementoccurred together episodically, but effectively continuously, for some 140 million years.Whether isoclinal folds originate as antiforms or synforms in steeply-dippingmultiply-deformed high metamorphic grade gneisses and schists is commonly difficult toascertain because sub-vertical stretching and high strain rotates fold axes towards the vertical.A succession of five FIAs (foliation intersection/inflection axes preserved withinporphyroblasts) trending SSW-NNE, SW-NE, W-E, NW-SE and NNW-SSE in cordieriteschist units interlayered with gneisses in Arkansas River region of South Central Coloradoreveal the timing and origin this fold. The asymmetry of curvature of the extremities offoliations preserved as inclusion trails within porphyroblasts, from gentle to steep pitches insections near orthogonal to the associated FIA, reveal that a steeply plunging isoclinal fold,which was originally interpreted to be a synform, formed as an antiform. It developed duringthe first stages of porpbyroblast growth. This approach can be applied to all similar rocks andcan be used to resolve stratigraphic successions in rocks containing no younging criteria.Fold axial plane traces and schistosity in the high temperature-low pressure regimePreCambrian rocks in Arkansas River region preserve the effects of a succession of 5 FIAtrends (foliation intersection or inflection axes in porphyroblasts). Similar fold axial planetrace 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 the5 sets that developed in this region. The other three have no equivalent fold axial planes. Thisdifference from shallow to deeper orogenic regimes reflects decreasing competency at greaterpressure with collapse and unfolding of earlier formed folds. The greater overlying load ofrocks tends to flatten folds preserving only those most recently developed or the foldsA similar succession of foliation inflection/intersection axis trends preserved withinporphyroblasts (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 bydeformation and metamorphism from approximately 1506 to 1370 Ma. A succession of fiveFIAs trending W-E, SSW-NNE, NNW-SSE, NW-SE and WSW-ENE has been distinguished inCentral Colorado that have been dated at 1506±15Ma, 1467±23Ma, 1425±18 Ma, not datedand 1366±20 Ma, respectively. To the south in Northern New Mexico, a succession of fiveFIAs trending SSW-NNE, WNW-ESE, NNW-SSE, NW-SE and WSW-ENE have beendistinguished dated at 1482±48Ma, 1448±12Ma, 1422±35Ma, not dated and 1394±22Ma.Correlation of these FIA succession, trends and ages reveals a six fold FIA succession acrossthe region with the first developed FIA not being present in Northern New Mexico and thethird not being present in Central Colorado. Preferential partitioning of W-E trendingdeformation into the Central Colorado region around 1506±15Ma was followed by SSW-NNEtrending deformation that affected both regions at 1470±20 Ma. However, preferentialpartitioning of WNW-ESE trending deformation into Northern New Mexico at 1449±12 Maleft Central Colorado unaffected. Both regions were then affected by the three remainingperiods of orogenesis, the first trending NNW-SSE at 1436±11 Ma followed by one trendingNW-SE that has not yet been dated, and then one trending WSW-ENE at 1390±19 Ma. Thissuggests that the Yavapai terrain was tectonized at approximately 1506 Ma, prior toamalgamation with the Mazatzal terrain around 1470 Ma. Subsequent orogenesis was initiallypartitioned preferentially into the Mazatzal terrain, but the following three periods oftectonism affected both terrains in a similar manner.