|THE PATRIARCH, OR IS IT MATRIARCH?|
Faceted spurs are common, fairly easy to recognize, landforms found along the flanks of mountains in the western U. S. where active, or recently active, faults are present. As the mountain ranges are uplifted, streams cut into the rocks and flow toward master streams located in the down-dropped and adjacent valleys. Ridges are formed between the mountain streams and usually slope gently toward the adjacent valleys. However, if bounding faults are active, a fault scarp (cliff) forms along the edge of the mountains. This scarp has the tendency to truncate the interfluve ridges (spurs) and create a triangular shaped “faceted spur”. Along the west flank of the Catalina Mountains north of Tucson, Arizona, the bounding Pirate Fault has created numerous faceted spurs that are quite evident from Catalina State Park. These triangular features seem common on outcrops of the Tertiary Wilderness Suite Granite.
In south central Colorado, the east side of the San Luis Valley is bounded by the Sangre de Cristo Fault along the western side of the Sangre de Cristo Range. The northern part of the range, usually referred to as the Culebra Range, is separated from the adjacent San Luis Basin by the Crestone Section (a large normal fault = mountains “up” and basin “down”) of the master Sangre de Cristo Fault (Ruleman and Machete, 2007). This fault extends from Poncha Pass, the northern-most boundary of the San Luis Basin, south to the Great Sand Dunes (McCalpin, 1982). The deepest part of the basin seems to lie adjacent to the Crestone Section where there may be 30,000 feet of sediment (Garca and Kind, 1965).
The Culebra Range and the adjacent Crestone Section Fault are characterized by a steep-walled mountain front with well-developed faceted spurs. Many of the fault scarps are developed on very recent late Pleistocene and Holocene alluvial deposits. (McCalpin, 1982, 1986). This would indicate movement along the fault within the last 10,000 years.
McCalpin, J.P., 1982, Quaternary Geology and Neotectonics of the West Flank of the Northern Sangre de Cristo Mountains, South-central Colorado: Colorado School of Mines Quarterly, v. 77, no. 3.
McCalpin, J.P., 1986, Quaternary Tectonics of the Sangre de Cristo and Villa Grove Fault Zones, in Rogers, W.P. and Kirkham, R.M., eds., Contributions to Colorado Seismicity and Tectonics—A 1986 update: Colorado Geological Survey Special Publication 28.
Ruleman, C. and M. N. Machette, 2007, An Overview of the Sangre de Cristo Fault System and New Insights to Interactions Between Quaternary Faults in the Northern Rio Grande Rift in Ruleman, C., M. N. Machette, M-M. Coates, and M. L. Johnson, eds., 2007 Rocky Mountain Section Friends of the Pleistocene Field Trip—Quaternary Geology of the San Luis Basin of Colorado and New Mexico, September 7–9, 2007: U. S. Geological Survey Open-File Report 2007-1193.
|WEST FACE OF SANTA CATALINA MOUNTAINS. THE PIRATE FAULT LIES UNDER THE SEDIMENT AS THE MOUNTAIN FRONT HAS RECEDED. THE FACETED SPURS ARE GEOLOGICALLY OLDER (TERTIARY) THAN SIMILAR FEATURES ALONG THE SANGRE DE CRISTO MOUNTAINS.|
|WEST FACE OF THE SANGRE DE CRISTO MOUNTAINS AT NORTHERN END OF THE SAN LUIS VALLY. MOVEMENT ALONG THE CRESTONE FAULT IS GEOLOGICALLY YOUNGER THAN THE PIRATE FAULT IN ARIZONA.|
|GOOGLE EARTH IMAGE OF SANTA CATALINA MOUNTAINS AND ADJACENT FEATURES.|
|GOOGLE EARTH IMAGE SHOWING EXTREME NORTHERN END OF SAN LUIS BASIN AND ADJACENT FEATURES.|