Details

Autor(en) / Beteiligte

• Litak, R. K
• Marchant, R. H
• Brown, L. D
• Pfiffner, O. A
• Hauser, E. C
• Meissner, Rolf
• Brown, Larry
• Franke, Wolfgang
• Seifert, Friedrich
• Dürbaum, Hans‐Jürgen
• Fuchs, Karl

Titel

Correlating Crustal Reflections with Geologic Outcrops: Seismic Modeling Results from the Southwestern USA and the Swiss Alps

Ist Teil von

• Continental Lithosphere: Deep Seismic Reflections, 1991, p.299-305

Ort / Verlag

Washington, D. C: American Geophysical Union

Erscheinungsjahr

1991

Link zum Volltext

Quelle

Wiley Online Library All Obooks

Beschreibungen/Notizen

• Outcropping geology can often provide direct and detailed information concerning the relationship of seismic reflections to specific geologic features, a crucial but difficult aspect of deep seismic interpretation. In this study we present two very different examples in which fortuitous geologic circumstances enable us to anticipate the nature and geometry of reflectors to substantial depths. The approach used is to construct geologic models based on surface mapping, for which synthetic seismograms are calculated and then compared to nearby deep seismic lines. In these cases, the geologic features observed at the surface lead to reflection patterns quite similar to those actually observed on the data. In the Buck Mts. of Arizona, southwestern USA, rotation above a Tertiary detachment fault has exposed an upper crustal block which now represents a virtual cross‐section of pre‐Tertiary crust. Seismic modeling based on this exposure reveals that the numerous (originally) subhorizontal Proterozoic diabase intrusions observed in the Buck Mts. generate strong synthetic reflections that are very similar in character, continuity, and distribution to the anomalous Bagdad Reflection Sequence (BRS) imaged by nearby COCORP and industry seismic lines. We infer that the BRS, which permeates the upper crust for distances of some 100 km and corresponds to depths of up to 15 km, is due to a series of subhorizontal mafic intrusions similar to those in the Buck Mts. Another area where surface geology is especially helpful in interpreting deep reflections is in the Penninic Alps of eastern Switzerland, where the combination of relatively steep dips, high relief, good exposures, and extensive fieldwork has permitted the construction of contour maps of several Penninic nappe surfaces. Extrapolating these contours to depth enables construction of a 3‐D model of the upper crust beneath the south end of Swiss seismic line NFP20‐East. Results of raytracing indicate that interfaces between nappe surfaces and metasedimentary cover rocks can account for most reflections in the upper six seconds, thereby constraining the 3‐D structure of the upper crust in this area. In both examples, reflections can be explained as caused by simple lithologic contrasts: between mafic and silicic rocks (in Arizona), and between silicic basement, metasediments, and ophiolites (in Switzerland). These results emphasize the importance of lithologie heterogeneity in producing deep crustal reflections, and provide possible analogs for reflection patterns in areas where geologic control is less certain.