Heterogeneous Continental Lithosphere Research Articles

The role of continental heterogeneity on the evolution of continental plate margin topography at subduction zones

The nature of the overriding plate plays a major role for subduction zone processes. In particular, the highly heterogeneous continental lithosphere modulates intra-plate tectonics and the surface evolution of our planet. However, the role of continental heterogeneity is relatively under-explored for the dynamics of subduction models. We investigate the influence of rheological and density variations across the overriding plate on the evolution of continental lithosphere and slab dynamics in the upper mantle. We focus on the effects of variations in continental plate margin and keel properties on deformation, topographic signals, and basin formation. Our results show that the thickness, extent, and strength of the continental plate margin and subcontinental keel play a crucial role for the morphology and topography of the overriding plate, as well as the retreat of the subducting slab. We show that this lateral heterogeneity can directly influence the coupling between the subducting and overriding plate and determine the partitioning of plate velocities across the overriding plate. These findings suggest that back-arc extension and subsidence are not solely controlled by slab dynamics but are also influenced by continental plate margin and keel properties. Large extended back-arc regions, such as the Pannonian and Aegean basins, may result from fast slab rollback combined with a weak continental plate margin and a strong and extended continental keel. Narrow margins, like the Okinawa Trough in NE Japan, may indicate a comparatively stronger continental plate margin and weaker or smaller continental keel. Additionally, continental keel properties may affect the overall topography of the continental lithosphere, leading to uplift of the deformation front and the formation of intermontane basins.

Gravity inversion for crustal thickness investigation of the continental and oceanic crusts of Brazil incorporating a lithosphere thermal gravity anomaly correction

The application of potential field methods in regional geophysical studies with orbital data has proven helpful in identifying, modeling, and dimensioning structural features and geophysical signatures in regions of interest. Potential field data, particularly those from gravity satellites, are robust and have a high resolution for regional studies. Therefore, gravity data enable in-depth identification of information from low-frequency sources, interpreted as features with relevant geotectonic significance. Furthermore, by containing wavelengths from low-frequency sources, these data allow investigation of lithospheric depths. However, it is necessary to apply thermal correction because gravity data are affected by thermal anomalies present in the lithosphere. Thus, the greater precision/robustness is attributed to these extensive and heterogeneous areas onshore. In this context, data from the compilation World Gravity Map – 2012 were applied to evaluate the density variation of features in depth and obtain the crust-mantle limit. The three-dimensional (3D) gravity inversion with seismic and seismological constraints was applied to obtain the crust-mantle limit for the onshore and offshore regions of Brazil. Furthermore, our objective was to apply and evaluate a gravity inversion methodology with seismic data constraints for the onshore portion of Brazil, which has an exotic and rich geological setting. The onshore portion is covered by seismographic stations, but has sparse coverage areas, mainly in the Amazonian Craton. In addition, the scarcity of seismic refraction data in the national territory makes it impossible to detail the structure of the base of the crust or lithosphere in a spatialized manner. To obtain the crust-mantle limit, the inversion permits corrections, such as topographic effects, the gravity effect of sediments, and the thermal gravity anomaly correction, which interferes with the investigation of crustal thickness using gravity data. Thus, it was possible to observe shallow Moho depths (∼10 km) in the offshore portion and deep (∼50 km) in the cratonic regions onshore, equivalent to the depths provided by the deep refraction seismic and receiver function data. Therefore, from the gravity inversion, it was possible to obtain the 3D depth of the Moho, adding information to the seismic interpretations, with the possibility of observing 3D features and profiles that present a resolution compatible with the seismic data. Moreover, we interpreted areas with obliterated signatures owing to thermal anomalies in the heterogeneous continental lithosphere.

Continental Magmatism: The Surface Manifestation of Dynamic Interactions Between Cratonic Lithosphere, Mantle Plumes and Edge‐Driven Convection

AbstractSeveral of Earth's intra‐plate volcanic provinces lie on or adjacent to continental lithosphere. Although many are believed to mark the surface expression of mantle plumes, our limited understanding of how buoyant plumes interact with heterogeneous continental lithosphere prevents further progress in identifying mechanisms at the root of continental volcanism. In this study, using a suite of 3‐D geodynamical models, we demonstrate that the magmatic expression of plumes in continental settings is complex and strongly sensitive to the location of plume impingement, differing substantially from that expected beneath more homogeneous oceanic lithosphere. Within Earth's continents, thick cratonic roots locally limit decompression melting. However, they deflect plume conduits during their ascent, with plume material channeled along gradients in lithospheric thickness, activating magmatism away from the plume conduit, sometimes simultaneously at locations more than a thousand kilometres apart. This magmatism regularly concentrates at lithospheric steps, where it may be difficult to distinguish from that arising through edge‐driven convection. At times, the flow field associated with the plume enhances melting at these steps long before plume material enters the melting zone, implying that differentiating geochemical signatures will be absent. Beneath regions of thinner lithosphere, plume‐related flow can force material downwards at lithospheric steps, shutting off pre‐existing edge‐related magmatism. Additionally, variations in lithospheric structure can induce internal destabilization of ponding plume material, driving intricate magmatic patterns at the surface. Our analysis highlights the challenges associated with linking continental magmatism to underlying mantle dynamics, motivating an inter‐disciplinary approach in future studies.

Neoproterozoic modification of heterogeneous continental lithosphere beneath the Yangtze interior: revealed from mafic dykes from the Huangling area, South China

Neoproterozoic modification of heterogeneous continental lithosphere beneath the Yangtze interior: revealed from mafic dykes from the Huangling area, South China

Heterogeneous modification and reactivation of a craton margin beneath the Korean Peninsula from teleseismic travel time tomography

Margins of old continental lithosphere are likely prone to ongoing modification processes. Therefore, constraining detailed structures beneath the margin can be essential in understanding the evolution of the continental lithosphere. The eastern margin of the Eurasian plate is a natural laboratory that allows us to study the strong effects from multiple episodes of continental collision and subduction of different oceanic plates since their formation. To reveal the detailed evolution of cratons at their margins, we describe, for the first time, the upper mantle structures beneath the southern Korean Peninsula (SKP) based strictly on teleseismic relative arrival time data from densely deployed local seismic arrays, which allows us to constrain the details of the lithospheric structures beneath the Archean-Proterozoic basement. We imaged a thick (~150 km) high-velocity anomaly mainly beneath the Proterozoic Yeongnam Massif with large velocity contrasts (dlnVp ≈ 4.0% and dlnVs ≈ 6.0%) at its boundaries, suggesting the presence of a long-lasting cratonic root in the southwestern SKP. On the other hand, low-velocity anomalies were found beneath the Proterozoic Gyeonggi Massif, Gyeongsang arc-back-arc basin, and along the eastern margin of the SKP, indicating significantly modified regions. The possible existence of a remnant cratonic root beneath the SKP and contrasting lithospheric structures across the different Precambrian massifs suggests the highly heterogeneous modification of cratonic lithosphere at the eastern Eurasian plate margin. Strong velocity reductions, which indicate a thermally elevated upper mantle potentially with partial melts, correspond to areas of Cenozoic basalts, high surface heat flow, and high topography along the eastern KP margin. We interpret this coincidence as a result of recent reactivation of a craton margin, which is controlled by intense interaction between the convective upper mantle and heterogeneous continental lithosphere.

Interactions between propagating rifts and linear weaknesses in the lower crust

AbstractPre-existing structures in the crust such as shear zones, faults, and mobile belts are known to exert a significant control on the structural evolution of continental rifts. However, the influence of such features when the extension direction progressively changes over time remains uncertain. Here we present new results from three-dimensional lithospheric-scale laboratory experiments of rotational extension that provide key insights into the temporal evolution of propagating rifts. We specifically test and characterize how rifts propagate and interact with linear crustal rheological heterogeneities oriented at variable angles with respect to the extension direction. Results show that approximately rift-parallel pre-existing heterogeneities favor the formation of long, linear faults that reach near-final lengths at early stages. Low angles between the heterogeneities and the propagating rift axis may result in strong strike-slip reactivation of the pre-existing structures if they are suitably oriented with respect to the stretching direction. When the linear heterogeneities are oriented at intermediate to high angles rift branches become laterally offset as they propagate, resulting in complex rhombic fault patterns. Rift-perpendicular crustal heterogeneities do not affect fault trends during rift propagation, but cause stalling and deepening of laterally growing rift basins. Similarities between the analogue experimental results and selected natural examples provide insights on how nature finds the preferential pathway to breakup in heterogeneous continental lithosphere.

Thinning mechanisms of heterogeneous continental lithosphere

The mechanisms responsible for the formation of extremely thinned continental crust (<10 km thick) and lithosphere during rifting remains debated. Observations from present-day and fossil passive margins highlight the role of deep-seated deformation, likely controlled by heterogeneities within the continental lithosphere, such as changing lithologies, mechanical anisotropies and inherited structures. We investigate the mechanisms of lithospheric thinning by exploring the role of pre-existing heterogeneities on the architecture and evolution of rifted margins. We estimate pre-rift pressure conditions (P0) vs. depth diagrams of crustal to lithospheric sections, to quantify rift-related modifications on inherited lithostatic pressure gradients. Two field examples from the Alpine Tethys margins in the Eastern and Southern Alps (SE Switzerland and N Italy) were selected to characterize: (1) the pre-rift architecture of the continental lithosphere; (2) the localization of rift-related deformation in distinct portions of the lithosphere; and (3) the interaction between pre-existing heterogeneities of the lithosphere and rift-related structures. These observations are compared with high-resolution, two-dimensional thermo-mechanical numerical models. The design of the models takes into account pre-existing mechanical heterogeneities representing the initial pre-rift architecture of the continental lithosphere. Extensional structures consist of high-angle and low-angle normal faults, anastomosing shear-zones and decoupling horizons. Such structures accommodate the lateral extraction of mechanically stronger levels derived from the middle to lower crust. As a result, the extremely thinned continental crust in Tethyan passive margins represents the juxtaposition and amalgamation of distinct strong levels of the crust separated by major extensional structures identified by sharp pressure gradients. Future work should determine the applicability of these results to other present-day and fossil rifted margins.

Lithosphere erosion and continental breakup: Interaction of extension, plume upwelling and melting

We present the results of thermo-mechanical modelling of extension and breakup of a heterogeneous continental lithosphere, subjected to plume impingement in presence of intraplate stress field. We incorporate partial melting of the extending lithosphere, underlying upper mantle and plume, caused by pressure–temperature variations during the thermo-mechanical evolution of the conjugate passive margin system. Effects of melting included in the model account for thermal effects, causing viscosity reduction due to host rock heating, and mechanical effects, due to cohesion loss. Our study provides better understanding on how presence of melts can influence the evolution of rifting. Here we focus particularly on the role of melting for the temporal and spatial evolution of passive margin geometry and rift migration. Depending on the lithospheric structure, melt presence may have a significant impact on the characteristics of areas affected by lithospheric extension. Pre-existing lithosphere heterogeneities determine the location of initial breakup, but in presence of plumes the subsequent evolution is more difficult to predict. For small distances between plume and area of initial rifting, the development of symmetric passive margins is favored, whereas increasing the distance promotes asymmetry. For a plume-rifting distance large enough to prevent interaction, the effect of plumes on the overlying lithosphere is negligible and the rift persists at the location of the initial lithospheric weakness. When the melt effect is included, the development of asymmetric passive continental margins is fostered. In this case, melt-induced lithospheric weakening may be strong enough to cause rift jumps toward the plume location.

Magnesium isotopic heterogeneity across the cratonic lithosphere in eastern China and its origins

Available data in the literature have demonstrated a broad magnesium (Mg) isotope range for mantle and lower continental crustal rocks, implying an isotopically heterogeneous continental lithosphere, but its origin has not been thoroughly understood. Here, to investigate the primary cause of lithospheric Mg isotopic heterogeneity, we report major-trace elements, Sr and Mg isotope data for thirty deep-seated mafic xenoliths, which sampled different lithospheric depths in the southeastern North China Craton (NCC). The xenoliths are classified into three types based upon petrology and mineralogy, sampling from middle continental crust (Group I), lower continental crust (Group II) and lithospheric mantle (Group III), respectively. The Group I xenoliths have mantle-like to slightly high δ26Mg values (−0.32‰ to +0.01‰), whereas some of the Group II xenoliths have very low δ26Mg values (−0.93‰ to −0.07‰), reflecting substantial reaction with intracrustal carbonate-derived fluids. Combined with data in the literature, the results suggest that the Mg isotopic composition of the lower continental crust is much more heterogeneous and lighter on average relative to the middle continental crust. Except for one sample, the Group III xenoliths have extremely low δ26Mg values (−1.23‰ to −0.73‰), the lightest among values already reported for mantle-derived rocks including peridotites and basalts. They also have highly variable 87Sr/86Sr ratios, of 0.70387 to 0.71675. The covariation of Mg and Sr isotopes in Group III xenoliths can be explained by Mg and Sr isotopic exchange reactions during mantle metasomatism, implying that the sub-continental mantle has been significantly modified by fluids derived from recycled carbonate-pelite bearing oceanic crust. Together with the metasomatism age of ∼400 Ma obtained for one Group III xenolith, the results provide new evidence for the presence of extremely low-δ26Mg rocks in the lithosphere and indicate ancient carbonate recycling during Paleo-Tethys slab subduction. Experimental and computational studies have demonstrated that the solubility of carbonate in aqueous fluids increases with increasing pressures. Therefore, we suggest that the decreasing δ26Mg values of mantle-derived rocks with increasing depths across cratonic lithosphere, as found in the NCC and other regions, have probably been caused by carbonate metasomatism to various extents at different lithospheric depths.

Isotopic shifts in the Cenozoic Andean arc of central Chile: Records of an evolving basement throughout cordilleran arc mountain building

The analysis of new and published Hf and Nd isotopic data of late Cenozoic Andean arc igneous rocks from central Chile, coupled with our improved knowledge of orogenic processes in the region, reveals a tight link between major magmatic isotopic shifts and different Andean basement domains and timing of the main uplifting event. Oligocene–Miocene magmas from the Western Principal Cordillera show a nearly constant and juvenile composition (e HfI : +5 to +10; e NdI : +2 to +7), while those from the Eastern Principal Cordillera, formed since early late Miocene, are variably more enriched (e HfI : −4 to +4; e NdI : 0 to +3). Post–4.8 Ma magmas from both belts share an enriched signature (e NdI : −2 to +2) reflecting source contamination from east to west, contrary to the eastward subduction direction, in a process that occurred toward the end of the main Andean uplifting event. This results from the deep western basement underthrusting the orogen, and thus accounts for the westward propagation of the eastern enriched isotopic signatures approximately coeval with thickening and uplifting events. The observed patterns highlight the strong control exerted by the continental lithosphere on the composition of arc magmas over deep controls from the subduction-modified asthenospheric mantle. Moreover, they dynamically represent both (1) the hybridization affecting magmas ascending from the mantle in a heterogeneous continental lithosphere, and (2) the evolution of such lithosphere resulting from the thermal weakening and mass transfer processes occurring underneath cordilleran arcs during mountain building.

Isotopic and geochemical characteristics of the late Miocene subalkali and alkali basalts of the southern part of the Russian Far East and the role of continental lithosphere in their genesis

Major oxides, trace elements (ICP-MS analysis), and Sr isotope ratios were analyzed in the late Miocene subalkaline and alkaline basaltoids of the southern part of the Russian Far East, which were formed during the final stage of the development of intraplate basaltic volcanism. Based on these data, variations in the main geochemical and mineralogical characteristics of various tectonomagmatic terranes were evaluated. The enriched and heterogeneous continental lithosphere modified to a varying extent by postaccretion subduction processes of different ages played a major role in the formation of the rocks. The first geochemical evidence was obtained for the subduction of the Solonker paleoceanic plate beneath the Amur microcontinent during the Permian.

Initiation and propagation of shear zones in a heterogeneous continental lithosphere

Continental plates represent rheologically heterogeneous media in which complex finite strain fields may develop due to interaction of plate tectonic processes with intraplate heterogeneities. Such a deformation pattern is displayed by the Borborema shear zone system in northeastern Brazil. It involves continental‐scale, curvilinear, E‐W trending right‐lateral transcurrent shear zones that branch off from a major NE trending, right‐lateral strike‐slip deformation zone formed at the northern termination of the São Francisco craton and that finally terminate in transpressive metasedimentary belts. We suggest that this strain field results from the compressional deformation of a highly heterogeneous continental lithosphère composed of a stiff domain (craton) and rheologically weaker domains (basins). The effect of a low‐viscosity domain on the deformation of a continental plate and, in particular, the perturbation induced by this domain on the development of a shear zone formed at the termination of a stiff block were investigated using numerical modeling. The low‐strength domain induces an enhanced strain localization, and the geometry of the shear zone is significantly modified. It is either split, forming a branched shear zone system in which one branch maintains its original orientation while the other rotates toward the low‐viscosity domain, or completely rotated toward the weak block. The perturbation of the finite strain field depends on the ability of the weak domain to accommodate deformation, which is controlled by its initial viscosity contrast relative to the surrounding lithosphere, its orientation relative to the convergence direction, and its distance from the shear zone initiated at the termination of the stiff block. The interaction between imposed boundary conditions (tectonic forces and plate geometry) and the internal structure of the plate may give rise to highly heterogeneous strain fields, as exhibited by the Borborema shear zone system, in which intraplate rheological heterogeneities induce the development of branched or sinuous shear zones. A heterogeneous continental plate subjected to a normal convergence may therefore display significant lateral variations in strain intensity, with shear zones bordering nearly undeformed blocks, and in deformation regimes and vertical strains that would result in contrasting metamorphic and uplift histories.

Self-indentation of a heterogeneous continental lithosphere

The Neoproterozoic Ribeira-Aracuai belt stretches along the southeastern edge of the Sao Francisco craton of southeastern Brazil and extends south of the cratonic domain for >1000 km. The termination of the craton is spatially correlated with a significant modification of the deformation pattern in the belt: (1) the structural trend bends from due north to northeast, (2) the dominant tectonic flow shifts from orogen-transverse to orogen-normal, and (3) the metamorphic conditions of deformation decrease southwestward from high to medium grade. Finite-element modeling suggests that the presence of a craton within a continent favors strain localization, initiation of continental-scale shear zones, and differential vertical deformations. The southward termination of the Sao Francisco craton may have triggered the development of the complex deformation pattern that characterizes the Ribeira-Aracuai belt.

Episodicity in foredeep basins

The stratigraphic record contained in several foredeep basins is incompatible with the formation of these basins by flexure of a homogeneously strong foreland lithosphere. For example, the stratigraphy of the Early Proterozoic Kilohigok foredeep basin of northern Canada indicates that the inner part of the basin deepened in time without migration of the flexural bulge toward the foreland, and that the flexural bulge remained fixed in time relative to the downgoing plate. One mechanism that may be responsible for the localization of the flexural bulge in the Kilohigok and similar basins is that of lateral strength variations in the flexing lithosphere. Our results show that lateral strength variations within the foreland lithosphere can produce temporal variations in basin geometry and also serve to localize the position of the flexural bulge relative to the foreland for extended periods of time. This produces basin episodicity on a time scale of millions to tens of millions of years. Perfectly elastic models require a large strength contrast (about 50% in elastic plate thickness), but a brittle-elastic-ductile lithosphere rheology reduces the magnitude of the strength contrast required for episodicity to about 10%-15% of the initial effective elastic plate thickness. We suggest that basin episodicity may induce comparable episodicity in the coeval thrust belt, because theoretical studies of thrust mechanics suggest that the deformation in thrust belts is highly sensitive to the dip of the basal thrust angle. Through mechanisms such as this, the laterally heterogeneous continental lithosphere may provide a natural filter that transforms the presumably continuous and smoothly varying processes of the mantle into the spatially irregular and temporally episodic deformation observed in the crust at time scales of tens of millions of years.