HR: 17:30h
AN: NS34A-07 INVITED    [Abstracts]
TI: Advances in Modeling Aeromagnetic Anomalies Related to Faults in Sedimentary Basins-- Lessons Learned From High-Resolution Surveys in the Central Rio Grande Rift, New Mexico
AU: * Grauch, V
EM: tien@usgs.gov
AF: U.S. Geological Survey, MS 964, Denver Federal Center, Denver, CO 80225-0046, United States
AU: Hudson, M R
EM: mhudson@usgs.gov
AF: U.S. Geological Survey, MS 980, Denver Federal Center, Denver, CO 80225-0046, United States
AB: High-resolution aeromagnetic data acquired over several basins in the central Rio Grande rift, north-central New Mexico, prominently display low-amplitude (5-15 nT), linear anomalies associated with faults that offset basin-fill sediments. The linear anomalies give an unprecedented view of concealed faults, with significant implications for understanding structure and ground-water flow in the top 500 m of sedimentary basins in general. These implications provide the impetus for understanding the sources of the anomalies in greater detail, using the central Rio Grande rift as a case study. Based on geophysical analysis combined with detailed magnetic- property studies, we have established that linear anomalies in the rift can be explained entirely by the tectonic juxtaposition of magnetically differing strata rather than by chemical processes focused at the fault zone. At the scale of ground profiles, tectonic juxtaposition can be adequately modeled by traditional step-like or offset-layers models. However, at a basin-wide scale typical of aeromagnetic surveys, geophysical models of multiple layers stacked on both sides of the fault are more appropriate. Computed curves from these models are complexly determined by differences in magnetic contrasts, stratal thicknesses, and depth to the strata. To better understand the primary geophysical parameters that drive the variability, we translate models of juxtaposed strata into equivalent geophysical models based on lateral magnetic contrast. The equivalent models more simply represent the juxtaposed strata as stacks of magnetic-contrast layers that alternate across the fault with depth. These models can be more easily compared to four end-member models consisting of only one or two truncated layers with variable magnetic-contrast and layer thickness. The end-member models can explain the observed anomaly variability. In particular, they can explain the presence of aeromagnetic lows over a fault zone, a counterintuitive situation that may previously have been mistaken as the expression of altered rocks.
DE: 0925 Magnetic and electrical methods (5109)
DE: 1517 Magnetic anomalies: modeling and interpretation
DE: 1540 Rock and mineral magnetism
DE: 8010 Fractures and faults
SC: Near-Surface Geophysics [NS]
MN: 2007 Fall Meeting