Continental Lower-crustal Flow: Channel Flow and Laminar Flow

EARTH SCIENCE FRONTIERS Volume 15, Issue 3, May 2008 Online English edition of the Chinese language journal Cite this article as: Earth Science Frontiers, 2008, 15(3): 130–139.

RESEARCH PAPER

Continental Lower-crustal Flow: Channel Flow and Laminar Flow LI Dewei Faculty of Earth Sciences and Center for Tibetan Plateau Studies, China University of Geosciences, Wuhan 430074, China

Abstract: Numerous geological, geophysical and geochemical investigations and finite element modeling indicate that crustal flow layers exist in the continental crust. Both channel flow model and laminar flow model have been created to explain the flow laws and flow mechanisms. As revealed by the channel flow model, a low-viscosity channel in middle to lower crust in orogen or plateau with thick crust and high elevation would flow outward from mountain root in response to lateral pressure gradient resulted from topographic loading or to denudation. However, according to the laminar flow model proposed based on investigation of the Qinghai-Tibet plateau, circulative movement of crustal lithologies with different rheological properties between basin and orogen would occur, under the driving forces resulted from dehydration and melting of subduction plate on active continental margin and from thermal energy related to upwelling and diapiring of intercontinental mantle plume or its gravitational interactions. Similarly, when driven by gravity, the softened or melted substances of the lower crust in a basin would flow laterally toward adjacent mountain root, which would result in a thinned basin crust and a thickened orogenic crust. Partially melted magma within the thickened orogenic lower crust would cause vertical movement of metamorphic rocks of lower to middle crust due to density inversion, and the vertical main stress induced by thermal underplating of lower crust would in turn lead to formation of metamorphic core complexes and low-angle detachment fault systems. Lateral spreading of uplifting mountain due to gravitation potential would result in thrust fault systems on the border between mountain and basin. Meanwhile, detritus produced synchronously by intense erosion of uplifting mountain would be transported and deposited along the marginal deep depression in the foreland basin dragged by lower crust flow. Channel flow is similar to laminar flow in a variety of aspects, covering continental intraplate deformation, ductile extrusion of middle to lower crust, synchronous transition from orogenic extrusion to extension, ductile deformation and exhumation of deep metamorphic rocks, partial melting, magmatism, etc. However, radical differences exist between the two models, such as in tectonic setting, flow domain, flow surface, flow scale, flow pattern, flow regime, flow direction, flow substance, flow behavior or flow effects, flow time, and flow mechanism. Channel flow can be regarded as part of a spatial-temporal structure of laminar flow, but lower crustal laminar flow is actually driven by thermal energy and gravity and not by surface processes such as denudation or topographic loading. Therefore, from a global viewpoint, laminar flow is only a small part of multi-grade or multi-scale circulative flow system of the earth. Key Words: lower crust; laminar flow; channel flow; circulative flow; basin-mountain coupling; the Qinghai-Tibet Plateau

1

Introduction

The plate tectonics theory established four decades ago has been extensively and successfully applied to the entire geological processes involving interactions of oceans and continents. However, in the last two decades remarkable

distinctions between continent and ocean have been realized, and more and more continental geological phenomena can not be reasonably explained simply with plate theory. The main geological phenomena include: the inhomogeneous structure of the continental lithosphere, the superimposed evolution of multi-stage continental tectonics, the synchronous coupling

Received date: 31-Mar-2008; Accepted date: 01-Apr-2008.

Corresponding author. E-mail: [email protected] Foundation item: Supported by the National Key Earlier Stage Basic Research Program of China (No. 2005CCA05600), the National Natural Science Foundation of China (No. 40572113), and the East Geophysical Corp. Research Program “Basin-mountain coupling and genesis in western China”. Copyright © 2008, China University of Geosciences (Beijing) and Peking University, Published by Elsevier B.V. All rights reserved.

LI Dewei / Earth Science Frontiers, 2008, 15(3): 130–139

and differential uplifting of basin-mountain system, the growth and migration of crustal blocks, low-velocity layer and high-conductivity layer in continental lower crust, the creation and transition of continental lower crust, the layered distribution of continental intraplate seismicity, intraplate ore-forming in continental orogenic belts, the late stage reservoir formation in oil basins, the integrated rapid uplifting and environmental variations of the Qinghai-Tibet Plateau since 3.6 Ma, etc. The contradictions that occur between the geological facts and the extant theory would require a new geological theory to be proposed beyond the traditional plate tectonics hypothesis. As revealed by a great number of geological, geophysical and geochemical investigations and finite element modeling results, crustal flow layers do exist in the continental lithosphere, but lack the rigidity of continental lithospheric plate. The lower crust in disintegrated continental lithosphere shows low strength and remarkable flow-like features, and is capable of solidification and hardening due to tectonic and thermal evolution. Recently, flow of continental lower crust has attracted great attention among the international geoscientists community, and rheology of continental lower crust has since become the focus of continental dynamics. As flow of continental lower crust involves composition, deformation, metamorphism, phase transformation, partial melting, geophysical anomalies of continental crust as well as climatic, ecological, environmental and topographical evolution, etc., it has become the hot topic in solid earth sciences at the GSA annual meetings held in Denver in October 2007, and the AGU annual meetings held at San Francisco in December 2007. Currently, knowledge on flow of continental lower crust was mainly obtained based on the channel flow model. The model, proposed by Bird specifically to explain the channel flow of continental lower crust[1], deems that the non-linearly deformed lithologies in the lower crust of orogens or plateau with thickened crust would extend outward from mountain roots in response to the lateral pressure gradient induced by topographical loading. Relationships between channel flow and orogenic tectonics, topography, faulting, seismicity, exhumation of metamorphic complexes, magmatism, foreland basin marginal deposition, thermal structure, low velocity layer and high conductivity layer, etc. have since been discussed by numerous scholars[2–27], and the emphasis is put on the channel flow in middle to lower crust controlled by gravitational potential and exhumation. Undoubtedly, the channel flow model has provided a new means for continental geodynamic research, as the model has been successfully applied to account for the topography, lithology, faulting, thermal chronology, shallow-focus earthquake distribution. However, many problems arise in application of the model to explanation of some complicated geological issues[18,26,28]. In order to address these problems,

the author proposed a tectonic model related to continental lower crust flow and basin-mountain coupling in 1992ķ, which gradually evolves into a hypothesis on laminar flow tectonics[29–31]. In this paper the author will attempt to make a systematic comparison between the channel flow model and the laminar flow model and to explore the distinctions and links between the two models.

2

Channel flow tectonics model

Channel flow is a concept developed in fluid mechanics and generally refers to flow of fluid along long and parallel channel as induced by pressure gradient[32]. Bird introduced the concept in fluid mechanics into geology and advanced the orogenic lower crust channel flow model[1], which holds that when gravitational gradient induced by topographical loading reaches effective lateral pressure gradient, lower crust lithology would flow outward from mountain root, which would in turn result in collapsing of mountain and flattening of earth surface. Obviously it is a ductile flow of lower crust lithology in response to mountain gravitational loading. Lower crustal flow occurs extensively in Mesozoic to Cenozoic orogens, as a great number of geophysical and seismic data indicate that ductile lower crust or partially melted middle crust was developed in the Qinghai-Tibet Plateauĸ,[33–40]. Royden et al. argued that no crustal shortening occurred in the east of the Qinghai-Tibet Plateau since Pliocene, and the eastward extrusion of the eastern crust of the Qinghai-Tibet Plateau from high plateau elevation is associated with the eastern flow of weakened lower crust lithology, which is a continuum media deformation process constrained by depth, while lower crustal channel flow plays only a minor role in tectonic deformation of the earth’s surface[2]. Later on, more emphasis was put on the point that no remarkable extrusion and shortening occurred in the east of the Qinghai-Tibet Plateau, lower crustal lithologies of varying viscosity flow outward from the Qinghai-Tibet Plateau through channels of homogeneous thickness, which results in the steep topography in the south and the slowly inclined topography in the east of the plateau. Meanwhile, the eastern flow of the lower crust of the Qinghai-Tibet Plateau wasblocked by the Sichuan Basin comprised of strong crustal lithologies, and was hence diversified along the two sides[3]. Enkelmann et al. performed a study of the Cenozoic fault system mainly of the strike-slip type in the northeast part of the Qinghai-Tibet Plateau and apatite fission track  ķ Li D W. Laminar flow of lower crust and related upliftingdepression—a continental dynamic model. First Symposium for Youth in Hubei Branch of China Association for Science and Technology, 1992. ĸ Li D W. Crustal tectonics of the Qiangtang block in Tibet and its formation and evolution. Doctoral. Dissertation. Wuhan: China University of Geosciences (Wuhan), 2000.

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thermochronology, and concluded that after the lower crust of the Qinghai-Tibet Plateau flows eastward and comes across the hard basement of Sichuan Basin, it diverts to northeast, and flows toward the West Qinling Mountains, via the channel of nearly E-W-trending Taibai active sinistral strike-slip fault and NEE-SWW-trending Qingchuan active dextral strike-slip fault at the north margin of Wudu-Chengxian Basin. During the eastern flow of the lower crust, the amount of erosion is decreased from the NE Qinghai-Tibet Plateau to the Qinling Mountains with the weakening in liquidity of the lower crust[4]. Schoenbohm et al. carried out a systematic study of the river system, thermochronology, geological mapping, and paleobotany of the Qinghai-Tibet Plateau, and came to reckon that the ground uplifting here evolves from NW to SE. For example, in Early Pliocene the Ailaoshan-Honghe shear zone basically ceases its activity while the Xianshuihe-Xiaojiang fault begins its activity. This kind of deformation transition is kinematically associated with the lower crustal flow, and was driven by the gravitational potential of the thickened crust at the NE margin of the plateau[5]. In the past several years, channel flow model has been used to study the Himalaya with a focus on its middle crustal flow. Grujic et al. studied the asymmetrical C-axis fabrics of deformed quartz in mylonites of ductile shear zones associated with South Tibet Detachment (STD) and Main Central Thrust (MCT), which are indicative of the ductile southward extrusion of metamorphic complexes, and concluded that the fold-like bending of the metamorphic isograds can be explained by the pure shear flow that occurred at the center of the flow channel in a channel flow model[6]. Thereafter, Grujic et al. further proved that the ductile-deformed metamorphic rocks of the Himalaya constitute a key part of the low viscosity, slowly inclined flow channel that extends to the heart of the Qinghai-Tibet Plateau, its lateral pressure gradient would induce pipe-flow effect, with the biggest flow speed being located at the central part of the flow channel[7]. Based on the study of a geological profile at the south margin of the Himalaya, Searle and Szulc thought that the ductile flow channel, comprised of the Himalaya metamorphic rocks and leucogranites and simultaneously bounded by STD and MCT, is 15–30 km thick, and is slowly inclined northward but extruded southward. The leucogranites in the flow channel didn’t cut across either its upper or lower margin. The high Himalaya metamorphic rocks consist of inverse metamorphic phase series, varying from silimanite + k-spar assemblage to kyanite + staurolite assemblage, garnet + biotite assemblage, and constitute a nearly prostrate inverted fold. The extensive occurrence of the high Himalayan metamorphic complexes, ductile shear zones, and partially melted leucogranites together reflect the southward channel flow and extrusion of the crust for the Qinghai-Tibet Plateau. The Qinghai-Tibet Plateau shows an internal elevation of ca. 5000 m and a crustal thickness of 65–70 km, while the Himalayan foreland

shows an elevation of less than 500 m and a crustal thickness of only about 35 km, indicating that variations in topography and crustal thickness control the channel flow[8]. Two viewpoints, i.e., gravitational potential and erosion, have been proposed to account for the formation mechanism of channel flow. As pointed out by Hodges et al., collision between Indian plate and Eurasian plate leads to crustal accretion, lithologic accumulation, plateau uplifting, and formation of an isolated system. In this system the Qinghai-Tibet Plateau stores excessive gravitational potential, which would transmit from the high elevation to low elevation. Induced by the gravitational potential, the ductile middle to lower crust bounded by both STD and MCT would flow laterally in channel from the heart of the Qinghai-Tibet Plateau to the south margin of the Himalayan orogen, while the middle to lower crust of the Qinghai-Tibet Plateau would be extruded southward[9]. The research further demonstrates that the Himalaya–Qinghai-Tibet orogenic system was jointly resulted from plate convergence, gravitational extension and erosion, while the latter two factors are closely associated with the channel flow and extrusion of the middle crust. Under the tectonic setting of the subducting Indian plate, the middle crustal channel flow of the Himalaya underwent three stages in its development[10]: (1) In Early Miocene weakened lateral flow of middle crust occurred in the hot and thickened crust in the south of the Qinghai-Tibet Plateau, and the intense erosion at the south margin of the Himalaya led to exposure of the middle crustal flow channel to the ground surface; (2) from Mid-Miocene to Early Pliocene the intense extension at the south of the Qinghai-Tibet Plateau led to development of secondary channels for the channel flow and subsequent formation of the gneiss dome in the northern Himalaya; (3) from Late Pliocene up to now the enhanced erosion at the south margin of the Himalaya led to the southward extrusion of the major flow channel. Beaumont et al. supported the erosional mechanism for channel flow. Based on thermodynamic modeling, Beaumont et al. found that the crustal temperature and pressure gradient are the boundary conditions, but not the determining factors, for middle to lower crustal channel flow. The rate and intensity of erosion related to intense rainfall in summer in the south slope of the Himalaya constrain the formation mechanism and direction of the middle to lower crustal channel flow in the south of the Qinghai-Tibet Plateau, while the fact that lower crustal flow channel in the east of the Qinghai-Tibet Plateau was not exposed to the ground surface is due to lack of topographic-related precipitation[11]. Recently, this group has performed numerous digital modeling aiming to study the relationship between erosion and channel flow[12–16]. By studying the material movement of the out-of-sequence thrust fault system related to channel flow extrusion tectonics in the south of the Himalayan orogen, Wobus et al. pointed out that gravitational potential is the major driving force for

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channel flow, while erosion is also an important factor[17]. A symposium titled “Channel Flow, Ductile Extrusion and Exhumation of Lower-mid Crust in Continental Collision Zones”, held in London in December 2004, was devoted to the study of channel flow in the Himalaya. The AGU annual meeting held in San Francisco in December 2007 was also focused on the same topic. A great number of related literature has since been published, with a majority of the scholars supporting the middle to lower crustal channel flow model, based on a variety of evidences in topography, geology, chronology, geophysics, tectonic modeling, etc.[18,19]. In summary, major research progresses are embodied in the following aspects: (1) The lower crustal channel of the Himalaya extends in a greater space and to a greater depth and is linked up with the partially melted body of the Gangdese middle crust that can be detected by INDEPTH[5–8], while the flow channel bounded between STD and Main Himalayan Thrust (MHT) is slowly inclined northward, extending to a greater depth showing a greater thickness. As a result, the original wedge-shaped channel that tapers toward the depth is now turned into a flow channel that thickens gradually toward the depth, with pure shear at the center and simple shear at the margin; (2) The thinned channel flow structure and processes appear to be more complicated that the toothpaste-type extrusion mode, the northern Himalayan domes and low Himalayan out-of-sequence thrust faults are secondary to the main channel flow and show multi-stage activities. The channel flow is mainly characterized by flow and ductile shearing at the early stage and shows syntectonic metamorphism and magmatism, but is characterized by weakened flow activity and enhanced deformation at the later stage. Searle et al. considered that the southward extension of the Himalayan channel flow mainly occurred from 20 to 16 Ma[20]; (3) Seismic and geophysical data support the channel flow model, as low-velocity and high-conductivity layers were extensively developed in the middle to lower crust of the Qinghai-Tibet Plateau[21]; (4) The flow and ductile extrusion of the middle to lower crustal lithologies is synchronized with the metamorphism and deformation in the Himalaya, and mainly occurred in Miocene[22–25]; (5) Many scholars have discussed the mechanisms for channel flow, which can be generalized as follows: The gravitational potential of the Qinghai-Tibet Plateau characterized by high elevation and thick crust and the intense surface erosion of the south Himalayan slope would drive forward the middle to lower crustal channel flow, while the pressure gradient would drive the low viscosity lithologies in the middle to lower part of the thickened crust forward along the oblique flow channel and transport them to the ground surface. Exhumation can facilitate the process, while ductile extrusion can enhance the width of the Qinghai-Tibet Plateau. Based on a summary of past research results, Harris outlined the formation mechanism for the channel flow in the Himalaya and in the south of the

Fig. 1

Himalayan channel flow model (after Harris, 2007)

1: Indian cover; 2: Low Himalayan rock system; 3: Indian shield; 4: Tethys Himalaya sedimentary rock system; 5: High Himalayan crystalline rock system; 6: Gangdese crust; 7: Partial melting

Qinghai-Tibet Plateau[26] (Fig. 1). Monsoon rainfall and topographic loading are two key factors in the mechanism for the low viscosity middle crustal channel flow, which would result in lateral extension of the Qinghai-Tibet Plateau. Bendick and Flesch emphasized the differences in crustal flow between the north and south of the Qinghai-Tibet Plateau, and pointed out that crust-mantle interactions shall be taken into account for crustal flow[27]. The channel flow model has received extensive support as well as some disputes[18,26,28]. For example, in regard to the requirement of a partially melted middle crustal layer in the south of the Qinghai-Tibet Plateau for the channel flow model, Harrison raised a series of scientific questions[28], e.g., (1) The geophysical traverse represented by INDEPTH mainly extends along the Guli-Yadong rift, here the middle to lower crustal partial melting as represented by the fluid activity and thermal structure of the rift cannot represent the entire south of the Qinghai-Tibet Plateau, and the bright spots limited in the rift may possibly reflect hydrous fluid rather than partially melted silicate melt; (2) The earthquake focuses are relatively concentrated in an area about 70–90 km away from Gangdese in the Qinghai-Tibet Plateau, and the temperature of the earthquake area is estimated at less than 700 °C. If extensive partial melting occurs in the middle crust of this area, then inverse geothermal gradient would occur; (3) The radon isotope data for the hot spring of the Yangbajing graben reveal existence of mantle components or mantle-derived heat sources, but no mantle roles were involved in the channel flow processes; (4) No records that show affinity with Gangdese can be found from zircon U-Pb dating results for more than1600 available samples from the high Himalaya and north Himalaya. As pointed out by Yin et al., before the Neotethys is opened the Lhasa block and the Indian plate share the common basement, therefore, the statement thatmetamorphic rocks and Cenozoic intrusives which were ductile-extruded from the Himalaya were originated from the deep crust of Ĺ Gangdese cannot be justified . In addition, some other scholars Ĺ Yin A, Dubey C S, Kelty T K, et al. Construction of the eastern Himalaya by thick-skinned thrust stacking of the Indian basement: no lower crustal flow from Tibet is needed (abstract). AGU Fall Meeting, 2007.

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proposed that the driving mechanism for channel flow is still uncertain, the time for start and termination for the Himalayan channel flow is not clear; The basal MCT and top STD for the flow channel are not identical in space size and incidence time, and the middle to lower crustal channel flow is not obvious in other orogens, and so on. The author would also like to raise several questions here in this article for discussion: (1) Spatially, the bulge of the crustal len in a basin-mountain system is where lower crustal lithologies get converged, so a great number of exotic lithologies would flow into the lower crust of the mountain root; (2) Temporally, channel flow occurs above the basement with high elevation and thickened crustal structure of an orogen, indicating that tectonic uplifting has taken place in the orogen. However, formation of the middle to lower crustal channel flow in the Himalayan orogen and the related STD and MCT in Miocene is resulted from crustal thickening and tectonic uplifting. If the channel flow is induced by intense erosion at high elevation, the Himalaya and the Qinghai-Tibet Plateau are far from reaching their highest heights in Miocene, however the large-scale integral uplifting of the Qinghai-Tibet Plateau occurs after 3.6 Ma. When the channel flow occurs in the Himalaya and the south of the Qinghai-Tibet Plateau in Miocene, the elevation is not high and the crust is not thick. But after Pliocene no remarkable channel flow occurs in the Himalaya and the Qinghai-Tibet Plateau characterized by high elevation and thick crust; (3) Materially, if channel flow originates from the heart of the Qinghai-Tibet Plateau, and the flow channel at the south of the Qinghai-Tibet Plateau cuts across the ophiolite belt along the Yarlung Zangbu River, then the complexes ductileextruded from the Himalaya shall consist of components of Neotethys oceanic crust in addition to the original Gangdese components. However, no such clues can be found at least up until now, as it is well known that the Himalayan leucogranites are basically resulted from in situ melting of the middle crust of the Himalayan orogen; (4) Mechanistically, the crust-mantle interactions for an orogen can never be regarded negligible, it appears to be very hard for lithologies under very high surrounding pressure and located at a depth of tens of kilometers in the crust to be uplifted to the ground surface simply by crustal surface processes such as erosion, obviously, the crustal basal processes and crust-mantle interactions shall have to be taken into account simultaneously.

3

Laminar flow model

Similar to channel flow, laminar flow is a term used in fluid mechanics, and refers to the laminar movement of viscous fluid within the boundary layers. It is time-dependent and is characterized by very small flow rate, remarkable layering

structure, stabilized status, clearly defined movement tracks, and intense momentum transmission. The concept of laminar flow, introduced into continental geodynamics by the author, is focused on the flow features of viscoplastic lithologies from thermally softened or melted continental crust, including layered structure, layer-wise movement pattern, movement continuity, non-linearity and rheology, etc. As continental lower crust of low strength generally shows slow and steady-state flow features, differences in density, viscosity, temperature and speed are remarkable between continental lower crust of low strength and upper crust and upper mantle of high strength, as a result, lower crustal flow layers of low viscosity would flow layer-wise under the influences of thermal power and gravity, which would result in tectonic detachment between the upper and lower boundaries of lower crust. The laminar flow model was initiated by the author in 1990, who was enlightened during a traverse along the geological profiles across the core and the north of the Himalayan orogen: The Miocene leucogranite-bearing metamorphic complexes in the high Himalaya and the Tethys-Himalayan sedimentary rock systems, which are in contact with the former via detachment faults, together constitute the world’s largest metamorphic core complexes characterized by thick crust, a series of northward-inclined secondary detachment faults developed in the Tethys-Himalaya sedimentary rock systems, a string of thermal domes in Lagrange constitute a chain-like thermal uplifting and extension belt, the collision tectonics of the Yarlung Zangbu ophiolite belt was reconstructed by the nearly east-trending graben, while these nearly east-trending extension tectonics were superimposed and reformed by the nearly north-trending grabens. These thermal uplifting and extension tectonics which occurred since Miocene and related magmatism, metamorphism as well as large-scale metallogeny verified later are synchronized with the crustal thickening and tectonic uplifting in the Himalaya and the Qinghai-Tibet Plateau, while no collision-related tectonic-thermal events occur inside the Himalaya. Therefore, it can be inferred that the asymmetric tectonics typified by ductile extrusion of the metamorphic complexes in the Himalaya, northward basement detachment at the top and southward overthrusting at the base is resulted from the lateral extension of rheological lithologies at the depth of the crust, and the uplifting of the Himalaya and the Qinghai-Tibet Plateau is an continental intraplate tectonic process, which is closely related to formation and evolution of its peripheral basins[41]. Thereafter, based on numerous past research results, a hypothesis on continental lower crust laminar flow[29–31,42] and a hypothesis on multi-level circulative earth system[43,44] were proposed systematically. Continental lower crust laminar flow refers to the circulative movement of crustal lithologies in a basin-mountain system as driven by dehydration and melting of subducting slab at continental margin or by upwelling

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thermal power of intracontinental branch of mantle plume. The thermally softened lower crustal lithologies in a basin would flow layer-wise toward adjacent mountain root under the gravity influence, leading to crustal thinning of the basin, while the partially melted lithologies in remarkably thickened orogenic lower crust and relic melt brought by laminar flow would cause upward irregular flow of deep metamorphic rocks under gravitational interactions, the vertical main stress induced by thermal power and gravity interactions would result in metamorphic core complexes that extend along thermal domes and low-angle detachment fault systems, the uplifted mountain body would extend laterally as induced by gravitational potential, leading to formation of thrust nappes and slip nappes at the basin-mountain boundaries. Meanwhile, orogen-derived coarse detritus as produced synchronously by intense erosion of uplifting mountains would be transported and deposited rapidly along the marginal deep depression in the foreland basin when dragged by lower crustal flow, and in this way a basin-mountain circulative system of crustal lithologies is formed (Fig. 2). Xing Jishan et al. studied the relationship between the eastern China lithosphereasthenosphere and crustal tectonics, and came to conclude that the thermal power derived from asthenospheric uplift can lead to softening and melting of the lower crust[45]. The hypothesis on continental lower crust laminar flow can be used to explain almost all natural phenomena currently recognized and related to continental intraplate tectonic processes, and particularly, key scientific issues and geological phenomena that has long perplexed the geoscientists, which mainly include the following[29–31,46,47]:

Fig. 2

Continental laminar flow model (asymmetric type or Himalayan type) (after Li Dewei, 1995[31])

A: Partial melting zone; B: Dynamic anatexis zone; 1: Flow direction for lower crustal laminar; 2: Flow direction for mantle-derived and crust-mantle-derived magma; 3: Flow direction for partially melted magma (crust-mantle-derived magma type) in lower crust; 4: Direction for nappe thrusting; 5: Flow direction for partially melted magma in middle crust; 6: Direction for erosional transportation; 7: Direction for slip thrusting; 8: Direction for detachment and settlement of basin sediments; 9: Direction for dragging in basin marginal depression; 10: Direction for orogenic extension and detachment. ķ: Crust-mantle shear zone along Moho; ĸ: Shear and detachment zone along Conrad discontinuity; Ĺ: Thrusting and gravity sliding system; ĺ: Metamorphic core complexes in thickened crust; Ļ: Mantle plume (diapir).

(1) The lensoid structure of continental basin-mountain crust; (2) The tectonic type typified by central uplifting and marginal depression of the basement for a sedimentary basin, particularly the structure and origin of foreland basin; (3) Thickening of continental orogenic crust and exhumation of metamorphic rocks at middle to lower crust on the core of continental orogenic belt; (4) Stratiform rheological structure of continental lithosphere, brittle block formation in upper crust, brittle-ductile shearing deformation of middle crust, ductile shearing and viscoplastic flow of lower crust; (5) Remarkable time difference and distinct tectonic movement features between continental orogenic uplifting and synchronized settlement of adjacent basins vs. plate collision; (6) The geometric structure and formation mechanism of low-angle normal fault, the thermal domes and metamorphic core complexes induced under the extensional setting of thickened crust in continental orogen, the upward movement of low density heat flow lithologies of lower crust that converge to orogenic root, upholding of middle to upper crust due to lower crustal thermal underplating, here thermal power and gravitational interactions combine to produce vertical main stress, which would result in thermal uplifting and extension of the upper crust, and formation of low-angle extension tectonics system and metamorphic core complexes in thickened crust. Through 2-D thermal modeling, Martinez et al. proved that inverse density distribution of ductile lower crustal lithologies and their vertical extrusion can lead to formation of metamorphic core complexes[48]; (7) Formation mechanism for non-Anderson faults in a strike slip fault system showing abnormal conjugate relationships in the basin-mountain transition zones along the two sides of a basin; (8) The non-Anderson mechanism for slip nappes in imbricated thrust fault system, related to gravitational spreading at the basin-mountain boundary under regional extrusion tectonic setting, and its nearly horizontal occurrence, as well as foreland-inclined thrust faults; (9) The lateral extension of middle to lower crust of intraplate-uplifted mountains resulted from transition between multi-level horizontal movement and vertical movement among layers or blocks, in a basin-mountain circulative system of crustal lithologies (Fig. 2). For example, Yang Weiran, et al. and Zeng Zuoxun, et al. explored the origin of the Himalayan arc structure respectively from the point of tectonic interpretation and digital modeling, and proved that it was resulted from southward active extrusion of crust[49,50]; (10) The formation background and mechanism for the crust-derived igneous rocks in 2D-distributed, continental intraplate orogens and mixed crust and mintle-derived igneous rocks in rift basins, for example, adakitic magmatites derived from partial melting of thickened lower crust, leucogranites

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derived from partial melting related to ductile-brittle shearing heat generation of middle crust (Fig. 2); (11) Phase transformation of rocks at high temperature and high pressure in continental basin-mountain crustal structure system, and metamorphic phase transition sequences in the flow process of basin-mountain lower crustal lithologies, including prograde sequence (e.g., transition between granulite facies and eclogite facies), retrograde sequence in the orogenic uplifting and erosion processes (e.g., eclogite facies ĺ granulite facies ĺ amphibolite facies ĺ greenschist facies) and burial metamorphic sequence for the sedimentary rock system in a basin. For example, in the processes of crustal thickening and pressure and strain enhancement induced by the basin-to-orogen flow of thermally softened lithologies in lower crust, high temperature granulite would be transformed into high pressure eclogite, partial melting would occur at the mountain root of a thickened orogen, then low density, low viscosity lithologies would cause elevation of high pressure eclogite and its subsequent retrograde metamorphism into assemblages of granulite facies, amphibolite facies, greenschist facies in turn; (12) Extensive development of seismic reflection layers, low velocity layers and high conductivity layers in the lower crust of continental active tectonic belts from Mesozoic to Cenozoic; (13) 3D inhomogeneous thermal structure of continental basin-mountain crustal system, thermal evolution at different tectonic development stages related to basin-mountain coupling, lower crust solidification and hardening in old tectonic units (e.g., shield areas in Africa, Australia, Canada) and lower crust thermal activation, tectonic transition and flow-induced thinning in old stable tectonic units (e.g., superimposition of basins in old North China orogen); (14) Tectonic transition, crustal growth, formation and reformation of Moho surface and Conrad surface in continental lower crust; (15) The upper and lower boundaries for laminar flow layers in continental lower crust, i.e., Conrad surface and Moho surface, are ductile-brittle shear zones or slip detachment zones, where tectonic transition and tectonic transformation would occur; the intracontinental earthquakes are concentrated in orogens and plateaus with weakened lower crust that was intensely thickened, shallow earthquakes are distributed diffusively along the top and basal surfaces of the lower crustal laminar flow layers; (16) During the circulation of basin-mountain lithologies, the continental crust would reconstruct the oceanic crust, which would help to improve the maturity of the crust, and lead to large-scale metallogeny related to thermal uplifting and extension of upper-middle crust at the stage of continental intraplate orogenesis; (17) Basin-mountain coupling as controlled by continental intraplate lower crustal flow constrains formation of

continental sedimentary basin s, transition from rift basins to foreland basins and formation of inverted structures, and related oil generation and late stage reservoir formation in terrigenous facies rocks; (18) Laminar flow in continental lower crust leads to remarkably vertical movement between basins and mountains, which would alter the atmospheric circumfluence state, lead to variations in ecology and environment. We have one typical example: the intense uplifting of the Qinghai-Tibet Plateau in Late Cenozoic and the synchronized settlement of the Ganges Basin induced monsoon circumfluence in East Asia, which has led to huge precipitation and intense erosion of the south slope of the Himalaya as well as drought, desertification, sharp decrease in forestation and fast accumulation of eolian loess in the Asian continent; (19) The tectonic topography characterized by interspacing of basins and mountains as generated by continental lower crustal flow has constrained the distribution patterns, combination types and evolution rules of the continental river systems.

4 Contrast between channel flow model and laminar flow model The channel flow model and the laminar flow model, being closely related to each other, are common in some aspects, but different in some other aspects. A detailed comparison will be outline below. 4.1 Common ground for channel flow model and laminar flow model The channel flow model and the laminar flow model share common grounds in the following aspects: (1) The two models are both based on layered rheology of continental lithosphere, as both models assume existence of a low strength crustal flow layer between upper crust and upper mantle and that the rigid continental lithospheric plate shall be disintegrated; (2) Both channel flow and laminar flow refer to continental intraplate tectonic processes, as flow channel and flow structure are resulted from superimposition and reconstruction of earlier stage structure derived from plate collision; (3) Both channel flow and laminar flow occur in continental active tectonics zones; (4) Flow channel exists inside the continental orogenic crust, and extension and transition of the boundary layers of a flow sheet, thrusting of basal layer, extension and detachment of top layer, can occur, in association with extrusion of ductile-brittle shear zones and crystalline basements; (5) Lateral flow of newly formed lithologies of low density and low viscosity can occur in continental crust; (6) Exhumation of middle to lower crustal metamorphic rocks is related to erosion to some degree; (7) Partial melting of rocks may occur in association with flow of crustal lithologies, which would result in crust-derived

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granite; (8) The structure and origin of gneiss dome in an orogen, e.g., the Lagrange chain-like dome system in the north Himalaya derived from thermal uplifting and extension[41]; (9) The ductile-brittle shearing deformation zone at the boundaries of a flow layer controls the stratiform distribution of shallow earthquakes; (10) Both channel flow and laminar flow are fluid mechanical behaviors typical of non-Newtonian continuum media. 4.2 Distinction between channel flow model and laminar flow model Channel flow is distinct from laminar flow in many aspects, such as in tectonic setting, development location, basic structure, flow layer scale, flow direction, flow materials, flow effect, flow time, driving force, etc. (Table 1). Currently, the channel flow model is mainly applied to the south and east of the Qinghai-Tibet Plateau, while the laminar flow model is applicable to all basin-mountain systems formed in various geological times during continental tectonic evolution. 4.3 Linkage between channel flow model and laminar flow model The key to understanding of the linkage between the channel flow model and the laminar flow model lies at recognition of position, marker, direction, time and mechanism of crustal flow. Previously the driving forces for middle to lower crustal flow were generally categorized as exogenic geological processes. Channel flow primarily refers to outward flow of the lower crust for an orogenic mountain root under the influence of gravitational potential related to high elevation[1], since in the east of the Qinghai-Tibet Plateau eastward channel flow of lower crust occurs as a result of gravitational potential[2–5]. Recently the channel flow model was extensively applied to study of the Himalayan orogen, with emphasis being placed on southward flow of middle crust as induced by erosion[6–26]. In digital modeling of the controlling effect of earth surface erosion upon intracontinental lower crustal flow, Avouac and Burov concluded that, under horizontal extrusion background, when intense erosion of a mountain body occurs and the rate of erosion surpasses the rate of compensation due to crustal thickening, erosional collapse of the mountain body would occur, lower crustal lithologies would flow toward the mountain as induced by erosion, and the mountain would be uplifted and grow due to isostatic compensation; In cases of no erosion or extremely weak erosion, the lower crustal lithologies at the mountain root would extend laterally and flow outward, which would result in subsurface collapse[51]. With finite element modeling, Burov and Poliakov modeled the flow process of ductile crustal lithologies as driven by pressure gradient induced by enhanced depositional loading at the ground surface, during

the synrifting and postrifting processes[52]. Yu Shaoli, et al. and Zhang Xinyu, et al. summarized relationship between lower crust flow and geomorphologic evolution[53,54]. As held by the laminar flow hypothesis, flow of thermally softened lithologies in continental lower crust from the basin basement toward the orogenic root is mainly caused by thermal erosion of the crustal base due to upwelling and thermal diapir of mantle plume or its branch, while exhumation of metamorphic rocks and syntectonic granites at the orogenic core is mainly driven by partially melted rocks of thickened lower crust in the orogen during the circulative movement of basin-mountain crustal lithologies, and to a minor extent or ground surface by surface erosion. In the temporal-spatial structure for the continental basin-mountain crustal system, channel flow is common to the entire process of systematic evolution, while laminar flow is temporal and generally occurs in localized sense. As exemplified by the Qinghai-Tibet Plateau, the continental intraplate basin-mountain coupling in the east of the Qinghai-Tibet Plateau and the lower crustal flow from basin to mountain occurred in Yanshanian Epoch[29,44]. In Miocene, thermal dissipation in Sichuan Basin came was close to end, and Sichuan Basin was turned into a craton, the northward heat flow in lower crust from the Ganges Basin to the Qinghai-Tibet Plateau was blocked by the stable Tarim basin. As a result, the remarkably thickened lower crust at the heart of the Qinghai-Tibet Plateau turns around and flows eastward, which results in replacement of the ductile lower crust created at the Yanshanian Epoch in the east of the Qinghai-Tibet Plateau by the Miocene new ductile lower crust. The stable Sichuan Basin after Miocene also blocked the eastward flow of the lower crust of the Qinghai-Tibet Plateau, which leads to linear flow and extrusion of the thermally activated lower crust with its new injection, along the two sides of the basineastern Himalayan-“Three Rivers” syntaxis and eastern kunlun–western Qinling syntaxis. The Ganges Basin is the dynamic source for lower crustal flow in Miocene. The loss and thinning of the lower crust in the Ganges Basin and the thickening of the lower crustal laminar flow in the Himalaya and the Qinghai-Tibet Plateau are the foundations for basin-mountain coupling and circulative movement of crustal materials. During transition from horizontal flow to vertical flow of continental lower crust, crustal thickening, tectonic uplifting, thermal uplifting and extension, magmatism, erosion and transportation, lateral extrusion occur in the Qinghai-Tibet Plateau, and constitute an integral part of the laminar flow system[29,31,46] (Fig. 2), and the channel flow, mainly occurring in middle crust, is also part of this system. However, the flow sheet is not a parallel channel or a wedge-shaped channel inclined northward and extending for long distance, the source is not located in the partially melted zone in Gangdese middle crust, and the main driving force is not erosion or gravity at high elevation in the Himalaya.

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Table 1 Comparison between channel flow tectonics and laminar flow tectonics Features

Channel flow model

Laminar flow model

Tectonic setting

Active orogen or plateau with high elevation and thickened crust

Melting and thermal uplifting of subducting slab at active continental margin, and diapir of intracontinental branch of mantle plume

Tectonic position

Weak middle to lower crust of continental orogen

Weak lower crust in intracontinental basin-mountain crustal system

Basic structure

Wedge or layered tectonics bounded by top detachment fault and basal thrust fault of the crystalline basement of an orogen, for general type (i.e., Himalayan type)

The main body is a layered structure bounded by Moho surface and Conrad surface, while the Conrad surface is reconstructed by the upwelling magma derived from partial melting of lower crust at the orogenic core

Flow layer scale

Regional flow of middle to lower crust in an orogen

Large-scale lower crustal flow in a basin-mountain system

Flow system

Extrusion structure system

Compression and extension tectonics system, multi-level 4D extension and compression in a basin-mountain crustal system

Flow layer boundary

Detachment fault and thrust fault at both the top and basal boundaries for basement metamorphic rocks

Conrad surface and Moho surface that show active tectonic features at the upper and lower boundaries of lower crust

Flow direction

Oblique middle to lower crustal flow from the root of orogen or plateau to surface, or into adjacent basin

Lateral flow from the intensely melted zone of lower crust near Moho at the basin center to the orogenic root, then upward flow of partially melted magma in thickened lower crust to shallow of central orogen

Flow materials

Himalaya consists mainly of middle crustal lithologies, together with lower crustal lithologies. No components from Neotethys and Gangdese were found in the exhumed area in outward flow channel in Himalaya

Flow materials mainly consist of lower crust-derived granulite. The lower crust channel from Ganges Basin to the Qinghai-Tibet Plateau cuts across the Yarlung Zangbu suture, takes the form of narrow bands at the south margin of Gangdese and contains huge amount of Cu, Mo metallogenic elements, as well as adakitic granite, derived from partial melting of lower crust mixed with Neotethys mantle-derived oceanic components

Flow type

Extrusive flow of low viscosity lithologies

Flow difference exists between layers and blocks, as persistent and slow steady state flow occurs in lower crust, unstable pulse-type upward flow occurs at basin center, debris flow occurs at the ground surface

Crustal effect

Channel flow shapes crustal len of mountain

Laminar flow forms continental basin-mountain crustal len

Mantle effect

Mantle process is not involved

Mantle heat and material input are necessary for startup of the laminar flow system, and crustal-mantle coupling and decoupling occur in laminar flow

Topographic effect

Channel flow will reduce the orogenic gravitational potential, flattening the topography

Laminar flow will enhance the orogenic gravitational potential, resulting in remarkable differentiation of basin-mountain topography

Flow effect

Wedge-shaped extrusion of middle to lower crust of orogen; combination of basement thrust fault and top detachment fault; ductile deformation of metamorphic complexes

Ductile-brittle shear zones at Conrad surface and Moho surface; ductile shear zone in lower crust; fault system related to thermal uplifting of middle to upper crust, e.g., the metamorphic core complexes at an orogenic core; the imbricated thrust fault system at basin-mountain boundaries; conjugate strike slip fault at basin-mountain transition zone, etc.

Magmatic effect

Partial melting and magmatism of middle crustal

Formation of adakitic rocks from partial melting of thickened lower crust in an orogen; formation of leucogranite in middle crust due to ductile shearing, extension, pressure decrease and dynamic anatexis; crust-mantle-derived igneous rocks due to high T melting at the crustal base of a basin

Metamorphic effect

Inverse metamorphic facies isograd facies in flow channel

arc-shaped

Regular combination of prograde series, retrograde series, burial metamorphic series in crustal laminar flow structure for a basin-mountain system

Deposition effect

Terrigenous facies deposition in foreland basin at late stage

Terrigenous facies deposition in continental sedimentary basin, especially molasses along marginal depression of basin

Continental intraplate earthquake

The base plate of a flow channel controls the distribution of shallow earthquakes

Shallow earthquakes are distributed in layers in the brittle-ductile shear zones along active Conrad surface and Moho surface

Continental intraplate metallogeny

No account

Metallogeny occurs in circulation of basin-mountain crustal lithologies in intracontinent, particularly the case with polymetallic metallogeny under the background of uplifting and extension in thermal orogen

Intraplate hydrocarbon reservoir formation

No account

It directly controls formation and evolution of continental hydrocarbon-bearing basins, and oil generation in terrigenous facies and late stage reservoir formation in these basins

Environmental effect

Monsoon shapes high mountain or plateau topography

Laminar flow leads to formation of basin-mountain system, modifies atmospheric circumfluence, affects ecological environment, while monsoon circumfluence is counteractive to continental topography

River system effect

Rivers migrate outward from plateau or orogen

It constrains the river systems on the entire continent, as erosion proceeds to sources at the basin-mountain transition zones

Flow time

Uncertain. Orogen is characterized by thickened crust and high elevation when channel flow occurs, estimated at later stage of orogenesis

Laminar flow occurs in the main process for intraplate orogenesis and basin formation, starting at thermal upwelling of central basin base and extension and rifting of its upper crust, terminated when clastic deposition stops at foreland basin

Basin-mountain coupling

In the adjustment of the continental basinmountain system

In the synchronized formation process of continental basin-mountain system

Flow mechanism

Induced by partial melting of thickened middle to lower crust and gravitational potential

Induced by diapiring and upwelling of mantle, thermal softening or melting of lower crust, upwelling of low density magma, lateral flow of relict lower crust

Driving force

Ultimately driven by ground topographic loading and its gravitational potential as well as erosion

Mantle heat input changes the structure of the system, thermal power is transformed into gravitational potential that plays a dominant role, and partially into stress at the middle to upper crust

and

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5

Discussion and conclusions

By comparing the channel flow model with the laminar flow model, some conclusions can be drawn and detailed as follows for discussion: (1) Conditions for continental lower crustal flow encompass low viscosity, flow channel and driving force[42]. As demonstrated by the modeling results of McKenzie and Jackson, when lower crust viscosity is less than mantle viscosity, rheology of lower crust will be induced[55]. The lower crust for continental active tectonic units is generally characterized by high heat flow and low viscosity, while the lower crust for the continental stable tectonic units is generally weak at the stage of orogenesis and basin formation, but will be strong via evolution with heat dissipation in crust[31]. As a result, continental lower crustal flow is generally significant to continental intraplate tectonic processes, and the layered rheology and continental lithosphere together with the ductile flow of lower crust, both key factors to establishment of continental geodynamics theory, have attracted high attention among the international community of geoscientists. (2) Both channel flow model and laminar flow model can be used to elucidate formation of low-angle detachment fault and its ductile to brittle transition, ductile deformation, middle to high temperature metamorphism and exhumation and partial melting of rocks and magmatism, related to crustal flow layer in continental orogen, in the continental intraplate tectonic process. (3) The channel flow model is remarkably distinct from the laminar flow model in tectonic setting, development position, basic structure, flow layer scale, flow direction, flow materials, flow effect, flow mechanism, flow time, driving force, etc. Also, the mechanism for erosion at the crustal surface in channel flow model is basically different from that for thermal alteration at the crustal base in the laminar flow model, as channel flow is a process of mountain adjustment in flow of continental middle to lower crustal, low viscosity lithologies from mountain root, as driven by erosion and gravitational potential, while laminar flow is a process of orogenesis and basin formation synchronized with flow of continental lower crustal, low viscosity, thermally softened lithologies from basin to orogen, as driven by mantle-derived vertical thermal power. (4) As discussed above, the channel flow of lower crust from mountain to basin in the east of the Qinghai-Tibet Plateau in Late Cenozoic is resulted from superimposition of orogenesis and basin formation by laminar flow in the east of the Qinghai-Tibet Plateau and Sichuan Basin in the Yanshanian Epoch. The revised Himalaya-type channel flow can be regarded as a component of the temporal and spatial structure of laminar flow, as channel flow occurs only at stages in transition from extreme imbalance to dynamic balance of the crustal basin-mountain lensoid structure and

from horizontal movement to vertical movement in laminar flow system, as well as in transition from deep crustal processes to supergenic and shallow crustal processes, during the circulation of crustal basin-mountain lithologies. The circulative system for continental basin-mountain crustal lithologies is only a small part of the non-linear movement of lithologies in the open and multi-level earth system[43,44]. Therefore, establishment of earth system dynamics theory will be a new really revolutionary event in geoscience.

Acknowledgements This research was jointly supported by the Special Program for Key Basic Research of the Chinese Ministry of Science (2005CCA05600AJ), China National Natural Science Foundation (40572113), and East Geophysical Corp.’s Comprehensive New Field Research Program “Basin-mountain coupling and genesis in the western China”.

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