GSA Medals & Awards

Arthur L. Day Medal

Mary Lou Zoback
Mary Lou Zoback
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Presented to Mary Lou Zoback

 Citation by Richard G. Gordon

Mary Lou Zoback has been exemplary in contributing to geologic knowledge through the creative application of physics to the solution of geologic problems. She is a rare individual who is at home both mapping in the field (notably in the Basin and Range province) and in understanding and elucidating the subtle relationships between buoyancy and the state of stress of the lithosphere.

Mary Lou’s early work on rifting in the Basin and Range province is widely recognized and highly cited, as is her ensuing work on the structure and evolution of the Colorado Plateau (Zoback & Thompson, 1978, 1979; Zoback et al., 1981). An important contribution of that early work was the recognition that the Pliocene and younger orientation (principal extensional incremental strain) of Basin and Range extensional deformation is rotated clockwise from the mid-Miocene orientation.

While this work was still in progress, she began the work that firmly established her international stature in geoscience, that of elucidating the state of stress first of the conterminous U.S. (Zoback & M.D. Zoback, 1980, 1981; M.D. Zoback et al., 1987; Zoback 1989) and eventually of the world (Zoback et al. 1989, Zoback 1992, Muller et al. 1992). Mary Lou Zoback has been the leader, a vital intellectual force, and the effective driving force behind the development of the world stress map. It is an exaggeration—but not that great of one—to claim that Mary Lou and colleagues developed an essential observational data set, the world stress field, which did not exist before they began their work. Reproductions of maps from that work now populate more textbooks than I can begin to count. It is impossible to go to a geoscience meeting without seeing talks that draw from it. The papers listed above have certainly not gone unnoticed by the community. The least cited of them has more than 90 citations, and the most cited are classics with many hundreds of citations.

There are many contributions made through Mary Lou’s U.S. and world stress maps including the following. First, she and colleagues showed that different types of indicators of stress orientation were consistent where they overlapped spatially and temporally, thus allowing many types of data to be combined in defining the world stress field. Second, they showed that stress orientations were regionally consistent, which permitted the definition of broad-scale regional stress patterns. Third, they showed that first-order midplate stress fields are largely the result of compressional forces (ridge push and continental collision) applied at plate boundaries, and that there is no evidence of the lateral stress gradients that would be expected if plates were driven or resisted significantly from below. Fourth, they showed that regions of deviatoric tensional stress are generally associated with high topography, as in the western U.S. Cordillera, the Andes, and the Tibetan plateau, and thus related to buoyancy.

Yet another profound contribution comes from a landmark paper that grew from the work in which her husband Mark is first author and she second-author (M.D. Zoback et al. 1989). In that paper, the Zobacks and colleagues showed that the principal compressive stress is nearly orthogonal to the San Andreas fault, with the implication that the fault is much weaker than widely supposed. This remarkable and surprising result has stimulated an enormous amount of thought and observation that still continues.

Mary Lou’s latest installment on understanding the pattern of world stress uses the USGS global data base of crustal structure determinations as well as surface heat-flow measurements to infer the thickness of the mantle lid. She infers thick roots beneath cratons and shows that they lead to strongly negative gravitational potential energy relative to surrounding regions. As a result, compressive stresses are superimposed on those from plate boundary forces, which in turn may explain the dominance of reverse-faulting earthquakes in cratons.

Other recent work has focused on the role of step-overs as the source and origin of the 1906 San Francisco earthquake. She has expanded this work to other earthquakes as well.

Mary Lou Zoback is a geoscientist who uses the tools of physics to delineate the forces that drive and deform the lithospheric plates. She is a scientist of international stature. She has made many profound, widely appreciated, and influential contributions. Please join me in congratulating her as the 2007 recipient of the Arthur L. Day Medal of the Geological Society of America.

 top 2007 Day Medal - Response by Mary Lou Zoback

Thank you Richard for that generous citation and for nominating me, along with your silent partner, Ken Kodama of Lehigh University, for the Day Medal. Ken, Richard, and I were graduate students together in the Geophysics Department at Stanford—an environment in which the students learned more from fellow students than they did from their professors, according to their advisor Alan Cox. It certainly was a rich and stimulating environment from which to launch.

It is truly an honor, and very humbling one, to receive this recognition in Arthur L. Day’s name from the Geological Society of America. I gave my first scientific talk at a GSA meeting in Denver, more years ago than I am willing to admit. I want to begin by thanking GSA for providing a friendly home to geophysicists such as myself, seeking to understand the physical forces driving very real geologic processes. It is a distinct pleasure and honor to share this podium with two giants of tectonics on whose shoulders I, and myriad of others have stood, Kevin Burke and Celal Sengor. Their bold thinking on regional, global, and even extra-terrestrial tectonics always challenged and inspired us to think creatively, beyond conventional wisdom.

Receiving a recognition like the Day Medal invokes many emotions: joy, embarrassment, as well as suspicion that the committee or someone at headquarters made a mistake in sending the email announcement. But most of all, I feel tremendous pride in this recognition of a number of wonderfully productive, and personally rewarding, collaborations throughout my career.

The first such collaboration, and an enduring one, was with my advisor and mentor, George Thompson at Stanford University. George’s guidance and inspiration provided me with a strong field geologic grounding and opened my eyes to the power of applying simple physical principles, such as force and mass balance, to constrain large-scale geologic processes. His gentle nudging and thought-provoking questions, not to mention his able service as “field assistant extraordinaire,” set me on the wonderful path of exploring the evolution of the active tectonic provinces of the Western U.S in the context of broader plate tectonic interactions.

As Richard indicated, this interest in active tectonics and changes in the state of stress in the Basin and Range expanded to a broader captivation with understanding the present day state of stress within the earth’s crust. Special thanks to Barry Raleigh, then a USGS branch chief, who first introduced me (as an undergrad technician) to the possibility of mapping the tectonic stress field.

The body of work on defining the state of stress in the earth’s crust resulted from the collaboration of which I am most proud, both in life and in my career—that with my husband Mark. Together we developed and tested a broad range of geologic and geophysical techniques to infer the present-day stress field. A major step came in quantifying quality criteria that would allow for comparison of the results of different techniques. This was truly a joint effort, often involving many late night, heated discussions, after the kids went to bed.

Applying our stress mapping techniques globally was the brainchild of Karl Fuchs, then President of the International Lithosphere Program and now Professor Emeritus at Karlsruhe University. Karl’s vision created the World Stress Map project, a collaboration of 40 eager and dedicated scientists in 30 different countries that I had the pleasure to lead. Each of these scientists made critical contributions to this global effort. Looking back, I am amazed at what we accomplished in a widely dispersed collaboration--all prior to the days of email. That the understanding we developed of global stress patterns has stood the test of time and has provided valuable constraints in many diverse areas of geology and geophysics is extremely gratifying. I am forever indebted each of those 40 scientists, especially Birgit Mueller, of Karlsruhe University, who was invaluable in the data compilation effort and whose good humor helped keep me sane. Special thanks too to Randy Richardson, for the knowledge and insights he shared in our collaborations quantifying relationships between forces and stress.

Finally, I want to acknowledge my amazing good fortune to have been able to spend much of my career at U.S. Geological Survey, the finest earth science agency in the world. The USGS remains a special and unique environment, consisting of a cadre of remarkably dedicated, world-class research scientists who always have time for discussion of ideas, regardless how crazy, and who willingly drop their own research to help out with a sticky problem. We understood the power, value and rewards of collaborative research long before “teamwork” became a management fad. I dare not mention these colleagues by name because it might begin to sound like a filibuster. I do want thank John Filson for many years of inspired leadership of the USGS earthquake program; his hands off leadership style allowed program scientists to pursue research directions we deemed important and productive. Special thanks to Bill Ellsworth for making it easy for me to establish a part-time schedule for much of my career.

I gratefully thank the GSA Council and Day Medal Committee for honoring me with this award which I accept on behalf of all those scientists I have had the privilege of working with. They stand beside me in receiving this honor. My heartfelt thanks to Mark, my collaborator in both life, and science, and to our children Eli and Megan—a continual source of joy, pride, and wonder.