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Breakup Without Borders
(2019)
Relative plate motions during continental rifting result from the interplay of local with far-field forces. Here we study the dynamics of rifting and breakup using large-scale numerical simulations of mantle convection with self-consistent evolution of plate boundaries. We show that continental separation follows a characteristic evolution with four distinctive phases: (1) an initial slow rifting phase with low divergence velocities and maximum tensional stresses, (2) a synrift speed-up phase featuring an abrupt increase of extension rate with a simultaneous drop of tensional stress, (3) the breakup phase with inception of fast sea-floor spreading, and (4) a deceleration phase occurring in most but not all models where extensional velocities decrease. We find that the speed-up during rifting is compensated by subduction acceleration or subduction initiation even in distant localities. Our study illustrates new links between local rift dynamics, plate motions, and subduction kinematics during times of continental separation.
role of subducted slabs
(2017)
Complex, time-dependent, and asymmetric rift geometries are observed throughout the East African Rift System (EARS) and are well documented, for instance, in the Kenya Rift. To unravel asymmetric rifting processes in this region, we conduct 2D geodynamic models. We use the finite element software ASPECT employing visco-plastic rheologies, mesh-refinement, distributed random noise seeding, and a free surface. In contrast to many previous numerical modeling studies that aimed at understanding final rifted margin symmetry, we explicitly focus on initial rifting stages to assess geodynamic controls on strain localization and fault evolution. We thereby link to geological and geophysical observations from the Southern and Central Kenya Rift. Our models suggest a three-stage early rift evolution that dynamically bridges previously inferred fault-configuration phases of the eastern EARS branch: (1) accommodation of initial strain localization by a single border fault and flexure of the hanging-wall crust, (2) faulting in the hanging-wall and increasing upper-crustal faulting in the rift-basin center, and (3) loss of pronounced early stage asymmetry prior to basinward localization of deformation. This evolution may provide a template for understanding early extensional faulting in other branches of the East African Rift and in asymmetric rifts worldwide. By modifying the initial random noise distribution that approximates small-scale tectonic inheritance, we show that a spectrum of first-order fault configurations with variable symmetry can be produced in models with an otherwise identical setup. This approach sheds new light on along-strike rift variability controls in active asymmetric rifts and proximal rifted margins.
Over the past few decades, azimuthal seismic anisotropy measurements have been widely used proxy to study past and present-day deformation of the lithosphere and to characterize convection in the mantle. Beneath continental regions, distinguishing between shallow and deep sources of anisotropy remains difficult due to poor depth constraints of measurements and a lack of regional-scale geodynamic modeling. Here, we constrain the sources of seismic anisotropy beneath Madagascar where a complex pattern cannot be explained by a single process such as absolute plate motion, global mantle flow, or geology. We test the hypotheses that either Edge-Driven Convection (EDC) or mantle flow derived from mantle wind interactions with lithospheric topography is the dominant source of anisotropy beneath Madagascar. We, therefore, simulate two sets of mantle convection models using regional-scale 3-D computational modeling. We then calculate Lattice Preferred Orientation that develops along pathlines of the mantle flow models and use them to calculate synthetic splitting parameters. Comparison of predicted with observed seismic anisotropy shows a good fit in northern and southern Madagascar for the EDC model, but the mantle wind case only fits well in northern Madagascar. This result suggests the dominant control of the measured anisotropy may be from EDC, but the role of localized fossil anisotropy in narrow shear zones cannot be ruled out in southern Madagascar. Our results suggest that the asthenosphere beneath northern and southern Madagascar is dominated by dislocation creep. Dislocation creep rheology may be dominant in the upper asthenosphere beneath other regions of continental lithosphere.
Continental rifting is responsible for the generation of major sedimentary basins, both during rift inception and during the formation of rifted continental margins. Geophysical and field studies revealed that rifts feature complex networks of normal faults but the factors controlling fault network properties and their evolution are still matter of debate. Here, we employ high-resolution 2D geodynamic models (ASPECT) including two-way coupling to a surface processes (SP) code (FastScape) to conduct 12 models of major rift types that are exposed to various degrees of erosion and sedimentation. We further present a novel quantitative fault analysis toolbox (Fatbox), which allows us to isolate fault growth patterns, the number of faults, and their length and displacement throughout rift history. Our analysis reveals that rift fault networks may evolve through five major phases: (a) distributed deformation and coalescence, (b) fault system growth, (c) fault system decline and basinward localization, (d) rift migration, and (e) breakup. These phases can be correlated to distinct rifted margin domains. Models of asymmetric rifting suggest rift migration is facilitated through both ductile and brittle deformation within a weak exhumation channel that rotates subhorizontally and remains active at low angles. In sedimentation-starved settings, this channel satisfies the conditions for serpentinization. We find that SP are not only able to enhance strain localization and to increase fault longevity but that they also reduce the total length of the fault system, prolong rift phases and delay continental breakup.
Flexural strike-slip basins
(2021)
Strike-slip faults are classically associated with pull-apart basins where continental crust is thinned between two laterally offset fault segments. We propose a subsidence mechanism to explain the formation of a new type of basin where no substantial segment offset or synstrike-slip thinning is observed. Such "flexural strike-slip basins" form due to a sediment load creating accommodation space by bending the lithosphere. We use a two-way coupling between the geodynamic code ASPECT and surface-processes code FastScape to show that flexural strike-slip basins emerge if sediment is deposited on thin lithosphere close to a strike slip fault. These conditions were met at the Andaman Basin Central fault (Andaman Sea, Indian Ocean), where seismic reflection data provide evidence of a laterally extensive flexural basin with a depocenter located parallel to the strike-slip fault trace.
Rock deformation at depths in the Earth’s crust is often localized in high temperature shear zones occurring at different scales in a variety of lithologies. The presence of material heterogeneities is known to trigger shear zone development, but the mechanisms controlling initiation and evolution of localization are not fully understood. To investigate the effect of loading conditions on shear zone nucleation along heterogeneities, we performed torsion experiments under constant twist rate (CTR) and constant torque (CT) conditions in a Paterson-type deformation apparatus. The sample assemblage consisted of cylindrical Carrara marble specimens containing a thin plate of Solnhofen limestone perpendicular to the cylinder’s longitudinal axis. Under experimental conditions (900 °C, 400 MPa confining pressure), samples were plastically deformed and limestone is about 9 times weaker than marble, acting as a weak inclusion in a strong matrix. CTR experiments were performed at maximum bulk shear strain rates of ~ 2*10-4s-1, yielding peak shear stresses of ~ 20 MPa. CT tests were conducted at shear stresses of ~ 20 MPa resulting in bulk shear strain rates of 1-4*10-4s-1. Experiments were terminated at maximum bulk shear strains of ~ 0.3 and 1.0.Strain was localized within the Carrara marble in front of the inclusion in an area of strongly deformed grains and intense grain size reduction. Locally, evidences for coexisting brittle deformation are also observed regardless of the imposed loading conditions. The local shear strain at the inclusion tipis up to 30 times higher than the strain in the adjacent host rock, rapidly dropping to 5times higher at larger distance from the inclusion. At both investigated bulk strains, the evolution of microstructural and textural parameters is independent of loading conditions. Ourresults suggest that loading conditions do not significantly affect material heterogeneity-induced strain localization during its nucleation and transient stages.
Observations of rift and rifted margin architecture suggest that significant spatial and temporal structural heterogeneity develops during the multiphase evolution of continental rifting. Inheritance is often invoked to explain this heterogeneity, such as preexisting anisotropies in rock composition, rheology, and deformation. Here, we use high-resolution 3-D thermal-mechanical numerical models of continental extension to demonstrate that rift-parallel heterogeneity may develop solely through fault network evolution during the transition from distributed to localized deformation. In our models, the initial phase of distributed normal faulting is seeded through randomized initial strength perturbations in an otherwise laterally homogeneous lithosphere extending at a constant rate. Continued extension localizes deformation onto lithosphere-scale faults, which are laterally offset by tens of km and discontinuous along-strike. These results demonstrate that rift- and margin-parallel heterogeneity of large-scale fault patterns may in-part be a natural byproduct of fault network coalescence.
The extent of continental rifts and subduction zones through deep geological time provides insights into the mechanisms behind supercontinent cycles and the long term evolution of the mantle. However, previous compilations have stopped short of mapping the locations of rifts and subduction zones continuously since the Neoproterozoic and within a self-consistent plate kinematic framework. Using recently published plate models with continuously closing boundaries for the Neoproterozoic and Phanerozoic, we estimate how rift and peri-continental subduction length vary from 1 Ga to present and test hypotheses pertaining to the supercontinent cycle and supercontinent breakup. We extract measures of continental perimeter-to-area ratio as a proxy for the existence of a supercontinent, where during times of supercontinent existence the perimeter-to-area ratio should be low, and during assembly and dispersal it should be high. The amalgamation of Gondwana is clearly represented by changes in the length of peri-continental subduction and the breakup of Rodinia and Pangea by changes in rift lengths. The assembly of Pangea is not clearly defined using plate boundary lengths, likely because its formation resulted from the collision of only two large continents. Instead the assembly of Gondwana (ca. 520 Ma) marks the most prominent change in arc length and perimeter-to-area ratio during the last billion years suggesting that Gondwana during the Early Palaeozoic could explicitly be considered part of a Phanerozoic supercontinent. Consequently, the traditional understanding of the supercontinent cycle, in terms of supercontinent existence for short periods of time before dispersal and re-accretion, may be inadequate to fully describe the cycle. Instead, either a two-stage supercontinent cycle could be a more appropriate concept, or alternatively the time period of 1 to 0 Ga has to be considered as being dominated by supercontinent existence, with brief periods of dispersal and amalgamation.