Project Goals: Hypotheses
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 Silver's 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 don't know how 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.