In a most general sense, all the research projects that my students and I have worked on, and are planning to work on in the future, involve normal faulting and crustal extension processes. Within this general framework, there are several specific topics that we have focused on over the last several years. These include: a research program that has made a significant contribution to a fundamental understanding of the normal fault growth process, a research program which has made significant progress toward understanding the relationship between mantle hotspot plumes and extension of the continental lithosphere, the study of faults zones in the brittle crust and what can be learned about faulting at the macroscale from microscale analysis, and a research program addressing the mechanical paradox of upper crustal low-angle normal faulting.

Our emphasis is field based studies. In this work we use a wide variety of techniques some of which include; paleomagnetic analysis, isotopic dating (especially Ar/Ar), seismic reflection interpretation, thinsection microscopy, microprobe analysis and mechanical and thermal modeling.

Some recent research topics

Fault growth

Continental hotspots (Yellowstone) and their thermal/mechanical effect on faulting

The paradox of normal faulting

Fault zone studies in the brittle crust

New projects and research directions

Other People at Lamont working in the general area of structure and tectonic

Roger Buck (Mechanical Modelling) - Theory of normal faulting including the mid-oceanic ridges

Chris Scholz (Rock Mechanics) - Faulting processes including fictional behavior of rocks in fault zones

Nick Christie-Blick (Stratigraphy) - Passive margins sedimentation and normal faulting

Nano Seeber (Seismotectonics) - Recent earthquake rupture patterns and future earthquake potential

Gary Karner (Mechanical Modelling) - Flexure of the lithosphere in response to faulting

Mike Steckler (Mechanical Modelling) - Mechanical behavior of the lithosphere in response of continental break-up

Paul Olsen (Paleontology) - Stratigraphy of normal fault basins including those associated with continental break-up

Fault Growth

This project, involving Chris Scholz, myself and a number of students, was an investigation of the scaling laws that govern faulting and of the physical mechanisms for fault growth that gives rise to these scaling laws. In several publications we have shown how knowledge of the scaling laws would allow the determination of strain from faulting in a sparse geological data set. Our work on the growth of faults was extended to the growth of normal faults and the understanding of how the scaling laws apply to such structures. We have concentrated on the nature of fracture near fault zones to test our fault growth models. We have also developed a theory of fault growth that explains how large normal faults grow and how such a growth history can be applied to an understanding of normal fault segmentation. This research project has resulted in four completed Ph.D.

EAR 90-04534, EAR 93-05175 (with C.H. Scholz)

Map view of normal fault system in the Tablelands of California, near Bishop. Faults were formed on a 738 ka surface formed by the Bishop Tuff (Figure from Dawers and Anders, 1995).

Figure showing possible modes of linkage of two normal faults growing toward each other. In order for scaling relationships between fault length and displacement to be maintained during linkage, displacement must be accelerated in the linkage region to account for the increased length of the fault. One way to do this is for a number of smaller faults to develop in the linkage regions, the sum total of displacement on these smaller faults can increase so that scaling relationships are maintained. (Figure from Anders and Schlische, 1994).

Displacement data from a normal fault system shown above. Note the large number of smaller faults in the region of linkage. The displacement on these faults when summed results in a pattern for the total fault system that is roughly representative of the displacement pattern of each of the smaller faults. This result is consistent with the faults within a fault system roughly being self-similar. There is however, some flattening of the total or summed profile with respect to the smaller faults that we suggest results from faults growing into a mechanically weak subsurface of volcanoclastic sediments. This can be thought of as analogous to large crustal-scale faults that extend to a weak ductile substrate. (Figure from Dawers and Anders, 1995).

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Continental Hotspots and their Thermal/Mechanical effect on Faulting

One of the first order questions concerning hotspots is how does the continental lithosphere respond when passing over one of them. One of the best examples of a continental hotspot is the Yellowstone hotspot. However, the Yellowstone hotspot appears not to fit the hotspot model sensu stricto. In other words, the assumed track rate is significantly different than would be expected assuming the location of the major rhyolite calderas represents the sublithospheric position of a stationary mantle plume. My students and myself have used the migrating extensional deformation field caused by the thermal effects of the hotspot rather than the assumed locations of rhyolitic volcanic calderas. Using the caldera location, several workers have estimated the migration rate of the Yellowstone volcanism to range from 2.9 cm/yr to 4.5 cm/yr. over intervals ranging from 10 m.y. to 16 m.y. This is a significantly greater rate than one would expect based on an assumed fixed plume source and plate motion based on NUVEL1 plate motion models. The difference may be due to the inability to accurately assess the migration rate because of the inability to determine the exact location of calderas because they are buried by 1 to 3 km of basalt.

Using the deformation field rather than caldera locations, we have determined an extension-corrected relative velocity of 2.2 ± 0.2 cm/yr. This value is significantly less than the previous extension-derived estimate of plate velocity of 3.7 cm/y.

In short, we have determined an independent estimate of the North American plate velocity of 2.2 ± 0.2 cm/yr and found that the many of the published caldera locations are in error suggesting that the hotspot, for the last 10 Ma, tracks exactly as expected for the "standard" hotspot model.

Publications

NSF EAR 94-18045, OYO Corporation, Dept of Energy

	Figure of western U.S showing the track of the Yellowstone hotspot as assumed by younging in a northeast direction of silicic calderas (yellow). Areas colored in shades of tan represent Neogene sedimentary deposits contemporaneous with the hotspot volcanism. Black dots are sampling locations of Yellowstone/Snake River Plain volcanic ash deposits. All radiometric ages for units in the interval including 4.49 Ma to 10.27 Ma are ages determined by Ar/Ar analysis at the Lamont-Doherty Earth Observatory Geochronology Laboratory and the at the Berkeley Geochronology Center. Many age determinations  are based on weighted averages of as many as 16 individual age determinations.
	Map showing the seismicity in the vicinity of Yellowstone and the eastern Snake River Plain. Three colors represent the respective positions of the seismic parabola as assessed by several authors. Note that is an interior parabola that defines a zone of aseismicity called the collapse shadow (Anders, 1983). The pattern defined by Pierce and Morgan (1992) is based primarily on the location of active normal faults whereas the two parabolas defined by Anders et al. (1989) are based solely on the distribution of seismicity.
Map showing the location of faults whose interval of accelerated displacement was determined. The accelerated  extension is thought to be related to arrival of the migrating seismic parabola. Distance is measured between sampling location and the position of the outer parabola along a line parallel to the migration path of the /Yellowstone/Snake River Plain volcanic center.	Plot of the interval of accelerated extension, determined at a particular location, versus the distance to the outer parabola alone a line trending N55°E (from Anders, 1994). Assuming the migration direction can reasonably be determined, the slope of the line reflects the migration rate of the thermomechanical effects of an assumed sub-lithospheric plume or hotspot. From these data the migration rate of the thermal source is determined to be 2.2 ± 0.2 cm/yr over the last 10 m.y. This is significantly less than the 2.9 cm/yr to 4.5 cm/yr values that have been suggested based on the location of volcanic centers.  If the thermal source is indeed fixed relative to the North American plate, as predicted by the standard hotspot model, then the relative North American plate velocity is 2.2 cm/yr in a roughly N55°E direction. This estimate is the same as that calculated for the last 3 m.y. using sources other than the Yellowstone hotspot (see DeMets et al. 1990).

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The Normal Fault Paradox

Geologists from around the world have reported finding normal faults that they suggest moved at low-angles(< 30°) in the brittle field. Our current understanding of fundamental rock mechanics suggests that normal faults should not move at such low angles. Moreover, there is no definitive evidence of a historic earthquake rupture on such a low-angle normal fault. Absent some special conditions, either the field evidence is incorrect or our understanding of fundamental mechanics is somehow misguided.

We at Lamont, Nick Christie-Blick, Kate Gregory-Wodzicki and my graduate students and myself, have undertaken an extensive reexamination of several of the key geologic examples of low-angle normal faults that played a significant role in advancing the concept of upper crustal low-angle normal faulting. One of the most prominent examples of an upper crustal low-angle normal fault is the Sevier Desert detachment of west-central Utah. The detachment is thought to be an 11° west dipping fault that has accommodate about 40 km of displacement with and a poorly defined length. Despite its size, prior to our work this feature, the fault itself had not been seen in outcrop, rather seen only on seismic reflections lines. The existence of this feature has strongly effected the thinking on the mechanical paradox of low-angle normal faulting. We at Lamont have come to questioned whether this feature seen on seismic reflection profiles is truly a fault. Our challenge of this concept is based on our examining material from well cuttings and core from the presumed fault surface. In examining the material from the location of the reflection, we found no evidence of either brittle or ductile deformation from either the upper or the lower contact which produces the prominent seismic reflection. We have, therefore, concluded that Tertiary/Paleozoic contact thought to be the fault surface is actually an unconformable contact. This interpretation of course presents many challenging questions such as how did the Tertiary Sevier Desert basin form and how might the communities interpretations of the Mesozoic thrust system be altered?

Fault Zone Studies in the Brittle Crust

One of the keys to understanding the faulting process in the brittle part of the crust is by examining the fault zone. Brittle failure produces a distinctive kind of deformation that characterizes the movement regiem of a fault. My students and myself have studing the growth of faults from incipinent ruptures to fault exhibiting significant displacement. From these studies we have been able to determine the changes in the stress field associated with the development of a process zone. Using microfractures, we have mapped that changes in the orientation of the principle stress axes yielding hitherto unobserved behavior that is consistent with an elastic/inelastic model of fault zone development.

We have also examined in detail the deformation patterns associated with shallow crustal normal fault and comparing these observation to the deformation style of deformation found at the base of large slide blocks. As one might expect a priori, there are significant differences. More importantly, we have found that the unique deformation style found at the base of landslides and slide blocks is almost identical to the deformation style found at the base of several low-angle detachment blocks thought to be associated with significant crustal extension.

Anders, M.H., Aharonov, E., Walsh, J.J., 2000, Stratification of granular media at the base of large slide blocks: implications for mode of emplacement, Geology v. 28, 971-974.

Anders, M.H., and D.V. Wiltschko, 1994, Microfracturing, paleostress and the growth of faults, Journal of Structural Geology, v. 16, p.795-815.

Anders, M.H., Wills, S., Christie-Blick, N., and Krueger, S.W., Rock deformation studies in the Mineral Mountains and the Sevier Desert of west-central Utah: Implications for upper crustal low-angle normal faulting: Geological Society of America Bulletin (in press).

From Anders et al., 2000. Photographs of the basal gravel layer found at the base of landslides.

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New Projects and Research Directions

Some of the broad research directions I view as potentially significant include topics such as how we distinguish between mechanical unroofing of a large mountain range and unroofing of that range by climatically driven processes? Similar questions include how rocks thought to be formed as much as 70 km beneath the Earth's surface make it to the Earth's surface? When mechanical processes are involved in unroofing, what are they? As with all of the previous research topics that my students I have addressed, we will take a multi-disciplinary approach. Areas we have drawn on in our past research as well as for futures endeavors include, field mapping, isotope geochronology, paleomagnetics, mechanical modeling, petrographic analysis to name just a few. Some of the new projects that I intend to undertake involve diverse projects such as determining the mechanic processes involved in opening of the Red Sea, the uplift history of the Andes, and the origin of eclogites in the Caledonides of western Norway.

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