Thermo-rheological Structure Research Articles

Heterogeneities of the lithospheric thermal structure and rheology control Cenozoic intracontinental deformation in southeast China

It still remains elusive how the rheology of the lithosphere influences intracontinental deformation. We constructed a thermo-rheological model of the lithosphere in southeast China, a typical intracontinental deformation area, to address this issue. The thermal model was derived from updated heat flow data assuming steady-state heat conduction, using xenoliths and Pn seismic velocity data as constraints. This thermal model was integrated with a compositional and structural model of the lithosphere to estimate the strength distribution. Remarkable heterogeneities in the thermo-rheological structure were obtained. Most of southeast China is characterized by a large mantle heat flow and high temperature, thin and weak lithosphere, except for the western Yangtze Craton where a cold and rigid lithosphere exists. The lithosphere thins southeastward from 200 km in the craton interior to 70 km in the northern South China Sea. Both the ‘jelly sandwich’ and ‘crème brûlée’ rheological profiles for the continental lithosphere exist in southeast China, depending on the geothermal condition. The seismicity in southeast China is restricted to areas cooler than 600 °C, which are rheologically weak and characterized by strength transition zones. The Mesozoic flat-slab subduction of the Paleo-Pacific Ocean and far-field effects of the Cenozoic Indo-Asia collision influence the thermo-rheological pattern in southeast China, and the thermo-rheological heterogeneities control the Cenozoic intracontinental deformation.

Differences in Thermo-Rheological Structure between Qiongdongnan Basin and Pearl River Mouth Basin: Implications for the Extension Model in the Northwestern Margin of the South China Sea

This study combines surface heat flow, multi-channel seismic reflection profiles, and ocean-bottom seismometer (OBS) profiles to determine the thermo-rheological structure of the Qiongdongnan Basin (QDNB) and Pearl River Mouth Basin (PRMB), with the aim of researching the west–east variation of the passive continental margin rifting. Based on the initial lithospheric rheological model of a jelly sandwich-1 (JS-1) regime, the current architecture of the continental margin is identified to be the result of a non-uniform extension. Due to the decoupled crust–mantle relationship caused by the weak lower crust, the non-uniform extension led to the rupture of the mantle lithosphere before the crust. The central Xisha Trough falls into the JS-2 regime with only one brittle load layer, which is close to the rigid oceanic lithosphere of the Northwest Sub-basin (NSB). The high-velocity layers (HVLs) and detachment faults beneath the Xisha Trough are considered to be the result of the cooling of a thinned lower crust with mantle underplating during the middle stage of continental margin rifting. A seaward-increasing trend of lithospheric rheological strength is exhibited across the PRMB, from the crème brûlée-1 (CB-1) regime at the continental shelf to the JS-2 regime at the NSB. Unlike the HVLs of the Xisha Trough, the lower crustal HVLs beneath the eastern PRMB formed during the late stage of continental margin rifting due to the mantle lateral flow. The absence of HVLs beneath the western PRMB may indicate that the mantle lateral flow demonstrates a limited impact.

Evolution Characteristics through Thermo-Rheological Lithosphere of the Liaonan Metamorphic Core Complex, Eastern North China Craton

Metamorphic core complexes are developed in crustal activity belts at the continental margins or within continents, and their main tectonic feature is that the ductile middle crust is exhumed at the surface. The deformation properties are closely related to the geodynamic process affecting the continental crust. However, the evolution of the metamorphic core complexes after their formation is still unclear. The Cretaceous Liaonan metamorphic core complex developed in the eastern North China craton provides an ideal environment to study its evolution. In this study, we estimate the paleo-temperature and paleo-stress at the time of formation of the metamorphic core complex dynamical recrystallization of quartz and calculate the thermo-rheological structure of the present Liaonan metamorphic core complex by one-dimensional steady-state heat conduction equation and power-creep law. The results show that compared with the Cretaceous period, the geothermal heat flow value of the present Liaonan metamorphic core complex decreases from 70–80 mW/m2 to 49.4 mW/m2, the thermal lithosphere thickness increases from 59–75 km to 173 km, and the brittle transition depth increases from 10–13 km to about 70 km, showing coupling of the crust–mantle rheological structure. We speculate that the evolution of the thermo-rheological structure of the Liaonan metamorphic core complex is possibly caused by rapid heat loss or lithospheric mantle flow in the Bohai Bay Basin.

Thermo-Rheological Structure and Passive Continental Margin Rifting in the Qiongdongnan Basin, South China Sea, China

Thermo-Rheological Structure and Passive Continental Margin Rifting in the Qiongdongnan Basin, South China Sea, China

Controls by rheological structure of the lithosphere on the temporal evolution of continental magmatism: Inferences from the Pannonian Basin system

Physical-chemical controls of continental magmatism evolution remain enigmatic. Of prime and controversial nature is the temporal transition from calc-alkaline magmas to alkali basalts, correlated with a switch in tectonic regime from extension to compression. We perform 1D thermo-kinematic modelling to analyze the evolution of the thermo-rheological structure of the lithosphere in such settings using the Northwestern Pannonian Basin as a test-bed. Given well-known evidence for major reduction of brittle deformation parameters due to melt-related softening, we use a relatively low internal angle of friction. We demonstrate that at the termination of extension, the presence of intra-crustal low-viscosity layers in the lithosphere provides optimal condition for emplacement and differentiation of intermediate crustal magmatic chambers along the pathway of deep-sourced basaltic melts. In contrast, subsequent lithosphere cooling after the end of extension combined with tectonic and magmatic thickening lead to a disappearance of the low-viscosity layers and formation of lithospheric-scale faults. The latter serve as conduits for rapid ascent of uncontaminated alkali basaltic melts from the mantle to the surface. These findings shed new light on the geodynamic controls of magmatism in extensional settings.

Differences in east-west thermo-rheological structure and lithospheric deformation in the Qaidam Basin, Northern Tibetan Plateau, China

ABSTRACT To investigate the differences in east-west lithospheric deformation in the Qaidam Basin, we present thermo-rheological models of two profiles across the western Qaidam Basin (WQB) and eastern Qaidam Basin (EQB). The differences in east-west geodynamic deformation styles are also described, involving GPS motions, focal mechanisms (P axes), seismic anisotropy (SKS-wave splitting), and low-velocity zones (LVZs). The WQB is characterized by a warm destabilized cratonic basin with a weak lower crust, and the rheological structure changes from a rather weak crème brûlée-1 regime into a strong jelly sandwich-1 regime, from the Altyn-Qaidam boundary to the northeastern corner of the WQB. Combined with the limited distribution of the LVZs and the strong crust-mantle decoupling revealed by the unmatched pattern between P axes (N20ºE) and SKS-wave splitting (N110°E), the crust-mantle mixing related to the under-thrusting of the Tarim Basin along the Altyn Tagh Fault is suggested as the primary tectonic dynamic inducement of the destabilized WQB craton, which weakens the lithospheric strength greatly and contributes to the shallow brittle deformation. However, the EQB performs as a typical cold and rigid cratonic basin characterized by a jelly sandwich-2 rheological regime, which is sufficiently strong to maintain crust-mantle coupling. The LVZs beneath the EQB revealed by recent wide-angle seismic profiles have only a small effect on the lithospheric strength drop. The EQB could be regarded as a solid basin, anchored in the NE Tibetan Plateau, which shows a strong resistance to NE extrusion of the weak plateau material, leading to clockwise rotation.

Three-dimensional thermo-rheological structure of the lithosphere in the North China Craton determined by integrating multiple observations: Implications for the formation of rifts

The lithosphere of the North China Craton (NCC) has experienced significant destruction and deformation since the Mesozoic, a notable feature of which is the widespread extensional structure and lithospheric thinning in the eastern NCC. Since the thermo-rheological structure of the lithosphere is one of the main factors controlling these dynamic processes, a three-dimensional thermo-rheological model of the present lithosphere in the NCC was developed based on a geophysical-petrological method using a variety of data, and its relationship with the extensional structures and the formation of rifts was further analyzed. Our results show that the western NCC is characterized by thick lithosphere, low Moho temperature (TMoho 1). The deformation of the western narrow rift is consistent with the localized deformation dominated by the strength of lithospheric mantle. On the other hand, the lithosphere in the eastern NCC is characterized by extensive thinning (with lithospheric thickness of about 80–110 km). However, the decrease of lithospheric strength is not uniform, with high strength (10×1012 Pa m) observed in some areas (such as the Bohai Bay Basin and Hehuai Basin). Most of the eastern lithosphere is characterized by high TMoho (600–750°C) and low Sm/Sc (<1), which is inconsistent with the widespread extensional structure in the eastern NCC. Incorporating results from palaeo-geothermal and petrological studies, we developed a thermo-rheological structure model of the lithosphere at different evolutionary stages of the NCC, and suggested that the eastern NCC had a significantly thinned and weakened lithosphere in the early stages of the formation of the rift, leading to a regional distributed extension deformation dominated by crustal strength, which eventually evolved into a series of wide rifts. However, the cooling and accretion of the lithosphere in the subsequent stages significantly increased the strength of the lithospheric mantle, resulting in the inconsistency between the present thermo-rheological structure of the lithosphere and the extensional structure formed in the past.

Thermo-rheological structure of the northern margin of the South China Sea: Structural and geodynamic implications

Rheological properties of continental lithosphere are key controls on the behavior of continental deformation. Using thermal structure, constrained by surface heat flow data and measured thermal properties of rocks, the present study calculates different thermo-rheological structure scenarios for the ocean–continent transition (OCT) at the northern margin of the South China Sea, using two different models: a conventional model, taking into account frictional sliding and power-law creep, and a model that additionally includes a high-pressure brittle-fracture mechanism. Two compositions of the lower part of the lithosphere are considered: a soft case with felsic granulite lower crust and wet peridotite lithospheric mantle, and a hard case with mafic granulite lower crust and dry peridotite lithospheric mantle. The former scenario shows a major rheological change from a “jelly sandwich” to a “Christmas tree” type of rheology from north to south along the margin. This complex rheological structure explains lateral changes in earthquake distribution and geometries of extensional faults of the OCT at the northern margin of the South China Sea. Further, our analyses indicate that the initial lithospheric rheology profile probably has only one ductile layer in the lower part of upper crust. Such an initial lithospheric rheology model predicts focused extension to form asymmetric margins, which is the case for the SCS.

Present-day geothermal regime of the Uliastai Depression, Erlian Basin, North China

In this study, we calculated the present-day terrestrial heat flow of the Uliastai Depression in Erlian Basin by using systematical steady-state temperature data obtained from four deep boreholes and 89 thermal conductivity measurements from 22 boreholes. Then, we calculated the lithospheric thermal structure, thermal lithospheric thickness, and lithospheric thermo-rheological structure by combining crustal structure, thermal conductivity, heat production, and rheological parameter data. Research from the Depression shows that the present-day terrestrial heat flow ( qs) is 86.3 ± 2.3 mW/m2, higher than the average of 60.4 ± 12.3 mW/m2 of the continental area of China. Mantle heat flow ( qm) in the Depression ranges from 33.7 to 39.3 mW/m2, qm/ qs ranges from 40 to 44%, show that the crust plays the dominant position in the terrestrial heat flow. The thermal thickness of the lithosphere is about 74–88 km and characterized by a “strong crust–weak mantle” rheological characteristic. The total lithospheric strength is 1.5 × 1012 N/m under wet mantle conditions. Present-day geothermal regime indicates that the Uliastai Depression has a high thermal background, the activity of the deep-seated lithosphere is relatively intense. This result differs significantly from the earlier understanding that the area belongs to a cold basin. However, a hot basin should be better consistent with the evidences from lithochemistry and geophysical observations. The results also show the melts/fluids in the study area may be related to the subduction of the Paleo-Asian Ocean. The study of the geothermal regime in the Uliastai Depression provides new geothermal evidence for the volcanic activity in the eastern part of the Central Asian Orogenic Belt and has significant implications for the geodynamic characteristics.

Varied thermo-rheological structure, mechanical anisotropy and lithospheric deformation of the southeastern Tibetan Plateau

To investigate the lithospheric structure and deformation of the Tibetan Plateau, we present thermo-rheological models for two transects across the southeastern Tibetan Plateau (SE Tibet), showing strong lateral heterogeneity. The geodynamic implications are also investigated in relation to GPS motions, seismic anisotropy, seismicity distribution and crustal isostatic state. The unmatched pattern between Pms and SKS splittings which even intersect at right angles to the south of 26°N, suggests the crust-mantle decoupling. Constrained by a brittle load-bearing layer at the uppermost mantle beneath the Indochina block, the decoupling may occur below the uppermost mantle. The strong crust beneath the South China plate and Indochina block implied by the seismicity distribution, has two thick brittle load-bearing layers, indicating a coupled system. The crust beneath the Emeishan large igneous province (ELIP) also has two brittle load-bearing layers, but the brittle deformation is restricted to the topmost 10 km of the upper and lower crust. Only one brittle load-bearing layer resides in the upper crust in the other part of the Chuandian block. Combining with GPS and Pms splitting, we can infer that the weak crust with seismicity restricted into the depth of <30 km beneath the Chuandian block becomes decoupled. The heterogeneous rheological behavior under SE Tibet is further supported by different crustal isostatic states among the blocks. Through the comparison with observations in the central-northern Tibetan Plateau and Chuandian block, we suggest that the layers with aqueous fluids and (or) partial melting are distributed only in some place/channels of the weak middle-lower crust.

Numerical models for continental break-up: Implications for the South Atlantic

We propose a mechanism that explains in one unified framework the presence of continental break-up features such as failed rift arms and high-velocity and high-density bodies that occur along the South Atlantic rifted continental margins. We used 2D and 3D numerical models to investigate the impact of thermo-rheological structure of the continental lithosphere and initial plume position on continental rifting and break-up processes. 2D experiments show that break-up can be 1) “central”, mantle plume-induced and directly located above the centre of the mantle anomaly, 2) “shifted”, mantle plume-induced and 50 to 200 km shifted from the initial plume location or 3) “distant”, self-induced due to convection and/or slab-subduction/delamination and 300 to 800 km off-set from the original plume location. With a 3D, perfectly symmetrical and laterally homogeneous setup, the location of continental break-up can be shifted hundreds of kilometres from the initial position of the mantle anomaly. We demonstrate that in case of shifted or distant continental break-up with respect to the original plume location, multiple features can be explained. Its deep-seated source can remain below the continent at one or both sides of the newly-formed ocean. This mantle material, glued underneath the margins at lower crustal levels, resembles the geometry and location of high velocity/high density bodies observed along the South Atlantic conjugate margins. Impingement of vertically up-welled plume material on the base of the lithosphere results in pre-break-up topography variations that are located just above this initial anomaly impingement. This can be interpreted as aborted rift features that are also observed along the rifted margins. When extension continues after continental break-up, high strain rates can relocalize. This relocation has been so far attributed to rift jumps. Most importantly, this study shows that there is not one, single rift mode for plume-induced crustal break-up.

Formation of metamorphic core complexes in non-over-thickened continental crust: A case study of Liaodong Peninsula (East Asia)

Pre-thickened hot orogenic crust is often considered a necessary condition for the formation of continental metamorphic core complexes (MCCs). However, the discovery of MCCs in the Liaodong Peninsula, where the crust has a normal thickness (~35km), challenges the universality of this scenario. Therefore, we implement a series of 2-D numerical thermo-mechanical modeling experiments in which we investigate the conditions of MCC formation in normal crusts, as well as the relationships between the underlying mechanisms and the syn-rift basin evolution. In these experiments, we explore the impact of the lithostratigraphic and thermo-rheological structure of the crust. We also examine the lithosphere thickness, strain softening, extension rate, and surface erosion/ sedimentation processes. The experiments demonstrate that high thermal gradients and crustal heterogeneities result only in a symmetric spreading dome, which is geometrically incompatible with the observations of the MCCs in the Liaodong Peninsula. According to our further findings, the strain softening should play a key role in the development of asymmetric strain localization and domal topography uplift, while synchronous surface erosion controls the polarity of the syn-rift basin. The synthetic model data are compatible with the geological observations and cooling history based on the thermo-chronology for the eastern part of the East Asia during the late Mesozoic to the early Cenozoic. The model-predicted P-T-t paths are essentially different from those inferred for the other known MCCs, confirming the exceptional character of the MCC formation in the wide rift system of the East Asia.

Seismically constrained thermo-rheological structure of the eastern Tibetan margin: Implication for lithospheric delamination

article i nfo The eastern Tibetan margin bordered by the Longmen Shan range exhibits significant lateral differences in the lithospheric structure and thermal state. To investigate the roles of these differences in mountain building, we construct a thermo-rheological model along a wide-angle seismic profile across the eastern Tibetan margin based on recent seismic and thermal observations. The thermal modeling is constrained by the surface heat flow data and crustal P wave velocity model. The construction of the rheological envelopes is based on rock me- chanics results, and involves two types of rheology: a weak case where the lower crust is felsic granulite and the lithospheric mantle is wet peridotite, and a strong case where the lower crust is mafic granulite and the litho- spheric mantle is dry peridotite. The results demonstrate: (1) one high-temperature anomaly exists within the uppermost mantle beneath eastern Tibet, indicating that the crust in eastern Tibet is remarkably warmer than thatinthe Sichuanbasin,and(2)therheologicalstrengthofthe lithosphericmantlebeneatheasternTibetiscon- siderably weaker than that beneath the Sichuan basin. The rheological profiles are in accord with seismicity dis- tribution. By combining theseresults with the observed crustal/lithospheric architecture, Pn velocity distribution and magmatism in the eastern Tibetan margin, we suggest that the delamination of a thickened lithospheric mantle root beneath eastern Tibet is responsible for the growth of the eastern Tibetan margin.

Surface topography as key constraint on thermo-rheological structure of stable cratons

Why some Archean cratons survived for billions of years while the rest of the lithosphere has been reworked, probably for several times, is both enigmatic and fundamental for plate tectonics. Craton longetivity is mainly explained by their buoyancy and analyzed by testing gravitational stability of hardly detectable cratonic mantle “keels” as a function of a hypothesized thermo-rheological structure. Destruction of some cratons suggests that buoyancy is not the only factor of their stability, and many previous studies show that their mechanical strength is equally important. The upper bounds on the integrated strength of cratons are provided by flexural studies demonstrating that Te values (equivalent elastic thickness) in cratons are highest in the world (110–150 km). Yet, the lower bounds on the integrated strength of stable cratons are still a matter of debate, as well as the question on how this strength is partitioned between crust and mantle, and which set of rheological parameters is most pertinent. We show that primary observed cratonic features – flat topography and “frozen” heterogeneous crustal structure – represent the missing constraints for understanding of craton longevity. The cratonic crust is characterized by isostatically misbalanced density heterogeneities, suggesting that the lithosphere has to be strong enough to keep them frozen through time without developing major gravitational instabilities and topographic undulations. Hence, to constrain thermo-rheological properties of cratons one should investigate the stability of their topography and internal structure. Our free-surface thermo-mechanical numerical models demonstrate that craton stability cannot be warranted even by very high crustal strength, so that dry olivine mantle and cold thick lithosphere are indispensable conditions. We establish lower-bound limits on the thermo-rheological structure of cratons. In particular, we find that minimal Te needed for long-term stability of continents is approximately 90–110 km of which at least 70 km should be “contributed” by the lithosphere mantle.

Modeling of thermo-rheological structure of lithosphere under the foreland basin and mountain belt of Taiwan

The system of Taiwan mountain belt and the West Taiwan foreland basin is a manifestation of the collision between eastern Asian continental margin and the Philippine volcanic arc. It provides an ideal place for the study of the flexure behavior of lithosphere under the mountain–basin system. The paper presents results of thermal and rheological modeling of the system along Profile B, which extends 200 km in a NW–SE direction from the Taiwan Strait across the foreland basin and mountain range to the Longitudinal Valley. Along the profile, the crustal structure is constrained by wide-angle seismic and gravity data as well as P-wave tomography, while the structure of the foreland basin is constrained by multichannel seismic and drilling. Assuming that a steady-state deformation has proceeded in Taiwan since the collision of the Luzon Arc with the SE Eurasian margin at 6.5 Ma, the present-day thermal and rheological structures of the lithosphere are modeled by finite element analysis. The base of lithosphere is assumed to be isothermal, with a temperature of 1300 °C. The interior heat source consists of radiogenic heating, frictional heating on thrust faults and basal decollement, and body heating by internal friction within thrust sheets. In addition to conduction, the heat convection is carried out by the subduction of the Eurasian margin, the exhumation and erosion in the mountain range, and the sedimentation in the foreland basin. Thermal parameters are carefully selected and updated based on available data on surface heat flow, downhole thermal gradient, and thermal conductivity from Taiwan and nearby regions. The mechanical parameters are determined based on regional geology. The resulting thermal model predicts well the elevated surface heat flow in the mountain range and the depressed surface heat flow in the thrust front. A high-temperature core of >500 °C appears in the bottom of thrust sheets and the uppermost upper crust. Sensitivity analysis indicates that this elevated temperature is mainly due to the frictional heating along thrust faults and basal decollement, and to the heat carried up by the exhumation of the deeper rocks and subsequent erosion. The depth-dependent strength envelopes were computed based on the thermal model and commonly used rheological parameters. Rheological stratifiction of the lithosphere along the profile is depicted. Although the estimation of the thickness of the rheological strata is affected by the uncertainty in lithosphere thickness, the modeling results indicate clearly that the rheological structure of the lithosphere under the mountain range is remarkably different from that in the foreland basin. While the foreland basin has a typical thermal and rheological structure of the rifted continental margin with three brittle layers above three ductile layers, the raised temperature under the mountain range has weakened the lithosphere dramatically. Under compressional stress, the lithosphere beneath the mountain range becomes ductile almost entirely, except a thin (about 6 km in the central part) brittle layer near the surface and perhaps a thin brittle layer in the uppermost mantle. Such a significant weakening of the lithosphere in the mountain range should not be overlooked while discussing the flexure behavior of the lithosphere in the area.