Crustal thickness, discontinuity depth, and upper mantle structure beneath southern Africa: constraints from body wave conversions

The technique of receiver function analysis is applied to the study of crustal and upper mantle structures beneath the Kaapvaal craton in southern Africa and its surroundings. Seismic data were recorded by the seismic array of 82 sites deployed from April 1997 to April 1999 across southern Africa, as well as a dense array of 32 sites near Kimberley, in operation from December 1998 to June 1999. Arrival times for phases converted at the Moho are used to determine crustal thickness. The Moho depth in the south–western section of the craton was found to vary between 37 and 40 km, except for one station that recorded a depth of 43 km (SA23). Farther north along the western block of the craton (into Botswana) the depth increases up to 43 km. The depth increases even further in the north–eastern section of the craton, where results vary from 40 to 52 km. Just north of the Kaapvaal craton, in the neighbouring Zimbabwe craton, the crustal thickness drops significantly. The results obtained there varied from 36 to 40 km. For the Kimberley area, using the dense array, the Moho depth was found to be 37.3 km. Arrivals of the Ps and Ppps phases were used to determine the Poisson’s ratio in the region. This was found to be 0.26±0.01. Arrivals of phases from the 410 and 660 km mantle discontinuities are used to interpret the relative positions of these discontinuities, as well as for comparison of mantle temperatures and seismic velocities in the region with global averages. In the Kimberley area the 410 and 660 km discontinuities were found at their expected depth, implying that mantle temperatures in the region are close to the global average. The seismic velocities above the ‘410’ were found up to 5% faster than the averages from the global iasp91 model, which is fast even by Precambrian standards. In other sections of the Kaapvaal craton, the velocities are also faster than global averages, but not as fast as beneath Kimberley. In these sections, the ‘410’ is also slightly elevated, while the ‘660’ is depressed, which implies a slightly lower mantle temperature relative to the global average. Beneath the Kaapvaal craton we find evidence suggesting the presence of a zone with a reduced wavespeed gradient at an upper bound of approximately 300 km, which may mark the lower chemical boundary of the craton.     

A review on developments in the electrical structure of craton lithosphere

Cratons have a long history of evolution. In this paper, applications of the magnetotelluric method used in the study of craton lithosphere over the past 30 years were reviewed, examining case studies of cratons in North America, South America, Asia, Australia, and Africa. The nuclei of the Archean cratons, for example the Kalahari Craton and Rae Craton, are usually characterized by thick and highly resistive lithospheric roots. During or after the formation of the cratons, tectonothermal events, such as collision, mantle plume, and asthenosphere upwelling led to the formation of high-conductivity zones in the craton lithosphere, which could be attributed to the increased hydrogen content (of nominally anhydrous minerals), higher iron content, and formation of graphite films or sulfides along the grain boundary of minerals. These conductive zones are characterized by resistivity discontinuities in craton lithosphere. In particular, the conductive zones include (1) large-scale lithospheric mantle conductors beneath the Slave Craton, Gawler Craton, and central part of North China Craton(Trans-North China Orogen); (2) near-vertical high-conductivity zone associated with the fossil subduction zone beneath the Dharwar Craton and Slave Craton; and (3) regional lateral electrical discontinuities, such as a conductive anomaly under the Bushveld Complex of the Kaapvaal Craton. The eMoho refers to the electrical discontinuity in the crust-mantle boundary. In existing research, this has been detected under the condition of extremely high lithospheric resistivity with only a slight decrease in the lower crust, and in the case of a very thin conductive lower crust or the lack thereof. In the resistivity model, the unique “mushroom-like” lower crust-lithosphere mantle conductor and very thin lower crust layer of the North China Craton may represent lithosphere destruction and/or thinning. We also find that some of the cratons are still not well understood. Therefore, extensive three-dimensional inversion and joint interpretation of geochemical, geophysical, and geologic data are necessary to understand the tectonic evolutionary history of craton lithosphere.

     

Nd-isotope systematics of ∼2.7 Ga adakites, magnesian andesites, and arc basalts, Superior Province: evidence for shallow crustal recycling at Archean subduction zones

An association of adakite, magnesian andesite (MA), and Nb-enriched basalt (NEB) volcanic flows, which erupted within ‘normal’ intra-oceanic arc tholeiitic to calc-alkaline basalts, has recently been documented in ∼2.7 Ga Wawa greenstone belts. Large, positive initial ?_Nd values (+1.95 to +2.45) of the adakites signify that their basaltic precursors, with a short crustal residence, were derived from a long-term depleted mantle source. It is likely that the adakites represent the melts of subducted late Archean oceanic crust. Initial ?_Nd values in the MA (+0.14 to +1.68), Nb-enriched basalts and andesites (NEBA) (+1.11 to +2.05), and ‘normal’ intra-oceanic arc tholeiitic to calc-alkaline basalts and andesites (+1.44 to +2.44) overlap with, but extend to lower values than, the adakites. Large, tightly clustered ?_Nd values of the adakites, together with Th/Ce and Ce/Yb systematics of the arc basalts that rule out sediment melting, place the enriched source in the sub-arc mantle. Accordingly, isotopic data for the MA, NEBA, and ‘normal’ arc basalts can be explained by melting of an isotopically heterogeneous sub-arc mantle that had been variably enriched by recycling of continental material into the shallow mantle in late Archean subduction zones up to 200 Ma prior to the 2.7 Ga arc. If the late Archean Wawa adakites, MA, and basalts were generated by similar geodynamic processes as their counterparts in Cenozoic arcs, involving subduction of young and/or hot ocean lithosphere, then it is likely that late Archean oceanic crust, and arc crust, were also created and destroyed by modern plate tectonic-like geodynamic processes. This study suggests that crustal recycling through subduction zone processes played an important role for the generation of heterogeneity in the Archean upper mantle. In addition, the results of this study indicate that the Nd-isotope compositions of Archean arc- and plume-derived volcanic rocks are not very distinct, whereas Phanerozoic plumes and intra-oceanic arcs tend to have different Nd-isotopic compositions.     

Lithosphere types in North China: Evidence from geology and geophysics

Deep-seated materials from lithosphere are the ba- sic parameters and the foundation for geodynamic and continental dynamic studies. Division of lithosphere types and their deep-seated materials and structure can provide important evidence in interpreting the com- plex phenomena derived from the processes of forma- tion and evolution of continents, in evaluating the mineral resource potential, in predicting geological disasters and in the research of the continental dy- namic process. Huge lit…     

Formation of Paleoproterozoic eclogitic mantle, Slave Province (Canada): Insights from in-situ Hf and U–Pb isotopic analyses of mantle zircons

In-situ Hf isotope analyses and U–Pb dates were obtained by laser ablation-MC-ICP-MS for a zircon-bearing mantle eclogite xenolith from the diamondiferous Jericho kimberlite located within the Archean Slave Province (Nunavut), Canada. The U–Pb zircon results yield a wide range of ages (2.0 to 0.8 Ga) indicating a complex geological history. Of importance, one zircon yields a U–Pb upper intercept date of 1989 ± 67 Ma, providing a new minimum age constraint for zircon crystallization and eclogite formation. In contrast, Hf isotope systematics for the same zircons display an intriguing uniformity, and corresponding Hf depleted mantle model ages range between 2.1 ± 0.1 and 2.3 ± 0.1 Ga; the youngest Hf model age is within error to the oldest U–Pb date.

The Jericho eclogites have previously been interpreted as representing remnants of metamorphosed oceanic crust, and their formation related to Paleoproterozoic subduction regimes along the western margin of the Archean Slave craton during the Wopmay orogeny. Hf isotope compositions and U–Pb results for the Jericho zircons reported here are in good agreement with a Paleoproterozoic subduction model, suggesting that generation of oceanic crust and eclogite formation occurred between 2.0 and 2.1 Ga. The slightly older Hf depleted mantle model ages (2.1 to 2.3 Ga) may be reconciled with this model by invoking mixing between ‘crustal’-derived Hf from sediments and more radiogenic Hf associated with the oceanic crust during the 2 Ga subduction event. This results in intermediate Hf isotope compositions for the Jericho zircons that yield ‘fictitiously’ older Hf model ages.     

Age and isotope geochemistry of the Archaean Pongola and Usushwana suites in Swaziland, southern Africa: a case for crustal contamination of mantle-derived magma

The igneous rocks of the Pongola Supergroup (PS) and Usushwana Intrusive Suite (UIS) represent a case of late Archaean continental magmatism in the southeastern part of the Kaapvaal craton of South Africa and Swaziland.

U-Pb dating on zircons from felsic volcanic rocks of the PS yields a concordia intercept age of 2940 ± 22Ma that is consistent with a Sm-Nd whole rock age of 2934 ± 114Ma determined on the PS basalt-rhyolite suite. The initial ε_Nd of−2.6 ± 0.9 is the lowest value so far reported for Archaean mantle-derived rocks. Rb-Sr whole rock dating of the PS yields a younger isochron age of 2883 ± 69Ma, which is not significantly different form the accepted U-Pb zircon age.

An internal (cpx-opx-plag-whole rock) isochron for a pyroxenite from the younger UIS yields an age of 2871 ± 30 Ma and initial ¹⁴³Nd/¹⁴⁴Nd that lies off the CHUR growth curve by ε_Nd −2.9 ± 0.2. However, Sm-Nd whole-rock data for the UIS yield an excessively high age of 3.1 Ga that conflicts with firm geological evidence showing the UIS to be intrusive into the PS.

The negative deviations of initialε_Nd from the chondritic Nd evolution curve suggest significant contamination of the PS and UIS melts by older continental crust. A mixing process with continental crust after magma segregation is supported by a high initial ⁸⁷Sr/⁸⁶Sr ratio of0.703024 ± 24 for a clinopyroxene sample from a UIS pyroxenite, compared with an expected value of 0.701 for the 2.9 Ga mantle. We therefore interpret the linear array of data points for the UIS gabbros as a mixing line between 2.87 Ga old magma and older continental crust.

Parallel LREE-enriched REE patterns, negative Nb-Ti anomalies, a distinctive and uniform ratio of Ti/Zr 46 and a narrow span of initial Nd indicate a common source for both the PS and UIS suites which is different from primitive mantle.     

四川盆地东西陆块中下地壳结构存在差异

四川盆地是中上扬子克拉通的主要组成部分.作为我国三大稳定克拉通之一,扬子克拉通经历了自太古代以来的长期演化,直到新元古代晚期与华夏板块发生碰撞拼合前,一直被认为是一个稳定的统一陆块.基底包括了新太古宙-新元古代岩层,其上广泛被新元古代晚期至显生宙地层覆盖,仅有~2.9—2.95Ga基底岩石零星出露于四川盆地的西缘、西南缘和三峡地区,使得对于沉积盖层之下的中下地壳的性质和分布规模的认识十分有限.重力异常则能够宏观揭示区域结构特征.本文通过刨除沉积盖层和莫霍面起伏引起的重力异常而获得了中下地壳的重力异常,反映了四川盆地东西陆块中下地壳存在结构差异,结合深地震反射资料、航磁异常和地球化学资料,证实了该分界线位于重庆—华蓥一线,故而推测中上扬子克拉通在太古宙-古元古代可能存在东西两个陆核.     

Tectonic controls on magma genesis and evolution in the northwestern United States

Field, chronologic, chemical, and isotopic data for late Cenozoic basaltic rocks from the northwestern United States illustrate the relationship between crustal structure and tectonic forces in controlling the genesis and evolution of continental volcanism. In the northwestern U.S., the first major episode of basaltic volcanism was triggered by crustal rifting in a “back-arc” environment, east of the westward-migrating volcanic arc created by the subduction of the Juan-de-Fuca plate beneath the North American plate. Rifting and volcanism were concentrated by pre-existing zones of crustal weakness associated with boundaries between the old Archean core of the continent and newly accreted terranes. Basalts erupted during this time (Columbia River, Steens Mountain) show evidence of significant fractionation histories including contamination by crust of varying age depending on the crustal structure at the eruption site. Presumably this reflects ponding and stagnation of primary magmas in the crust or at the crust-mantle interface due to their encounter with thick crust, not yet extended and still containing its low-density, easily fusible component. Continued rifting of this crust, and modification of its composition through extraction of rhyolitic partial melts and deposition of the fractionation products from primary basaltic melts, coupled with a shift in stress orientation roughly 10.5 Ma ago, allowed relatively unfractionated and uncontaminated magmas to begin reaching the surface. In the western part of the region (Oregon Plateau), these magmas tapped a mantle source similar to that which produced most of the ocean island basalts of the northern hemisphere. To the east (Snake River Plain), however, the mantle sampled by basaltic volcanism has isotopic characteristics suggesting it has preserved a record of incompatible element enrichment processes associated with the formation of the overlying Archean crustal section some 2.6 Ga ago.     

秦岭造山带与邻域华北克拉通和扬子克拉通的壳、幔精细速度结构与深层过程

秦岭造山带与其南北两侧华北克拉通和扬子克拉通属三大构造单元,不论其各构造单元体还是其界带构造均甚为复杂,并受到多期次构造运动的制约,形成了大陆内部特异的造山过程.尽管在这一地域曾做过大量的地表地质工作和一些相关的地球物理工作,但对其壳、幔精细结构、深层动力过程,特别是同步穿越华北克拉通、秦岭-大巴造山带和扬子克拉通系统的耦合研究甚少.为了研究和探索该地域的壳、幔精细速度结构和其形成的深层过程,专门布置了一条北起榆林,向南经咸阳、宁陕直抵涪陵长达1000 km的高精度地震宽角反射、折射波场探测剖面.通过剖面辖区高分辨率的数据采集,数据处理、反演和壳、幔层、块精细速度结构,发现剖面辖区深部壳、幔结构存在特异的速度和结构变化,并厘定了一系列的新认识.研究结果表明:(1)秦岭—大巴造山带具有同一基底,其形成乃为结晶基底隆升所致,即它的形成仅涉及到上地壳的受力变形和空间状态.造山带与其南、北两侧的前陆盆地为陆内造山过程中同一深层过程的产物,但其沉积速率和形态却不相同.华北克拉通与秦岭造山带之间前陆盆地B_fc拉张为该区Moho界面的局部隆升所致.(2)首次提出了沿1000 km长剖面连续的沉积建造、结晶基底、上地壳、下地壳和上地幔顶部的层、块速度结构和各界面的起伏变化与空间状态.基于地震波边界场响应厘定了华北克拉通、秦岭—大巴造山带和扬子克拉通的分区界带.论述了三大构造单元各自的内部结构和其相邻界域的速度变化特征.(3)该区大陆内部速度结构和不同类型断裂分布及层序在华北克拉通、秦岭—大巴造山带、扬子克拉通三大块体地域存在显著差异.不同规模、层次与产状的断裂分布反映出它们在变形行为和机制上及所受构造运动的制约上均存在明显的差异.     

The influence of crustal structure on compositions of subduction-related magmas

A survey of Sr isotopic ratios and other compositional features of subduction-related magma suites reveals significant correlations between these averaged parameters and characteristics of the underlying crust (i.e., thickness, composition, and age). These observations lead to the conclusion that crust and(or) mantle rocks in the hanging walls of subduction zones are involved in modification of primary mafic magmas (typically basalt or basaltic andesite). It is proposed that mafic magmas will stagnate within the crust or uppermost mantle where they may differentiate and react with wall rocks. The extent to which such processes manifest themselves will depend upon details of the local crustal structure. In particular, the composition and age of the crust will strongly influence such parameters as Sr, Nd and Pb isotopic compositions. Such data strongly indicate the involvement of crustal rocks in locales underlain by old sialic crust (e.g., central Andes). Depending upon the level of magma stagnation and evolution within the crust, different trends in isotopic composition may result. These isotopic trends may be enhanced by partial melting of the wall rocks to produce relatively silicic anatectic magmas, and locally they may reflect subduction of continental sediments. Interpretation of the isotopic data may be more ambiguous in locales underlain by younger and more mafic continental crust (Cascades, E Eleutians) and those underlain by oceanic crust owing to the similarity in isotopic composition of primary magmas and the latter crustal materials. Yet some degree of crustal involvement in magmatic evolution seems highly probable even in these more primitive terranes. Consequently, most island arc magmas, and especially those more evolved than basalt, are probably not primary in the sense that they do not represent direct melts of the upper mantle. Studies of arc volcanic rocks may yield misleading conclusions concerning processes of magma generation related to subduction unless evolutionary processes are defined and their effects considered. It appears that modern volcanic arcs provide a poor analog for models of early crustal development because the modern mantle-derived magmatic components are more mafic in composition than average continental crust.     

Nb/Ta variations of mafic volcanics on the Archean- Proterozoic boundary: Implications for the Nb/Ta imbalance

Hofmann[1] found that the depleted mantle (DM) and bulk continental crust (BCC) are simply compen-satory in terms of many elements and elemental ratios. High-field-strength elements (HFSE) (e.g., Zr, Hf, Nb, Ta and Ti) are immobile and thus useful to track the primary features of the ancient mafic volcanics. It has been long considered that Nb-Ta behave as identical twins during magmatic processes, based on their matching atomic radii and valence state and hence coherent geochemical affi…     

Age dependence of the composition of continental crust: evidence from Nd isotopic variations in granitic rocks

The Nd isotopic systematics of the sources of crustal granitic rocks are used to estimate the Sm/Nd ratio of the continental crust as a function of its age. It is found that the Sm/Nd value of granite magma sources in continental crust increases from about 0.47 to 0.64 times the chondritic value with decreasing age from the Early Archean to the Late Proterozoic. This trend is opposite to that inferred for the crust from rare earth element patterns in sedimentary rocks. The observed trend may apply strictly only to the felsic portions of the crust, but unless older crust contains a much higher percentage of mafic material than young crust (50% versus 0%), the direction of the trend also applies to the bulk crust. Because some portion of the earth's oldest crust has probably been destroyed by subsequent processes, the trend could conceivably be the result of preservational bias rather than a real shift in crustal composition with time. The isotopic data, combined with the crustal age distribution, indicate that the Sm/Nd value of the bulk continental crust is not lower than 0.60 times the chondritic value. This limit and estimates of the Nd concentration of the crust are consistent with the mass balance that equates the Nd in the continents to that missing from the upper mantle down to a depth of about 700 km.     

Thinning and destruction of the cratonic lithosphere:A global perspective

It has been proposed that the North China Craton(NCC)was thinned up to a thickness of100 km during the Phanerozoic,and underwent an associated craton destruction.Evidently,it is an important topic worthy of future study to understanding the mechanism of cratonic destruction and its role played in the continental evolution.After synthesized the global cratons of India,Brazil,South Africa,Siberia,East Europe(Baltic)and North America,we found that lithospheric thinning is common in the cratonic evolution,but it is not always associated with craton destruction.Most cratons was thinned by thermal erosion of mantle plume or mantle upwelling,which,however,may not cause craton destruction.Based on the studies of the North American and North China Cratons,we suggest that oceanic subduction plays an important role in caton destruction.Fluids or melts released by dehydration of the subducted slabs metasomatize the mantle wedge above and trigger extensive partial melting.More importantly,the metasomatized mantle lost its original rigidity and make craton easier to be deformed and then to be destoyed.Therefore,we suggest that the widespread crust-derived granite and large-scale ductile deformation within the continental crust can be regarded as the petrological and structural indicators of craton destruction,respectively.     

Bundelkhand mafic dykes, Central Indian Shield: Implications for the role of sediment subduction in Proterozoic crustal evolution

Abstract The Archean to Paleo–Proterozoic Bundelkhand massif basement of the central Indian shield has been dissected by numerous mafic dykes of Proterozoic age. These dykes are low‐Ti tholeiites, ranging in composition from subalkaline basalt through basaltic‐andesite to dacite. They are enriched in light rare earth elements (LREE), large ion lithophile elements (LILE) and depleted in high field strength elements (HFSE: Nb, P and Ti). Negative Sr anomaly is conspicuous. Nb/La ratios of the dykes are much lower compared with the primitive mantle, not much different from the average crustal values, but quite similar to those of continental and subduction related basaltic rocks. Bulk contamination of the mantle derived magma by crustal material is inadequate to explain the observed geochemical characteristics; instead contamination of the mantle/lithospheric source(s) via subduction of sediment is a better proposition. Thus, in addition to generating juvenile crust along the former island arcs, subduction processes appear to have influence on the development of enriched mantle/lithospheric source(s). The Bundelkhand massif basement is inferred to represent subduction related juvenile crust, that experienced lithospheric extension and rifting possibly in response to mantle plume activities. The latter probably supplied the required heat, material (fluids) and extensional environment to trigger melting in the refractory lithospheric source(s) and emplacement of the mafic dykes. Proterozoic mafic magmatic rocks from Bundelkhand, Aravalli, Singhbhum and Bastar regions of the Indian shield and those from the Garhwal region of the Lesser Himalaya display remarkably similar enriched incompatible trace elements characteristics, although limited chemical variations are observed in all these rocks. This may indicate the existence of a large magmatic province, different parts of which might have experienced similar petrogenetic processes and were probably derived from mantle/lithospheric source(s) with similar trace element characteristics. The minor, less enriched to depleted components of the Jharol Group of the Aravalli terrane and those from the Singhbhum terrane may represent protracted phases of rifting, that probably caused thinning and mobilization of the lithosphere, facilitating the eruption/emplacement of the asthenospheric melts (with N‐ to T‐types mid‐oceanic ridge basalts signatures) and deposition of deep water facies sediments in the younger developing oceanic basins. In contrast, Bundelkhand region did not experience such protracted rifting, although dyke swarms were emplaced and shallow water Bijawar Group and Vindhyan Supergroup sediments were deposited in continental rift basins. All these discrete Proterozoic terranes appear to have experienced similar petrogenetic processes, tectonomagmatic and possibly temporal evolution involving subduction processes, influencing the lithospheric source characteristics, followed by probably mantle plume induced ensialic rifting through to the development of oceanic basins in the Indian shield regions and their extension in the Lesser Himalaya.     

俯冲带的后撤与弧后扩张

西太平洋地壳年龄较老,因而岩石层较冷和比重较大,俯冲带的角度也较大,活动和成熟的弧后盆地则较多;条件与之相反的东太平洋弧后盆地则较少.本文探讨这种相关关系的力学成因,计算了俯冲板块诱生的弧后上涌地幔流动.计算表明,俯冲角度大及存在后撤俯冲时,有利于在弧后地区产生明显的上涌地幔流,这种深部热物质的上涌会导致弧后扩张.反之,年龄较轻的海洋地块较热和较轻,俯冲角度一般也较小,不易诱生上涌地幔物质流动和弧后扩张.大陆地壳密度小于地幔物质,大陆碰撞区就更不具备弧后扩张的条件.     

Evolution of continental crust in southern Africa

Nd isotopic data from the Zimbabwe and Kaapvaal cratons and the Limpopo, Kalahari, Namaqualand and Damara mobile belts imply that over 50% of present-day continental crust in this region had separated from the mantle by the end of the Archaean and that< 10% of continental crust of southern Africa has formed in the last 1.0 Ga. Such a growth rate implies that average erosion rates through geological time were high and that evolution of continental crust has been dominated by crustal growth prior to 1.4 Ga, and crustal reworking since that time. The evolution of average crust is not represented directly by clastic sediment samples but may be determined from sediment analyses if both the time of orogeneses and the average erosion rate are known. Both trace element data from southern Africa granitoids and the high erosion rates implied by the isotopic study suggest that growth of continental crust in the Archaean was by underplating rather than lateral accretion, but arc accretion was the dominant mechanism after 2.0 Ga.     

Evidence for ∼80–75 Ma subduction jump during Anatolide–Tauride–Armenian block accretion and ∼48 Ma Arabia–Eurasia collision in Lesser Caucasus–East Anatolia

Orogens formed by a combination of subduction and accretion are featured by a short-lived collisional history. They preserve crustal geometries acquired prior to the collisional event. These geometries comprise obducted oceanic crust sequences that may propagate somewhat far away from the suture zone, preserved accretionary prism and subduction channel at the interplate boundary. The cessation of deformation is ascribed to rapid jump of the subduction zone at the passive margin rim of the opposite side of the accreted block. Geological investigation and ⁴⁰Ar/³⁹Ar dating on the main tectonic boundaries of the Anatolide–Tauride–Armenian (ATA) block in Eastern Turkey, Armenia and Georgia provide temporal constraints of subduction and accretion on both sides of this small continental block, and final collisional history of Eurasian and Arabian plates. On the northern side, ⁴⁰Ar/³⁹Ar ages give insights for the subduction and collage from the Middle to Upper Cretaceous (95–80 Ma). To the south, younger magmatic and metamorphic ages exhibit subduction of Neotethys and accretion of the Bitlis–Pütürge block during the Upper Cretaceous (74–71 Ma). These data are interpreted as a subduction jump from the northern to the southern boundary of the ATA continental block at 80–75 Ma. Similar back-arc type geochemistry of obducted ophiolites in the two subduction–accretion domains point to a similar intra-oceanic evolution prior to accretion, featured by slab steepening and roll-back as for the current Mediterranean domain. Final closure of Neotethys and initiation of collision with Arabian Plate occurred in the Middle-Upper Eocene as featured by the development of a Himalayan-type thrust sheet exhuming amphibolite facies rocks in its hanging-wall at c. 48 Ma.     

Thickness and thermal history of continental crust and root zones

The survival to the present of the Archean nuclei of Precambrian shields requires special explanation if, as seems likely, the rate of heat flow out of the earth was two or three times greater in the late Archean (2.5 b.y. ago) than at present, since such a high heat flux would have melted the base of the Archean crust. It is proposed that there must have existed beneath stable continental crust a root zone (or lithosphere, or tectosphere) at least 200 km thick which has acted as a thermal buffer between the crust and the convecting mantle; this is virtually the same model as has been proposed to explain the present distribution of heat flow between continents and oceans. The strong temperature dependence of silicate rheology insures that the mantle temperature at the base of the root zone was no more than about 150°C higher in the late Archean than at present; the greater Archean heat flux would have been removed mainly through faster sea-floor spreading. To have survived, the root zone must be mechanically and chemically distinct from the rest of the mantle, and its formation was probably intimately related to the differentiation and stabilization of the continental crust.     

Metallogenesis within continental collision zones: Comparisons of modern collisional orogens

Modern collisional orogens represent the natural laboratory for the study of metallogeny in continental collision zones. The Pyrenees, Alps, Zagros and Himalaya are all associated with Neo-Tethyan subduction and represent the youngest collisional orogens on Earth. Here, we compare these four orogens in terms of their composition, architecture, tectonic evolution, and metallogenic systems. The four orogens can be divided into simple and composite types. Simple orogens are represented by the Pyrenees and the Alps, and are characterized by narrow linear shapes in plain view and symmetric structures in cross-section, are free of arc magmatism, and are associated with the Mississippi Valley Valley-type Pb-Zn and orogenic gold deposits. The mineral deposits that form in these simple collisional orogens are generally related to processes that occur in the middle and upper crust. In contrast, composite orogens, as exemplified by the Zagros-Iranian and Himalayan-Tibetan Plateaus, are associated with broad orogenic plateaus in plain view and asymmetrical structures in cross-section, record extensive arc magmatism in continental margins, and are associated with a variety of deposit types including carbonatite-related rare earth element (REE), porphyry Cu-Mo, orogenic Au, Mississippi Valley type Pb-Zn, and detachment-fault-related polymetallic deposits. Although the subduction of Neo-Tethys oceanic crust occurred before the creation of simple collisional orogens in the Pyrenees and the Alps, these areas do not show the record of continental arc magmatism. In contrast, the composite collisional orogens are associated with the development of huge continental margin arcs prior to continental subduction, and the subduction was followed by reactivation of the subduction-modified arc lithospheric material, generating the ore-forming systems in these regions.     

Plate tectonics in relation to mantle temperatures and metamorphic properties

When plate tectonics emerged and how it has evolved over Earth history are two of the most fundamental challenges in Earth Sciences. These questions are tackled using a holistic approach to analyze tectonic styles in the history of Earth, giving rise to the interpretation of two styles of plate tectonics since the Archean. In these interpretations, there are different styles of deformation and metamorphism between early times dominated by warm subduction, and later times preferring cold subduction.The two styles of plate tectonics are recorded by different properties of regional metamorphism at convergent plate boundaries,which are linked to the differences in mantle temperature between the Archean and Phanerozoic. A transition to modern plate tectonics is recorded by the signature of blueschist facies metamorphism developed in the Neoproterozoic. This is consistent with geological evidence for the operation of ancient plate tectonics since the early Archean. The temporal cooling of the mantle explains the geochemical trends of mantle-derived melts, the likely change from numerous small plates to fewer but larger plates,changes in thickness and preservation of oceanic crust and lithosphere in accretionary and collisional orogens, and led to the oxygenation of the surface environment providing the environments needed to foster life.