Pre-existing Zones Of Weakness Research Articles

Crustal‐Scale Pop‐Up Structure at the Junction of Two Continental‐Scale Deformation Zones in the Southern Baltic Sea

AbstractThe southern Baltic Sea is a peculiar area, where the Sorgenfrei‐Tornquist Zone (STZ), stretching from Bornholm into the North Sea, connects to the Teisseyre‐Tornquist Zone (TTZ) that continues SE up to the Black Sea. In this study, we show the structure and evolution of this controversially debated area, both on crustal and basin scale, by using three seismic reflection profiles combined with 2‐D potential field data. The results demonstrate that the southern Baltic Sea is underlain by a thick crust of the East European Craton with a Moho depth in the range of 38–42 km. The overall crustal architecture is shaped by three phases of localized stretching in the early Paleozoic, Devonian‐Carboniferous, and Permian‐Mesozoic. The most spectacular feature of the southern Baltic Sea is a zone of thick‐skinned compressional deformation produced by Late Cretaceous‐early Paleogene inversion, including a system of thrusts and back thrusts penetrating the entire crust in an 80–90 km wide inversion zone. ENE‐vergent thrusts are traced from the top of the Cretaceous down to the Moho and they are accompanied by back thrusts of opposite vergence, also reaching the Moho. Inversion tectonics resulted in the uplift of a block of cratonic crust as a pop‐up structure, bounded by thrusts and back thrusts, and the displacement of the Moho within the STZ and TTZ. The similar mechanism of intra‐cratonic inversion was recognized for the Donbas Foldbelt in eastern Ukraine, and it may be characteristic of rigid cratons, where deformation is localized in a few preexisting zones of weakness.

Deep lithosphere-coupled surface deformation beneath Southeastern Brazil from amphibious magnetotellurics

The southeastern Brazilian onshore-offshore margin and underlying lithosphere are investigated by three-dimensional magnetotelluric imaging using 202 high-quality broadband and long-period amphibious MT-dataset, orthogonally traversing the regional surface deformation hills of Serra da Mantiqueira and Serra do Mar, as well as the coast-parallel onshore-offshore lineaments. Three-dimensional conductivity model reveals an electrically heterogeneous crust (up to 40 km) in the onshore Brasília and Ribeira Orogenic Belts, and ∼ 2–10 km thick-conductive-sedimentary-wedge beneath the offshore Santos Basin laying on top of a stretched and thinner resistive crust. Regionally, the lithosphere is found ∼130 km thick beneath the most northwestern part of the Brasília Orogenic Belt which is thinning out towards the deep-water Santos Basin (∼70 km). This thinning coincides with the lithosphere-asthenosphere boundary in recent seismic tomography results, reflecting the current state of the uplifted asthenosphere post-West Gondwana break-up. A major steeply dipping sub-lithospheric conductor, in conjunction with a confined asthenosphere upwelling (or lithospheric thinning), has been observed underneath Embu-Paraíba do Sul domain (Ribeira belt), which may be somehow associated with the continental scale rifting, surface uplift and the Cretaceous São Paulo-Rio de Janeiro dykes swarm. Our findings provide valuable insights into the formation of southeast Brazilian highlands and its association with mantle plume-induced lithospheric delamination (or removal) and the occurrence of alkaline magmatism, after reactivation of some pre-existing weak zones.

Decoupled Oligocene mylonitic shearing and Miocene detachment faulting in the East Humboldt Range metamorphic core complex, northeast Nevada, USA

Abstract The relationships between brittle detachment faulting and ductile shear zones in metamorphic core complexes are often ambiguous. Although it is commonly assumed that these two structures are kinematically linked and genetically related, direct observations of this coupling are rare. Here, we conducted a detailed field investigation to probe the connection between a detachment fault and mylonitic shear zone in the Ruby Mountain–East Humboldt Range metamorphic core complex, northeast Nevada. Field observations, along with new and published geochronology, demonstrate that Oligocene top-to-the-west mylonitic shear zones are crosscut by ca. 17 Ma subvertical basalt dikes, and these dikes are in turn truncated by middle Miocene detachment faults. The detachment faults appear to focus in preexisting weak zones in shaley strata and Mesozoic thrust faults. We interpret that the Oligocene mylonitic shear zones were generated in response to domal upwelling during voluminous plutonism and partial melting, which significantly predated the middle Miocene onset of regional extension and detachment slip. Our model simplifies mechanical issues with low-angle detachment faulting because there was an initial dip to the weak zones exploited by the future detachment-fault zone. This mechanism may be important for many apparent low-angle normal faults in the eastern Great Basin. We suggest that the temporal decoupling of mylonitic shearing and detachment faulting may be significant and underappreciated for many of the metamorphic core complexes in the North American Cordillera. In this case, earlier Eocene–Oligocene buoyant doming may have preconditioned the crust to be reactivated by Miocene extension, thus explaining the spatial relationship between structures.

Origin of the Sierras Pampeanas, Argentina: Flat-slab subduction and inherited structures

Abstract The Sierras Pampeanas (27°–33°S) in South America are characterized by basementcored uplifts and shortening that occurs >500 km from the nearest convergent margin. The deformation correlates spatially and temporally with an area of flat-slab subduction of the Nazca plate in the last 10 m.y. We use two-dimensional thermal-mechanical models to study the dynamics of Pampean flat-slab subduction and the origin of the Sierras Pampeanas. Models examine a geological time from ca. 12 Ma to present day, during which time the Juan Fernández Ridge subducted beneath South America. Models show that the buoyant ridge triggers slab flattening, resulting in regional continental compression through end loading at the plate margin. Deformation in the continental interior depends on the inherited structure of the continent, where surface uplifts and shortening are concentrated at preexisting weak zones. The inboard migration of deformation is controlled by surface topography caused by the buoyant ridge rather than basal shear from the growing flat slab. Deformation occurs prior to the passage of the ridge and is inhibited when the ridge is beneath the region owing to dynamic uplift.

How do pre-existing weak zones and rheological layering of the continental lithosphere influence the development and evolution of intra-continental subduction?

• The depth of a preexisting weak zone is one of the key factors in initiating intra-continental subduction. • Crustal internal friction angle plays an important role in altering the deformation type of a continental collision system. • The reactivation of the suture may be an auxiliary condition for the evolution of the south-dipping continental subduction underneath the North Pamir. Intra-continental subduction is of special importance for studying the formation of intra-continental orogens, crust-mantle structural evolution, and the far-field effects of continental collision, whose mechanism is still a matter of discussion. In this work, we investigated the role of pre-existing weak zones and the continental lithospheric rheological layering in the formation and evolution of the intra-continental subduction based on a 2D finite element numerical technique. The model results indicate that the deeper the intra-continental weak zone is and the faster the convergence velocity is, the more likely it is to develop into a new intra-continental subduction. Altering the rheological strength of the overriding plate may not have a substantial impact on the intra-continental subduction mode when the depth of the pre-existing weak zone is larger than half of the lithospheric thickness. In contrast, the lithospheric rheological strength is closely related to the continental collision system’s deformation style: Models with a weaker overriding plate are inclined to delaminate continuously under collision, whereas a strong overriding plate results in the subducting plate’s roll-back. The reactivation of the suture that runs deep into the lithosphere as a result of the Indian-Asian continental collision could be one of the crucial factors controlling the formation of the south-dipping subduction under the North Pamir.

How do pre-existing weak zones and rheological layering of the continental lithosphere influence the development and evolution of intra-continental subduction?

How do pre-existing weak zones and rheological layering of the continental lithosphere influence the development and evolution of intra-continental subduction?

What causes the persistent seismicity below the eastern flank of Piton de la Fournaise (la Réunion Island)? Elasto-plastic models of magma inflation

Identifying the causes of flank destabilization of active volcanic edifices is key to prevent catastrophic events. The persistent seismicity recorded below the eastern flank of Piton de la Fournaise shield volcano (la Réunion Island), both in between and during eruptive events, may give indications on the mechanical stability of this edifice. Whether this asymmetric “cup” shaped seismicity is linked to magma injections and whether it sparks a gravitational flank slide motivates this study. Here we model the elasto-plastic behavior of this volcanic edifice at crustal scale, with the 3D finite-element code Adeli. First, we test the influence of tensile failure, recently implemented in combination to a Drucker-Prager shear failure criterion; a pressurized cavity below a flat top surface triggers shear failure in general, with tensile failure restricted to the surface and cavity tip. Then we include the topography of Piton de la Fournaise in the gravity field. Considering first only elasticity, deviatoric stresses attain about 35 MPa below the volcanic edifice and displacements are maximum in the horizontal east–west direction, reaching 30 m near sea-level. Introducing plastic behavior produces a rather symmetric cup shape plastic domain around the volcano’s summit, that extends at depth with reducing bedrock effective friction (which acts is a proxy for reduced standard friction due to pore fluid pressurization). An asymmetric listric shear zone develops down to −3 km (bsl) only if the tensile strength, cohesion and friction angle are set as low as 1.5 MPa, 3 MPa and 3°, respectively; these values hence provide a lower bound for the edifice’s effective strength. The second part of this study explores the influence of an internal overpressure, which is either applied as a vertical inflation source located about 500 m below the surface of the eastern flank, simulating a distal dike, or from a deeper ellipsoid simulating the magma reservoir located at depth ca. 0 km (near sea level) below the summit. The resulting strain pattern forms a cup-shaped shear zone dipping down below the eastern flanks of the edifice, reaching depth −2 km (bsl) if effective friction angle is ⩽5°. Whereas the deep base of the dike and the eastern edge of the magma reservoir coincide geometrically in the models, the inflating dike produces a shear zone 1 km shallower than does the inflating magma reservoir, the latter coinciding better with the shape of the observed seismic cup. Hence, we propose that this structure is a mechanical consequence of continuous magma supply in the reservoir, coherent with previous interpretations. This means that at least originally it did not need to form as a pre-existing weak zone or a magma-filled structure. However, this shear zone delimits an underlying domain in dilatation relative to a constricted hanging-wall; it may thus promote magma sills. It also branches to the surface with planar radial shear zones comparable to some observed eruptive fissures. The 3D kinematics of this shear zone does not rule out the possibility of a giant flank slide, although it does not appear today as imminent.

The Role of Tectonic Discontinuities in Triggering Large Seismic Landslides

Abstract Rock landslides are generally controlled by a series of discontinuous interfaces formed by damage to complex structural rock masses that is related to folds, faults, and shear zones, which provide potential failure boundaries. Based on a detailed case study, the authors illustrate that structural damage not only affects the 3D boundaries of landslides but also significantly controls the initiation mode. In this paper, six large landslides triggered by the 2008 Wenchuan earthquake (Ms 8.0) in the carbonate strata within two wings of an anticline are selected as examples. A detailed geological engineering investigation, rock mass testing, and rock mass quality assessment are performed. The results of the research show that landslides near the core of the anticline may be closely related to the geological strength index, while landslides far away from the core of the anticline are more likely to be controlled by the preexisting weak zone within the slope. In addition, the combination of structural planes and weak zones further determines the initiation mode. A comparison of earthquake-induced landslides with gravity-induced landslides reveals that strong earthquakes accelerate the process of geomorphic erosion by reducing the geological strength index (GSI) values of landslides. Finally, this study proposes that the damage to rock masses in complex tectonic environments is of great importance to assess the location, initiation mechanism, and mode of potential large-scale landslides caused by strong earthquakes.

Impact of Timanian thrust systems on the late Neoproterozoic–Phanerozoic tectonic evolution of the Barents Sea and Svalbard

Abstract. The Svalbard Archipelago consists of three basement terranes that record a complex Neoproterozoic–Phanerozoic tectonic history, including four contractional events (Grenvillian, Caledonian, Ellesmerian, and Eurekan) and two episodes of collapse- to rift-related extension (Devonian–Carboniferous and late Cenozoic). Previous studies suggest that these three terranes likely accreted during the early to mid-Paleozoic Caledonian and Ellesmerian orogenies. Yet recent geochronological analyses show that the northwestern and southwestern terranes of Svalbard both record an episode of amphibolite (–eclogite) facies metamorphism in the latest Neoproterozoic, which may relate to the 650–550 Ma Timanian Orogeny identified in northwestern Russia, northern Norway, and the Russian Barents Sea. However, discrete Timanian structures have yet to be identified in Svalbard and the Norwegian Barents Sea. Through analysis of seismic reflection, as well as regional gravimetric and magnetic data, this study demonstrates the presence of continuous thrust systems that are several kilometers thick, NNE-dipping, deeply buried, and extend thousands of kilometers from northwestern Russia to northeastern Norway, the northern Norwegian Barents Sea, and the Svalbard Archipelago. The consistency in orientation and geometry, as well as apparent linkage between these thrust systems and those recognized as part of the Timanian Orogeny in northwestern Russia and Novaya Zemlya, suggests that the mapped structures are likely Timanian. If correct, these findings would imply that Svalbard's three basement terranes and the Barents Sea were accreted onto northern Norway during the Timanian Orogeny and should hence be attached to Baltica and northwestern Russia in future Neoproterozoic–early Paleozoic plate tectonics reconstructions. In the Phanerozoic, the study suggests that the interpreted Timanian thrust systems represent major preexisting zones of weakness that were reactivated, folded, and overprinted by (i.e., controlled the formation of new) brittle faults during later tectonic events. These faults are still active at present and can be linked to folding and offset of the seafloor.

Plume-Induced Subduction Initiation: Revisiting Models and Observations

Subduction initiation induced by a hot and buoyant mantle plume head is unique among proposed subduction initiation mechanisms because it does not require pre-existing weak zones or other forces for lithospheric collapse. Since recognition of the first evidence of subduction nucleation induced by a mantle plume in the Late Cretaceous Caribbean realm, the number of studies focusing on other natural examples has grown. Here, we review numerical and physical modeling and geological-geochemical studies which have been carried out thus far to investigate onset of a new subduction zone caused by impingement of a mantle plume head. As geological-geochemical data suggests that plume-lithosphere interactions have long been important - spanning from the Archean to the present - modeling studies provide valuable information on the spatial and temporal variations in lithospheric deformation induced by these interactions. Numerical and physical modeling studies, ranging from regional to global scales, illustrate the key role of plume buoyancy, lithospheric strength and magmatic weakening above the plume head on plume-lithosphere interactions. Lithospheric/crustal heterogeneities, pre-existing lithospheric weak zones and external compressional/extensional forces may also change the deformation regime caused by plume-lithosphere interaction.

Two-phase intracontinental deformation mode in the context of India–Eurasia collision: insights from a structural analysis of the West Kunlun–Southern Junggar transect along the NW margin of the Tibetan Plateau

The convergence of India and Eurasia, which began in the early Cenozoic, established the Tibetan Plateau and the Circum-Tibetan Plateau Basin and Orogen System. When and how the convergence-driving strain propagated into this system is important in deciphering the growth processes of the Tibetan Plateau. We conducted a structural analysis of the West Kunlun–southern Junggar transect along the NW margin of the Tibetan Plateau to establish the propagation of deformation and, through this, to determine the plateau growth processes. Our results suggest a two-phase deformation mode. The first-stage features deformation confined to pre-existing weak zones (e.g. the West Kunlun orogen, the Buchu Uplift and the Tian Shan orogen) during the Paleogene, when the intracontinental strain is speculated to be mainly consumed by shortening of these weak zones. The second stage is characterized by deformation propagating into the foreland regions since the early Miocene, where shortening along the foreland fold–thrust belts on a scale of tens of kilometres and decreasing basinwards had a key role in absorbing intracontinental strain. We suggest that this two-phase deformation mode may reflect a shift in the governing mechanism of the expansion of the Tibetan Plateau from a rigid block to a critical wedge taper style.Thematic collection: This article is part of the Fold-and-thrust belts collection available at: https://www.lyellcollection.org/cc/fold-and-thrust-belts

Post-Miocene topographic rejuvenation in an elevated passive continental margin not characterized by a sharp escarpment (northern end of the Mantiqueira Range, Brazil)

A sharp escarpment lying close to the coast or more in the continental interior defines the morphology of Elevated Passive Continental Margins (EPCMs). Most studies on the dynamics and evolution of EPCMs, including numerical modeling exercises, concentrate on the geometry and evolution of escarpments located at or near a continental drainage divide. However, topographic relief varies considerably along the length of an EPCM, as one can observe in southeastern Brazil and southeastern Australia, and in some cases, the steep, wall-like escarpment may not be present. Here we investigate the post-rift geomorphic evolution in an EPCM lacking a sharp escarpment (southeastern Brazil), focusing on exploring topographic rejuvenation and links with potential controls such as the arrangement of pre-existing structures and lithological variability. We present topographic data showing a regional zone of high channel steepness and local relief extending continuously through the middle part of all seaward-dipping catchments. In this regional “belt” of high topography lies many knickpoints, and all rivers crossing it are marked by non-linear shapes in elevation-χ space. The structural data show a large number of NE-SW and NW-SE oriented brittle structures with slickensides recorded on filling materials following the orientation of pre-existing basement structures, suggesting a multiphase brittle deformation with at least two main paleostress tectonic regimes between the Miocene and Pleistocene. Our results indicate post-Miocene topographic rejuvenation and brittle deformation, probably driven by the accumulation of far-field stresses in pre-existing weakness zones. Nevertheless, the origin of the regional drainage divide and adjacent areas, located considerably upstream of the regional knickzone, must predate the topographic resurgence we infer, implying that older and more recent topographic elements coexist in the landscape. Our study demonstrates that post-rift topographic rejuvenation (and brittle deformation) in an EPCM is not necessarily linked to the formation or persistence of a “Great Escarpment”.

Origin of theater-headed valleys in the Valles Marineris region, Mars

This paper reports high-resolution observations of theater-headed valleys in the Valles Marineris region and proposes an integrated model of their formation and localization across the walls of the canyons of Valles Marineris. These theater-headed valleys have formed over a long period of time from Middle-Hesperian to Early-Amazonian. The valleys show several common features, namely, proximity to areas of high ejecta mobility on the Tharsis plateau; alignment along the major tectonic features such as wrinkle ridges, grabens, pit crater chains; association with fluvial channels and layered deposits on the plateau adjacent to the valley heads; and occurrences of mass wasting features at the valley mouths. Elevation profiles of the walls of Valles Marineris display ramp and cliff topography indicating mechanical stratification in the wall rocks. These observations reveal multiple controls over the formation and localization of theater-headed valleys in Valles Marineris. However, the major controls appear to be strong-over-weak stratigraphy in the canyon walls and the pre-existing zones of structural weakness in the Valles Marineris region. The source of water could likely be groundwater; however, snowmelt accumulated on the upper parts of the wallrock would additionally facilitate the process. Based on the above observations, we propose a sequence of events for the evolution of theater-headed valleys in the Valles Marineris region.

Plate motion and plume-induced subduction initiation

Impingement of a hot buoyant mantle plume head on the lithosphere is one of the few scenarios that can initiate a new subduction zone without requiring any pre-existing weak zones. This mechanism can start subduction and plate tectonics on a stagnant lid and can also operate during active plate tectonics where plume-lithosphere interactions is likely to be affected by plate motion. In this study, we explore the influence of plate motion on lithospheric response to plume head-lithosphere interaction including the effect of magmatic weakening of lithosphere. Using 3d thermo-mechanical models we show that the arrival of a new plume beneath the lithosphere can either (1) break the lithosphere and initiate subduction, (2) penetrate the lithosphere without subduction initiation, or (3) spread asymmetrically below the lithosphere. Outcomes indicate that lithospheric strength and plume buoyancy control plume penetration through the lithosphere whereas the plate speed has a subordinate influence on this process. However, plate motion may affect the geometry and dynamics of plume-lithosphere interaction by promoting asymmetry in the subduction zone shape. When a sufficiently buoyant plume hits a young but subductable moving lithosphere, a single-slab modern-style subduction zone can form instead of multiple subduction zones predicted by stagnant lid models. In the case of subduction initiation of older moving oceanic lithosphere, asymmetrical cylindrical subduction is promoted instead of more symmetrical stagnant lid subduction. We propose that the eastward motion of the Farallon plate in Late Cretaceous time could have played a key role in forming one-sided subduction along the southern and western margin of the Caribbean and NW South America.

Dip angles of active faults from the surface to the seismogenic zone inferred from a 2D numerical analysis of visco-elasto-plastic models: a case study for the Osaka Plain

The geometries (i.e., dip angles) of active faults from the surface to the seismogenic zone are the most important factors used to evaluate earthquake ground motion, which is crucial for seismic hazard assessments in urban areas. In Osaka, a metropolitan city in Japan, there are several active faults (e.g., the Uemachi and Ikoma faults), which are inferred from the topography, the attitude of active faults in surface trenches, the seismic reflection profile at shallow depths (less than 2 km), and the three-dimensional distribution of the Quaternary sedimentary layers. The Uemachi and Ikoma faults are N–S-striking fault systems with total lengths of 42 km and 38 km, respectively, with the former being located ~ 12 km west of the latter; however, the geometries of each of the active faults within the seismogenic zone are not clear. In this study, to examine the geometries of the Uemachi and Ikoma faults from the surface to the seismogenic zone, we analyze the development of the geological structures of sedimentary layers based on numerical simulations of a two-dimensional visco-elasto-plastic body under a horizontal compressive stress field, including preexisting high-strained weak zones (i.e., faults) and surface sedimentation processes, and evaluate the relationship between the observed geological structures of the Quaternary sediments (i.e., the Osaka Group) in the Osaka Plain and the model results. As a result, we propose geometries of the Uemachi and Ikoma faults from the surface to the seismogenic zone. When the friction coefficient of the faults is ~ 0.5, the dip angles of the Uemachi and Ikoma faults near the surface are ~ 30°–40° and the Uemachi fault has a downward convex curve at the bottom of the seismogenic zone, but does not converge to the Ikoma fault. Based on the analysis in this study, the dip angle of the Uemachi fault zone is estimated to be approximately 30°–40°, which is lower than that estimated in the previous studies. If the active fault has a low angle, the width of the fault plane is long, and thus the estimated seismic moment will be large.

Pre-existing lithospheric weak zone and its impact on continental rifting – The Mid-Polish Trough, Central European Basin System

The large portion of Central and NW Europe was subjected to widespread continental rifting since the Carboniferous-Permian transition. This process led to nucleation of several large depocentres, coalescing from the late Permian onwards into the extensive Central European Basin System (CEBS), that grew and evolved until the regional-scale Late Cretaceous inversion. Development of deep grabens from Germany to the Netherlands and the North Sea was connected with volcanism and crustal thinning. The exception makes the Mid-Polish Trough (MPT), trending NW-SE across Poland along the Teisseyre-Tornquist Zone (TTZ) at the margin of the East European Craton (EEC). The MPT is underlain by relatively thick lithosphere and its formation was not associated with volcanic activity in the direct neighbourhood. To investigate this anomaly, we compiled grids for key horizons defining the crustal architecture, separately for NW European and Polish sectors of the CEBS. Seismic reflection and well data were used to build the base Permian grid, whereas deep seismic refraction and reflection data along with constrained gravity inversion were employed to construct the top basement and Moho grids. Based on a dense net of seismic refraction profiles, the additional top Ediacaran grid was created solely for the Polish part of the CEBS. Our data show distinct crystalline crust thinning SW of the TTZ (β factor = 3.68) expressed in both shallowing of Moho and deepening of the crystalline basement. We interpret this feature as a fossilised crustal neck inherited after an Ediacaran rifting preceding the break-up of Rodinia. The early Permian rifting moderately contributed to the crustal thinning (β factor = 1.4), overprinting the primary rifted margin geometry. The 8 km deep MPT developed in front of a pre-existing crustal neck that obstructed further propagation of extensional deformation. Lithospheric mantle beneath a rift axis was probably too strong to accommodate extension but the thinner lithosphere farther west was weak enough to be stretched. Therefore, the location of the pre-existing weakness zone relative to a change in lithosphere thickness may localise a rift graben and shift the magmatism away from the surface expression of the rift. Thus, our study demonstrates that fossil crustal architecture was a key factor controlling the amount, distribution, and style of continental rifting.

Crustal velocity structure of the Northwest Sub-basin of the South China Sea based on seismic data reprocessing

The deep crustal structure of the Northwest Sub-basin (NWSB) of the South China Sea (SCS) is of great importance for understanding the tectonic nature of the continent-ocean transition (COT) and magmatism in this oceanic basin. The 2-D wide-angle reflection/refraction seismic profile OBS2006-2 is almost parallel to the extinct spreading ridge (ESR) of the NWSB. In addition to the original data, we added the data of two reprocessed OBS stations, and carried out seismic phase re-picking and travel-time imaging to obtain the crustal velocity structure along this profile. Resolution tests demonstrate that the newly acquired velocity structure is more reliable than the prior interpretation. The depth of the Moho (23.5–11.8 km) and crustal thickness (20.5–6.5 km) systematically changes from continental crust of the Xisha Block to the oceanic crust within the NWSB. The COT zone has a width of ∼20 km and the depth of the Moho decreases from 15.0 to 11.0 km, corresponding to a ∼4 km decrease in crustal thickness (6–10 km). A high velocity layer (HVL, 7.2–7.4 km s−1) exists at the bottom of the crust at the location where the sharp lateral transition of the continental crust to the oceanic crust occurs. Age dating shows that the Double-peak Seamount was formed at ∼23 Ma, after the cessation of the NWSB seafloor spreading (∼32-25 Ma). The crust beneath the Double-peak Seamount is oceanic with a thickness of 9 km. We infer that this oceanic crust was formed by magmatic upwelling and decompression melting along a pre-existing zone of weakness.

The role of inherited crustal and lithospheric architecture during the evolution of the Red Sea: Insights from three dimensional analogue experiments

The Red Sea represents an incipient ocean basin that is in transition from continental rifting to ocean initiation. In this paper we assess how pre-existing mantle and crustal heterogeneities within the African and Arabian lithospheres influenced rift development and propagation using three-dimensional (3D) brittle-ductile analogue laboratory experiments with a rotational boundary condition. Here we investigate the effect of three different configurations: (i) pre-existing rheological heterogeneities in the crust, (ii) in the lithospheric mantle, and (iii) in both. The experimental outcomes show that a linear weak zone in the lithospheric mantle layer exerts a first-order control on the overall mechanical behaviour of extending lithosphere, rift architecture and style of rifting. Crustal heterogeneities control second-order features, such as the trends of the main boundary escarpments along the rift axis. Our experiments reproduce the geological history of the Red Sea and provide key insights on its deformation history in space and time. We provide, for the first time, 3D dynamic constraints on how pre-existing weak zones in the lithosphere have controlled the temporal evolution of the Red Sea and determined its distinctive present-day morphology. The results find that certain morphological elements of the Red Sea can only be explained by the combined effects of crustal and mantle lithosphere heterogeneities. When applied to other natural examples, our findings provide significant new insights on the role of crustal and lithospheric heterogeneities during continental rifting.

The role of pre-existing weak zones in the formation of the Himalaya and Tibetan plateau: 3-D thermomechanical modelling

SUMMARY The Tibetan crust is sliced by several east–west trending suture zones. The role of these suture zones in the evolution of the Himalayan range and Tibetan plateau remains unclear. Here we use 3-D thermomechanical simulations to investigate the role of pre-existing weak zones within the Asian Plate in the formation of orogen and plateau growth during continental collision. Our results show that partitioning of deformation along the convergent margin leads to scraping off of crustal material into an orogenic wedge above the margin and crustal thickening in the retro-continent, eventually forming a large orogenic plateau in front of the indenter. Pre-existing weak zone(s) within the retro-continent is reactivated at the early stage of convergence, and facilitates the northward propagation of strain and widening of the orogenic plateau. The northernmost weak zone sets the northern limit of the Tibetan plateau. Our models also show rheological weakening of the congested buoyant crust within the collisional zone drives wedge-type exhumation of deeply buried crust at the southern flank of the plateau, which may explain the formation of the Greater Himalayan Sequence.

Plume‐Induced Subduction Initiation: Single‐Slab or Multi‐Slab Subduction?

AbstractInitiation of subduction following the impingement of a hot buoyant mantle plume is one of the few scenarios that allow breaking the lithosphere and recycling a stagnant lid without requiring any preexisting weak zones. Here, we investigate factors controlling the number and shape of retreating subducting slabs formed by plume‐lithosphere interaction. Using 3‐D thermomechanical models we show that the deformation regime, which defines formation of single‐slab or multi‐slab subduction, depends on several parameters such as age of oceanic lithosphere, thickness of the crust and large‐scale lithospheric extension rate. Our model results indicate that on present‐day Earth multi‐slab plume‐induced subduction is initiated only if the oceanic lithosphere is relatively young (<30–40 Myr, but >10 Myr), and the crust has a typical thickness of 8 km. In turn, development of single‐slab subduction is facilitated by older lithosphere and pre‐imposed extensional stresses. In early Earth, plume‐lithosphere interaction could have led to formation of either episodic short‐lived circular subduction when the oceanic lithosphere was young or to multi‐slab subduction when the lithosphere was old.