Crustal Ductile Flow Research Articles

Stretching–buoyancy interplay in the magma-poor rifting process

Abstract The Cauvery Basin in East India represents a failed rift zone in the west and transform-related termination in the east. The deformation associated with a westward-propagating rift zone involves stretching and necking-related deformation from west to east. The rift axis and the flanks exhibit maximum and minimum deformation. In this study, we document the increasing role of buoyancy-driven processes and the development of the rift asymmetry during the advanced stages of rifting in a magma-poor setting. We use a series of reflection seismic profiles intersecting the failed rift zone maturing eastward. The onset of buoyancy-controlled extension correlates with the localized extension. It creates a relatively symmetrical axial dome, and exhibits rift flank rotations and central up-warping. This permanent uplift is associated with lower crustal ductile flow. Notably, the deep-seated syn-rift buoyancy forces progressively operate eastward. We deduce the associated transient dome uplift and its subsequent dissipation using a seismic flattening technique. The axial dome formation is associated with an upwelling asthenosphere and lower lithospheric mantle. This correlates with localized contraction within flat-lying fault blocks at its flanks, concurrently forming the typical hanging-wall and footwall geometry. The multiple shallow- to deep-seated mechanisms promote strain acceleration in the uplifted regions along the rift zone.

Orogen-parallel mid-lower crustal ductile flow during the late Triassic Qinling orogeny: structural geology and geochronology

ABSTRACT The North and South China Block collision in Triassic resulted in the Qinling orogeny with massif batholith intrusion. In this study, we investigated the deformation structures of the Foping domal zone, the Ningshan bedding-parallel shear zone, and the Yangtian-Tongchewan strike-slip shear zone in East Qinling, aiming at unravelling orogen-parallel mid-lower crustal ductile flow during this late-stage orogeny. Field measurements of foliated felsic migmatites in the Foping domal zone show a subhorizontal stretching lineation associated with top-to-east shearing, and the quartz c-axis fabrics of rock samples from leucosome display characteristic prism <c> slip system (>650°C), consistent with granulite-facies metamorphic condition. The ductile fabrics observed along the W-E extending Yangtian-Tongchewan mylonitic zone along the southern Foping domal zone indicate dextral strike-slip shearing. The Ningshan bedding-parallel shear zone was mapped in the Palaeozoic sedimentary strata along the southern rim of the Foping domal zone and it exhibits subhorizontal stretching lineation. Zircon U-Pb dating results of migmatite from the Foping domal zone, mylonite from the Yangtian-Tongchewan shear zone and syntectonic granitic dyke from the Ningshan bedding-parallel shear zone, together with mica 40Ar/39Ar dating results, allow to constrain the timing of coeval development of these structures in Late Triassic at 220–208 Ma, broadly consistent with the pluton intrusion. The overall pattern is interpreted to result from a local indentation of the Hannan Massif with a salient boundary geometry, which possibly controlled the lateral extrusion of the orogenic wedge, orogen-parallel crustal ductile flow as well as the Late Triassic plutonic emplacements in East Qinling.

A new crustal shear-velocity model in Southwest China from joint seismological inversion and its implications for regional crustal dynamics

SUMMARY Southwest (SW) China is located in a transition site from the active Tibetan Plateau to the stable Yangtze craton, which has complicated tectonic deformation and severe seismic hazards. We combine data from ambient noise, teleseismic body and surface waves, and petroleum wells to better constrain the crustal shear-velocity structure in SW China. We jointly invert the Rayleigh wave dispersion (5–40 s period), Rayleigh wave ZH ratio (20–60 s period), and P-wave receiver function for 114 permanent stations with a stepwise linearized joint inversion method. Compared to previous tomography results, we observe higher shear velocity in the sedimentary rocks within the Sichuan Basin, which is consistent with sonic logging measurements. Our model reveals widespread low-velocity zones in the mid-lower crust, and their boundaries correlate well with major fault systems. Between two main mid-crustal low-velocity channels, a prominent high-velocity region surrounded by earthquakes is observed in the inner zone of the Emeishan large igneous province (ELIP) and around the Anninghe-Zemuhe fault zone. These observations are comparable to regional tomography results using very dense arrays. Based on the results, we suggest that mid-lower crustal ductile flow and upper-crustal rigid fault movement play equally important roles in controlling the regional deformation styles and earthquake distribution in SW China. Our results also resolve thick crust-mantle transition zones beneath the eastern Tibetan Plateau and the inner zone of the ELIP due to ‘top-down’ and ‘bottom-up’ crust-mantle interactions, respectively. Our new model can serve as a reference crustal model of future high resolution model construction in SW China.

Archaean continental spreading inferred from seismic images of the Yilgarn Craton

On the early Earth, oceanic plateaux similar to present-day Iceland are thought to have evolved into less dense microcontinents as they thickened by continued melt intrusion and crustal fractionation. These earliest continents may have been so weak on a hotter Earth that they collapsed laterally in response to thickening by further magmatic growth or tectonic imbrication. This continental spreading is likely to have resulted in the development of pervasive ductile strain fabrics in the deeper crust, which, if preserved, could generate seismic reflections. Here we present seismic images from the ancient core of the Archaean Yilgarn Craton of Australia that reveal shallowly dipping to horizontal reflections that pervade the middle and lower crust. We interpret these reflective fabrics as the result of widespread lateral crustal flow during the late stage of craton evolution approximately 2.66 to 2.61 billion years ago, which coincided with the widespread intrusion of high-temperature crustal melts, as thickened early continental crust collapsed. The consequent subsidence of large regions of the upper crust, including volcanic and sedimentary greenstone rocks, in the hanging walls of listric mid-lower crustal ductile flow fabrics caused these rocks to drop beneath the granitic melts rising towards the surface, and did not involve Rayleigh–Taylor instabilities within a mostly mobile crust. Seismic images of giant crustal-collapse structures preserved in the Yilgarn Craton, Australia, reveal that these structures may have formed over 2.5 billion years ago when the cores of continents were hot and weak.

Mountain building at northeastern boundary of Tibetan Plateau and craton reworking at Ordos block from joint inversion of ambient noise tomography and receiver functions

We have obtained a high resolution 3-D crustal and uppermost mantle velocity model of the Ordos block and its surrounding areas by joint inversion of ambient noise tomography and receiver functions using seismic recordings from 320 stations. The resulting model shows wide-spread low velocity zone (Vs ≤ 3.4 km/s) in the mid-to-lower crust beneath northeastern Tibet Plateau, which may favor crustal ductile flow within the plateau. However, our model argues against the eastward crustal ductile flow beneath the Qinling belt from the Tibetan Plateau. We find high velocities in the middle part of Qinling belt which separate the low velocities in the mid-to-lower crust of the eastern Qinling belt from the low velocity zone in eastern Tibetan Plateau. More importantly, we observe significant low velocities and thickened lower crust at the Liupanshan thrust belt as the evidence for strong crustal shortening at this boundary between the northeastern Tibetan Plateau and Ordos block. The most important finding of our model is the upper mantle low velocity anomalies surrounding the Ordos block, particularly the one beneath the Trans North China Craton (TNCO) that is penetrating into the southern margin of the Ordos block for ∼100 km horizontally in the depth range of ∼70 km and at least 100 km. We propose an on-going lithospheric mantle reworking at the southernmost boundary of the Ordos block due to complicated mantle flow surrounding the Ordos block, that is, the eastward asthenospheric flow from the Tibet Plateau proposed by recent SKS study and mantle upwelling beneath the TNCO from mantle transition zone induced by the stagnant slabs of the subducted Pacific plate.

Moho depth and crustal thinning in the Marmara Sea region from gravity data inversion

AbstractThe free‐air gravity in the Marmara Sea reveals that the low density of sedimentary basins is partly compensated in the lower crust. We compiled geophysical upper crust studies to determine the sediment basin geometries in and around the Marmara Sea and corrected the gravity signal from this upper crust geology with the Parker method. Then, assuming long wavelength anomalies in the residual gravity signal is caused by variations in the Moho topography, we inverted the residual to build the Moho topography. The result shows that the Moho is uplifted on an area greater than the Marmara Sea with a maximum crust thinning beneath the basins where the Moho is at about 25 km, 5 km above the reference depth. We then evaluated the Neogene extension by comparing the surface covered by our 3‐D thinned model with the surface covered by an unthinned model with same crustal volume. Comparing this surface with areal extension rate from GPS data, we found a good compatibility indicating that the extension rate averaged over the Sea of Marmara area probably remained close to its present‐day value during major changes of tectonic regime, as the incursion of the North Anatolian Fault system during the Pliocene leads to the establishment of the dominantly strike‐slip present‐day system. We also show that crustal extension is distributed over a wider domain in the lower crust than in the upper crust, and that this may be accounted for by a relatively minor component of lower crustal ductile flow.

Coeval thrusting and extension during lower crustal ductile flow – implications for exhumation of high‐grade metamorphic rocks

AbstractNumerical models of tectonic processes in large hot orogens offer insight into the styles, controls and consequences of ductile flow of middle and lower orogenic crust. We present results from crustal‐scale thermal‐mechanical models illustrating contrasting styles of syn‐ and post‐convergent ductile flow. Crustal viscosity in the models is controlled by temperature‐dependent flow laws, scaled to represent lithological strength variations. An additional viscosity decrease at high temperature can be imposed to represent weakening during incipient partial melting. Two end‐member tectonic styles, in which lower crustal flow is driven either by gravity or tectonics, develop in response to the initial properties and evolving strength of the model crust. Homogeneous channel flow, driven by the gravitational potential energy difference between a plateau and its foreland, is characteristic of Himalayan–Tibetan (HT‐series) models with relatively weak, laterally homogeneous crust and moderately high heat production. In contrast, Grenville Orogen (GO‐series) models with relatively strong, laterally heterogeneous crust do not reach the low effective viscosity (ηeff≤1019 Pa.s) required for gravity‐driven channel flow. Instead, tectonically driven lateral flow and explusion of lower crust is triggered by underthrusting of a strong indentor. The dominant structures are ductile fold nappes that may be transported hundreds of kilometres towards the foreland. When tectonic convergence is turned off, the model orogens undergo gravitational spreading, in which ductile middle and lower crust flow laterally from the plateau towards the foreland, driving thrusting on the orogenic flanks and ductile thinning in the orogenic core. The resulting geometry can resemble that produced by syn‐convergent ductile flow, except for the development of regional‐scale normal‐sense shear zones in the post‐convergent stage. Model feasibility is tested against data from the western Grenville Orogen in Ontario. The comparison suggests that high‐grade metamorphic rocks in the orogenic core record the effects of protracted syn‐ and post‐convergent ductile flow, whereas those on the flanks record mainly post‐convergent thrusting and extension. The models suggest a number of ways to distinguish among different styles of syn‐ and post‐convergent ductile flow; metamorphic P–T–t paths alone are not diagnostic of tectonic process without additional information on crustal‐scale structure and timing. In combination with erosion, both types of flow may contribute to exhumation of regionally extensive high‐grade gneiss terranes.

Crustal ductile flow and its contribution to tectonic stress in Southwest China

Knowledge of tectonic crustal stress in highland region can provide important insights into relative plate motion, mountain building and genesis of larger crustal earthquakes underneath tectonically active regions. Here we focus on the highland regions in Southwest (SW) China, including southeast Tibet and Sichuan and Yunnan provinces. To better understand the correlation between crustal stress and structural heterogeneities, three-dimensional (3-D) velocity (Vp, Vs) structures in the crust are imaged using a large number of P- and Pn-wave, and S- and Sn-wave arrival times simultaneously. A slow-velocity layer at depths of 16–26 km was clearly imaged in most regions of SW China, suggesting the presence of a ductile layer that may be associated with partial melting and/or aqueous fluids in the crust. The azimuths of the maximum compressional stresses estimated using 147 high-quality stress data revealed a characteristic regional stress field along the fault belts in the study region. The trajectories of the stress axes along the southeastern Tibetan plateau vary from NE–SW in southwest Tibet to E–W in southeast Tibet. Our study indicates that, apart from the influence from the collision between the Indian and Eurasian plates, the present-day crustal stress field along the tectonic zones in SW China is significantly affected by ductile deformation in the crust, suggesting that crustal structural heterogeneity plays an important role in the tectonic stress.

Rheology of the lithosphere in space and time

Abstract The rheology of the lithosphere is a factor of primary importance in the kinematics and dynamics of mountain belts. This paper attempts to clarify the role of rheology in orogenesis, by applying simple physical principles to the analysis of tectonic processes. The emphasis is on broad generalizations leading to order-of-magnitude estimates. Since the rheology of lithospheric materials is strongly dependent on temperature, the discussion opens with a review of continental and oceanic geotherms and an assessment of their reliability. Then the brittle (frictional) and ductile (high-temperature creep) properties of the lithosphere are considered. In the brittle field, particular attention is paid to the problem of fault reactivation, which is shown to be more likely in extensional than in compressional regimes. In the ductile field, a summary of creep parameters for the most common lithospheric materials is presented. The central concept of rheological profiles (strength envelopes), essential to the estimation of the depth variations of lithospheric rheology, is discussed with reference not only to its applicability but also to its limitations (a two-dimensional example from the Canadian Cordillera is given). Processes related to the rheological properties and layering of the lithosphere — gravitational collapse in thickened and softened crust, tectonic inversion following the detachment of a lithospheric root, lower crustal ductile flow with consequent relaxation of Moho topography — are analysed semi-quantitatively, mainly to show that they are indeed likely to be important players in geodynamics. Finally, in a brief Archaean detour, some possible important differences between present-day and Archaean oceanic lithosphere are examined, and the conclusion is reached that in all likelihood plate tectonics, although at different rates, has been the main agent of orogenesis during most of the history of the planet.

Reflectivity and seismic properties of the deep continental crust

The recovery of drill core from a section of upper amphibolite facies terrane located in the Inner Piedmont of South Carolina provides a rare opportunity to study in detail the seismic character of rocks which were once part of the deep continental crust Seismic reflection profiles over the Inner Piedmont in the vicinity of the drill hole reveal subhorizontal high‐amplitude reflections. The origin of these reflections has been investigated by measuring compressional wave velocities as a function of confining pressure and densities in 150 samples obtained from 305 m of drill core. Synthetic seismograms generated from the velocity and density measurements demonstrate that reflections originate from subhorizontal metamorphic layering, probably originating from lower crustal ductile flow. Fine tuning of the metamorphic layering, which is temperature and pressure related, can produce bright spots in the deep crust. Anisotropy, resulting from preferred mineral orientation, is another important property of this section. Velocities are low for vertical wave propagation, whereas horizontal velocities are relatively high. Horizontal velocities show only a slight variation with azimuth, which is unlikely to be detected using standard refraction techniques. Reflectivity decreases with increasing anisotropy. Critical to this study has been the continuous coring of a major section of high‐grade metamorphic terrane, which has provided the necessary stratigraphic and lithologic control for the generation of accurate synthetic seismograms.