Necking of the Lithosphere: A Reappraisal of Basic Concepts With Thermo‐Mechanical Numerical Modeling

Abstract

We investigate lithosphere necking using two‐dimensional thermo‐mechanical numerical simulations without strain softening or weakening mechanisms. The models have an initial small sinusoidal perturbation of the Moho depth, whose wavelength corresponds to the model width. Applied boundary conditions (constant extension velocity or bulk extension rate) and initial model width significantly impact the necking dynamics. For constant bulk extension rates, wider models generate more intense necking with locally higher strain rates, whereas for constant velocity extension, models evolution is similar independent on their initial width. However, the width of the final necking zones ranges consistently between 45 and 105 km, independent on the type of applied boundary conditions and the initial Moho wavelength. The modeled widths are similar to along dip necking zones widths of natural rifted margins that formed during a single, unidirectional, and relatively continuous extensional event (e.g., Iberia‐Newfoundland margins, Porcupine Basin, Gulf of Aden). When the crust is mechanically decoupled from the mantle by a weak ductile lower crust, models exhibit three characteristic stages: (1) distributed thinning and extension associated with progressive subsidence; (2) upper mantle necking compensated by flow of the weak lower crust, which hampers both crustal thinning and subsidence at the rift center; and (3) crustal necking associated with fast subsidence after the mantle has necked. Decoupled models display regions of relatively thick crust on one or both sides of the rift center, comparable to the Galicia, Rockall, Hatton, and Porcupine Banks along the North Atlantic rifted margins.

Controls on the Thermomechanical Evolution of Hyperextended Lithosphere at Magma‐Poor Rifted Margins: The Example of Espirito Santo and the Kwanza Basins

“…Until this point, viscoelastoplastic boudinage of the crust dominates lithospheric deformation. H‐Block formation and coupling imposes a deformation wavelength that is dependent on the strength of the mantle and the width of H‐Block and is generating a necking instability at the lithosphere asthenosphere boundary (LAB; Chenin et al, ). This instability leads to the accelerated thinning of the mantle lithosphere by buoyant upwelling.…”

“…This is followed by a phase of rapid subsidence down to 2,000 below sea level and rift flank uplift up to 1,000 m during the generation of H‐Block and the initiation of DDT through lithospheric necking (Huismans & Beaumont, ; Chenin et al, ; from 5 to 10 Myr [140 to 135 Ma]; Figures b and c). Individual basins in half‐graben structures form on both sides of the central basin floored by the top of H‐Block.…”

“…This is followed by a phase of rapid subsidence down to 2,000 below sea level and rift flank uplift up to 1,000 m during the generation of H-Block and the initiation of DDT through lithospheric necking (Huismans & Beaumont, 2008;Chenin et al, 2018; Figures 7c and 7d) the crust is wholly thinned, and mantle is exhumed along with detachments that are evolving within the serpentinized lithospheric mantle. Due to the extreme crustal thinning in the distal margin, subsidence increases from~1000 m to~2500 m below sea level (Figures 7e to 7f).…”

“…This exercise is critical at a time when new thermochronological data of fossil magma‐poor margins in the Alps and the Pyrenees have shown that reheating of the crust and the sediments occurs during lithospheric necking (thinning; Smye & Stockli, ; Beltrando et al, ; Seymour et al, ; Hart et al, ; Smye et al, ). Some authors suggest that these thermal events can be explained by rapid, active mantle upwelling during necking (Smye et al, ) and suggest that structures coupling fault in the crust and mantle are linked to both hyperextension and an actively upwelling mantle that could increase heat flow at the base of the extending crust and mantle lithosphere (Chenin et al, ; Gallacher et al, ; Hart et al, ; Huismans & Beaumont, , ; Royden & Keen, ; Svartman Dias et al, , ).…”

“…Numerical models can simulate the evolution of magma‐poor hyperextended margins for first‐order features and be used as experimental tools to test mechanisms by which crustal and mantle deformation becomes coupled or decoupled (e.g., Brune et al, ; Chenin et al, ; Davis & Lavier, ; Huismans & Beaumont, , , ; Lavier & Manatschal, ; Naliboff et al, ; Pérez‐Gussinyé & Reston, ; Ranéro & Pérez‐Gussinyé, ; Ros et al, ; Svartman Dias et al, , ; Van Avendonk et al, ). These models did verify that rifting can be polyphase (Lavier & Manatschal, ) and involves the thinning of the continental lithosphere by brittle faulting and ductile shearing eventually leading to embrittlement of the whole lithosphere and the coupling of crustal and mantle deformation.…”