Rift migration explains continental margin asymmetry and crustal hyper-extension

Rift-induced disruption of cratonic keels drives kimberlite volcanism

Kimberlites are volatile-rich, occasionally diamond-bearing magmas that have erupted explosively at Earth's surface in the geologic past1-3. These enigmatic magmas, originating from depths exceeding 150 km in Earth's mantle1, occur in stable cratons and in pulses broadly synchronous with supercontinent cyclicity4. Whether their mobilization is driven by mantle plumes5 or by mechanical weakening of cratonic lithosphere4,6 remains unclear. Here we show that most kimberlites spanning the past billion years erupted about 30 million years (Myr) after continental breakup, suggesting an association with rifting processes. Our dynamical and analytical models show that physically steep lithosphere-asthenosphere boundaries (LABs) formed during rifting generate convective instabilities in the asthenosphere that slowly migrate many hundreds to thousands of kilometres inboard of rift zones. These instabilities endure many tens of millions of years after continental breakup and destabilize the basal tens of kilometres of the cratonic lithosphere, or keel. Displaced keel is replaced by a hot, upwelling mixture of asthenosphere and recycled volatile-rich keel in the return flow, causing decompressional partial melting. Our calculations show that this process can generate small-volume, low-degree, volatile-rich melts, closely matching the characteristics expected of kimberlites1-3. Together, these results provide a quantitative and mechanistic link between kimberlite episodicity and supercontinent cycles through progressive disruption of cratonic keels.

Ridge Jumps and Mantle Exhumation in Back-Arc Basins

Back-arc basins in continental settings can develop into oceanic basins, when extension lasts long enough to break up the continental lithosphere and allow mantle melting that generates new oceanic crust. Often, the basement of these basins is not only composed of oceanic crust, but also of exhumed mantle, fragments of continental crust, intrusive magmatic bodies, and a complex mid-ocean ridge system characterised by distinct relocations of the spreading centre. To better understand the dynamics that lead to these characteristic structures in back-arc basins, we performed 2D numerical models of continental extension with asymmetric and time-dependent boundary conditions that simulate episodic trench retreat. We find that, in all models, episodic extension leads to rift and/or ridge jumps. In our parameter space, the length of the jump ranges between 1 and 65 km and the timing necessary to produce a new spreading ridge varies between 0.4 and 7 Myr. With the shortest duration of the first extensional phase, we observe a strong asymmetry in the margins of the basin, with the margin further from trench being characterised by outcropping lithospheric mantle and a long section of thinned continental crust. In other cases, ridge jump creates two consecutive oceanic basins, leaving a continental fragment and exhumed mantle in between the two basins. Finally, when the first extensional phase is long enough to form a well-developed oceanic basin (>35 km long), we observe a very short intra-oceanic ridge jump. Our models are able to reproduce many of the structures observed in back-arc basins today, showing that the transient nature of trench retreat that leads to episodes of fast and slow extension is the cause of ridge jumps, mantle exhumation, and continental fragments formation.

Rheological inheritance controls the formation of segmented rifted margins in cratonic lithosphere

Observations from rifted margins reveal that significant structural and crustal variability develops through the process of continental extension and breakup. While a clear link exists between distinct margin structural domains and specific phases of rifting, the origin of strong segmentation along the length of margins remains relatively ambiguous and may reflect multiple competing factors. Given that rifting frequently initiates on heterogenous basements with a complex tectonic history, the role of structural inheritance and shear zone reactivation is frequently examined. However, the link between large-scale variations in lithospheric structure and rheology and 3-D rifted margin geometries remains relatively unconstrained. Here, we use 3-D thermo-mechanical simulations of continental rifting, constrained by observations from the Labrador Sea, to unravel the effects of inherited variable lithospheric properties on margin segmentation. The modelling results demonstrate that variations in the initial crustal and lithospheric thickness, composition, and rheology produce sharp gradients in rifted margin width, the timing of breakup and its magmatic budget, leading to strong margin segmentation.

The evolution of rifts and rifted margins is controlled by the rheology of the lithosphere. Thus, pre-existing lateral rheological variations can dominate the rifting process and lead to margin segmentation, with along-strike changes in crustal structure and nature and timing of continental breakup.

Melt volume at Atlantic volcanic rifted margins controlled by depth-dependent extension and mantle temperature

Breakup volcanism along rifted passive margins is highly variable in time and space. The factors controlling magmatic activity during continental rifting and breakup are not resolved and controversial. Here we use numerical models to investigate melt generation at rifted margins with contrasting rifting styles corresponding to those observed in natural systems. Our results demonstrate a surprising correlation of enhanced magmatism with margin width. This relationship is explained by depth-dependent extension, during which the lithospheric mantle ruptures earlier than the crust, and is confirmed by a semi-analytical prediction of melt volume over margin width. The results presented here show that the effect of increased mantle temperature at wide volcanic margins is likely over-estimated, and demonstrate that the large volumes of magmatism at volcanic rifted margin can be explained by depth-dependent extension and very moderate excess mantle potential temperature in the order of 50-80 °C, significantly smaller than previously suggested.

Rifted margins classification and forcing parameters

Rifted margins are the result of the successful process of thinning and breakup of the continental lithosphere leading to the formation of new oceanic lithosphere. Observations on rifted margins are now integrating an increasing amount of multi-channel seismic data and drilling of several Continent-Ocean Transitions. Based on large scale geometries and domains observed on high-quality multi-channel seismic data, this article proposes a classification reflecting the mechanical behavior of the crust from localized to diffuse deformation (strong/coupled to weak/decoupled mechanical behaviors) and magmatic intensity leading to breakup from magma-rich to magma-poor margins. We illustrate a simple classification based on mechanical behavior and magmatic production with examples of rifted margins. We propose a non-exhaustive list of forcing parameters that can control the initial rifting conditions but also their evolution through time. Therefore, rifted margins are not divided into opposing types, but described as a combination and continuum that can evolve through time and space.

Complex fault interaction controls continental rifting

Rifted margins mark a transition from continents to oceans and contain in their architecture a record of their rift history. Recent investigations of rift architecture have suggested that multiphase deformation of the crust and mantle lithosphere leads to the formation of distinct margin domains. The processes that control transitions between these domains, however, are not fully understood. Here we use high-resolution numerical simulations to show how structural inheritance and variations in extension velocity control the architecture of rifted margins and their temporal evolution. Distinct domains form as extension velocities increase over time and deformation focuses along lithosphere-scale detachment faults, which migrate oceanwards through re-activation and complex linkages of prior fault networks. Our models demonstrate, in unprecedented detail, how faults formed in the earliest phases of continental extension control the subsequent structural evolution and complex architecture of rifted margins through fault interaction processes, hereby creating the widely observed distinct margin domains.

Continental rifting and break up processes are poorly constrained in the early stages. Here, the authors using high-resolution numerical simulations to show how early formed faults in continental extension can then control subsequent structure evolution of rifts.

Chapter 4 Cenozoic rifting, passive margin development and strike-slip faulting in the Andaman Sea: a discussion of established v. new tectonic models

The Andaman Sea evolved from near-pure extension (WNW–ESE) during the Late Palaeogene, to highly oblique extension (NNW–SSE) during the Neogene, to strike-slip-dominated deformation (Late Miocene–Recent). These changes in extension direction and deformation style probably reflect the switch from slab rollback-driven extension to India coupling with Myanmar and driving oblique extension/dextral strike-slip. The East Andaman, Mergui–North Sumatra and Martaban Shelf basins, along with the Alcock and Sewell rises and Central Andaman Basin (CAB), were all involved with this deformation which became increasingly focused on the CAB and the rises with time. Possible revisions to traditional models for the Andaman Sea include: (1) the Alcock and Sewell rises are hyper-extended continental or island arc crust, not Miocene oceanic crust; (2) the East Andaman Basin is predominantly underlain by strongly necked to hyper-extended continental crust, not oceanic crust; or (3) CAB oceanic crust is of Miocene, not Pliocene–Recent age. At present a number of major issues can be addressed but not fully resolved, including: (1) the distribution, timing, volume and origin of magmatism in the basins; (2) the causes and significance of strong crustal reflections imaged on 2D and 3D seismic data; (3) implications for crustal thinning geometries, upper crustal extensional patterns and distribution of igneous intrusions for current models of passive margin development (i.e. volcanic v. non-volcanic margins), and how the back-arc setting modifies these models. Elements of both volcanic and non-volcanic margins are present in the East Andaman Sea, with well-developed necking of continental crust (perhaps due to dry mafic, granulite facies lower crust) and extensive igneous intrusions in the lower and middle crust.

The importance of structural softening for the evolution and architecture of passive margins

Lithospheric extension can generate passive margins that bound oceans worldwide. Detailed geological and geophysical studies in present and fossil passive margins have highlighted the complexity of their architecture and their multi-stage deformation history. Previous modeling studies have shown the significant impact of coarse mechanical layering of the lithosphere (2 to 4 layer crust and mantle) on passive margin formation. We built upon these studies and design high-resolution (~100–300 m) thermo-mechanical numerical models that incorporate finer mechanical layering (kilometer scale) mimicking tectonically inherited heterogeneities. During lithospheric extension a variety of extensional structures arises naturally due to (1) structural softening caused by necking of mechanically strong layers and (2) the establishment of a network of weak layers across the deforming multi-layered lithosphere. We argue that structural softening in a multi-layered lithosphere is the main cause for the observed multi-stage evolution and architecture of magma-poor passive margins.

Abrupt plate accelerations shape rifted continental margins

Rifted margins are formed by persistent stretching of continental lithosphere until breakup is achieved. It is well known that strain-rate-dependent processes control rift evolution, yet quantified extension histories of Earth's major passive margins have become available only recently. Here we investigate rift kinematics globally by applying a new geotectonic analysis technique to revised global plate reconstructions. We find that rifted margins feature an initial, slow rift phase (less than ten millimetres per year, full rate) and that an abrupt increase of plate divergence introduces a fast rift phase. Plate acceleration takes place before continental rupture and considerable margin area is created during each phase. We reproduce the rapid transition from slow to fast extension using analytical and numerical modelling with constant force boundary conditions. The extension models suggest that the two-phase velocity behaviour is caused by a rift-intrinsic strength--velocity feedback, which can be robustly inferred for diverse lithosphere configurations and rheologies. Our results explain differences between proximal and distal margin areas and demonstrate that abrupt plate acceleration during continental rifting is controlled by the nonlinear decay of the resistive rift strength force. This mechanism provides an explanation for several previously unexplained rapid absolute plate motion changes, offering new insights into the balance of plate driving forces through time.