The Late Cenozoic geodynamic evolution of the central segment of the Andean subduction zone

The presented model of the Late Cenozoic geodynamic evolution of the central Andes and the complex tectonic, geological, and geophysical model of the Earth’s crust and upper mantle along the Central Andean Transect, which crosses the Andean subduction zone along 21°S, are based on the integration of voluminous and diverse data. The onset of the recent evolution of the central Andes is dated at the late Oligocene (27 Ma ago), when the local fluid-induced rheological attenuation of the continental lithosphere occurred far back of the subduction zone. Tectonic deformation started to develop in thick-skinned style above the attenuated domain in the upper mantle and then in the Earth’s crust, creating the bivergent system of the present-day Eastern Cordillera. The destruction of the continental lithosphere is correlated with ore mineralization in the Bolivian tin belt, which presumably started at 16° S and spread to the north and to the south. Approximately 19 Ma ago, the gently dipping Subandean Thrust Fault was formed beneath the Eastern Cordillera, along which the South American Platform began to thrust under the Andes with rapid thickening of the crust in the eastern Andean Orogen owing to its doubling. The style of deformation in the upper crust above the Subandean Thrust Fault changed from thick- to thin-skinned, and the deformation front migrated to the east inland, forming the Subandean system of folds and thrust faults verging largely eastward. The thickening of the crust was accompanied by flows at the lower and/or middle crustal levels, delamination, and collapse of fragments of the lower crust and lithospheric mantle beneath the Eastern Cordillera and Altiplano-Puna Plateau. As the thickness of the middle and lower crustal layers reached a critical thickness about 10 Ma ago, the viscoplastic flow in the meridional direction became more intense. Extension of the upper brittle crust was realized mainly in gliding and rotation of blocks along a rhombic fault system. Some blocks sank, creating sedimentary basins. The rate of southward migration estimated from the age of these basins is 26 km/Ma. Tectonic deformation was accompanied by diverse magmatic activity (ignimbrite complexes, basaltic flows, shoshonitic volcanism, etc.) within the tract from the Western Cordillera to the western edge of the Eastern Cordillera 27–5 Ma ago with a peak at 7 Ma; after this, it began to recede westward; by 5 Ma ago, the magmatic activity reached only the western part of the Altiplano-Puna Plateau, and it has been concentrated in the volcanic arc of the Western Cordillera during the last 2 Ma.     

A Preliminary study on deep geodynamic process of Xar Moron tectonic belt in eastern Central Asia Orogenic BeltSCIEI北大核心CSCD

板块汇聚边缘的陆壳厚度变化与构造和岩浆过程的动态相互作用有着错综复杂的联系,也是对深部地球动力学背景的直接响应。西拉木伦构造带是中亚造山带东部重要的汇聚板块边界,查明其浅部构造变形及深部动力学过程对于理解中亚造山带构造演化具有重要意义。本文通过野外地质工作查明晚二叠世-早三叠世西拉木伦构造带的上地壳发育一系列北东向、轴面向南东倾的宽缓褶皱以及向南东逆冲的断层,变形样式属于薄皮构造,显示出由北西向南东挤压的单向构造应力背景,平衡剖面恢复显示此时期构造变形造成地表~30%的缩短以及~4km的浅部地壳增厚。利用林西地区火成岩全岩La/Yb比值和锆石Eu/Eu*参数构建的年龄-地壳厚度曲线揭示,二叠纪早-中期地壳厚度从49km连续减薄到33km,反映此时期整体处于伸展环境。二叠纪晚期至三叠纪初期,地壳厚度增加了~15km,峰值厚度达~48km,这个迅速的地壳增厚过程可能是岩浆作用导致的地壳垂向增生和构造作用产生的造山带物质堆叠综合作用的结果。本文根据构造带同汇聚期岩浆岩面积和地壳厚度估算造山作用形成了~11%的新生陆壳。同时,两个时期的深部壳幔相互作用方式也有不同,二叠纪早期西拉木伦构造带火成岩锆石的εHf(t)值相对较高(6.1~19.9;均值10.1),δ^(18 )O值较分散(5.1‰~8.3‰),指示岩浆在形成过程中有幔源物质的加入,示踪了林西地区深部与软流圈上涌有关的伸展过程;而晚二叠世至三叠纪初期花岗岩锆石εHf(t)值相对下降(-1.1~17.2;均值9.3),δ^(18 )O值仍高于幔源值(>5.9‰),揭示同源地幔岩浆的持续重融改造过程。综合沉积环境、地壳厚度变化、岩浆岩同位素变化、地壳增生量及地表单向构造应力背景等特征,本文提出西拉木伦构造带可能经历了地幔俯冲;而早-中二叠世的软流圈上涌和晚二叠世-早三叠世的下地壳及岩石圈地幔密度增大,可能是发生地幔俯冲的深部地球动力学原因。     

Seismic, gravity and petrological evidence for partial melt beneath the thickened Central Andean crust (21–23°S)

On the basis of seismic refraction investigations and gravimetric data we have modelled the crustal structure of the southern Central Andes (21–23°S). A pronounced variation in crustal parameters is seen in N-S- and W-E-crossing seismic profiles over the entire Andean orogene, characterized by a crustal thickness of up to 70 km under the magmatic arc and backarc, strongly reduced seismic velocities and a Bouguer minimum of −450 mGal. Anomalously low velocities of 5.9–6.0 km/s in the deeper crust of the Western Cordillera and Altiplano regions lead to an over-compensation of the Bouguer minima resulting in values of crustal densities higher than estimates based purely on seismic velocity measurements. In an attempt to reconcile these differences, the behavior of crystalline rocks based on published laboratory data was studied under varying pressure and temperature conditions up to the range of partial melting. If the temperature is increased above the melting point, a rapid decrease in seismic velocity is accompanied by a slow decrease in density. For the Central Andes, a good fit of the observed and calculated Bouguer anomalies is obtained if the densities of the rocks from the low-velocity zone (LVZ) beneath the Western Cordillera and the Altiplano are varied. Model calculations lead to a velocity-density relation for partial molten rocks that allows the melt proportions of rocks to be estimated. Model calculations indicate that 15–20 vol.% of basaltic to andesitic melt at depth is necessary to explain the LVZ and Bouguer anomaly beneath the arc and parts of the backarc. High heat flow values (100 mW/m²) support the idea that large areas of the deeper Andean crust are strongly weakened by the presence of partially molten rocks, resulting in reduced seismic velocities, with the Western Cordillera, the active volcanic arc of the Andean mountain range, acting as a ductile buffer between the two more rigid crustal blocks of the forearc and backarc regions.     

Crustal thickening processes in the Central Andes and the different natures of the Moho-discontinuity

New seismic data from the Central Andes allow us to clarify the crustal structure of this mountain chain and to address the problem of crustal thickening. Evidence for the deep crustal root can be observed in both gravimetric and seismological data. Crustal structure and composition change significantly from east to west. In the eastern part of the backarc the Moho discontinuity is clearly recognisable. However only poor Moho arrivals are observed by active seismic measurements beneath the Altiplano and the Western Cordillera where broad-band seismology data indicate such a discontinuity. In the Precordillera, a pronounced discontinuity is detected at a depth of 70 km. Along the coast, the oceanic Moho is developed at a depth of 40 km. There are several processes which can change the petrological and petrophysical properties of the rocks forming the crust. Variations of the classical Moho discontinuity are presented which do not correspond to the petrological crust/mantle boundary. Tectonic shortening in the backarc is the dominant process contributing to at least 50–55% to the root formation along 21°S. In the forearc and arc, hydration of the mantle wedge produced ≈15–20% of crustal thickening. Magmatic thickening and tectonic erosion contributed only ≈5%. The other ≈25% is not yet explained.     

Crustal thickening in an active margin setting （Philippines）： The whys and the hows

A synthesis of crustal thickness estimates was made recently utilizing available field, geochemical, seismicity, shear wave velocity and gravity data in the Philippines. The results show that a significant portion of the Philippine archipelago is generally characterized by crust with a thickness of around 25 to 30 kilometers. However, two zones, which are made up of a thicker crust (from 30 to 65 km) have also been delineated. The Luzon Central Cordillera region is characterized by thick crust. Another belt of thickened crust is observed in the Bicol-Negros-Panay-Central Mindanao region. This paper examines the interplay of tectonic and magmatic processes and their role in modifying Philippine arc crust. The processes, which could account for the observed crustal thicknesses, are presented. The contributions of magmatic arcs as compared to the contribution of the emplacement and accretion of ophiolite complexes to crustal thickness are also discussed.     

How does the elevation changing response to crustal thickening process in the central Tibetan Plateau since 120 Ma?

When and how the Tibetan Plateau formed and maintained its thick crust and high elevation on Earth is continuing debated. Specifically, the coupling relationship between crustal thickening and corresponding paleoelevation changing has not been well studied. The dominant factors in crustal thickness changing are crustal shortening, magmatic input and surface erosion rates. Crustal thickness change and corresponding paleoelevation variation with time were further linked by an isostatic equation in this study. Since 120 Ma crustal shortening, magmatic input and surface erosion rates data from the central Tibetan Plateau are took as input parameters. By using a one-dimensional isostasy model, the authors captured the first-order relationship between crustal thickening and historical elevation responses over the central Tibetan Plateau, including the Qiangtang and Lhasa terranes. Based on the modeling results, the authors primarily concluded that the Qiangtang terrane crust gradually thickened to ca. 63 km at ca. 40 Ma, mainly due to tectonic shortening and minor magmatic input combined with a slow erosion rate. However, the Lhasa terrane crust thickened by a combination of tectonic shortening, extensive magmatic input and probably Indian plate underthrusting, which thickened the Lhasa crust over 75 km since 25 Ma. Moreover, a long-standing elevation >4000 m was strongly coupled with a thickened crust since about 35 Ma in the central Tibetan Plateau.©2021 China Geology Editorial Office.     

Lithospheric evolution of the Andean fold–thrust belt, Bolivia, and the origin of the central Andean plateau

We combine geological and geophysical data to develop a generalized model for the lithospheric evolution of the central Andean plateau between 18° and 20° S from Late Cretaceous to present. By integrating geophysical results of upper mantle structure, crustal thickness, and composition with recently published structural, stratigraphic, and thermochronologic data, we emphasize the importance of both the crust and upper mantle in the evolution of the central Andean plateau. Four key steps in the evolution of the Andean plateau are as follows. 1) Initiation of mountain building by 70 Ma suggested by the associated foreland basin depositional history. 2) Eastward jump of a narrow, early fold–thrust belt at 40 Ma through the eastward propagation of a 200–400-km-long basement thrust sheet. 3) Continued shortening within the Eastern Cordillera from 40 to 15 Ma, which thickened the crust and mantle and established the eastern boundary of the modern central Andean plateau. Removal of excess mantle through lithospheric delamination at the Eastern Cordillera–Altiplano boundary during the early Miocene appears necessary to accommodate underthrusting of the Brazilian shield. Replacement of mantle lithosphere by hot asthenosphere may have provided the heat source for a pulse of mafic volcanism in the Eastern Cordillera and Altiplano at 24–23 Ma, and further volcanism recorded by 12–7 Ma crustal ignimbrites. 4) After 20 Ma, deformation waned in the Eastern Cordillera and Interandean zone and began to be transferred into the Subandean zone. Long-term rates of shortening in the fold–thrust belt indicate that the average shortening rate has remained fairly constant (8–10 mm/year) through time with possible slowing (5–7 mm/year) in the last 15–20 myr. We suggest that Cenozoic deformation within the mantle lithosphere has been focused at the Eastern Cordillera–Altiplano boundary where the mantle most likely continues to be removed through piecemeal delamination.     

Local earthquake tomography of the Andes at 20°S: Implications for the structure and building of the mountain range

Arrival-times of local events recorded in northern Chile and southern Bolivia were used to determine the P velocity structure above the subducted Nazca plate. The data were recorded between June and November 1994 by the French “Lithoscope” network: 41 vertical and 14 three-component short-period seismic stations were installed along a 700 km long profile crossing the main structures of the Andean chain, from the Coastal Cordillera to the Subandean Zone. The inversion method used is a modified version of Thurber’s 3D iterative simultaneous inversion code. The results were compared with a model obtained from previous German nearby refraction seismic studies and supplemented by field geological observations.The relocated seismicity is consistent with an ∼30° dipping slab between 0 and 170 km depth. We found a variation of about 30 km of the Moho depth along the profile. The crustal thickness is about 47 km under the Coastal Cordillera, 70 km under the Western Cordillera and the western part of the Eastern Cordillera, and 60–65 km beneath the Altiplano. Close to the surface, a good agreement between the velocity model and the geological structures is observed. Generally, in the upper crust, high velocities coincide with zones where basement is present near the surface. Low velocities are well correlated with the presence of very thick sedimentary basins or volcanic material. At greater depth, the trend of the velocity model is consistent with the existence of asymmetrical west-dipping imbricated blocks, overthrusting toward the east, which explain the asymmetrical pattern of the sedimentary basins. Beneath the Western Cordillera, the active volcanic arc, a large zone of low velocity is observed and interpreted to be due to partially molten material. A clear velocity contrast appears between the western and eastern parts of the upper mantle beneath the Andes; this geometry suggests the existence of a low velocity wedge in the mantle above the slab and the presence of a thick old lithosphere in the eastern part of the Andes.     

Crustal scale balanced cross-sections of the Pyrenees; discussion

From surface and subsurface data, line-length and area balancing were used to construct four balanced and restored sections of the Pyrenees. In the Mesozoic cover, a thin-skinned tectonic model is used. In the basement an anticlinal stack geometry is applied for the foreland part of the thrust nappes. We present and discuss three possible models for the deep structures of the belt: a thin-skinned tectonic model, a thick-skinned tectonic model and an inhomogeneous strain model. The thrusts steepen downwards and the displacements die out in ductile deformation deep in the section. Therefore, we use the inhomogeneous strain model and we equal-area balance the surface of the continental crust.Hanging-wall sequence diagrams are constructed taking into account (1) the strong N-S thickness variations of the Mesozoic cover related to the Cretaceous drift of Spain and (2) the related crustal thinning of the North Pyrenean Zone superimposed upon a previous late Hercynian rise of the lower crust.The Moho step at the vertical of the North Pyrenean Fault results from the thinning of the North Pyrenean Zone. The thickening of both the Axial Zone and the North Pyrenean Zone during the Eocene compressional event preserved the step geometry.Calculated values of the minimum shortening range from 55 km in the western part of the belt to 80 km in the eastern part. Most of the shortening occurs south of the North Pyrenean Fault in the eastern part (Axial Zone) and north of the North Pyrenean Fault in the western part (Labourd thrust).     

Recent mountain building of the central Alpine-Himalayan Belt

From the end of the Eocene through the Pliocene, the Alpine-Himalayan Belt underwent collisional shortening induced by convergence of the Gondwana plates with the Eurasian Plate and varied in orientation from the north-northwestern to the northeastern directions. The collisional shortening was expressed in folding, thrusting of continental crustal tectonic sheets over one another, and closure of the residual basins of Neotethys and its backarc seas; it resulted in local thickening of the crust and its isostatic uplifting. As a rule, the uplifts were not higher than ∼1.5 km. In other words, before the Pliocene, the growth of local mountain edifices was caused by collisional shortening of the belt. Isostatic uplifting of the thickened crust was continued in the Pliocene and Quaternary even more intensely than before, but the general rise of the mountain systems was superposed on this process. The rise substantially exceeded in amplitude the contribution of the uplift caused by shortening and did not depend on the preceding Cenozoic history of either territory. Not only the mountain ridges but also most adjacent basins were involved in rising, which eventually led to the contemporary mountain topography of the belt. The spread of the hot and fluidenriched asthenosphere of the closed Tethys beneath the orogenic belt could have been a cause of such additional rising. The uplift was an isostatic reaction to decompaction of the lithospheric mantle partly replaced with asthenosphere and of the lower crust subject to retrograde metamorphism under the effect of cooled asthenospheric fluids. The deep transformations are also probably responsible for deepening of some basins in the Pliocene-Quaternary and more contrasting transverse segmentation of the belt.     

青藏高原整体隆升与地壳短缩增厚的物理－力学机制研究（上）

本文综合研究了青藏高原大地构造格局、地壳与地幔结构、地球物理场特征,对青藏高原整体隆升的物理一力学机制,进行了总结,并提出了隆升、地壳短缩和增厚的动力学模式。论文对以下五个问题进行了研究和讨论：第一,青藏高原巨厚的地壳、薄的岩石图结构、不同产状深大断裂以及推覆、切割和碰撞造山带的基本模式;第二,地震活动、断层面解与区域应力场;第三,板块运移与地体拼贴和大陆增生;第四,青藏高原隆升的物理一力学机制分析;第五,青藏高原隆升的地球动力学模式。研究结果表明,南部印度板块向北运移并与欧亚大陆板块碰撞,北部则受古亚洲板块阻隔并向南推移。在长期的碰撞与挤压作用下,造成了高原地区异常的地震活动和应力场,Ｌｇ波能量向南快速衰减和Ｑ值向南递增,水热活动强烈和地壳“南热”、“北冷”及岩石围中“壳热”、“慢冷”的格局。喜马拉雅南、北麓重力未达均衡,高山仍在上升,沿雅鲁藏布江由深部上涌的蛇绿岩套长达１７００ｋｍ,一系列走滑断层的形成和强烈的形变,形成了南界恒河平原北缘、北抵雅鲁藏布江的宽约３００～５００ｋｍ的碰撞挤压过渡带。基于此,青藏高原的隆升和地壳短缩增厚的物理一力学机制为软流圈的拖曳作用,促使印度板块与欧亚板块的碰撞和长期的挤     

Crust restoration for the western Lachlan Orogen using the strain-reversal,area-balancing technique: implications for crustal components and original thicknesses

Strain reversal of structural/stratigraphic profiles at different scales in the western Lachlan Orogen provides a perspective on original crustal thickness estimates, the former depositional basin width of the proto-western Lachlan Orogen, the original sedimentary-fan thickness, and the possible length extent of lower crust lost by subduction. Retrodeformation using strain-reversal techniques allows basin reconstruction giving an original width of the western Lachlan Orogen basin receptor of between 800 km (minimum) and ～1150 km (maximum), depending on the amount of stratal duplication allowed in the turbidites. Crude area balancing of the regional cross-section, adding in sectional volume lost by erosion and assuming strain compatibility between the upper and lower crust, suggests that the predeformation crustal thickness ranges between 15 km and ～21 km, with a lower crustal thickness (oceanic lithosphere) of ～9 km and a turbidite fan thickness of ～6 km (minimum) and ～12 km (maximum allowable), respectively. Disparity between the calculated fan thickness and that derived from measured stratigraphic sections adjusted for strain (～6 km) indicates that some form of crustal stacking must be important in structural thickening of the turbidite crustal component. By varying shortening due to fault stacking, mass balance dictates the mismatch of the upper crustal (uc) and lower crustal (lc) retrodeformed lengths, and therefore provides an estimate of lower crustal loss by subduction. End members range from: (i) a 12 km-thick fan without fault duplication, a basin width of ～800 km where uc = lc giving no lower crustal loss by subduction; to (ii) a ～6 km fan, requiring duplication by faulting, a basin of ～1150 km where uc > lc, and ～360 km of lower crust length (～30%) lost by subduction. This suggests that the total thickness of underplated igneous material in the western Lachlan Orogen is low, probably < ～2 km.     

Late Pleistocene equilibrium-line reconstructions in the northern Peruvian Andes

Equilibrium-line-altitude (ELA) reconstructions using the toe-to-headwall-altitude ratio method for paleoglaciers in the Cordilleras Blanca and Oriental, northern Peruvian Andes (7–10°S; 77°20'–77°35'W), indicate that ELAs during the last glacial maximum (LGM; marine isotope stage 2) were c . 4300 m in the Cordillera Blanca, c . 3900–3600 m on the west side of the Cordillera Oriental, and c . 3200 m on the east (Amazon Basin) side of the Cordillera Oriental. Comparison with estimated modern ELAs and glaciation thresholds indicate that ELA depression ranged from c . 700 m in the Cordillera Blanca to c . 1200 m on the east side of the Cordillera Oriental. This augments data from many mountain ranges in middle- and low-latitude regions that indicate that ELAs during the LGM were depressed by c . 1000 m. Published palynological evidence for drier conditions during the LGM in the tropical Andes suggests that ELA depression of this amount involved a temperature reduction (> 5–6°C) that greatly exceeded the tropical sea-surface temperature depression estimates of CLIMAP (< 2°C). The west to east increase in ELA depression during the LGM indicates that the steep modern precipitation gradients may have been even steeper during the LGM.     

Lithospheric transition from the Variscan Iberian Massif to the Jurassic oceanic crust of the Central Atlantic

A 1000-km-long lithospheric transect running from the Variscan Iberian Massif (VIM) to the oceanic domain of the Northwest African margin is investigated. The main goal of the study is to image the lateral changes in crustal and lithospheric structure from a complete section of an old and stable orogenic belt—the Variscan Iberian Massif—to the adjacent Jurassic passive margin of SW Iberia, and across the transpressive and seismically active Africa–Eurasia plate boundary. The modelling approach incorporates available seismic data and integrates elevation, gravity, geoid and heat flow data under the assumptions of thermal steady state and local isostasy. The results show that the Variscan Iberian crust has a roughly constant thickness of 30 km, in opposition to previous works that propose a prominent thickening beneath the South Portuguese Zone (SPZ). The three layers forming the Variscan crust show noticeable thickness variations along the profile. The upper crust thins from central Iberia (about 20 km thick) to the Ossa Morena Zone (OMZ) and the NE region of the South Portuguese Zone where locally the thickness of the upper crust is <8 km. Conversely, there is a clear thickening of the middle crust (up to 17 km thick) under the Ossa Morena Zone, whereas the thickness of the lower crust remains quite constant (6 km). Under the margin, the thinning of the continental crust is quite gentle and occurs over distances of 200 km, resembling the crustal attitude observed further north along the West Iberian margins. In the oceanic domain, there is a 160-km-wide Ocean Transition Zone located between the thinned continental crust of the continental shelf and slope and the true oceanic crust of the Seine Abyssal Plain. The total lithospheric thickness varies from about 120 km at the ends of the model profile to less than 100 km below the Ossa Morena and the South Portuguese zones. An outstanding result is the mass deficit at deep lithospheric mantle levels required to fit the observed geoid, gravity and elevation over the Ossa Morena and South Portuguese zones. Such mass deficit can be interpreted either as a lithospheric thinning of 20–25 km or as an anomalous density reduction of 25 kg m⁻³ affecting the lower lithospheric levels. Whereas the first hypothesis is consistent with a possible thermal anomaly related to recent geodynamics affecting the nearby Betic–Rif arc, the second is consistent with mantle depletion related to ancient magmatic episodes that occurred during the Hercynian orogeny.     

Morphologic evolution of the Central Andes of Peru

In this paper, we analyze the morphology of the Andes of Peru and its evolution based on the geometry of river channels, their bedrock profiles, stream gradient indices and the relation between thrust faults and morphology. The rivers of the Pacific Basin incised Mesozoic sediments of the Marañon thrust belt, Cenozoic volcanics and the granitic rocks of the Coastal Batholith. They are mainly bedrock channels with convex upward shapes and show signs of active ongoing incision. The changes in lithology do not correlate with breaks in slope of the channels (or knick points) such that the high gradient indices (K) with values between 2,000–3,000 and higher than 3,000 suggest that incision is controlled by tectonic activity. Our analysis reveals that many of the ranges of the Western Cordillera were uplifted to the actual elevations where peaks reach to 6,000 m above sea level by thrusting along steeply dipping faults. We correlate this uplift with the Quechua Phase of Neogene age documented for the Subandean thrust belt. The rivers of the Amazonas Basin have steep slopes and high gradient indices of 2,000–3,000 and locally more than 3,000 in those segments where the rivers flow over the crystalline basement of the Eastern Cordillera affected by vertical faulting. Gradient indices decrease to 1,000–2,000 within the east-vergent thrust belt of the Subandean Zone. Here a correlation between breaks in river channel slopes and location of thrust faults can be established, suggesting that the young, Quechua Phase thrust faults of the Subandean thrust belt, which involve Neogene sediments, influenced the channel geometry. In the eastern lowlands, these rivers become meandering and flow parallel to anticlines that formed in the hanging wall of Quechua Phase thrust faults, suggesting that the river courses were actively displaced outward into the foreland.     

Late Triassic thickening of the Songpan–Ganzi Triassic flysch at the edge of the northeastern Tibetan Plateau

Growing geologic evidence documents incremental Mesozoic and early Cenozoic shortening and thickening of the Tibetan crust prior to the onset of the main Cenozoic orogenic event. The Tibetan crust shows spatial and temporal variability in thickness, style, and timing of thickening, and in plateau-forming processes. The Songpan–Ganzi area of northeastern Tibet provides evidence for shortening and thickening of the crust in Late Triassic time. An oil exploratory well (HC-1) of 7012.4 m located in the area shows at least six tectonic repetitions, resulting in more than ～46% thickening of the Triassic sequence. It indicates that the true thickness of the Songpan–Ganzi Triassic flysch is not 10–15 km as previously assumed, but not more than 3–5 km. Based on this evidence, combined with prior tectonostratigraphic studies, we propose that substantial crustal shortening and thickening, leading to initial plateau formation in the northeastern Tibetan Plateau, had already occurred during the Late Triassic.     

An Andean tectonic cycle:From crustal thickening to extension in a thin crust(34°-37°SL)

Several orogenic cycles of mountain building and subsequent collapse associated with periods of shallowing and steepening of subduction zones have been recognized in recent years in the Andes.Most of them are characterized by widespread crustal delamination expressed by large calderas and rhyolitic flare-up produced by the injection of hot asthenosphere in the subduction wedge.These processes are related to the increase of the subduction angle during trench roll-back.The Payenia paleoflat-slab,in the southern Central Andes of Argentina and Chile(34°—37°S) recorded a complete cycle from crustal thickening and mountain uplift to extensional collapse and normal faulting,which are related to changes in the subduction geometry.The early stages are associated with magmatic expansion and migration,subsequent deformation and broken foreland.New ages and geochemical data show the middle to late Miocene expansion and migration of arc volcanism towards the foreland region was associated with important deformation in the Andean foothills.However,the main difference of this orogenic cycle with the previously described cycles is that the steepening of the oceanic subducted slab is linked to basaltic flooding of large areas in the retroarc under an extensional setting.Crustal delamination is concentrated only in a narrow central belt along the cordilleran axis.The striking differences between the two types of cycles are interpreted to be related to the crustal thickness when steepening the subducting slab.The crustal thickness of the Altiplano is over 60-80 km,whereas Payenia is less than 42 km in the axial part,and near 30 km in the retroarc foothills.The final extensional regime associated with the slab steepening favors the basaltic flooding of more than 8400 km~3 in an area larger than 40,000 km2,through 800 central vents and large fissures.These characteristics are unique in the entire present-day Andes.     

GEOLOGIC CONDITIONS OF EARTHQUAKE OCCURRENCES

The definition and characterization of the Oligocene-Miocene Crucero Supergroup of the southern Peru Inner Arc domain provide a previously unavailable lithostratigraphic framework for the clarification of the geodynamic context of magmatism across the southern Peruvian transect. We integrate new lithostratigraphic, petrologic, and ⁴⁰Ar/³⁹Ar geochronologic data for these volcanic and hypabyssal rocks with published information to establish a geotectonic model for the transect since 55 Ma.

The early Eocene to Early Miocene tectono-magmatic history of the region was controlled by the evolution of a flat-subduction regime, which was initiated at ～52 Ma with the onset of rapid convergence, terminating the magmatic and hydrothermal activity of the Toquepala superunit of the Coastal Batholith. Collision of the flat slab with the leading edge of the Brazilian Shield lithospheric mantle at 39-40 Ma resulted in catastrophic failure of the overriding plate and crustal-scale, NE-directed ramping, generating the “Incaic” Zongo-San Gaban tectonothermal zone in the proto-Cordillera Oriental. Failure of the flat slab (～37 Ma), its progressive foundering into the asthenosphere (from ～37 to 20 Ma), and the upwelling of asthenosphere in its wake are inferred to have generated an eastward-migrating thermal anomaly that severely weakened the lower crust. This resulted in the resumption of Main Arc magmatism in the proto-Cordillera Occidental (～32 Ma) in an axial regime of transtension, followed at 29 Ma by quasi-instantaneous broadening of the arc to a width of ～200 km. Further continentward arc expansion into the Inner Arc at ～25 Ma, generating the mixed crust- and mantle-derived suites of the Picotani Group, resulted in a 360-km-wide swath of magmatism. Termination of magmatism in the Inner Arc at ～22 Ma reflected the waning stages of anomalous mantle flow associated with foundering of the slab, and was accompanied by substantial uplift and crustal thickening.

The subsequent evolution of the transect was initiated and prompted by the broad zone of thermally weakened lower crust. Ductile shortening of the lower crust from ～22 to 17 Ma generated widespread crustal thickening and uplift across the orogen, and was accompanied by the antithetic oceanward subduction of Brazilian Shield lithosphere beneath the Inner Arc and the initiation of thrusting and folding along the Sub-Andean zone. Delamination and, probably, detachment of the Brazilian Shield from its lithospheric mantle root, as well as partial melting of this root, yielded enriched mafic melts that ascended into the orogenic crust and contributed both thermally and through the release of volatiles to the generation of the Quenamari Group.

We emphasize the spatial association of the Crucero Supergroup and comparable suites of the Bolivian Cordillera Oriental with the Bolivian Orocline, and delimit a ~250-km amagmatic longitudinal zone separating the sites of anatexis in southern Peru and northern Bolivia. This is inferred to represent the zone of maximum orthogonal convergence throughout the later Cenozoic, analogous to a “tectonic indentation” regime. Since ～22 Ma, magma ascent has been possible only in zones marginal to the indentor.     

造山带的伸展作用及其地壳演化意义

造山带的伸展作用大致可以分为两种类型:(1)喜马拉雅型伸展,伸展限于上地壳,表现为规模有限的伸展断层,发生于俯冲—碰撞阶段;(2)科迪勒拉型伸展,整个地壳发生伸展,涉及拆离断层、沉积盆地、变质核杂岩的形成,发生于碰撞后阶段。对加厚地壳的热力学模拟,可以解释造山带挤压终止到伸展开始的时序与岩浆活动的关系。喜马拉雅型伸展伴随高压变质作用,并使变质岩系近等温减压;科迪勒拉型伸展与高温变质作用关系密切,伴随花岗质岩体的侵位,并使变质岩系近等温减压之后近等压冷却。     

TECTONIC TRANSFER ON THE EASTERN EDGE OF PAMIR

TECTONIC TRANSFER ON THE EASTERN EDGE OF PAMIR