Flipping through field notebooks…
The Paleozoic strata and Precambrian crystalline rocks of the Matchless Mountain area northeast of Gunnison in Central Colorado, show deformation unique among Laramide structures in Colorado. These structures are part of a northeast-striking belt of deformed crystalline and sedimentary rocks recognizable for 25 miles along strike in the West Elk, and eastern Sawatch Ranges. Field and geometric studies show the area contains at least five southwest verging thrust faults which root into Precambrian rocks, separated by basement cored, shallowly plunging, northeast-trending recumbent folds. Folding shows a roughly east-west gradient with intensity increasing towards the east. Within the overturned limbs of these folds, Paleozoic strata show up to 70% shortening. Microstructural studies show some small cataclastic shear zones and some evidence for pressure solution in the quartzite samples examined. However, the observed mechanisms cannot account for the stratigraphic attenuation. Field relationships indicate that recumbent folding was followed by and probably coeval with thrusting and associated drag folding, followed by extensional faulting. Deformed 72 Ma andesitic porphyries define a probable minimum age for initiation of contraction while cross-cutting Tertiary rhyolite porphyries define a minimum age for deformation. These observations are consistent with the localized involvement of upper Cretaceous Dakota Formation in themore » structurally highest thrust sheet of the area. These data indicate that the rocks of the Matchless Mountain area were complexly deformed by southwest-northeast contraction during Laramide times. The nature of the roughly west to east gradient of increasing deformation indicates that this area is transitional from typical Laramide drape fold deformation to the west.« less
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