VOLUME 41 NUMBER 2
Montana transform: A tectonic cam surface linking thin- and thick-skinned Laramide shortening across the Rocky Mountain foreland
James W. Sears
Department of Geosciences, University of Montana, Missoula, MT 59812, U.S.A.
e-mail: james.sears@umontana.edu
The Montana transform is here defined as the northern boundary of the Wyoming Laramide foreland. This sinistral-transpressional shear zone followed a crustal-scale structure that defined the southern margin of the Mesoproterozoic Belt basin of western Montana. The structural zone transferred clockwise tectonic rotation of the thick-skinned Wyoming foreland to clockwise rotation of thin-skinned thrust plates of the Montana thrust belt. Traction above the Farallon plate, which rotated northeastward beneath the Wyoming foreland, may have driven the rotation of the basement-involved Laramide ranges. The ancestral crustal structure that controlled the transform did not coincide exactly with a small circle of rotation about the Euler pole for the Wyoming foreland. Instead, the transform rotated eccentrically, like a cam surface, and generated sufficient sinistral transpression against the Belt Supergroup to thrust it out of its basin.
Key Words: Belt basin • Laramide • Rocky Mountains • tectonic rotation • Wyoming foreland
Proterozoic tectonic evolution of the Sangre de Cristo Mountains, southern Colorado, U.S.A.
James V. Jones, III1,*,*
and James N. Connelly1
1 Department of Geological Sciences, Jackson
School of Geosciences, The University of Texas at Austin, Austin, TX 78712,
U.S.A.
* Correspondence should be addressed to: jonesjv@morris.umn.edu
Field studies and U-Pb geochronology in the Sangre de Cristo Mountains, southern Colorado, provide new constraints on the Proterozoic tectonic evolution of southern Laurentia. Protoliths for basement gneisses and amphibolites were formed in an arc environment and underwent early penetrative deformation and metamorphism (D1 and M1) during formation of the Yavapai province. D1 deformation produced penetrative, subvertical, northwest-striking fabrics (S1) in rocks exposed throughout the range and is interpreted to have occurred during long-lived arc formation and accretion across northwest-striking tectonic boundaries. The ages of D1 and M1 are constrained by a suite of 17501730-Ma calc-alkaline intrusions in the southern part of the range and might have occurred as late as ca. 1710 Ma in the northern part of the range. The northeast-striking tectonic grain that was developed regionally during the culmination of the Yavapai orogeny is not recognized locally. Post-orogenic granitoid plutons were emplaced at 16952 Ma and 16823 Ma, broadly coeval with deposition of locally derived quartzite at the surface. Magmatism and sedimentation during this time are interpreted to represent contemporaneous responses to crustal extension during the ca. 60-m.y. inter-orogenic period between the Yavapai and Mazatzal orogenic events. D2 deformation is interpreted to represent the Mazatzal orogeny locally, and involved northwest-directed shortening and dextral shear localized along subvertical, northeast-trending high-strain zones. D2 was accompanied by amphibolite-facies metamorphism (M2) at 16376 Ma, and the quartzite is inferred to have been deformed during this time. Mesoproterozoic deformation (D3) produced a northeast-striking, subvertical tectonic foliation and localized shear zones between 1420 and 1412 Ma. D3 deformation was bracketed by the emplacement of two newly dated granitic intrusions at 14342 Ma and 14076 Ma. The map-scale geometry of these intrusions and coeval deformational fabrics suggest that ca. 1.4-Ga granites were emplaced into a broadly compressional stress field during subhorizontal northwestsoutheast shortening. These new data and observations indicate that ca. 1.4-Ga granites are not anorogenic, consistent with tectonic models suggesting that widespread magmatism was broadly synchronous with intracontinental orogenesis at ca. 1.4 Ga.
Key Words: Colorado • Laurentia • Proterozoic • Sangre de Cristo Mountains • tectonics • U-Pb geochronology