LATE CENOZOIC UPLIFT OF THE PIKES PEAK STRUCTURAL BLOCK

Pikes Peak is the summit of a composite uplifted block of late Cenozoic age. The base of the block is a large (23x26 mi) platform at the south end of the Front Range. The northeast part of the platform supports a fault-bounded protrusion (10x20 mi) topped by Pikes Peak. A recurrently active (Laramide and later) fault borders the platform to the northeast; elsewhere the platform is bounded by an outward-dipping ramp marked by dissection and by offset of physiographic features and volcanic units. Uplift across the west part of the ramp, from 8,000 ft altitude west of Fourmile Creek to near Cripple Creek is about 2,000 ft; 4,000 ft more took place to the top of Pikes Peak. Asymmetric, intrenched Fourmile Creek canyon west of Cripple Creek, low rim to the west and high rim to the east, emphasizes eastward rise of the ramp. Incised meanders are common in the outer ramp area; tributary canyons with incised meanders east of Fourmile Creek have gradients of 200-500 ft/mi, well above 5-10ft/mi gradients along modern meandering streams in the area. Incised meanders with steep gradients are evident elsewhere around the south margin of the platform. Hilly topography west of the ramp (altitude about 7,500-8,500 ft) is similar to terrain on the platform near Cripple Creek (altitude 9,000-10,000 ft). Oligocene volcanics west of Fourmile Creek lie at about 8,000 ft, and near Cripple Creek at about 9,000-10,000 ft. The high protrusion topped by Pikes Peak rises steeply from the platform, and is marked by a complex mosaic of fault blocks. The mosaic faults are random in trend, but they bound blocks that generally rise toward the peak's summit. The faults mark abrupt scarps that separate blocks capped by relics of early Miocene paleotopography (Steven and Shawe, this volume) we recognize elsewhere in the southern Front Range. Segments of paleostream courses within the fault-block mosaic indicate that erosion progressed along with uplift, and Pikes Peak is capped by a paleotopographic relic, showing that the mountain was formed by uplift (mantle diapirism?), and is not just an eroded stump of a formerly very high mountain.