The proposed research program tests a number of
hypotheses related to crust-mantle interactions operating across a variety of
scales during continental accretion. Specifically we will address the following
questions:
1)
Continental
growth: We hypothesize that modern
continental growth results from accretion of intermediate-felsic arcs that
form on oceanic plateaus, like the Leeward Antilles-Caribbean type, rather
than Aleutian-type arcs. If this hypothesis is true then the
process by which
modern continental crust forms can produce a new continental area roughly
the size of the accretion zone, ~106 km2,
in 50Ma. One
such process occurring somewhere on the globe in the Phanerozoic
would produce
continental material equivalent to ~ 5% of the global continental
mass.
2)
Crustal
Mass Redistribition: How crustal
bouyancy and mantle driving forces influence crustal mass
redistribution during
subduction polarity reversal along an oblique collision zone is
unclear. Preliminary
geodynamic modeling suggests that not only is the interplay between crustal
buoyancy and the negative thermal buoyancy of the subducting
mantle lithosphere
a key to mass redistribution, but so too is the rheology of the lower crust
of the continent and arc and the degree to which the rheology does or does
not allow for partial crust-mantle decoupling. Thus, not only is
there a potential
to constrain the chemical buoyancy of arc crust, which relates directly to
its composition, but also to constrain the rheology of the lower
crust within
specific regions.
3) HP/LT Rock Exhumation: We conjecture that HP/LT rocks are exhumed in a two stage process: Arc parallel strike slip and extension first permits large-scale lateral transport and gradual shallowing of previously subducted sediments within strike-slip fault systems. Final exhumation then occurs by obduction of these rocks during transpression. The proposed studies will provide the timing and geometry to test this hypothesis, and validate it with constrained geodynamic modeling. As an alternative we can ask if deeply buried rocks travel more vertically than horizontally? The mechanism for largely vertical flow has been identified for relatively shallowly buried accretionary wedge rocks in the Olympic peninsula (Brandon et al., 1998). Since plate tectonics in the shallow Earth is largely a horizontal phenomena, it is reasonable to look for mechanisms that can exhume deeply buried rocks through horizontal transport. This is not unlike, but requires far more vertical and horizontal travel, than exhumation of metamorphic core complexes.
4)
Neogene
Basin Formation as a Geodynamic Phenomenon: Three different hypotheses aris in regard to the Neogene
basins in the
strike slip margin: 1) The Neogene basins along this margin
formed primarily
as pull-apart basins in a purely strike-slip or transpressional
plate boundary
and are therefore a function of strike slip fault offsets
resulting from pre-existing
structures along, i.e. the paleogeography of, the South American
passive margin.
2) They formed by arc-parallel extension arising as the arc straightens out
upon entering the transform boundary during arc accretion. 3) They are the
result of extension during orogenic collapse of the Caribbean Mountain belt
caused by relative changes in crustal buoyancy and mantle driving
forces spatially
correlated with subduction polarity reversal along the
boundary.
5)
Lithosphere
and sub-Lithospheric Mantle Interplay:
Detailed study of the mantle in this fascinating region can answer a number
of first order questions posed by current knowledge: For the
sublithospheric
mantle Russo and Silverâs model implies a broad zone of West to East
mantle flow beneath this entire plate boundary resulting from
subslab corner
flow around the northern end of the Nazca plate, generating the Caribbean
plate. However, in this region a number of complicating factors
arise: First,
the local subduction polarity reversal along this boundary should strongly
modulate the local mantle flow fields. Second the strike-slip
system may represent
localized rather than distributed mantle shear. Third although we
donât
know far north the continental mantle of the craton extends,
plate reconstructions
since the Cretaceous show continental and arc fragments being swept around
the northwest corner of South America, clearly influenced by
cratonic mantle
structure. We have an excellent situation to determine the lateral extent
of the craton both in a "natural" unperturbed setting in the East
as well as in a tectonically perturbed setting (the large scale,
perhaps deep)
strike-slip systems in the West. A
general conjecture for this region is that the local subduction features,
the local shear boundary, and the northern edge of the craton will severely
modulate the pattern of West to East flow generated by Nazca
subduction.
To test these we will develop geodynamic models
that require that we understand the timing of a number of events that occurred
along this plate boundary, as well as a number of fundamental geometries in the
plate boundary system. Geologic studies will focus on the 1) sequence and
timing of arc volcanism and cessation, 2) the pressure-temperature uplift
histories of HP/LT metamorphic rocks in the Caribbean mountain system, and 3)
timing the formation of the strike-slip fault system and formation of basins in
the plate margin to understand the dominant forms of development. The geologic
investigations will focus on the time transgressive development of the margin,
and will particularly emphasize the active source seismic corridors.
To understand
the evolution of the plate boundary and the accretion of the arc we will
determine modern geometries across a large range of scales to constrain the
geometry of the plate boundary: The largest scale is the mantle structure of
the entire plate boundary and surrounding plates. We will image the complex
geometry of the two lithospheric plates as subduction polarity reverses across
the margin, and examine the flow field in the deeper mantle beneath the
Caribbean plate boundary and South America. The lithospheric structures
illuminated by teleseismic imaging will be tied to the crustal and uppermost
mantle structures imaged by a reflection/refraction seismic program along the
length of the Leeward Antilles arc and along 3 N-S and 1 NW-SE corridors,
hereafter referred to as the 4 NS corridors, which extend across the Caribbean
deformed belt, the Leeward Antilles arc and into the Caribbean Mountain system.
In particular we will image 1) The sublithospheric mantle flow field. 2) The
structure of the lithospheric plates and their relation to crustal deformation,
3) The structure of the crust in the accretion zone of the arc and the
metamorphic belts as it evolves along the plate boundary, 4) The structure of
the crust in the incipient folded belt developing in the Trinidad-Gulf of Paria
region.