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Fall 2015 Colloquia

Thursday, October 8th, 2015

Dr. Carol Stein, University of Illinois at Chicago
Title: New Insights Into the Tectonic Evolution of North America’s Midcontinental Rift
Host: Julie Bowles

Rifts are segmented linear depressions, filled with sedimentary and igneous rocks, that form by extension and often evolve into plate boundaries. Flood basalts, a class of Large Igneous Provinces (LIPs), are broad regions of extensive volcanism due to sublithospheric processes. Typical rifts are not filled with flood basalts, and typical flood basalts are not associated with significant crustal extension and faulting. North America’s Midcontinent Rift (MCR) is an unusual combination. Its 3000-km length formed as part of the 1.1 Ga rifting of Amazonia (Precambrian NE South America) from Laurentia (Precambrian North America) and became inactive once seafloor spreading was established, but contains an enormous volume of igneous rocks. MCR volcanics are significantly thicker than other flood basalts, due to deposition in a narrow rift rather than a broad region, giving a rift geometry but a LIP’s magma volume. Structural modeling of seismic reflection data shows an initial rift phase where flood basalts filled a fault-controlled extending basin, and a postrift phase where volcanics and sediments were deposited in a thermally subsiding basin without associated faulting. The crust thinned during rifting and rethickened during the postrift phase and later compression, yielding the present thicker crust. The coincidence of a rift and LIP yielded the world’s largest deposit of native copper. This combination arose when a new rift associated with continental breakup interacted with a mantle plume or anomalously hot or fertile upper mantle. Integration of diverse data types and models will give insight into questions including how the magma source was related to the rifting, how their interaction operated over a long period of rapid plate motion, why the lithospheric mantle below the MCR differs only slightly from its surroundings, how and why extension, volcanism, and compression varied along the rift arms, and how successful seafloor spreading ended the rift phase.

Thursday, October 15th, 2015

Dr. Robert Hodyss, Jet Propulsion Laboratory, California Institute of Technology
Title: Organic Geochemistry on Titan
Host: Lindsay McHenry

The Cassini-Huygens mission has revealed Saturn’s largest moon Titan to be a remarkably Earth-like world, with a diverse landscape of rivers, lakes and seas, vast dune fields, plains and mountains. Unlike Earth, however, Titan’s visible surface is dominated by solid organic materials at cryogenic temperatures, and altered by the action of liquid methane and ethane.

Our work at JPL focuses on understanding the fundamental chemical processes that lead to this varied landscape. Dissolution and solubility of solid organics in the lakes and seas leads to erosion of Titan’s surface. The precipitation of dissolved materials as the lakes dry can result in organic evaporite deposits, which may contain novel crystalline forms only possible at cryogenic temperatures.

Even at Titan’s low surface temperature (94 K), rapid chemical reactions are still possible. We are studying the reaction of carbon dioxide with primary amines to form carbamic acids, a reaction that occurs relatively rapidly at temperatures as low as 40 K. Titan’s assorted environments and array of organic molecules are intriguing as a venue for prebiotic chemistry, and make Titan an important target for future astrobiology missions.

Thursday, October 22nd, 2015

Dr. Heather Cunningham, The McCrone Group
Title: Life After Academia: Problem Solving in the Private Sector
Host: Tina Hill

Ever wonder what life in the private sector is like or what skill sets are in demand? In this presentation, I will focus on my personal journey, in particular the transition from volcanologist to senior research scientist. When I made this transition, I believed geologists working for small analytical laboratories participated in dull routine projects that resulted in very little intellectual reward. Fortunately I was very wrong. Although the scope of the problems can be smaller, a large toolbox of skills from sample preparation to data interpretation are necessary to solve scientific projects. Teamwork, communication, time management and flexibility are key (and valued) employee attributes. I will highlight several high profile projects conducted at McCrone Associates and the analytical skill sets employed to solve them. Since demonstrated mastery of practical skills and a breadth of experience are more important than publications in the private sector, take this opportunity in academia to build up your analytical resume.

Thursday, November 12th, 2015

Dr. Lucas Zoet, University of Wisconsin-Madison
Title: Experimentally Testing How Glaciers Slide
Host: Julie Bowles

In order to accurately estimate the contribution of ice flux from glaciers to sea level change as well as the erosive capabilities of glaciers it is imperative that ice sheet and landscape evolution models use constitutive rules that are physically based and empirically constrained. Constitutive rules exist in various forms for glacier sliding, but identifying the correct form has proven difficult. Investigation of the complex physical processes that form the basis for sliding rules at the ice-bed interface from field observations alone has proven insufficient. Difficulties in field observations arise from a relative lack of ice-bed interface accessibility, difficulty in extrapolating borehole measurements and, most importantly, an inability to isolate the effect of relevant variables.

Experiments were conducted to study glacier sliding using a ring-shear apparatus specially designed to simulate the ice-bed interface. The device rotates a ring of ice 90 cm in outer diameter across a deformable or rigid bed. Sliding speed or shear stress is set, while the other of these parameters is allowed to attain a steady state. Data are collected at sliding speeds of 7-1500 m/yr and shear stresses of 30-100 kPa, while a vertical (normal) stress of 150 -500 kPa is applied to the ice ring. The temperature of the ice is held at the pressure melting point, and in response to strain, ice quickly develops a fabric similar to that observed in ice at the base of glaciers.

Results indicate that hard beds demonstrate a rate-weakening effect while deformable beds behave as a coulomb rheology. A sinusoidal bed exhibits a double valued sliding response in which shear stress first increases and then decreases with progressively higher sliding speeds. A stepped bed demonstrates only rate weakening with sliding speed. A deformable bed with clasts exhibits rate dependence over a small range of shear stress values less than the yield strength of the till. At shear stresses equal to the yield strength, the bed shears in a thin layer near the ice-bed interface with resistance independent of slip velocity. At stresses below the yield strength, the rate dependence is likely the result of classical sliding mechanisms allowing flow of ice around clasts partially lodged in the bed. These results provide the first experimental guidance for flow models that seek to include a physically based constitutive rule for basal slip.

Thursday, November 19th, 2015

Dr. Eva Enkelmann, University of Cincinnati
Title: Tectonics and Surface Process Interaction at the Yakutat Plate Corner Southeast Alaska
Host: Erik Gulbranson

Investigations of tectonic and surface processes have shown a clear relationship between climate-influenced erosion and long-term exhumation of rocks. Evaluation of the driver for exhumation and the understanding of key feedback mechanisms are complicated partly because climate has changed significantly in the late Cenozoic. The effects of climate change are particularly pronounced at high latitude where modern climate change is largest and where the growth and decay of enormous ice masses during the last ~2.5 yr have profoundly affected the landscape. Numerical models suggest most orogens are in a transient state, but observational evidence of a spatial shift in mountain building processes due to tectonic climate interaction are missing.

Thursday, December 3rd, 2015

Dr. Marcia Bjornerud, Lawrence University
Title: Decoding the record of ancient earthquakes:
Pseudotachylytes from Norway, New Zealand and northern Wisconsin
Host: Gina Szablewski

Pseudotachylyte, or frictionally-generated melt rock, is considered the only rock type diagnostic of ancient earthquakes.The comparative rarity of pseudotachylyte in ancient fault zones is surprising in light of estimates that ca. 90% of the energy budget of an earthquake is expended in frictional heating. One explanation is that frictional melting (pseudotachylyte generation) is suppressed after the initial rupture on a fault zone because fluids infiltrate the zone and thermal pressurization of these fluids inhibits melting in subsequent seismic events. While this seems plausible for many of the iconic occurrences of pseudotachylyte in otherwise undamaged crystalline rocks, some pseudotachylytes clearly formed in host rocks in which permeability was apparently high and fluids were present at the time of frictional melting. In these fault zones, cataclasites and pseudotachylyte commonly have mutually cross cutting relationships, and both types of fault rock have been complexly intruded into the surrounding damage zone. In contrast, cataclasites associated with pseudotachylyte in pristine crystalline rocks occur in smaller volumes and have simpler geometries, typically limited to the margins of fault veins or in dilational jogs.