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Rupturing Continental Lithosphere: A process-oriented approach to studying active extensional margins

A two-day workshop funded by NSF will be held at Snowbird, Utah on 27-28 January, 2000 to formulate a science plan for the focused investigation of faulting, strain partitioning, and magma emplacement at sites of active continental rifting where there is a transition to initial seafloor spreading. This science plan will serve as one of the templates for NSF funding of the MARGINS Program. It will be used to nurture and accelerate important directions for future research and will contain strategies for accomplishing this research. Like the science plans for the three other MARGINS initiatives, it will specify sites for detailed (and possibly allied) interdisciplinary studies based on site criteria agreed upon at the workshop.

Five process-oriented objectives and associated notional experiments comprised the MARGINS Initial Science Plan of 1996, which was based on community workshops and selected according to scientific merit, societal relevance, and feasibility (EOS, v.79, 137 & 142-143, 1998). Two of these objectives relate to: 1) The low-strength paradox of lithospheric deformation, and 2) Strain partitioning during deformation. These two objectives were combined into one of the four MARGINS Program Initiatives presented to NSF in 1997: Rupturing Continental Lithosphere and the Birth of an Ocean, described as follows.

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.

The following (draft) Focus-Site Criteria list desirable characteristics of focused study regions to be chosen to address the two fundamental questions posed by the MARGINS Initial Science Plan. While each of these criteria is deemed important, failure by a site to meet one or more criteria may not necessarily disqualify it from consideration. Defining the Site Criteria and evaluating the candidate focus sites are primary objectives of the upcoming Rupturing Continental Lithosphere Workshop.

Draft Focus-Site Criteria

  • Extension needs to culminate in sea-floor spreading
  • Identifiable conjugate margin segments
  • Identifiable, active low-angle normal fault
  • Post-rift and syn-rift packages can be imaged and sampled.
  • Basement and stratal geometry are not obscured by post-depositional deformation (e.g., minimal salt and shale tectonics)
  • Must know the timing of margin break-up (hence, margins formed with magnetic quiet zones may present problems to be avoided?).
  • Involves break-up of old (> or = Paleozoic) continental lithosphere
  • Access to geological and geophysical data (reflection and refraction seismics, potential field, drilling and logging data, and field observations)
  • Accessibility (logistically and politically)

Potential Focus Sites

A fundamental and appreciated aspect of the MARGINS program includes the need to focus on active and "complete" systems. The need to concentrate on active systems is because once they are non-active, it becomes more difficult to completely characterize the boundary conditions and the physical and chemical states of materials in the system. Furthermore, one or more of its characteristics may have changed during the active-passive transition, with paleo-conditions being potentially difficult to infer from the rock record. The "complete" system approach is deemed critical because of the need to study the extensional margin as a large, complex, interactive dynamic system. Given this necessity to concentrate on both active and complete extensional systems, in addition to the above Focus Site Criteria, there is actually only a handful of potential candidates around the World, such as (listed alphabetically):

  • Gulf of Aden / Gulf of Tadjura (Arabia-Somalia)
  • Gulf of California / Salton Sea (Mexico/USA)
  • Lena delta / Gakkel Ridge (Russia)
  • Red Sea / Gulf of Suez (Arabia-Nubia)
  • western Woodlark Basin (Papua New Guinea)

Both the "Draft Focus-Site Criteria" and the "Potential Focus Sites" will be a major part of the Snowbird workshop deliberations, and will be used to frame and focus the discussions during the two-day workshop. We present this information only as a preamble and as a source of background information to help seed thought and discussion prior to the meeting, in addition to generating feedback and comment on the criteria, topics for discussion and additional sites of investigation.