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Gang Lu
March 30, 2016 @ 12:15 pm - 1:15 pm
Magmatism at passive margins: Effect of depth-dependent rifting and depleted continental lithospheric counterflow
Some passive margins exhibit complex depth-dependent rifting that are not explained by the classical uniform-extension model. Typical examples are found at the Iberia-Newfoundland conjugate margins (Type I) with earlier breakup of crust, and at some ultra-wide South Atlantic margins (Type II) with wide thin crust beneath which the continental mantle lithosphere has been removed. Moreover, the southern segment of South Atlantic margins are volcanic, while margins of the central segment are contrastingly non-volcanic, although they are both wide margins. The dynamics of such structural styles and magmatism activities are not fully understood.
In this study, we perform 2-D thermo-mechanical numerical experiments to investigate magmatism at various passive margins. End-member models with strong (Model I) or weak (Model II) crust are designed to account for depth-dependent rifting. In addition, lithospheric counterflow are investigated by models with thick depleted mantle lithosphere (C models), compared with normal lithosphere (A models).
Results show that depth-dependent rifting and lithospheric counterflow strongly affect magmatism during rifting. Strong crust (model I-A) results in the formation of narrow margins and early establishment of seafloor spreading, while weak crust (model II-A) forms wide margin with early magmatism but late establishment of seafloor spreading. Thick (>20 km) igneous crust may form in the continent-ocean transition.
The presence of lithospheric lithosphere strongly delays the onset of magmatism. It results in large traction of exhumed continental mantle with limited magmatism in model I-C, and forms non-volcanic wide margin in model II-C.
We illustrate that model I-A and I-C are comparable with the Gulf of Aden conjugate margins and the Iberia-Newfoundland non-volcanic margins, respectively. Model II-C is consistent with central South Atlantic non-volcanic margins while model II-A likely contributes to the formation of extrusive igneous rocks at southern South Atlantic volcanic margins.