This year's geodynamics field trip went to the Pacific Northwest to visit the sites of numerous past "Catastrophic Events" including volcanic eruptions, tsunamis, earthquakes, lahars and pyroclastic flows. This region allowed us to visit examples of nearly all the types of catastrophic instabilities that speakers discussed during the semester.
The field trip to the Pacific Northwest was led by Jeff McGuire, Greg Hirth, and Karen Hanghoj from WHOI. Brian Atwater from the United States Geological Survey (USGS) led the excursion to the muddy realm of Lynch Cove in Puget Sound. The complete list of participants is below:
Joint Program students enrolled in the Geodynamics seminar are required to complete a project for the class. This includes research, an oral presentation during the last two or three seminar meetings, and a written paper due at the end of the semester. For first and second year students, the project must be on a topic related to the theme of the seminar and must be different from their main research interest. For more advanced students, the topic may be closely related to their dissertation research.
The following topics are suggested as potential student projects (advisor listed in bold):
Jeff McGuire and Jian Lin
Stress Triggering Analysis of the 2002 sequence of 4+ M>5 earthquakes on the Gofar Transfrom Fault (EPR) within a span of 15 minutes. These events may have rupture a series of adjacent strands of the Gofar transform, which makes the geometry interesting for evaluating stress triggering models.
The NOAA t-phase catalogs indicate that the Quebrada, Discovery, and Gofar transform faults on the EPR have the highest rate of foreshocks in the 1000 s before large (M>5.5) earthquakes of any oceanic transform fault. However, because of the acoustic nature of the catalog it is difficult to evaluate whether the likelihood of a foreshock increases with the size of the eventual mainshock, a result that would have profound implications for earthquake nucleation. This project would use a Rayleigh wave based match filter to determine the seismic moments of many of the "moderate" magnitude M4.5-5.5 events that are neccessary to evaluate whether foreshocks are more likely before big earthquakes than moderate ones.
Interaction between earthquakes at oceanic transform faults and mid-ocean ridges.
- Origin of 'structural holes' in the flanks of the slow-spreading Mid-Atlantic Ridge at 25 -27 deg. N and 0-26 Ma.
- Structure and origin of the Southeast Newfoundland Ridge: Origin of seaward-dipping reflections and character of magmatism associated with formation of this edifice and its conjugate in the eastern North Atlantic about the time of ~Aptian breakup between Newfoundland and Iberia (based on morphological, multichannel seismic reflection, and possibly gravity data)
- Can authigenic bedforms form on manganese nodules?: Analysis of literature and bottom photographic data (and possibly nodules) from the abyssal-current erosion zone around Agulhas Plateau off South Africa. with Brian Tucholke.
- Structure/origin of shallow thrust faults in the Nares Abyssal Plain. with Brian Tucholke./li>
- Late Eocene - early Oligocene initiation of abyssal circulation in the North Atlantic: Seismic stratigraphic study of the Horizon Au unconformity in the Newfoundland Basin gateway. with Brian Tucholke.
- We have worked with a hydraulic jump which we see is meta-stable in a certain parameter range. Such metastable states are characterized by two stable modes of hydraulic jump, and one unstable mode. In our experiments, we can manually "balance" the unstable mode of the hydraulic and show it to viewers, or we can place the flow in one or the other of the stable modes and flip it back and forth between the two. I think that we may be able to make an experiment where the unstable mode is balanced by elastic forces. The transition from one stable mode to another can appear suddenly (as a catastrophe). Applications are made to ash flows or turbidity currents encountering topography. Citation: Recent work by Baines and Whitehead in Physics of fluids January 2003.
- A cup sitting in the bottom of a basin that is flushed with fresh room temp water receives salty water at a constant rate. The cavity fills and overflows and the overflow is swept away. Then the bottom of the cavity is heated. Either it remains filled in the state with hot, salty relatively stagnant water, or the water has a mixed state where it overturns in the cavity and the cavity is flushed convectively with fresh water that becomes heated and mixes with the salty water. The transition from one state to the other is sudden even when the heating temperature is increases very slowly. New experiments are needed with electrical heating, which I believe might oscillate between the two states.
- A few years ago, we (Peter Kelemen, Kelsey Jordahl, and I) studied flow of water that was saturated with a salt that has a high temperature coefficient of solubility. We had it flow in a narrow gap over a cold plate so the salt deposits accumulated. The accumulations would block the flow that produced them, so as time went on, many structures are formed as interactions between the crystals and the flow channels. A particularly interesting effect happened right at the beginning, when a growing circle of water from a suddenly started point source developed a striking star-shaped pattern of crystals. We need to understand the dynamics that produce such structures. Applications are magma encountering colder regions, river delta regions and other places where material is moved in by liquid motion and then allowed to solidify.
Constraints on the depth of seismicity in the oceanic crust: Use microstructural observations, including SEM analyses, to constrain the deformation processes responsible for strain localization, faulting and earthquakes in the oceanic crust.
Catastrophic Volcanic Controls on Climate Change in the Middle Miocene.
Statistical analysis of the fossil record of Mass Extinctions.
Thermobarometry in peridotites and pyroxenites from the base of an arc crustal section in south central Alaska, via electron probe analysis of mineral compositions. Thermobarometry of gabbroic samples in the same section, plus data from the analogous Kohistan arc section in Pakistan, show that Moho temperatures are ~ 400°C hotter than predicted in any arc thermal model. There is some question about whether these are cooling temperatures set during the waning of arc activity, contact metamorphic temperatures, or cooling temperatures set during igneous rock solidification. Temperatures from residual peridotites and cumulate pyroxenites will help to resolve these questions. Rhea Workman did a few last spring. You can talk to her about her results. An ambitious student could help with theoretical development of a "new" plagioclase-olivine-pyroxene-spinel barometer, to be used not only on Talkeetna rocks, but also on plagioclase peridotites worldwide (Trinity and Josephine ophiolites, Kane Fracture zone peridotites (see (1), below), and many others.
Greg Hirth and Peter Kelemen
- OMAN AND OCEANIC SAMPLES
Do thermobarometry and mineral fabric studies on "impregnated", deformed peridotite samples from the Kane Fracture Zone, Mid-Atlantic Ridge. The textures of these samples suggest that localized deformation of mantle peridotite beneath the Fracture Zone extended to depths where migrating melt was present, at a temperature of perhaps 1200 °C. This is important in understanding the transition from localized deformation, in faults and ductile shear zones near the Earth's surface, to homogeneously distributed deformation in lower viscosity, higher temperature upper mantle.
- Make grain boundary images of plagioclase in foliation parallel thin sections of lower crustal gabbros from a full crustal section through the Oman ophiolite,in order to determine the relative strength of foliation and lineation as a function of depth in the crust. This, in turn, will constrain the mechanisms of deformation, recrystallization, and lower crustal accretion at oceanic spreading ridges. Guaranteed first author publication. These samples have already been studied for Crystal Size Distribution in foliation perpendicular sections (Garrido et al., G-cubed, 2001), and analyzed chemically by ICP-MS, XRF, electron probe and ion probe. (Maybe Brian deMartin is doing this already?).
- Determine variation of mineral composition and/or texture with height on the scale of individual outcrops in the Oman ophiolite. Korenaga & Kelemen (JGR, 1997) showed that vertical variation i gabbro lenses in the Moho Transition zone of the Oman ophiolite was on the scale of centimeters, thus constraining crystallization and diffusion kinetics. Korenaga & Kelemen (EPSL, 1998) showed how (poorly known) vertical compositional variation in the lower crust could be used to constrain melt transport and crustal accretion (this was a Geodynamics Seminar project). We used data on samples with a vertical spacing of 10 meters or more, which clearly is larger than the actual scale of vertical variation. Better data would greatly improve this constraint.
- Measure olivine sub-grain spacing in gabbro samples from a complete crustal section in the Oman ophiolite, in order to constrain stress and therefore viscosity during deformation as a function of depth in the crust. An initial, qualitative approach to this is described by Yoshinobu & Hirth (J. Struc. Geol., in press), but quantitative data are needed.
Comparing relationships between topography and seismicity in compressional and extensional settings.