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:
- 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.
- 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.
- 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.
