Rheology and Deformation

Garry Karner¹, Dave Kohlstedt², Neal Driscoll³, and Brian Taylor⁴

¹Lamont-Doherty Earth Observatory, Palisades, NY, 10964
²University of Minnesota, Morris, MN 56267
³Woods Hole Oceanographic Institution, Woods Hole, MA, 02543
⁴SOEST, University of Hawaii, Honolulu, HI, 96822

Rheology and Deformation of the Lithosphere at Continental Margins
The primary goal of the MARGINS Program is "to understand the complex interplay of processes that govern continental margin evolution". The plan is to investigate active systems as a whole, viewing a margin not so much as a "geological" entity of divergent, translational or convergent type, but more in terms of a complex physical, chemical and biological system, subject to a variety of influences. One approach that has been adopted by MARGINS to promote progress toward this goal is the organization of Theoretical and Experimental Institutes. These Institutes are designed to foster stronger interaction between observationalists, experimentalists, and theoreticians, and to give researchers and their students the required background to address complex, interdisciplinary problems.

NSF has funded the first MARGINS Theoretical and Experimental Institute (TEI) in January of 2000 to investigate the "Rheology and Deformation of the Lithosphere at Continental Margins". Traditionally, such investigations have taken place at one scale in the laboratory and at entirely different scale in the field. Laboratory experiments are generally restricted to centimeter-size samples and day/year-length times, while geological processes occur over tens to hundreds of kilometers and millions of years. Application of laboratory results to geological systems necessitates extensive extrapolation in both time and spatial scales, as well as a detailed understanding of the dominant physical mechanisms. Development of an understanding of large-scale processes requires an integrated approach. Communication between experimentalists and theoreticians is essential in order to design experiments to address the implications raised by macroscopic field observations. One of the principal objectives of the MARGINS TEI will be to stimulate cross-disciplinary inquiry into the rheology and deformation of lithosphere, which will provide a better understanding of the varying margin architectures observed and a framework within which laboratory, field, and modeling experiments can be posed.

The R&D TEI will consist of a four-day Short Course followed by a two-day workshop. The first day of the Short Course will provide an overview of the setting and nature of deformation at extensional and compressional continental margins. Day 2 will concentrate on: a) observations supporting, and models explaining, strain partitioning within the crust and lithosphere and b) numerical and analogue modeling experiments that address the scaling problem of comparing physical experiments with natural systems. Day 3 will focus on laboratory observations related to frictional sliding and crack healing along fault surfaces. Day 4 will center on experimental studies of the rheology of crustal rocks. The Workshop will last two days and will be focused on the Rupturing Continental Lithosphere (RCL) initiative of the MARGINS Program: "a comprehensive investigation of faulting, strain partitioning, and magma emplacement at sites of active continental rifting where there is a transition to initial seafloor spreading". With the input from the Short Course, the Workshop is designed to flesh out the RCL science plan (fieldwork, modeling and experiments) and to select field areas for focused investigation (as has been done previously at workshops for two of the other MARGINS Initiatives: the Seismogenic Zone Experiment and the Subduction Factory).

Timetable and Applications
The meeting will be held at Snowbird, Utah, January 23-30, 2000. One of the broad goals of MARGINS is to involve numerous researchers and students from a variety of fields in interdisciplinary research aimed at the complex interplay of processes that govern the formation and evolution of continental margins. Consequently, participation in the Short Course will be open to all researchers and students. Nevertheless, in an effort to maximize participation and effectiveness of communications, MARGINS supported attendance will be limited to 95 participants. For these participants, the MTEI will provide full travel and lodging costs for keynote speakers, convenors, students, and an additional 30 participants. Approximately 50% travel and lodging costs will be paid for an additional 20 participants. Registration fees will be reimbursed for keynote speakers, convenors, and students. High priority will be given to supporting students. Workshop participation will be limited to 30 people who will make the commitment to stay for the entire Workshop.

Applications to attend the TEI should be submitted to the MARGINS Office by 31 October. The MARGINS Steering Committee will evaluate these and make recommendations for the award of travel funds in November. To apply, send a one page e-mail message to margins@soest.hawaii.edu containing (1) address and contact information, including web site, (2) description of research interests, and (3) statement of potential contributions to the meeting. Inquiries may be addressed to either Garry Karner (garry@ldeo.columbia.edu) or Dave Kohlstedt (dlkohl@maroontc.umn.edu). For further information, see RheologyDeformation.html.

The MARGINS Steering committee is excited at the prospect of bringing together a number of research communities that traditionally have had little interaction. Prior to the meeting, participants will be asked to express thoughts about criteria for choosing topics and sites of investigation. These will be used to frame and focus the discussions during the two day workshop. Outlines of the short course lectures with key references and figures will be sent to all participants well in advance of the meeting. Following the meeting, we plan to publish the review papers derived from the lectures and associated participant research as a high-quality publication for broad distribution.

Rationale for the TEI
Rheology is the branch of Physics dealing with the deformation and flow of materials. Macroscopic observations of margins using remote sensing (e.g., seismics, gravity, magnetics) examine the style and wavelength of the deformation from which predictions are made concerning the "mode of deformation" (rheology) and how it varies throughout the deformational history. Laboratory experiments place constraints on the physical conditions required for materials to deform and illustrate how the deformational style varies as a function of temperature, strain rate, and material. Modeling efforts that incorporate and build on the results from laboratory experiments and make predictions of margin architecture that can be tested provide a potential vehicle to bridge the scaling problems of comparing physical experiments with natural systems.

• educate researchers and students in rheology and deformation processes and to concentrate on aspects of theory that observations can test.
• enhance communication and interaction between modelers, experimentalists, and observationalists.
• bridge the scaling problems associated with comparing physical experiments with natural systems.
• foster interdisciplinary studies required to make substantial advances in understanding how the earth deforms at margins.

Rupturing Continental Lithosphere
With the input from the Short Course, the two-day Workshop is designed to flesh out the Rupturing Continental Lithosphere science plan (fieldwork, modeling and experiments) and to choose field areas for focused investigation. The Rupturing Continental Lithosphere initiative derived from two of the five science foci identified in the MARGINS Initial Science Plan, 1996: (1) The Low-Stress Paradox and (2) Strain Partitioning (see http://www soest.hawaii.edu/margins/Science_Plan.html), and is briefly summarized here.

The mechanisms that allow continental lithosphere to be deformed by weak tectonic forces are not understood, nor is the manner in which strain is partitioned and magma distributed. These processes control the fundamental margin architecture and hence the location and magnitude of resources and geologic hazards. One way to solve these problems is to focus a comprehensive investigation on faulting, strain partitioning and magma emplacement at sites of active continental rifting where there is a lateral transition to initial seafloor spreading. The along strike variation will provide a spatial proxy for temporal variability. The effects of, and consequences for, hydrous fluids and sediments will be included in these integrated observational, laboratory and modeling experiments. The objectives of these experiments are to:

1. Determine the local and regional states of stress, the distribution and rate of strain, the pressures and temperatures, and the physical and chemical properties of rocks and fluids associated with a well-imaged and seismically active low-angle normal detachment (the extreme case of the weak fault paradox). Measurements of these in situ parameters made by drilling, instrumenting and long-term monitoring will be used to determine how such faults move at resolved shear stresses far smaller than those expected based on laboratory observations and Coulomb rheologies.

2. Determine the spatial and temporal distribution of strain by (i) mapping the geometry and offset of faults, (ii) inverting and modeling the stratigraphic and structural record to resolve the history of strain variation and its control on topography/erosion/deposition, (iii) using seismic, gravity/geoid and geothermal methods to obtain an integrated sum of the deformation and a measure of the ductile thinning of the lower crust, and (iv) evaluating the heterogeneity of the continental lithosphere prior to rifting.

3. Determine the pattern of mantle flow, the extent of melt generation, and the style of melt migration and emplacement during continental rifting and the early stages of seafloor spreading by imaging with seismic and electromagnetic methods an active rift-spreading transition, by measuring the heat flow distribution, and by analyzing the chemistry of magmas emplaced in these regions.