Exhumation Of Lower Crust Research Articles

Extension Discrepancy of the Hyper‐Thinned Continental Crust in the Baiyun Rift, Northern Margin of the South China Sea

AbstractAt rifted continental margins, it has been widely reported that the amount of extension measured from faulting is less than the one observed from crustal and lithospheric thinning, referred to as “extension discrepancy.” Here, we use observations from high‐resolution seismic reflection data over the Baiyun Rift to explore the discrepancy between upper and whole crustal thinning factors when the crust of the Northern South China Sea margin thinned from 30 to <10 km. We first restore the rift system of the Baiyun Rift in the absence of post‐rift sediments and water loading. Subsequently, we calculate thinning factors based on fault geometries and crustal thickness ratios to compare the deformation of the upper and whole crust. Our results show (1) in the eastern and western basins, upper crustal thinning factors exceed that of whole crust, showing an inverse discrepancy and (2) upper crustal faulting was sufficient to explain the whole crustal thinning in a narrow area of the basin center, indicating no extension discrepancy. The inverse discrepancy in the western basin results from relatively weak ductile shearing in the lower crust, while in the eastern basin, it is caused by intense sequential detachment faulting in the upper crust. Moreover, lower crust exhumation may trigger magma underplating, which further decreases whole crustal thinning factors. Overall, the hyper‐thinning process of the continental crust beneath the Baiyun Rift is dominated by sequential detachment faulting and is characterized by upper crustal thinning factors equaling or significantly exceeding whole crustal thinning factors.

Reappraisal of Variscan tectonics in the southern French Massif Central

The comprehensive kinematic analysis of the Variscan fold nappes and associated wildflysch in the Southeastern Montagne Noire shows that the nappes were thrust northeastwardly along the Cévennes fault as a result of strike-slip-dominated sinistral transpression between 340 and 323 Ma. Macro- and map-scale recumbent folding and nappe stacking were achieved by ENE-directed, strike-parallel thrust shearing. The recumbent folds evolved into sheath folds by interplay of continued strike-parallel thrust shearing and strike-normal shortening manifested by north-verging folds that grew during wildflysch sedimentation. These results call for a tectonic reassessment of the southern side of the Variscides. The foreland basin was decoupled from the hinterland along orogen-scale sinistral fault(s) trending at a high angle to the orogenic trend, which interacted with a variety of hinterland-ward thrust systems. The shift from sinistral transpression to dextral transtension along the Cévennes fault after 317 Ma accompanied the closure of the Iberian-Armorican syntax by accommodating northeastward escape and gravitational collapse of the northern limb of the syntax. These results illustrate the impact of the shift from plate convergence-controlled to escape-controlled kinematics on shear sense reversal and lower crust exhumation along long-lived strike-slip fault systems.

Episodic collisional orogenesis and lower crust exhumation during the Palaeoproterozoic Eburnean Orogeny: Evidence from the Sefwi Greenstone Belt, West African Craton

New constraints on the architecture, deformation sequence and tectonic evolution of the sparsely exposed Palaeoproterozoic Sefwi Greenstone Belt (ca. 2200–2070 Ma) in SW Ghana are derived from the integration of field observations, structural mapping and interpretation of regional airborne geophysical data. Our findings provide new insights into the tectonic evolution of the West African Craton during the Palaeoproterozoic Eburnean Orogeny (ca. 2150–2070 Ma). Domains of contrasting metamorphic grade comprising high-grade paragneisses and meta-volcanic rocks are tectonically juxtaposed against low-grade volcano-sedimentary packages. The earliest deformation event is characterised by a bedding-parallel high-grade metamorphic foliation (S1) that is axial planar to isoclinal folds formed during NNW-SSE shortening and late-D1 partial melting. This crustal thickening event generated biotite ± muscovite granites emplaced parallel to the NW margin of the Sefwi Belt between ca. 2092 and 2081 Ma, providing a minimum age for D1. D2 deformation is characterised by a moderate to steeply dipping, ENE-WSW to NE-SW striking, penetrative S2 mineral foliation. Within the Sefwi Belt, coaxial F2 fold axes and L2 stretching lineations pitch shallowly ENE-WSW to NE-SW, oblique to the major shear zones, indicative of ENE-WSW transtension and associated constrictional deformation. Differential exhumation of middle and lower crustal segments along NNE-striking D2 extensional detachments was coincident with sinistral oblique reactivation of NE-SW regional-scale shear zones. The timing of exhumation is constrained by metamorphic monazite U-Pb ages at ca. 2073 Ma. Subsequent E-W shortening is associated with dextral reactivation of regional NE-SW striking shear zones and refolding of earlier structures away from the major shear zones. We propose that the NW margin of the Sefwi Belt represents the collision (D1) of southern Ghana and central and NW Ghana/Ivory Coast which record different tectonic histories. Furthermore, we suggest the amalgamation of the West African Craton is the product of episodic collisional orogenesis, providing new insights into the evolution of orogenic processes during the Palaeoproterozoic.

Lower crust exhumation during Paleoproterozoic (Eburnean) orogeny, NW Ghana, West African Craton: Interplay of coeval contractional deformation and extensional gravitational collapse

We present new litho-structural and metamorphic maps of the Paleoproterozoic (2.25–2.00Ga) West African Craton in northern Ghana, based on the interpretation of field observations and airborne geophysical datasets. The study reveals contrasting metamorphic domains consisting of high-grade gneisses and low-grade volcano–sedimentary belts, separated by shear zones and assembled during the Paleoproterozoic Eburnean orogeny (2.15–2.07Ga). Supracrustal rocks were buried and metamorphosed at conditions corresponding to the amphibolite–granulite facies transition, during a (D1) deformation event, consistent with N–S horizontal shortening, which generated reverse shear zones. High and low metamorphic grade rocks were brought in contact along extensional shear zones formed during N–S extension (D2). These structures were overprinted by contractional deformation associated with E–W shortening and N to NNE stretching (D3), coeval with amphibolite-facies metamorphism. The late-stage tectonic evolution (D4–D7) is characterised by strain localisation in successive generations of narrow shear zones, and the re-activation of inherited structures in a dominantly transcurrent regime. U–Pb dating of zircon and monazite from magmatic and metamorphic rocks reveals that D1-D3 deformations form a continuous and overlapping time sequence between ca. 2140 and 2110Ma. The changes in deformation style from D1 to D3 are interpreted to reflect a shift from dominant horizontal tectonic forces to an interplay between tectonic and gravitational forces, which allowed for the exhumation of the lower crust in an anatectic migmatite dome. We suggest that doming is accommodated by lateral extensional sliding of the upper crust and amplified by coeval orthogonal shortening. The abrupt rotation of shortening directions points to a change in boundary conditions applied to the orogen. We hypothesise that it is due to the collision of northern Ghana with the Paleoproterozoic province in modern-day south-western Burkina Faso, which shows a contrasting geological history. The results reveal that the Eburnean orogeny in NW Ghana shared some thermo-mechanical similarities with modern orogenic belts. The findings bring new insight in Paleoprotezozoic plate tectonics, at the transition between archaic and modern geodynamics.

Modes of continental extension in a crustal wedge

We ran numerical experiments of the extension of a crustal wedge as an approximation to extension in an orogenic belt or a continental margin. We study the effects of the strength of the lower crust and of a weak mid-crustal shear zone on the resulting extension styles. A weak mid-crustal shear zone effectively decouples upper crustal extension from lower crustal flow. Without the mid-crustal shear zone, the degree of coupling between the upper and the lower crust increases and extension of the whole crust tends to focus on the thickest part of the wedge. We identify three distinct modes of extension determined by the strength of the lower crust, which are characterized by 1) localized, asymmetric crustal exhumation in a single massif when the lower crust is weak, 2) the formation of rolling-hinge normal faults and the exhumation of lower crust in multiple core complexes with an intermediate strength lower crust, and 3) distributed domino faulting over the weak mid-crustal shear zone when the lower crust is strong. A frictionally stronger mid-crustal shear zone does not change the overall model behaviors but extension occurred over multiple rolling-hinges. The 3 modes of extension share characteristics similar to geological models proposed to explain the formation of metamorphic core complexes: 1) the crustal flow model for the weak lower crust, 2) the rolling-hinge and crustal flow models when the lower crust is intermediate and 3) the flexural uplift model when the lower crust is strong. Finally we show that the intensity of decoupling between the far field extension and lower crustal flow driven by the regional pressure gradient in the wedge control the overall style of extension in the models.

Petrological and geochronological constraints on lower crust exhumation during Paleoproterozoic (Eburnean) orogeny, NW Ghana, West African Craton

AbstractNew petrological and geochronological data are presented on high‐grade ortho‐ and paragneisses from northwestern Ghana, forming part of the Paleoproterozoic (2.25–2.00 Ga) West African Craton. The study area is located in the interference zone between N–S and NE–SW‐trending craton‐scale shear zones, formed during the Eburnean orogeny (2.15–2.00 Ga). High‐grade metamorphic domains are separated from low‐grade greenstone belts by high‐strain zones, including early thrusts, extensional detachments and late‐stage strike‐slip shear zones. Paragneisses sporadically preserve high‐pressure, low‐temperature (HP–LT) relicts, formed at the transition between the blueschist facies and the epidote–amphibolite sub‐facies (10.0–14.0 kbar, 520–600 °C), and represent a low (~15 °C km−1) apparent geothermal gradient. Migmatites record metamorphic conditions at the amphibolite–granulite facies transition. They reveal a clockwise pressure–temperature–time (P–T–t) path characterized by melting at pressures over 10.0 kbar, followed by decompression and heating to peak temperatures of 750 °C at 5.0–8.0 kbar, which fit a 30 °C km−1 apparent geotherm. A regional amphibolite facies metamorphic overprint is recorded by rocks that followed a clockwise P–T–t path, characterized by peak metamorphic conditions of 7.0–10.0 kbar at 550–680 °C, which match a 20–25 °C km−1 apparent geotherm. These P–T conditions were reached after prograde burial and heating for some rock units, and after decompression and heating for others. The timing of anatexis and of the amphibolite facies metamorphic overprint is constrained by in‐situ U–Pb dating of monazite crystallization at 2138 ± 7 and 2130 ± 7 Ma respectively. The new data set challenges the interpretation that metamorphic breaks in the West African Craton are due to diachronous Birimian ‘basins’ overlying a gneissic basement. It suggests that the lower crust was exhumed along reverse, normal and transcurrent shear zones and juxtaposed against shallow crustal slices during the Eburnean orogeny. The craton in NW Ghana is made of distinct fragments with contrasting tectono‐metamorphic histories. The range of metamorphic conditions and the sharp lateral metamorphic gradients are inconsistent with ‘hot orogeny’ models proposed for many Precambrian provinces. These findings shed new light on the geodynamic setting of craton assembly and stabilization in the Paleoproterozoic. It is suggested that the metamorphic record of the West African Craton is characteristic of Paleoproterozoic plate tectonics and illustrates a transition between Archean and Phanerozoic orogens.

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We ran numerical experiments of the extension of a crustal wedge as an approximation to extension in an orogenic belt or a continental margin. We study the effects of the strength of the lower crust and of a weak mid-crustal shear zone on the resulting extension styles. A weak mid-crustal shear zone effectively decouples upper crustal extension from lower crustal flow. Without the mid-crustal shear zone, the degree of coupling between the upper and the lower crust increases and extension of the whole crust tends to focus on the thickest part of the wedge. We identify three distinct modes of extension determined by the strength of the lower crust, which are characterized by 1) localized, asymmetric crustal exhumation in a single massif when the lower crust is weak, 2) the formation of rolling-hinge normal faults and the exhumation of lower crust in multiple core complexes with an intermediate strength lower crust, and 3) distributed domino faulting over the weak mid-crustal shear zone when the lower crust is strong. A frictionally stronger mid-crustal shear zone does not change the overall model behaviors but extension occurred over multiple rolling-hinges. The 3 modes of extension share characteristics similar to geological models proposed to explain the formation of metamorphic core complexes: 1) the crustal flow model for the weak lower crust, 2) the rolling-hinge and crustal flow models when the lower crust is intermediate and 3) the flexural uplift model when the lower crust is strong. Finally we show that the intensity of decoupling between the far field extension and lower crustal flow driven by the regional pressure gradient in the wedge control the overall style of extension in the models.

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We ran numerical experiments of the extension of a crustal wedge as an approximation to extension in an orogenic belt or a continental margin. We study the effects of the strength of the lower crust and of a weak mid-crustal shear zone on the resulting extension styles. A weak mid-crustal shear zone effectively decouples upper crustal extension from lower crustal flow. Without the mid-crustal shear zone, the degree of coupling between the upper and the lower crust increases and extension of the whole crust tends to focus on the thickest part of the wedge. We identify three distinct modes of extension determined by the strength of the lower crust, which are characterized by 1) localized, asymmetric crustal exhumation in a single massif when the lower crust is weak, 2) the formation of rolling-hinge normal faults and the exhumation of lower crust in multiple core complexes with an intermediate strength lower crust, and 3) distributed domino faulting over the weak mid-crustal shear zone when the lower crust is strong. A frictionally stronger mid-crustal shear zone does not change the overall model behaviors but extension occurred over multiple rolling-hinges. The 3 modes of extension share characteristics similar to geological models proposed to explain the formation of metamorphic core complexes: 1) the crustal flow model for the weak lower crust, 2) the rolling-hinge and crustal flow models when the lower crust is intermediate and 3) the flexural uplift model when the lower crust is strong. Finally we show that the intensity of decoupling between the far field extension and lower crustal flow driven by the regional pressure gradient in the wedge control the overall style of extension in the models.

Dating granulite-facies structures and the exhumation of lower crust in the Moldanubian Zone of the Bohemian Massif

Deformation of granulite-facies rocks in the Moldanubian Zone of the southern Bohemian Massif is expressed in two intersecting planar fabrics – steeply disposed (S1) and flat-laying (S2) – which correspond to two deformation stages (D1) and (D2). The existing Sm-Nd garnet ages from banded granulite gneisses, new U-Pb zircon data from deformed granite intrusions within the granulite gneisses, and the P–T and field structural relations constrain the ages and P–T conditions of the two deformation phases. The early deformation (D1) was associated with a HP-HT metamorphic stage with a minimum age of ca. 354 Ma which was followed by a near-isothermal decompression. A concordant U-Pb zircon age of 318±1 Ma dates the emplacement of intrusions of deformed granite into the granulite gneisses and constrains deformation phase (D2). This phase was associated with an LP–HT metamorphism dated in the region at ca. 340–330 Ma. The available structural and isotopic data indicate that granulites in the southern Bohemian Massif were exhumed from lower to middle crust during compression. The structural relations and P–T–t data for the studied granulites are consistent with their exhumation by near-vertical extrusion of the softened orogenic root.

Collision tectonics in the Swiss Alps: Insight from geodynamic modeling

This paper compares results from two‐dimensional finite element dynamic modeling with the kinematic evolution of the Swiss Alps during the collision phase. In particular, we investigate the role of inherited lateral strength heterogeneities on orogenesis. A number of first‐order characteristics are directly comparable at crustal scales. In the models the entry of continental crust into the convergent margin marks the end of near‐perfect subduction. Accretion of material of the subducting plate to the upper plate creates an orogenic wedge on the incoming (pro)side and initiates a retroshear zone (or model backthrust). The addition of material to the upper plate builds a bivergent orogen. Heterogeneities in the pro‐crust focus shear and lead to the development of “nappe structures” The combined action of pro‐shear (nappe stacking) and retroshear (backthrusting) uplifts a plug between the two shear zones. Subsequent focusing of shear along the retroshear zone results in rotation of the plug and overlying units, leading to crustal‐scale backfolds as observed in the Swiss Alps. The model experiments predict features relevant to Alpine dynamics, including (1) similar crustal thicknesses and exhumation patterns to those observed in the Swiss Alps today for erosion rates comparable to natural ones (1 mm yr−1), (2) continued accretion and subduction of upper crustal fragments allowing high‐pressure metamorphic conditions, (3) tilting and exhumation of lower crust when a midcrustal weak zone is present, and (4) “shunting” of material across the strong lower crustal wedge of the upper plate.