Initial Publication Date: May 1, 2006

Enigmas in Structural Geology

In early January 2005, we emailed those on the TSG email list and asked what they thought the biggest enigmas are currently in structural geology. We were looking for topics where all of the models/ideas that have been tested are still clearly off the mark. Kind of like Taylor's Axiom and Taylor's Corollary for the Moon ("The best models for lunar origin are the testable ones; the testable models for lunar origin are wrong.") Below, you'll see the responses. I think you'll agree that they are fascinating and provocative!

If you would like to contribute a topic, or add a comment to any one of the topics, please send an e-mail to Barb Tewksbury at

The weak fault problem

Seems like the anomalous weakness of the San Andreas (and possibly other large-scale or `mature' strike slip faults might well be one remaining enigma. It takes many forms, going back to Escher von der Linth. It's not that there has been no progress, but it's a multifaceted problem and it's hard to get data on certain key aspects of the problem, in contrast with Thom's axiom. The San Andreas is a current focus, but is just a symptom of the larger problem. There is a session at EGU on this.

There are important cases in which static excess fluid pressures clearly do not dominate the problem, including the Chelungpu fault that slipped in the Chi-Chi earthquake in Taiwan (which has hydrostatic fluid pressures) and the toe of the Nankai trough wedge (lambda = 0.7, but given the taper should be 0.9 with Byerlee's law).(from Jan Tullis, Brown University, and John Suppe, Princeton University)

Deformation rates

My own favorite enigma is the relationship(s) between deformation rates at different temporal sampling windows. I would pose the general question as: "how does instantaneous deformation integrate over 105 to 107 years to produce the mountain belts and basins we see today?"This is a key question for anyone dealing with active tectonics and late Cenozoic mountain building, and with techniques that allow one to measure deformation velocity fields in real time (GPS, InSAR). (from Rick Allmendinger, Cornell University) More thoughts from Rick on this topic.

Strain partitioning along fault systems

Why is it that plate boundaries with oblique convergence seem to accommodate the motion on two separate faults - one with pure thrust motion and one with pure strike slip motion - separate but both seismic. This is also the enigma here in (finally!) sunny SoCal, where thrust and strike-slip motion is accomodated on separate faults, even when they're in close proximity. (from Jan Tullis, Brown University, and Joan Fryxell, Cal State San Bernardino)

Lack of orthorhombic symmetry in structures from a single continuous deformation

I think that there is something fundamentally enigmatic about the fact that we commonly see fault slickenline data sets and earthquake focal mechanism solution data sets with symmetry that is of lower order than orthorhombic. The stress tensor is constructed as a symmetric tensor. If stress is taken as the 'cause' for the structures that we see, shouldn't the structures for a single continuous deformation have orthorhombic symmetry? Using symmetry arguments the answer is probably yes. This raises the question of whether it is appropriate to interpret ancient and active structures in the context of stress? To me this remains a fundamental question - one that is increasingly important as the geophysics, geodesy and structural geology communities begin to work more collaboratively than ever (thanks in part to GPS, INSAR, EarthScope etc). If this pushes any buttons, I urge you to see Twiss and Unruh (JGR, 1998) for additional and original thoughts on this. This paper will take you back to additional key papers on the subject. (from John Lewis, Indiana University of Pennsylvania)

Magmatism and tectonism

At some point in the evolution of most large orogens, magmatism and tectonism become spatially and temporally coincident. Therefore, at the risk of a public stoning, I offer that the REAL role between magmatism (e.g. melting, segregation, migration and emplacement) and tectonism is not well defined. For example, what are the rheological implications of melting, melt migration and emplacement of plutons within continental crust? Although headway has been made, melt or partial-melt rheologies are not well understood (Jan, others, please correct me if I'm wrong). (from Tom Kalakay, Rocky Mountain College) More thoughts from Tom on this topic.

Added by Rick Allmendinger (Cornell University): I agree with Tom Kalakay's post that the relation between magmatism and tectonism is a true enigma in part, I would argue, because it is difficult to document this relationship without resorting to circular reasoning. More often than not, we date igneous rocks to constrain the age of deformation (this is certainly true in active orogens such the Andes where I work; it is less true in deeply exhumed terrains). Therefore, the age of tectonism and that of magmatism are not independent measurements. More generally stated, we date rocks, not deformation so temporal correlations are always suspect. This in no way diminishes the importance of the the point that Tom raised, only that it is not so easy to demonstrate!

The Baja British Columbia controversy

I use the Baja-BC controversy as a case study. Lots of conflicting results, and papers expressing strong opinions one way or the other about terrane translation. The overall problem is regional tectonics but many of the issues are structural (e.g., the question of whether paleomagnetic poles are systematically tilted). Dave quotes one published source as saying, "We contend that our results, while robust and reproducible, are tectonically unreasonable." Sounds a bit like Taylor's Axiom...(from David Greene, Denison University) References from Dave on this topic.

Absence of petroleum in the Basin and Range

This is more of a regional enigma than a general one, but there's ongoing head-scratching over the scarcity of petroleum in the Basin and Range, in spite of all the basins, thrust faults, and organic-rich formations. In my judgement, it all hinges on structural configurations. (from Joan Fryxell, Cal State San Bernardino)

Mechanism for deep focus earthquakes

Deep focus earthquakes can't have the same cause as shallow quakes, yet focal mechanism solutions are typical double-couple solutions. Models involving transformational faulting (generation of spinel-filled anticracks in metastable olivine and linking of anticracks followed by catastrophic failure) seem to make good sense both rheologically and in terms of models for metastable persistence of olivine in subducted slabs. When the model has been tested with data from recent quakes in Tonga and Chile, though, it's clear that transformational faulting doesn't adequately explain the data. (from Charlie Onasch, Bowling Green State University, and Barb Tewksbury, Hamilton College)

Plate rates and seismic moment release

How about the discrepancy between plate tectonic rates and seismic moment release; i.e. how is strain in the upper crust accommodated when earthquake moment release accounts for only a fraction of the measured rates? (from Doug Yule, Cal State Northridge)

Mechanism of exhumation of UHP rocks

I think the exhumation mechanisms of ultra-high pressure supracrustal rocks is a true enigma. This is especially true for rocks exhumed during the same mountain building event that led to their great burial. I think it is fairly easy to imagine mechanisms (although exotic) for how rocks are taken to very great depths (100 km +) but it is very difficult to imagine how you get them back to the surface. (from Chris Andronicos, University of Texas, El Paso)

Development of modern plate tectonics

I would add the very fundamental question of when modern-style plate tectonics actually began. There is a tendency to see the world (and mountain belts) through ultrauniformitarian spectacles, and we haven't really explained how we got from an early lunar or martian-type tectonic regime to our current earthly one. For that matter, we've never really documented the birth of a subduction zone even in the modern realm. (from Marcia Bjornerud, Lawrence University)

Tectonic regime and growth of continental crust during the Proterozoic

An examination of 'basement' rocks in many continents suggests (through U/Pb and Tdm data) that a significant proportion of continental crust was generated during the Proterozoic (from ~2.0 to 1.0 Ga). This is especially true for Laurentia, and it suggests that the Proterozoic tectonic regime was fundamentally different from what came before and what has come after. The present-day volume of Archean crust is less than Proterozoic crust, although that doesn't necessarily mean that it has always been that way, and Phanerozoic crust doesn't appear to be accumulating at anywhere near the Proterozoic rate. So what was different about Proterozoic tectonics? And what were the tectonic processes that facilitated the remarkable increase in continental crust? (from Steve Whitmeyer, University of Tennessee)

Strength of the upper mantle

I would add that the strength of the upper mantle is unresolved, if not an enigma. James Jackson's recent publications bring the old Brace andd Kohlstedt model into question, and the strength of the upper mantle is very important for understanding deformation of the continents. (from Michelle Markley, Mt. Holyoke College)