Lower Crustal Deformation Research Articles

The continental extension discrepancy and anomalous subsidence pattern in the western Qiongdongnan Basin, South China Sea

Passive margins are first-order tectonic features on earth, formed by polyphase extension and breakup of continental lithosphere. It is accepted that mildly extended basins in the proximal margin form largely by uniform continental thinning, but the continental extension in deepwater is much debated. Here we choose a line from the western Qiongdongnan Basin in the magma-poor South China Sea margin to further explore the style of continental extension and the corresponding subsidence patterns, where high resolution seismic and heat flow data are available. Structural analysis reveals that the basin architecture is extremely asymmetric and mainly controlled by the SE directed normal faults, soling into a low-angle detachment fault. However, fault timings nicely show that the faults form through polyphase rifting and from inside outwards, and do not follow a simple sequential faulting like the West Iberia margin, where the upper and lower crustal deformation are strongly coupled. Perhaps the fault order here reflects a much weaker and decoupled lower crust. Then we divide the basin into five zones and calculate the average fault derived extension factors. Moreover, the whole crustal extension factors are also estimated after assessing the initial crustal thickness on basis of the backstripped subsidence. And the lithospheric extension factors are numerically modeled using a coupled lithosphere and basin scale model constrained by the heat flow and sedimentation data. Results show that along the profile, the overall lithospheric extension is much less than that of the whole crust, and only at the south end of the profile does lithospheric extension exceeds the whole crust, showing lithospheric depth-dependent thinning. The whole crustal extension is roughly equal to the faults derived extension in the northern part, displaying no extension discrepancy. In the middle part, the whole crustal extension is less than the faults derived extension, denoting inverse extension discrepancy. While the whole crustal extension is in far excess of the faults derived extension in the southern part, showing obvious positive extension discrepancy. We infer that the inverse and positive discrepancy of crustal extension was likely to be caused by landward directed lower crustal flow, driven by the horizontal pressure arising from the combination of the differential extension in the faulted upper crust, in the mantle lithosphere, and the differential sedimentation during the syn-rift period.

GEOPHYSICS. Layered deformation in the Taiwan orogen.

The underthrusting of continental crust during mountain building is an issue of debate for orogens at convergent continental margins. We report three-dimensional seismic anisotropic tomography of Taiwan that shows a nearly 90° rotation of anisotropic fabrics across a 10- to 20-kilometer depth, consistent with the presence of two layers of deformation. The upper crust is dominated by collision-related compressional deformation, whereas the lower crust of Taiwan, mostly the crust of the subducted Eurasian plate, is dominated by convergence-parallel shear deformation. We interpret this lower crustal shearing as driven by the continuous sinking of the Eurasian mantle lithosphere when the surface of the subducted plate is coupled with the orogen. The two-layer deformation clearly defines the role of subduction in the formation of the Taiwan mountain belt.

Lower crustal deformation takes a turn

Geophysics Collisions creating mountain belts frequently involve a tectonic plate plunging into the mantle. Huang et al. connect the deformation of rock from the subducting plate to the surface topography in Taiwan (see the Perspective by Long). Subsurface deformation mapping required interpreting certain seismic wave velocities as they travel through the crust. The subsequent images of Taiwan's deep crust show two distinct layers of deformation. The bottom layer comprises the subducting slab, which is being pulled into the mantle. This mechanically couples with the upper layer of crust, compressing it into a mountain range. Science , this issue p. [720][1]; see also p. [687][2] [1]: /lookup/doi/10.1126/science.aab1879 [2]: /lookup/doi/10.1126/science.aac8671

A tectonic model for hyperextension at magma-poor rifted margins: an example from the West Iberia–Newfoundland conjugate margins

Abstract Hyperextended, magma-poor margins are characterized by a wide continent–ocean transition and anomalously small fractions of magmatism during mantle exhumation prior to oceanic spreading. Here, I bring together several aspects of their rift to drift transition and give a coherent picture of their evolution from platform to deep-sea environments. I focus mainly on the West Iberia Margin (WIM)–Newfoundland (NF) conjugates in the North Atlantic Ocean. The architectural evolution of these margins is characterized by upper-crustal faulting and lower-crustal deformation that are tightly coupled, resulting in fault displacement that is accompanied by underlying, equal and coeval crustal thinning. Lower crust deforms first ductilely, but then progressively switches to brittle due to enhanced conductive cooling at very slow extension velocities (< c. 6 mm a −1 half-rate). The switch from ductile to progressively brittle lower crust is accompanied by the emergence of a dominant basinwards fault dip and oceanwards younging of fault activity. It is shown that these processes, acting in concert: (1) reconcile the horizontal extension on faults with crustal thinning without the need of lower-crustal flow; (2) explain, within one common Andersonian framework (faults active at 65°–30°), the change in fault geometry from planar to listric to detachment-like with increasing extension; and (3) generate the tectonic asymmetry observed between conjugate pairs. This work also discusses a high-resolution seismic section of the WIM showing that the ‘detachment-like’ fault S is truncated prior to the peridotite ridge where mantle exhumation first takes place. This suggests that serpentinized mantle rises due to its own buoyancy, separating and pulling the thinned crustal blocks apart. Once the crust has been separated, further mantle exhumation takes place by magma-poor extension of the underlying mantle. I show with numerical models that either a small reduction of mantle potential temperature ( c. 25–50 °C), a mantle depletion of more than 10% or very slow half-extension velocities ( c. 6 mm a −1 ) are required to reproduce the small amount of magmatism inferred. Available observations support either a very slow extension velocity or a smaller than normal mantle temperature; however, estimation errors may be large. Ultimately, unravelling which of these factors most contribute to magma-poor mantle exhumation will provide an improved understanding of the mantle lateral homogeneity and the three-dimensional nature of the rifting to drifting process.

Details of the Doublet Moho Structure beneath Lhasa, Tibet, Obtained by Comparison of P and S Receiver Functions

The collision of the Indian and Asian plates since about 55 Ma has created the largest plateau with the thickest crust on Earth. There is, however, no general agreement on the modes of the crustal thickening. The crustal thickness of the Tibetan plateau still remains poorly determined. It is generally accepted that the Tibetan crust has roughly double normal thickness and that it thins somewhat toward the north, but individual observations of Moho depth vary spatially and for different techniques at the same place by greater than 20 km. In this work we compare P and S receiver functions at station LSA, located in the southern Lhasa terrane, to determine the crustal thickness beneath the station. A doublet Moho structure with two significant interfaces at about 60 and 80 km depth in the lower crust is seen clearly in the P receiver functions. The deeper phase (Moho) is, however, absent in the S receiver function data. Here we model the observed P and S receiver functions by a strong Moho topography that dips to the east at an angle of 30°. This result may indicate that the Moho structure beneath Tibet can be very complicated and has strong lateral variations and suggests that a detailed map of Moho depth is only possible with 2D dense seismic experiments. The Moho dips locally perpendicular to the direction of the Indian plate motion, suggesting that the lower crustal deformation is decoupled from the underlying Indian mantle lithosphere. The extremely strong Moho variation beneath and east of station LSA may also imply a crust-mantle interaction, specifically delamination or foundering of lower crust down to the upper mantle.

Characteristics of a kilometer-scale high strain zone in the lower continental crust: Mt. Hay block, central Australia

A granulite facies high strain zone was studied on Capricorn Ridge, part of the Mt. Hay block granulites in the Arunta inlier, central Australia. Deformation occurred in these primarily mafic composition granulites at lower crustal conditions (∼8 kb, ∼800 °C) at 1740–1690 Ma. Pervasive and consistently oriented foliation (striking SSW and dipping steeply) and lineations (pitching steeply eastward) occur within the shear zone. The high strain zone is recognized by the presence of SL-tectonites, parallelism of grain shape fabrics and lithologic boundaries, transposition of lithologic boundaries into parallelism with the foliation, and the local occurrence of sheath-like folds. Field-based fabric mapping uses compositional foliation intensity as a proxy for finite strain to document strain variations within the Capricorn Ridge high strain zone. There is no observed microstructural grain size reduction that corresponds to these strain variations. Fabric intensity increases from the centers of the major lithologic units towards the contacts. 100 m-scale strain localization within Capricorn ridge occurs adjacent to major lithologic contacts, indicating competency contrast during lower crustal deformation.

Crustal architecture of basin inversion during the Proterozoic Isan Orogeny, Eastern Mount Isa Inlier, Australia

The folded and metamorphosed supracrustal rocks of the eastern margin of the Mount Isa Inlier, NW Queensland, Australia, provide a record of basin inversion processes at high temperatures, from the foreland toward the core of a developing orogen. Deep water, turbiditic metasedimentary rocks of the lower Maronan Supergroup (ca. 1.71–1.66 Ga) occupy the hanging wall of a crustal-scale thrust, the Cloncurry Overthrust, which separates them from calc-silicate and meta-evaporitic rocks of the Doherty Formation (ca. 1.74–1.72 Ga) in the footwall. During the early stages of the Isan Orogeny (ca. 1.60–1.58 Ga) the rocks of the Maronan Supergroup were expelled from their basin and thrust to the north-northwest. Shortening was restricted to the hanging wall and produced a 10 km scale north to northwest verging nappe with a crumpled frontal zone of upright east–west trending folds. This morphology is consistent with a push-from-the-rear mechanism of nappe emplacement with the simple shear component of deformation increasingly accommodated within the hanging wall, down dip toward the core of the orogen. Metamorphism associated with this folding reached upper amphibolite facies temperatures at moderate to low pressures (4–6 kbar). Metamorphic zones in the hanging wall of the Cloncurry Overthrust were antiformally folded with continuing deformation. The relationship between faulting, folding and metamorphic zoning across the Cloncurry Overthrust are consistent with a thrust system, despite that fact that younger rocks are juxtaposed over older. Basin inversion provides the best explanation for this phenomenon. Within this context metamorphism during the early stages of orogenesis can be explained by the inheritance of elevated geothermal gradients produced by lithospheric thinning and sediment loading during formation of the basin. The overall orogenic architecture was controlled by a combination of thermal softening, which focussed early deformation within the basin, and weak zones at the base of the basin and the base of the crust. This resulted in crustal-scale thrusts of opposite vergence that partitioned upper crustal and lower crustal deformation.

Combined GPS and InSAR Models of Postseismic Deformation from the Northridge Earthquake

Models of combined Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar (InSAR) data collected in the region of the Northridge earthquake indicate that significant afterslip on the main fault occurred following the earthquake. Additional shallow deformation occurred to the west of the main rupture plane. Both data sets are consistent with logarithmic time-dependent behavior following the earthquake indicative of afterslip rather than postseismic relaxation. Aftershocks account for only about 10% of the postseismic motion. The two data sets are complimentary in determining the postseismic processes. Fault afterslip and shallow deformation dominate the deformation field in the two years following the earthquake. Lower crustal deformation may play an important role later in the earthquake cycle.

Stress And Seismicity In The Lower Continental Crust: A Challenge To Simple Ductility And Implications For Electrical Conductivity Mechanisms

The lower crust is generally considered to be an aseismic, weak zone where fluid distribution might be governed by textural equilibrium geometries. Saline fluids below the transition from brittle to ductile rheology have been advanced as a joint explanation for deep crustal conductivity and seismic reflectivity, the depth of onset of both phenomena being apparently bounded by isotherms in the 300-450 °C temperature range. Some petrologists, meanwhile, contest that the deep crust should be devoid of extensive fluid networks. This review exposes some geophysical exceptions to the statistical norm suggested by global geophysical data compilations and presents counter-arguments that the lower crust in places may be both dry and strong, that fluids if at all present at such depths may not necessarily be connected and that fluid mobility in the lower crust may be more limited and heterogeneous than commonly assumed. Laboratory data on crustal rocks implies that the transition from brittle to ductile rheology actually occurs over a much broader range of temperatures than 300-450 °C, and the apparent association of deep crustal conductive horizons with a temperature field of 300-450 °C may be interpretable in terms of formation temperatures of graphite, rather than fluids and brittle-ductile transition rheology. High v P /v S ratios from a 6 km thick, seismically layered zone below the Weardale granite, NE England can be explained by underplated mafic material. They are unlikely to be explained by fluids in an area where deep crustal conductance has been shown to be relatively low, unless conventional assumptions regarding deep crustal fluid distribution are inadequate or false. Perusal of the literature reveals that lower crustal seismicity is less seldom than generally appreciated. Interpretation of earthquakes nucleating at lower crustal depths is ambiguous, but in some tectonic regimes may indicate preservation of brittle rheology to the Moho and a lower crust that is predominantly mafic and dry. A better understanding of lower crustal deformation mechanisms and history may provide better insight into deep crustal conductivity mechanisms. Recent rock mechanical experiments suggest that permeability (and thus fluid connectivity) may be decreased by ductile shearing, whereas ductile shearing may aid graphitisation at lower crustal temperatures. If the lower crust in some regions is strong, this may explain the apparent preservation of both extant- and palaeostress orientations in interpretations involving electrical anisotropy.

A first coincident normal-incidence and wide-angle approach to studying the extending Aegean crust

The Aegean Sea is a broad area of submerged continental crust undergoing active extension to varying degrees. A combined near-normal incidence and wide-angle seismic recording programme was conducted in the western Aegean Sea in 1993, with the principal objective of testing the popular hypothesis that lower crustal deformation (particularily extension) is expressed as a seismically “layered lower crust” (LLC). Across the southern margin of the Cretan trough (i.e. North Cretan offshore margin), a LLC was indicated by wide-angle arrivals that was not apparent on either the coincident near-normal-in-cidence profile or on older low-frequency refraction records. North of the northern margin of the Cretan Trough, beneath the Cyclades, a domain of strong reflectivity is recorded from the middle to lower crust. Here, the near-normal incidence sections also show this typical LLC reflectivity. On the wide-angle sections, a distinct interface is suggested in addition, at a larger depth than that previously assumed for the Moho discontinuity. The structural images and interpretations derived from the new seismic data so far do not clearly support either a pure-shear crustal stretching or an asymmetric simple-shear extension model for the Aegean Sea. Our results appear to be consistent with a tectonic model, where middle crust mobilised by flow coincides spatially with upper crust that has been thinned by active extension of an orogenically thickened crust and expressed near the surface as an exhumed metamorphic core complex.

Lower-crustal deformation during terrane dispersion along strike-slip faults

After accretion, terranes caught between obliquely converging plates are often displaced several hundreds of kilometers. How this motion is accommodated at depth is not well known. Spurred by our Trans-Alaskan Crustal Transect (TACT) seismic studies of the Yukon-Tanana terrane, Alaska, I propose that the lower crust responds ductilely to accommodate differential movement between upper crust and upper mantle during margin-parallel translation. The locations of these ductile regions are dependent on temperature, composition and strain rate. For geotherms based on moderate surface heat flow (60–90 mWm −2) and reasonable geological strain rates (100% in 3 Ma), typical lower-crustal rocks can deform ductilely. The resultant subhorizontal layered fabric should be detectable with seismic techniques. Two terranes that have experienced hundreds of kilometers of margin-parallel translation, the Yukon-Tanana terrane sampled by our seismic refraction/wide-angle reflection profiles and the Salinian block (California) sampled by both seismic refraction and reflection, have reflective lower crust. Both of these terranes have moderate to high surface heat flows, have relatively thin crust, and are interpreted seismically to be underlain by a quartzo-feldspathic lower crust; conditions favorable for ductile deformation in the lower crust. Interpretations of lower-crustal layering for these two terranes is based on an increase in lower-crustal reflectivity. This layering is attributed to tectonic banding consistent with ductile deformation.

Seismic images and evolution of the Iberian crust in the Pyrenees

Crustal images of the Pyrenean range based on coincident wide-angle and vertical deep seismic profiles are presented in this study. Wide-angle experiments in the thickened Iberian crust along strike lines have provided no evidence for reflections from interfaces between layers of different velocities in the middle or lower crust. In contrast, reversed sounding on a line across the Pyrenean range shows wide-angle reflectivity. Its unusual reflectivity pattern points to widespread impedance contrasts of localized continuity embedded in the crust. This observation is consistent with a new image of vertical-incidence reflectivity, obtained by coherency-weighted migration of the coincident Ecors traverse. This profile shows numerous discontinuous and dipping reflections from the middle to the base of the crust. The reflections dip towards the axis of the range. The upper limit and the attitude of the reflective zones are so strongly variable that the crust can hardly be envisaged as the simple, continuous three-layered model previously used for geodynamical reconstructions. The reflective zones cannot be brought back to the horizontal image of a layered-lower crust of proposed extensional origin by plate unbending to the pre-Alpine stage. The dipping reflections may result from compressional deformation, as the south Pyrenean deep crustal domain has undergone both Hercynian and Alpine orogenic deformation. If part of the present reflectivity pattern was due to Alpine remobilization, the corresponding lower crustal deformation would have spread out, thereby accomodating convergence by thickening, even outside the axial zone, where the upper crustal deformation appears to be concentrated.