Weak Continental Crust Research Articles

Continental Deep Subduction Versus Subduction Cessation: The Fate of Collisional Orogens

AbstractThe contrasting fates of collisional orogens, that is, continental deep subduction or subduction cessation, are widely recognized by petrological, paleomagnetic and geophysical observations. However, the mechanisms of such different collisional modes, especially the dynamics of continental deep subduction, are controversial. In this study, we integrate the phase transition‐induced density evolutions into a thermo‐mechanical numerical model. Combing the systematic petrological‐thermo‐mechanical models with force balance analysis, we find that the high metamorphic transformation degree, mildly depleted mantle composition of the subcontinental lithosphere, and a long preceding oceanic slab, increase the driving force for continental deep subduction. Additionally, the rheologically weak continental crust and asthenospheric mantle decrease the resistance force and promote deep subduction. Otherwise, the continental subduction cessation mode is favored. The calculations of slab negative buoyancy indicate that the phase transition‐induced metamorphic densification of the subducted continental crust and the mildly to moderately depleted lithospheric mantle can provide a great slab pull force to sustain the continued continental deep subduction; however, the positive buoyancy of highly depleted Archean lithospheric mantle impedes deep subduction and causes subduction cessation. Based on systematic numerical models, we also evaluate the crustal mass balance or deficit in continental collisional system, which indicates that ∼12%–47% of pre‐collisional felsic crust could be subducted deeply with the sinking slab in the regime of continental deep subduction. In contrast, the recycled felsic crust is negligible in the regime of subduction cessation. Thus, the different modes of continental collision play a crucial role in the global crustal recycling and related mantle heterogeneities.

Strain Partitioning Within Bending Orogens, New Insights From the Variscan Belt (Chiroulet‐Lesponne Domes, Pyrenees)

AbstractThis study investigates how strain is partitioned within hot orogenic belts in a context of plate reorganization. We document crustal‐scale strike‐slip shear zones, plate‐scale oroclinal bending, and widespread exhumation of deep crustal domains in the core of nascent Pangea plate during late‐Variscan times (ca. 310–290 Ma). The role of strain partitioning in the finite strain patterns is first highlighted at regional scale by a new detailed structural, kinematic, and petrologic analysis of the Chiroulet and Lesponne gneiss domes in the Pyrenees. Vertical strain partitioning in response to N‐S shortening‐dominated dextral transpression is secondly documented at the scale of the Variscan domains of the Pyrenees, based on a synthesis of recent geochronological, thermobarometrical, and structural data. We show that the mid‐lower crust is affected by eastward lateral flow and the upper crust by penetrative thickening and localized thrusting. Local transverse extension in the mid‐lower crust illustrates gravity‐driven instability favored by the accumulation of anatectic melts in gneiss domes. Finally, we highlight the respective position of extended and shortened area within the Variscan belt during late‐Carboniferous/early‐Permian that suggests large‐scale lateral strain partitioning and reorganization of the weak continental crust in the core of the Pangea. This distribution appears significantly different from other orogens in a similar context of oroclinal bending, resulting in localization of the shortened zones in the core of the belt. This highlights the influence of the initial and boundary conditions, such as the obliquity of the belt to the applied stress field and the location of free‐boundaries, on finite strain patterns.

Fast dismantling of a mountain belt by mantle flow: Late-orogenic evolution of Pyrenees and Liguro-Provençal rifting

The Pyrenean Belt ends against the Gulf of Lion passive margin. The mechanism responsible for dismantling the mountain belt during Oligocene rifting has not yet found a proper explanation. The Late Eocene and Oligocene period is characterized by a first order change in subduction dynamics in the Mediterranean and the subduction zones started to retreat with back-arc basins forming at the expense of mountain belts build earlier. The slab subducting below Provence and Sardinia-Corsica started a fast south-eastward retreat, forming the Liguro-Provençal Basin. This syn-rift period in the Gulf of Lion margin is coeval with exhumation of the eastern Pyrenean basement, while underthrusting of Iberia continued until Early Miocene. Based on interpretation of seismic lines, we propose a tentative model in which the mantle flow related with Apennine slab retreat has (1) exhumed and thinned the continental mantle below the Gulf of Lion and the eastern Pyrenees and (2) exhumed the lower crust, leading to crustal thinning and subsidence of the Gulf of Lion margin. The wide distribution of syn-rift volcanism in the transition between the Gulf of Lion and Valencia Basin is in line with the geometry observed in the margin suggesting ductile deformation of a weak continental crust, typical of volcanic margins. The direction of SKS-waves seismic anisotropy below the Pyrenees and the observed migration of exhumation toward the west fit this simple model. The concentration of syn-rift magmatism and the ductile behaviour of the crust during rifting can be explained by the high heat flow above the slab tear that separates the future Apennines and Maghreb branches of the West Mediterranean subduction zone. Finally, removal of upper mantle, inducing uplift and an increase of potential energy, may explain why thrusting continued in the Pyrenees while rifting was still active nearby along strike.

Feedback between synrift lithospheric extension, sedimentation and salt tectonics on wide, weak continental margins

Numerical modelling in 2D is used to explore interactions between synrift lithospheric extension, salt deposition and deformation, and pre- and post-salt sedimentation, for wide rifted margins with weak continental crust. Distributed aggrading synrift sedimentation enhances listric normal faulting of the sediments and crust in the mid and distal margin. In contrast, localized prograding sedimentation initiates a positive feedback between sedimentation, faulting and mid- to lower-crustal flow. This feedback causes localized crustal extension at the proximal margin, and leads to thick sediments in deep proximal basins. The feedback is more pronounced when more sediment is deposited, and does not develop in models with stronger, narrower rifted margins. Later initiation of the post-salt prograding sediments leads to a less pronounced feedback with lower-crustal flow and a more significant advancement of the prograding wedge over the salt body. We compare our model results with the rifted Nova Scotia Atlantic margin, contrasting margin evolution and salt tectonics between the northeastern region, which experienced significant post-salt synrift sedimentation, and the central region, where less post-salt sediment was deposited. We show that the northeastern margin may have experienced enhanced proximal graben development owing to prograding synrift sedimentation. Thematic collection: This article is part of the Mechanics of salt systems: state of the field in numerical methods collection available at: https://www.lyellcollection.org/cc/mechanics-of-salt-systems

The Role of Crustal Strength in Controlling Magmatism and Melt Chemistry During Rifting and Breakup

AbstractThe strength of the crust has a strong impact on the evolution of continental extension and breakup. Strong crust may promote focused narrow rifting, while wide rifting might be due to a weaker crustal architecture. The strength of the crust also influences deeper processes within the asthenosphere. To quantitatively test the implications of crustal strength on the evolution of continental rift zones, we developed a 2‐D numerical model of lithosphere extension that can predict the rare Earth element (REE) chemistry of erupted lava. We find that a difference in crustal strength leads to a different rate of depletion in light elements relative to heavy elements. By comparing the model predictions to rock samples from the Basin and Range, USA, we can demonstrate that slow extension of a weak continental crust can explain the observed depletion in melt chemistry. The same comparison for the Main Ethiopian Rift suggests that magmatism within this narrow rift zone can be explained by the localization of strain caused by a strong lower crust. We demonstrate that the slow extension of a strong lower crust above a mantle of potential temperature of can fit the observed REE trends and the upper mantle seismic velocity for the Main Ethiopian Rift. The thermo‐mechanical model implies that melt composition could provide quantitative information on the style of breakup and the initial strength of the continental crust.

Numerical modeling of extensional sedimentary basin formation with MATLAB: Application to the northern margin of the South China Sea

The coupled simple-shear/pure-shear model (CSSPSM) with broad application for studying the evolution of continental extensional sedimentary basins was proposed by Kusznir and co-workers. It can be used to determine the geometry of sedimentary basins and their crustal structure by integration of the rheological, thermal and isostatic response of lithosphere to various loads caused by lithosphere extension. We developed a MATLAB code, MODBAS, to model extensional sedimentary basin formation based on the CSSPSM. The validity of the code was tested with a single listric fault model and a multiple-fault model. The application of the code in the Pearl River Mouth Basin (PRMB) on the northern margin of the South China Sea demonstrates (1) the effective elastic thickness of the lithosphere beneath the PRMB is very low (<5km), which may support the idea of a very weak continental crust beneath the northern margin of the South China Sea; and (2) there are significant misfits between predicted and observed basements beneath the two high-standing areas on the profile, which was interpreted as an indication of substantial dynamic support from small-scale mantle convection.

The three-dimensional lithospheric structure of the Falkland Plateau region based on gravity modelling

The lithospheric structure of the Falkland Plateau region has been modelled by making an initial estimate on the basis of local isostasy and then refining the geometries by inversion of gravity anomalies. The model predicts a crustal thickness beneath the Falkland Plateau Basin that is more than twice that previously inferred from seismic evidence. The preferred explanation is that the seismic survey detected high-velocity (?underplated) lower crust rather than upper mantle. This is compatible with the results of deep seismic experiments over the conjugate Filchner Block of Antarctica and also appears likely on the basis of the position of the Falkland Plateau Basin in relation to the Karoo–Ferrar magmatic province at the time of extension. Continental crust is inferred to be continuous beneath the northern part of the Falkland Plateau, as there are consistent magnetic and flexural anomalies associated with the continent–ocean boundary. Flexural modelling of the southern margin of the plateau indicates lateral variations in strength, with the strongest lithosphere beneath the Falkland Plateau Basin. This may indicate thick oceanic crust beneath the southern part of the basin or the effect of thinning relatively weak continental crust and replacing it with relatively strong underplating.

Tectonic plate coupling and elastic thickness derived from the inversion of a steady state viscoelastic model using geodetic data: Application to southern North Island, New Zealand

A steady state viscoelastic model of deformation at an oblique convergence zone is used to analyze crustal velocities deduced from Global Positioning System (GPS) observations in southern North Island, New Zealand. The model is physically more reasonable than elastic dislocation theory because the tectonic plates have finite elastic thicknesses. In an inversion that makes use of Green's functions derived from finite element calculations, we solve for depth‐dependent fault backslip rates. The associated chi‐squared goodness of fit parameter depends on the values of the elastic thicknesses of the overriding Australian and subducting Pacific Plates. These thicknesses are systematically varied in order to find the chi‐squared minimum. We find that: (1) the plates have coupling coefficient between 0.8 and 1.0 to a depth of about 22 km; (2) elastic dislocation theory appears to adequately fit the observations because the effects of viscoelastic flow are small; (3) viscoelastic results depend on the contrast between the elastic moduli of the plates, (4) the trench normal, rather than the trench parallel component of motion is more diagnostic for choosing between models with different parameters; (5) for the favored model (one with a weak continental crust), the estimated value of the Pacific Plate thickness is 40–60 km. Although the estimates of the plate thickness are not tightly constrained, those deduced from geodetic data tend to be larger than those deduced from geologic data, consistent with the idea that thickness estimates depend on the time scale of the loading process.

Evidence of low flexural rigidity and low viscosity lower continental crust during continental break-up in the South China Sea

The South China Sea was formed by seafloor spreading in the Late Oligocene at ∼30 Ma following a series of extensional events within crust formed by Mesozoic continental arc. In this study, we interpreted faults along seismic reflection profiles from both the northern and southern conjugate margins of the South China Sea, and forward modeled these using a flexural cantilever model to predict modern basin geometries. When compared with the observed structure, the models based on upper crustal faulting consistently underpredicted the amount of subsidence, especially towards the continent–ocean transition (COT). We interpret this to indicate preferential extension of the continental lower crust along the COT on both margins, extending up to ∼80 km landward from COT. The regional slope of the South China continental shelf indicates lower crustal viscosities of 10 19–10 18 Pa s, representing an offshore continuation of the weak crust documented onshore on the eastern flanks of the Tibetan Plateau. Only in the region of Hainan Island in the western South China Shelf does lower crustal viscosity increase (10 21–10 22 Pa s) and the preferential loss of lower crust become less pronounced and limited to <40 km from COT. This western area represents a rigid block analogous to the Sichuan Basin onshore. Forward models based on upper crustal faulting support the idea of a very weak continental crust because models where the effective elastic thickness of the plate ( T e) exceeds 5 km fail to reproduce the geometry of the sub-basins within the Pearl River Mouth Basin (PRMB) of the South China Margin. The observed basins are too deep and narrow to be consistent with models invoking high flexural rigidity in the upper crust or mantle lithosphere. The fact that rifting and seafloor spreading seem to co-exist for ∼5 my. adjacent to the PRMB is consistent with very weak continental crust during break-up.

Ontong Java Plateau and late Neogene changes in Pacific plate motion

Aided by a new geometrical technique called hot spotting, we recently proposed a new model for the absolute motion of the Pacific plate. This model, called WK97, assumed the Hawaiian hotspot is under Loihi and that there was a change in plate motion at 3–6 Ma. The model located the Louisville hotspot close to the Hollister Ridge, a shallow volcanic ridge associated with a geoid high, seismicity, and recent volcanism. Most of the tectonic implications of WK97, however, were not addressed. Furthermore, although several recent studies suggest that the geochemistry of Hollister lavas may indicate a mixture between Louisville melt material and Pacific‐Antarctic Ridge mid‐ocean ridge basalt, these studies strongly disagree with WK97 and consider it seriously flawed. Here we discuss numerous tectonic predictions of WK97 and present a large amount of observational evidence in support of it. We find that a late Neogene collision between the Ontong Java Plateau and the northern margin of the Australia plate appears to have altered the motion of the Pacific plate, as inferred from hotspot volcanism, by forcing it to rotate counter‐clockwise. This rotation, about a low‐latitude pole, appears to have induced right‐lateral shear stress along the entire Pacific plate divergent boundary, which resulted in the formation of extensional transform faults, microplates, and propagating ridges. This change in motion also appears to have triggered concomitant circum‐Pacific tectonism, including trench migration and back arc rifting. The WK97 suggests that long‐lived, plume‐fed volcanism on the Pacific plate appears very limited, perhaps only to five to six hotspots: Hawaii, Louisville, Cobb, Caroline, and perhaps Marquesas and Bowie; the geometry of these chains are consistent with our model. Despite some evidence for volcanic age progressions, most other Pacific Neogene hotspots are most likely “crack spots”, i.e., sites of extensional volcanism at preexisting zones of weakness reactivated by plate stresses. Indeed, several volcanic ridges, such as Hollister Ridge near the Louisville hotspot, appear to have formed in response to stresses following the change in absolute plate motion. WK97 also obviates the need for simultaneous, different directions of mantle flow in the south central Pacific. Transpression at the San Andreas and Alpine strike‐slip faults and Aleutian Arc explosivity since 6 Ma also appear to be consequences of the changes in Pacific plate motion. Furthermore, both the WK97 model and observed relative plate motions suggest that adjoining plates have changed their absolute motions as well and that large changes in absolute plate motion do not necessarily imply large changes in relative plate motion. Our findings support slab pull as the dominant plate tectonic driving force, highlight the rheological difference between strong oceanic lithosphere and weak continental crust, and suggest that WK97 presently provides a unifying, albeit preliminary, framework for relating both intraplate and circum‐Pacific tectonism and volcanism in the late Neogene.

Structural controls on the evolution of the Kutai Basin, East Kalimantan

The Kutai Basin formed in the middle Eocene as a result of extension linked to the opening of the Makassar Straits and Philippine Sea. Seismic profiles across the northern margin of the Kutai Basin show inverted middle Eocene half-graben oriented NNE–SSW and N–S. Field observations, geophysical data and computer modelling elucidate the evolution of one such inversion fold. NW–SE and NE–SW trending fractures and vein sets in the Cretaceous basement have been reactivated during the Tertiary. Offset of middle Eocene carbonate horizons and rapid syn-tectonic thickening of Upper Oligocene sediments on seismic sections indicate Late Oligocene extension on NW–SE trending en-echelon extensional faults. Early middle Miocene (N7–N8) inversion was concentrated on east-facing half-graben and asymmetric inversion anticlines are found on both northern and southern margins of the basin. Slicken-fibre measurements indicate a shortening direction oriented 290°–310°. NE–SW faults were reactivated with a dominantly dextral transpressional sense of displacement. Faults oriented NW–SE were reactivated with both sinistral and dextral senses of movement, leading to the offset of fold axes above basement faults. The presence of dominantly WNW vergent thrusts indicates likely compression from the ESE. Initial extension during the middle Eocene was accommodated on NNE–SSW, N–S and NE–SW trending faults. Renewed extension on NW–SE trending faults during the late Oligocene occurred under a different kinematic regime, indicating a rotation of the extension direction by between 45° and 90°. Miocene collisions with the margins of northern and eastern Sundaland triggered the punctuated inversion of the basin. Inversion was concentrated in the weak continental crust underlying both the Kutai Basin and various Tertiary basins in Sulawesi whereas the stronger oceanic crust, or attenuated continental crust, underlying the Makassar Straits, acted as a passive conduit for compressional stresses.