Impact is a unique, short-time, high-energy geological process. The importance of impact cratering on terrestrial planets (Mercury, Venus, Mars), our Moon, and the satellites of the outer planets is obvious from the abundance of craters on their surfaces. On most bodies of the solar system that have a solid surface, impact cratering is the most important surface-modifying process even today. On Earth, active geological processes rapidly obliterate the cratering record. To date only about 170 impact structures have been recognized on the Earth’s surface. The come in various forms, shapes and sizes, from 300 km to less than 100 m in diameter, from Recent to 2 billion years in age. Mineralogical, petrographic, and geochemical criteria are used to identify the impact origin of such structures or related ejecta layers. The two most important criteria are the presence of shock metamorphic effects in mineral and rock inclusions in breccias and melt rocks, as well as the demonstration, by geochemical techniques, that these rocks contain a minor extraterrestrial component. In impact studies there is now a trend towards the use of interdisciplinary and multi-technique approaches to solve open questions. In this lecture we will take a look at impact craters on the Earth (and some other planets), and discuss how they formed and how they can be recognized. An aspect of impact cratering that may be underestimated is the influence of impacts on the geological and biological evolution of our own planet. Even the impact of relatively small asteroids or comets can have disastrous consequences for our civilization. There is a 1 in 10,000 chance that a large asteroid or comet 2 km in diameter (corresponding to a crater of about 25-50 km in diameter) may collide with the Earth during the next century, severely disrupting the ecosphere and annihilating a large percentage of the Earth's population. The biological evolution of our planets is punctuated by mass extinction events, of which the one 65 million years ago, which marks the Cretaceous-Tertiary boundary, is probably the best known one. Abundant impact debris marks this boundary, providing a clear link with a major impact event. The Chicxulub impact structure in Mexico, about 200 km in diameter, which resulted from the impact of an about 10-km-diameter asteroidal body, has been identified as the culprit. Understanding of impact structures, their formation processes, and their consequences should be of interest not only to earth and planetary scientists, but also to society in general.

The Himalaya mountains are the product of the largest continental collision in the world today, and are home to large and deadly earthquakes, such as the Pakistan earthquake of October 8, 2005. To understand how the mountains were created and to help quantify the earthquake hazards of this vulnerable region, Dr. Sheehan led a National Science Foundation funded project that included placement of ground motion recorders (seismometers) throughout eastern Nepal and southern Tibet. The seismic stations were installed in areas that are remote and logistically difficult, with challenges including the mountains, weather, scorpions, cobras, and political unrest and guerrilla warfare in Nepal. Much like a medical CT scan, ground motion recordings from earthquakes provide a detailed image of the Earth beneath the seismic stations. The earthquake recordings collected in Nepal and Tibet produce a first-ever glimpse of the earthquake faults beneath the Himalayan mountains, and can be used to determine details of the earthquake faulting processes.

Pacific-North America relative motion is accommodated north of San Francisco on a series of sub-parallel strike-slip faults. From GPS data, we understand the broad distribution of slip between these stuctures, but data are too sparse to map the deformation in detail. However, using an advanced form of InSAR processing - the Permanent Scatterer method - we can generate a dense spatial dataset of surface velocity measurements. There now exist three such datasets for the Bay Area, each from a different viewing geometry/satellite track.
We find a variety of nontectonic and tectonic signals in these data, ranging from ground subsidence and landsliding to strain accumulation and fault creep. I will prsent a series of case studies from around the Bay Area, showing how the different observation geometries can be used to make first order inferences of horizontal and vertical velocities in deforming areas, how the data were used to identify creep on a fault previously considered locked, and how using the pattern of creep on the Hayward fault - currently considered the most dangerous structure in the region - a series of locked asperities can be imaged geodetically.

The “hotspot” hypothesis predicts that time-progressive linear chains of oceanic intraplate volcanoes (OIV) are formed on tectonic plates that pass over buoyantly rising plumes from fixed deep mantle sources. However, this hypothesis has been called into question by an alternate mechanism, which explains OIV chains by lithospheric extension and mantle melts rising to the surface along lithospheric fractures. Distinguishing between these models is very important because they imply a profoundly different dynamic and chemical state of the mantle that is likely to substantially influence the chemical evolution of the Earth. I will present new geochemical data from a geochemical study of several seamount chains in the Western Pacific, all likely erupted over the seismically anomalous Pacific mantle. These data provide a >100 myr geochemical record that can be related to three geochemically distinct active OIVs in the Cook-Austral region with the help of plate motion models. In a geophysical and geodynamic context these volcanoes should most likely be viewed as the result of deep mantle plumes, while lithospheric cracks are probably a secondary factor.

We have been developing methodology that allows us to simulate long histories of seismic and aseismic fault slip while accounting for slow tectonic loading and all inertial effects. Our 3D models incorporate laboratory-derived rate and state friction laws. We have used 3D models to simulate interaction of seismic and aseismic slip, to reproduce abnormal scaling of moment and recurrence time for small repeating earthquakes, and to study interaction of dynamic rupture with fault heterogeneities over many earthquake cycles. In 2D models, additional dynamic weakening mechanisms due to shear heating are incorporated, When combined with defect regions to nucleate ruptures, fault models with high static friction strength and low dynamic strength operate under low average shear stress and low heat production, while producing earthquakes that have typical stress drops and pulse-like mode of rupture propagation. Hence the models reconcile several laboratory and observational constraints. Decrease in dynamic weakening causes the fault to operate at higher average levels of shear stress and results in systematic change of rupture mode from pulse-like to crack-like. Such change of dynamic rupture mode with fault prestress has been recently documented in laboratory experiments.

Richard Norris studies periods of warm climates in the geologic past that may be analogs for the biological and climatological effects of future global warming. He is particularly interested in how global warming affects ocean circulation (potentially producing abrupt climate change) and how warming has affected life in the oceans and on land. Recently, he has become interested in how the addition of greenhouse gases to the atmosphere affects the acidity of the ocean with possibly catastrophic effects on organisms such as corals and calcareous plankton.

Analyses of a 3-D seismic survey and well logs in the southwestern Sacramento basin, San Joaquin County, California, have revealed a subsurface, circular, ~5.5-km-diameter anomaly that may represent a previously unrecognized complex impact crater. This unique anomaly, buried 1,490–1,600 m below sea level under the southwestern part of the Sacramento-San Joaquin Delta, is provisionally named the Victoria Island structure for an overlying surface geographic feature.
The Victoria Island structure is characterized by a concentric, annular, terraced rim and trough surrounding a structurally uplifted central peak. Well logs tied to seismic data show that the upper surface of the structure occurs stratigraphically near the top of the siliciclastic, continental to shallow-marine Domengine Formation, indicating a middle Eocene age. Overlying fill material, which reaches an estimated thickness of at least 80 m in the trough, is primarily deep-marine, middle Eocene Nortonville Shale. Both well and seismic data indicate thinned Domengine and thickened Nortonville sections across the center of the feature. A disturbed stratigraphic sequence under the structure includes upper to lower Domengine and underlying lower Eocene Capay Formation and Cretaceous-Paleocene Mokelumne River Formation siliciclastic units. Characterized by discontinuous seismic reflectors, the central peak is estimated to be ~600 min diameter with at least 35 m of structural uplift. The seismic data demonstrate that the feature is ‘rootless’, being underlain by gently dipping, relatively undeformed strata. The 3-D data further suggest the presence of a series of discontinuous, inward dipping, concentric normal faults with minor offset surrounding the trough and outer rim areas. Estimates of the dimensions of the structure indicate a circularity ratio (short-to-long axes) of 0.91 and a depth-todiameter ratio of ~0.02.

Mesozoic-Cenozoic circum-Pacific batholiths are a prime example of convergent continental margin magmatic processes thought to drive growth and maturation of continental crust. Although it's now clear that non-steady-state magmatic flare-ups of ~5-15 Ma duration account for the bulk of magmatic addition in long-lived continental margin arcs, understanding of deep crust & mantle processes related to the build-up and triggering of flare-up events remains elusive. “La Posta-type” intrusions that dominate the eastern Peninsular Ranges batholith of southern and Baja California provide an important perspective on this issue. These rocks are chemically similar to high-Al tonalite-trondhjemite-granodiorite gneiss terrains of Archean crust produced by high pressure melting of mafic source regions. New zircon U-Pb ages (n=43) from most of the major La Posta intrusive centers document synchronous and surprisingly brief emplacement at 96±3 Ma throughout this 1600-km long batholith segment. High del18O values of La Posta-type zircon (~7-11 per mil) indicate large components of supracrustal contaminants into deep crustal magma source regions. Because the volume La Posta-type crust in the Peninsular Ranges may easily exceed a million cubic kilometers, simple mass balance considerations require supracrustal recycling on a massive scale. The fact that high del18O La Posta-type zircon are from rocks with Sri values mostly <0.706 constrains the nature of the supracrustal contaminant. Assimilation of high Sri Julian Schist-type metasedimentary wallrock to account for La Posta zircon oxygen isotope compositions is unrealistic on several counts. We speculate that large-scale sediment underplating of isotopically primitive accretionary prism material may have played an important role leading up to La Posta-type melt generation. This view is supported by a sediment deficit in adjacent forearc basin & accretionary prism belts of the California borderland.

Triton is a large satellite in retrograde orbit around Neptune, the most distant (40AU) of the giant gaseous planets. Surface temperatures average about 40K and are cold enough to condense all of the heavier gases, including nitrogen. A pronounced tilting of Triton's spin axis gives rise to a strongly seasonal climate and the available imagery indicates a deteriorating south polar cap made of nitrogen. Evidently the released nitrogen is being transferred to the north pole and condensing there during the long winter season. Distant though the sun is, it evidently supplies enough energy to provide seasonal transfer of nitrogen much in the same manner that water alternately collects and melts at the earth's poles with the seasons. Triton has been resurfaced and shows little evidence of cratering. Probably the satellite has been heated, evaporating the surface ices and then the released gases have recondensed to provide a new surface. The energy source for the heating would appear to be tidal friction following capture by Neptune. The retrograde orbit of Triton would indicate capture and the lack of surface craters would suggest that the capture was a fairly recent event. Numerous irregular surface textures indicate repeated expansion and contraction and are compared with somewhat similar features on earth. Although the Earth/Triton surface features resemble one another, they would be composed of totally different material.

Abstract

We have thoroughly studied the properties of electromagnetic radiation (EMR) emitted from fracturing materials. A model was suggested to explain these phenomena, which helped us relate EMR measured parameters with crack sizes and velocities. Since EMR appears when fracturing only starts it might be useful as a tool to predict earthquakes in their latent period.

Author of the Tensile Fracturing in Rocks: Tectonofractographic and Electromagnetic Radiation Methods by Dov Bahat, Avinoam Rabinovitch, Vladimir Frid. Understanding tensile fracture in rocks provides an important key for the interpretation of many problems in structural geology. This book presents a multidisciplinary approach to tensile fracture in rocks (faulting is briefly addressed), starting with an introduction to fracture physics and progressing through tectonofractographic features, characterized both in experimental settings and in geological outcrops. Four examples of sedimentary rocks and two of granites have been chosen to demonstrate the principles and problems in fracture geology. Principles of fracture mechanics and rock mechanics are applied throughout the book, which also explores current understanding about electromagnetic radiation induced by fractures and how such radiation can be used to monitor and predict earthquakes and hazardous collapses in mines. The monograph serves not only as a manual on how to handle specific problems and their solutions in fractual geology but also as a starting point for researchers and graduate students interested in the field of rock fracturing.

Join us as department graduate students Jared Warner, Adam Cosentino, Melanie Biggs and Matt Burgess share their summer internship and research experiences.