Alan Levander
Department Chair and Carey Croneis Professor

Research:

Lithospheric Seismology and Wave Propagation

My research has largely been guided by two interests: seismic wave propagation and imaging, and understanding the formation of mountain belts. These two intellectual interests complement each other nicely in the field of lithosphere seismology. All aspects of seismology are important to understanding the evolution of mountain belts, but the images from deep crustal reflection seismology have profoundly changed the way we view the formation of mountain belts. The early images from deep crustal reflection seismology showed both the large scale horizontal transport of crustal packages in mountain belts, and highly complicated lower crustal reflections in many tectonics environments, particularly regions of orogenic collapse. To understand the source of deep crustal seismic signatures my colleagues and I have gone from a few simple numerical experiments, to rather elaborate numerical models of seismic velocity distributions which are based on mapped exposures of deep crustal terranes and petrophysical data. My research has made contributions to seismic acquisition methods, seismic modeling, and seismic imaging. All of these areas have a large number of scientists actively engaged in research, and much of my research frequently has been done in parallel and in collaboration with others efforts.

In seismic acquisition my research group was among the first to develop dense onshore-offshore profiling as a standard seismic acquisition technique; a now standard acquisition method in continental margin studies (Levander et al., 1987; Lafond and Levander, 1995, Henstock et al, 1997). The Brooks Range seismic investigation, conducted with the crustal seismology group at the USGS, merged reflection and refraction seismology into a single acquisition method. (Levander et al., 1994, Fuis et al., 1995; Wissinger et al 1997, 1998), a technique that has been widely used in other field studies (e.g., Mendocino, LARSE I and II, New Zealand).

To complement the combined reflection/refraction datasets we have acquired, my students, postdocs and I have developed processing methods for these data, in particular for migrating wide-aperture seismic data. This extends the imaging methods developed for near-vertical incidence to the far more complicated ray geometries that arise for large source-receiver offsets in wide-angle seismic investigations. (Lafond and Levander, 1993, 1995; Henstock et al., 1997; Henstock and Levander 2000). Interestingly this imaging method also appears to be very effective for very shallow seismic data acquired for environmental investigations, and lately we have been applying and extending the techniques developed for crustal exploration to investigating the upper few 10's of meters of the earth.

My colleagues and I have developed outcrop based stochastic models of crustal heterogeneity for predicting the seismic response of complex geologic structures. (Holliger and Levander, 1992; Holliger et al. 1993, Levander et al, 1994a, 1994b; Holliger and Levander 1994; Lendl et al, 1997). These models capture both the statistical properties of outcrop data and when translated to seismic models capture the statistical properties of deep crustal seismic data.

In specific seismic acquisition campaigns I have been involved in large projects that have contributed to the understanding of the structures and processes operative in the central, southern and northern San Andreas transform plate boundary zone, the Brooks Range fold and thrust belt, and the western United States orogenic plateau.

In the San Andreas transform we identified and described the importance of remnant fragements of the Farallon plate as a part of the structural architecture of the margin (Meltzer and Levander, 1991; Henrys et al., 1993; Lafond and Levander, 1995). We have observed whole crustal strike slip faulting in both central California at the Hosgri fault (a proto-San Andrea) and near the Mendocino Triple Junction at the present San Andreas fault (Henstock et al., 1997; Henstock and Levander, 2000). The seismology group at Caltech has subsequently identified whole crustal strike slip faulting on the San Andreas between the San Gabriel Mountains and the Mojave. This observation places severe limits on models of tectonically resetting the Moho and on the rates of lower crustal flow and Moho topography.

The Brooks Range seismic experiment images demonstrated whole crustal detachment faulting, whole crustal duplexing, and lower crustal subduction beneath an intact Rocky Mountain type fold and thrust belt. (i.e., unlike in the conterminous US the central Brooks Range has not experienced much post-orogenic extension, and is therefore intact). The observation of detachment faults reaching the Moho supports models of orogenic float in which the entire crustal column is transported during orogenesis. Rthe observation of lower crustal subduction beneath the Brooks Range provides one of the few direct images of lower crustal delamination and recycling to the mantle within the world's orogenic belts.

The Deep Probe experiment examined the large scale lithospheric differences between three provinces: The Proterozoic terranes of the US southwest that have been involved in Phanerozoic plate boundary tectonics, the Archean Wyoming province which has been tectonically altered by the Laramide orogeny, and the Hearne province, a stable Archean province. The large scale differences are profound: The Proterozoic terranes are underlain by tectonically active (i.e. "Cenozoic") mantle, whereas both of the Archean terranes appear to be underlain by tectospheric (i.e. "Archean") mantle. The transition from one mantle type to another is abrupt, occurring over a distance of 150km. The large scale crustal structure reflects more the original accretion of the continent than modern tectonism, underscoring the overriding importance of mantle control on surface tectonics.

The CDROM project (Continental Dynamics of the Rocky Mountains) is a large scale geologic and geophysical investigation that builds on the results of the Deep Probe experiment. Coordinating the efforts of 20 investigators at 14 institutions, the CDROM project, led by Karl Karlstrom at the University of New Mexico, combines seismic, structural geology, geothermometry, geobarometry, geochronology, and tectonic geomorphology studies to investigate the Proterozoic accretion of the terranes of the southwestern US to the North American continent, and to understand the Cenozoic-Mesozoic evolution of the Rocky Mountains. My part of the project is reflection seismic investigations of the Jemez Lineament, a zone of Cenozoic magmatism that appears to coincide with one of the ancient accretionary sutures.

Selected Publications

S.P.S. Gulick, A.S. Meltzer, T.J. Henstock, and A. Levander, "Internal deformation of the southern Gorda plate: Fragmentation of a weak plate at the Mendocino triple junction," Geology, 29 (2001): 691-694.

T.J. Henstock and A. Levander, "Lithospheric evolution in the wake of the Mendocino Triple Junction: Structure of the San Andreas Fault system at 2 Ma," Geophysical Journal International, 140 (2000): 233-247.

T.J. Henstock and A. Levander, "Impact of a complex overburden on analysis of bright reflections: A case study from the Mendocino Triple Junction," Journal of Geophysical Research, 105 (2000): 21,711-21726.

Levander, A., E.D. Humphreys, G. Ekstrom, A.S. Meltzer, P.M. Shearer, "Continental Assembly, Stability, and Instability: USArray: An Earth sciences tool for investigating North America," EOS, 80: 245-249.

Henstock, T.J., and A. Levander, "Lithospheric evolution in the wake of the Mendocino Triple Junction: Structure of the San Andreas Fault system at 2 Ma," Geophysical Journal International, 140 (2000): 233-247.

A. Levander, T.J. Henstock, A.S. Meltzer, B.C. Beaudoin, A.M. Trehu, and S.L. Klemperer, "Fluids in the lower crust following Mendocino Triple Junction migration: Active basaltic intrusion? ," Geology, 26 (1998): 171-174.

E.S. Wissinger, A.R. Levander, J.S. Oldow, G.S. Fuis, and W.J. Lutter, "Seismic profiling constraints on the evolution of the central Brooks Range, Arctic Alaska, in Architecture of the Central Brooks Range Fold and Thrust Belt," GSA Special Paper 324, J.S. Oldow and H.G. Ave Lallemant editors, (1998): 269-292.

T.J. Henstock, A. Levander, and the Deep Probe Working Group, "Probing the Archean and Proterozoic lithosphere of western North America," GSA Today, 8 (1998): 1-5, 16-17.

C.M. Snelson, T.J. Henstock, G.R. Keller, K.M. Miller, and A. Levander, "Crust and uppermost mantle structure along the Deep Probe seismic profile," Rocky Mountain Geology, 33 (1998): 181-198.

Robertsson, J.O.A., K. Holliger, and A. Levander, "Simulation of a deep ocean scattering experiment (Test Case No. 6) using HARVEST - A hybrid wave propagation technique," , Benchmark Solutions in Reverberation and Scattering: Proceedings of the Reverberation and Scattering Workshop, (1997). D.B. King, S.A. Chin-Bing, R.B. Evans, J.A. Davis, editors, U.S. Government Printing Office

Akerberg, P., D. Dana, A. Levander, T. Henstock, and C.A. Zelt, "High resolution reflection surveying in an open pit copper mine, Tyrone, New Mexico," , Geophysics, (1999). submitted 8 March 1999

Henstock, T.J., and A. Levander, "Imaging and analysis of melt in the lower crust of the San Andreas Fault system at 2 Ma," , J. Geophys. Res., (1999). submitted 27 April 1999

Ekstrom, G., E.D. Humphreys, and A. Levander, "USArray - a tool for probing the continent," , IRIS Newsletter, v.16, n.2, 2 & 4-6 (1999).

Orcutt, J.A., R. Detrick, F. Duennebier, A. Levander, P. Stoffa, A. Trehu, "The future of ocean bottom seismology," , National Science Foundation Report published by the Scripps Institution of Oceanography, Series 99-3, 55 pages (1999).

USArray Steering Committee, "The USArray Initiative," , GSA Today, v.9, pgs. 8-10, (1999).

USArray Steering Committee, "The USArray Initiative," , GSA Today, v.9, pgs. 8-10, (1999).

Pending Publications

L. Nielsen, H. Thybo, A. Levander, and A.V. Egorkin, "Origin of Upper Mantle Seismic Scattering - Evidence From Russian PNE Data," Geophysical Journal International, (in press).

I.B. Morozov and A. Levander, "Depth image focusing in travel-time map based wide-angle migration," Geophysics, (accepted).

A.R. Gorman, R.M. Clowes, R.M. Ellis, T.J. Henstock, G.D. Spence, G.R. Keller, A. Levander, C.M. Snelson, M.J.A. Burianyk, E.R. Kanasewich, I. Asudeh, Z. Hajnal, and K.C. Miller, "Deep Probe: Imaging the roots of western North America," Canadian Journal of Earth Sciences, (in press).

T.J. Henstock and A. Levander, "Structure and Seismotectonics of the Mendocino Triple Junction," Journal of Geophysical Research, (submitted).

A. Levander, A.S. Meltzer, T.J. Henstock, and S.P.S. Gulick, "Deformation, mass transfer, and crustal recycling at the Mendocino Triple Junction: What Gorda give up North America receives," Tectonics, (submitted).