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Open-File Report 2009–1156

Prepared in cooperation with the Colorado Governor’s Energy Office

High-Resolution Aeromagnetic Survey To Image Shallow Faults, Poncha Springs and Vicinity, Chaffee County, Colorado

By V.J.S. Grauch and Benjamin J. Drenth

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High-resolution aeromagnetic data were acquired over the town of Poncha Springs and areas to the northwest to image faults, especially where they are concealed. Because this area has known hot springs, faults or fault intersections at depth can provide pathways for upward migration of geothermal fluids or concentrate fracturing that enhances permeability. Thus, mapping concealed faults provides a focus for follow-up geothermal studies. Fault interpretation was accomplished by synthesizing interpretative maps derived from several different analytical methods, along with preliminary depth estimates. Faults were interpreted along linear aeromagnetic anomalies and breaks in anomaly patterns. Many linear features correspond to topographic features, such as drainages. A few of these are inferred to be fault-related. The interpreted faults show an overall pattern of criss-crossing fault zones, some of which appear to step over where they cross. Faults mapped by geologists suggest similar crossing patterns in exposed rocks along the mountain front. In low-lying areas, interpreted faults show zones of west-northwest-, north-, and northwest-striking faults that cross ~3 km (~2 mi) west-northwest of the town of Poncha Springs. More easterly striking faults extend east from this juncture. The associated aeromagnetic anomalies are likely caused by magnetic contrasts associated with faulted sediments that are concealed less than 200 m (656 ft) below the valley floor. The faults may involve basement rocks at greater depth as well. A relatively shallow (<300 m or <984 ft), faulted basement block is indicated under basin-fill sediments just north of the hot springs and south of the town of Poncha Springs.

First posted August 18, 2009

For additional information contact:

Team Chief Scientist, USGS Crustal Imaging and Characterization
Box 25046, Mail Stop 964
Denver, CO 80225

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Suggested citation:

Grauch, V.J.S., and Drenth, B.J., 2009, High-resolution aeromagnetic survey to image shallow faults, Poncha Springs and vicinity, Chaffee County, Colorado: U.S. Geological Survey Open-File Report 2009-1156, 31 p.




Geophysical Background

Survey Design

Data Acquisition and Processing

Methods of Analysis


Gradient Window Method

Analysis of Terrain Effects

First Vertical Derivative

Preliminary Depth Estimation


Faults and Linear Geologic Contacts

Anomaly Near Hot Springs



Digital Files

References Cited

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