CURRENT RESEARCH:
Advisor: Dr. Randy Keller

High Resolution Aeromagnetic Survey of the Big Bend National Park Region, Texas


The Big Bend National Park region of west Texas has experienced a complex geologic history, including such events as the Paleozoic Ouachita orogeny, Laramide compression, Tertiary magmatism, and Basin and Range/Rio Grande rift extension. During late fall of 2002, a high resolution aeromagnetic survey was flown over this region for the purposes of improving present geologic maps and developing a better understanding of igneous stratigraphy in the region. Total-field anomaly data was acquired with a 400 meter flight-line spacing and continued to a draped surface at a constant level of 400 meters above the ground surface. Reduction to pole, pseudogravity, and analytic signal filters were applied to the total-field anomaly grid, revealing many features that correlate well with mapped geology and many additional, unmapped features. A set of northeast trending, long wavelength anomalies correlate well with mapped trends of rocks of the Ouachita orogenic belt, especially a magnetic low that correlates with the Ouachita interior zone. A prominent magnetic and pseudogravity high indicates the presence of a large (30 by 25 km) igneous body underlying the Chisos Mountains. Superimposed on this anomaly is a set of smaller anomalies that correlate with mapped locations of the Pine Canyon and Sierra Quemada calderas. Numerous short wavelength anomalies correlate with mapped locations of Tertiary volcanoes and intrusions, plus many similar anomalies indicate the presence of unmapped and buried intrusions. In the Christmas Mountains, a large negative anomaly indicates that a magnetic reversal is recorded in the rocks of this area. Northwest-trending normal faults produced by Tertiary extension produce magnetic anomalies where they cut and offset magnetic rocks of the South Rim and Chisos Formations, most notably within the Terlingua Abaja and Punta de la Sierra fault belts. These faulting trends are enhanced in the analytic signal map, which will allow delineation of unexposed faults by extrapolation from their surface exposures. (Abstract for AGU Fall Meeting, 2003)



reduced to pole total-field magnetic anomalies at 400 meters above the ground surface




Geophysical Signature of the San Juan Mountains batholith, Colorado


One of the largest gravity lows in North America is spatially related to the volcanic San Juan Mountains in southwestern Colorado. The accepted interpretation for this has been the effect of a low density granitic batholith underlying the mountain range's caldera complex. However, there are other effects that probably contribute to the huge gravity low, including the very low densities of caldera fill rocks. This alone puts the validity of 2.67 for a reduction density into question. An even greater effect may be caused by the lack of proper terrain corrections, especially considering that the San Juans have some of the most severe relief on the continent. I will be using a three-dimensional forward modeling program that takes digital topography and variable densities (varying laterally and vertically) into account to model the gravity anomaly more comprehensively than has been done in the past. Also, a new high-resolution aeromagnetic dataset exists for this region. Extraction of longer wavelength magnetic features will provide an additional constraint on the depth extent and geometry of the batholith.

PRESENTATIONS AND PUBLICATIONS:

Drenth, B.J. and Finn, C.A., High Resolution Aeromagnetic Survey of the Big Bend National Park Region, Texas; Eos Trans. AGU, 84(47), Fall Meet. Suppl., Abstract T31E-0875, 2003

Drenth, B.J., Moore, D.E., and Young, C.T., Geophysical Response of the Mission Creek Fault near Desert Hot Springs, California; Symposium on the Application of Geophysics to Engineering and Environmental Problems, San Antonio, Texas; April 2003

Reno, B.L., Drenth, B.J., Miller, K.C., Harder, S., Kaip, G., Results from Shallow Seismic Refraction Surveys across Range-Bounding Faults in the Southern Rio Grande Rift; Eos Trans. AGU, 83(47), Fall Meet. Suppl., Abstract S61C-1147, 2002

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