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Syn-collisional delamination in convergent orogens: insights from lithospheric buoyancy and physical modeling
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Delamination of the lower crust and lithospheric mantle has occurred in several orogens during convergence, most notably the Alps and Pyrenees. The factors responsible for initiating the delamination are not clear. Some workers suggest this process to be mechanically similar to tectonic wedging and triangle-zone development, well known at the scale of kilometers in fold and thrust belts. In a search for controlling factors, >35 convergent orogens explored by deep geophysical techniques were examined. No correlation between crustal composition and a propensity for delamination is evident. Delamination of subducting passive margins is more common than tectonically and/or volcanically active over-riding margins. Estimates of lithospheric density show that the lithospheric mantle of a cool passive margin may contain greater net negatively buoyant mass than a warmer active margin. The top of the lower lithospheric layer with the greatest net negatively buoyant mass is aligned with a weak zone in the lithosphere near the Moho. A greater net negatively buoyant mass indicates a propensity for delamination to initiate in a passive margin versus an active margin during a subsequent collisional event. For delamination to initiate above the Moho, the lower crust must be subducted or depressed to eclogite grade conditions. Transformation of the mafic lower crust to mafic eclogite also generates a lower lithospheric layer containing net negatively buoyant mass aligned with a weak zone in the lithosphere. Scaled physical models with a model continental crust weakly coupled to a lithospheric mantle containing net negatively buoyant mass delaminate during shortening. In contrast, weakly coupled models containing net positively buoyant mass within the model lithospheric mantle exhibit no tendency for delamination or wedging, despite a weak horizon at the base of the model crust. This contrast in behavior indicates that the mass distribution within the model lithosphere is a primary control on the process of syn-collisional delamination. Syn-collisional delamination is likely to occur in response to an increase in the lateral tectonic stress on a weak horizon already stressed by net negatively buoyant mass in the lower lithosphere. Thus, delamination and tectonic wedging is not a similar mechanical process at all scales.
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Includes bibliographical references (leaves 145-166).
Issued also on microfiche from Lange Micrographics.
Moore, Vernon Michael (2002). Syn-collisional delamination in convergent orogens: insights from lithospheric buoyancy and physical modeling. Master's thesis, Texas A&M University. Available electronically from
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