Continent-continent Collision Zone Research Articles

Petrology and geochronology of a suite of meta-supracrustal rocks from Madukkarai, Tamil Nadu: Implications for the Ediacaran-Cambrian orogenesis of the Granulite Terrane of South India

The Granulite Terrane of South India (GTSI) has two major components. An orthogneiss dominated northern block of Archaean/early Palaeoproterozoic age and an Ediacaran-Cambrian, high to ultra-high temperature meta-supracrustal dominated southern block (the Madurai Block, MB). It is commonly perceived that the two blocks of GTSI came into juxtaposition along the Palghat-Cauvery Shear/Suture Zone (PCSZ) during a continent-continent collision. However, the timing and the nature of the contact relations between the two major components remains controversial. In this work, detail petrological and geochronological study of two metapelites along with a meta-psammite (only for geochronological study), from the Madukkarai Supracrustal Unit (MSU) within the PCSZ, are presented. The studied metapelites show two distinct assemblages:A.Garnet + biotite + sillimanite (both idioblasts and pseudomorphs after kyanite) + K-feldspar + plagioclase + quartz + ilmenite (rutile, zircon and monazite as accessory phases).B.Mineralogy is similar to A but contains cordierite and minor spinel, but no zircon.Both the metapelites develop a gneissic fabric that is defined by centimetre thick laterally discontinuous garnet-bearing leucosomes. ‘Frozen in’ reaction textures, phase equilibria modelling and mineralogical thermobarometry yielded “peak” temperature and pressure of metamorphism in the range of ~730–790 °C, ~5.5–7 kbar (considering sillimanite and ilmenite to be stable). Subsequently, the rocks followed a decompressive retrograde P-T path recorded till ~630 °C and ~4 kbar. The geometry of the construed retrograde P-T path is similar to that commonly found in rocks of a continent-continent collision zone. LA-ICP-MS UPb zircon dating and U-Th-total Pb monazite dating constrain the major high-grade metamorphism and deformation during ~550–520 Ma with vestiges of ~600 Ma old tectonothermal/ thermal event. The geological history of the MSU shares many characteristics with the Ediacaran-Cambrian meta-supracrustal rocks lying on both sides of the PCSZ, in terms of lithological association, timing of protolith formation, deformation and metamorphism. The variation in intensity of metamorphism in the meta-supracrustal rocks of the GTSI could be linked to differential upliftment of the GTSI during the neotectonic activities, followed by erosion. Integrating all the geological, geochronological and geophysical information, it has been argued that the GTSI behaved as a coherent continental block that was involved in continent-continent collision during the formation of the Ediacaran-Cambrian supercontinent Gondwanaland. The findings of this study do not corroborate the view that PCSZ is an Ediacaran-Cambrian terrane boundary.

Constraining the Moho Depth Below Bhutan With Global-Phase Seismic Interferometry

We use a novel technique named global-phase seismic interferometry (GloPSI) to image the lithospheric structure, and in particular the Moho, below two parallel north-south transects belonging to the GANSSER network (2013–2014). The profiles cross the Himalayan orogenic wedge in Bhutan, a tectonically important area within the largest continent-continent collision zone on Earth that is still undergoing crustal thickening and represents a challenging imaging target for the GloPSI approach. GloPSI makes use of direct waves from distant earthquakes and receiver-side reverberations with near vertical incidence. Reflections are isolated from earthquake recordings by solving a correlation integral and are turned into a reflectivity image of the lithosphere below the arrays. Our results compare favorably with first-order features observed from a previous receiver function (RF) study. We show that a combined interpretation of GloPSI and RF results allows for a more in-depth understanding of the lithospheric structure across the orogenic wedge in Bhutan.

Boron isotope insights into the origin of subduction signatures in continent-continent collision zone volcanism

We present the first boron abundance and δ11B data for young (1.5-0 Ma) volcanic rocks formed in an active continent-continent collision zone. The δ11B of post-collisional volcanic rocks (−5 to +2‰) from the Armenian sector of the Arabia-Eurasia collision zone are heavier than mid-ocean ridge basalts (MORB), confirming trace element and isotope evidence for their derivation from a subduction-modified mantle source. Based on the low B/Nb (0.03-0.25 vs 0.2-90 in arc magmas), as well as low Ba/Th and Pb/Ce, this source records a subduction signature which is presently fluid-mobile element depleted relative to most arc settings. The heavier than MORB δ11B of post-collision volcanic rocks argues against derivation of their subduction signature from a stalled slab, which would be expected to produce a component with a lighter than MORB δ11B, due to previous fluid depletion. Instead, the similarity of δ11B in Plio-Pleistocene post-collision to 41 Ma alkaline igneous rocks also from Armenia (and also presented in this study), suggests that the subduction signature is inherited from Mesozoic-Paleogene subduction of Neotethys oceanic slabs. The slab component is then stored in the mantle lithosphere in amphibole, which is consistent with the low [B] in both Armenian volcanic rocks and metasomatic amphibole in mantle xenoliths. Based on trace element and radiogenic isotope systematics, this slab component is thought to be dominated by sediment melts (or supercritical fluids). Previously published δ11B of metasediments suggests a sediment-derived metasomatic agent could produce the B isotope composition observed in Armenian volcanic rocks. The lack of evidence for aqueous fluids preserved over the 40 Myr since initial collision supports observations that this latter component is transitory, while the lifetime of sediment melts/supercritical fluids can be extended to >40 Myr.

The role of lateral strength contrasts in orogenesis: A 2D numerical study

In this paper we present a series of thermo-mechanical experiments on continent-continent collision zones to investigate the role of lateral strength contrasts in terms of collision dynamics and orogen geometries. Our results show that differences in crustal rheology, may lead to a variety of different patterns of deformation and crustal geometries. Upper plate indentation forms a sequence of foreland propagating thrust units made of brittle upper crust on the lower plate. The strong deformation of the lower plate stands in stark contrast to the undeformed upper plate. In contrast, subduction of strong lithosphere beneath a weak upper plate forms a complex pattern of deformation. Deformation initiates on the lower plate and forms an antiformal stack made of brittle upper crust, before it indents the upper plate. Hence, deformation is present on both, the lower and upper plate, despite differences in initial strength. The outcome of this study has direct implications to natural collision zones, where differences in strength between the upper and lower plate exist. For example, along strike variability of upper vs. lower plate shortening in the Alps are likely to be controlled by contrasts in crustal strength. Furthermore, we suggest that the antiformal stack in the axial zone of the Pyrenees might indicate a strong lower plate at the onset of collision. Our results may therefore provide important constraints for the rheological state of continents during collision.

Characteristics and 40Ar/39Ar Geochronology of the Erdenet Cu-Mo Deposit, Mongolia

The Early to Middle Triassic Erdenet porphyry Cu-Mo deposit, in northern Mongolia, developed in a continent-continent arc collision zone, within the Central Asian orogenic belt. The porphyry system is related to multiple intrusions of crystal-crowded biotite granodiorite porphyry, which formed a composite stock about 900 m in diameter, with multiple porphyritic microgranodiorite dikes. Wall rocks are Late Permian to Early Triassic, medium-grained granodiorite, with similar whole-rock geochemistry, mineralogy, and composition to the granodiorite porphyry. Whole-rock analysis of the granodiorite porphyry and wall rocks shows that these rocks cannot be discriminated, but both have depleted middle heavy rare earth elements and Y, typical of fertile porphyry magmatic suites. At the current pit level (1,250 m elev), early porphyry-style quartz veins (A and B type) are locally infilled by pyrite-chalcopyrite, with subordinate bornite, but most of the chalcopyrite occurs in D veins that constitute more than 50% of the Cu grade (~0.5 wt % Cu). The 0.3 wt % Cu shell resembles a molar tooth, enveloping the granodiorite porphyry, with deeper roots extending down the wal-rock contacts. Molybdenite occurs in monomineralic veins, and in finely laminated to massive quartz-molybdenite veins. The most important alteration is quartz-muscovite, which occurs as relatively coarse (100–500 μ m) alteration selvages (1–5 cm) that envelop D veins. The D veins cut illite ± kaolinite-smectite (or intermediate argillic) alteration. Intermediate argillic alteration, together with abundant pink anhydrite (commonly hydrated to gypsum), extends from at least 1,300- to 900-m elevation in the deepest drill holes, and has overprinted early potassic alteration, or relatively unaltered red granodiorite. Meter-wide zones of kaolinite cut the anhydrite-gypsum at all levels. There is an abrupt transition outward from the intermediate argillic alteration to chlorite-epidote (propylitic) alteration, at 50 to 200 m from the granodiorite porphyry contact, although D veins (and chalcopyrite) extend outward to the propylitic zone. The Erdenet porphyry system, was overprinted by advanced argillic alteration, which outcrops 2 km northwest of the pit, and forms a lithocap that extends over 10 × 2.5 km. It is characterized by residual quartz, andalusite, Na-Ca and K-alunite, diaspore, pyrophyllite, zunyite, topaz, dickite, and kaolinite. The upper part of the porphyry Cu-Mo deposit (removed by mining), comprised a bornite-chalcocite enriched zone up to 300 m thick with an average grade of 0.7 wt % Cu and up to 5 wt % Cu locally. Based on hypogene bornite-chalcocite mineral textures and high-sulfidation state mineralogy, the enriched zone is inferred to be of hypogene origin, but modified by supergene processes. Consequently, it may be related to formation of the lithocap. Previous Re-Os dates of 240.4 and 240.7 ± 0.8 Ma for molybdenite in quartz veins are comparable to new 40 Ar/ 39 Ar dates of 239.7 ± 1.6 and 240 ± 2 Ma for muscovite that envelops D veins. One 40 Ar/ 39 Ar date on K-alunite from the lithocap of 223.5 ± 1.9 Ma suggests that it may be about 16 m.y. younger than Erdenet, but this result needs to be verified by further dating.

동중국과 한반도 경상분지의 백악기초기 화성활동의 성인 고찰

동중국과 우리나라 경상분지에 분포하는 백악기초기 화성활동의 지구화학적 특징을 고찰하였다. 동중국에 분포하는 암상은 피크라이트-현무암-안산암-조면암-유문암 및 황반암의 분출암과 반려암-섬록암-몬조니암-섬장암-화강암 및 휘록암의 관입암체로 다양하다. 이들은 고-칼륨 칼크-알칼리 내지는 쇼쇼나이트 계열에 속한다. 경상분지의 하양층군 속에 협재되어 있는 화산암은 같은 계열의 현무암질 조면안산암이다. 미량원소(Zr, Nb, Y)를 이용한 현무암류의 생성 지구조환경 분류도에서, 이들은 대개 판내부환경 현무암류의 범주에 도시된다. Sr-Nd 동위원소상관도에서 북중국지괴와 북 남중국지괴의 충돌대에 분포하는 현무암류는 대개 맨틀배열 보다 매우 부화된 동위원소비를 가진다. 남중국지괴 현무암류의 <TEX>$^{87}Sr/^{86}Sr$</TEX> 비는 북중국지괴의 범위와 유사하나 <TEX>${\varepsilon}_{Nd}$</TEX> 값은 북중국지괴에 비하여 상대적으로 높은 편이다. 북중국지괴와 충돌대의 현무암류들은 대개 낮은 <TEX>$^{206}Pb/^{204}Pb(t)$</TEX> 비를 가지는 것이 특징이며, <TEX>$^{207}Pb/^{204}Pb(t)$</TEX>-<TEX>$^{206}Pb/^{204}Pb(t)$</TEX> 상관도에서 지오크론의 왼쪽에 도시된다. 남중국지괴 현무암류는 지오크론의 오른쪽에 도시되며, 상대적으로 높은 방사기원 Pb 동위원소비를 가진다. 하양층군 현무암류는 Sr-Nd과 Pb-Pb 동위원소 상관도에서 북중국지괴 현무암류의 범주 내에 도시된다. 오랜 기간 동안 변성교대작용에 의해 지구화학적으로 부화된 암석권맨틀이 위 현무암류를 생성한 근원물질로 추정된다. 백악기초기의 확장응력장에서 발생한 연약권의 용승이 열원이 되어 암석권맨틀의 부분용융이 가능하였을 것이다. 암석권맨틀을 부화시킨 매체로는 엽렬되어 침몰된 하부지각이 재활성화되어 생성된 액, 섭입된 양쯔지괴 대륙지각 기원 액 내지는 섭입된 고태평양판 기원의 유체/액 등을 들 수 있다. Geochemical characteristics of the Early Cretaceous igneous rocks from eastern China and the Gyeongsang Basin, Korean Peninsula has been summarized. They have wide range of lithological variation with extrusive picrite-basalt-andesite-trachyte-rhyolite and lamprophyre, and intrusive gabbro-diorite-monzonite-syenite-granite and diabase in eastern China, mostly belonging to the high-K calc-alkaline or shoshonitic series. The volcanic rocks intercalated with the Hayang Group sedimentary assemblages in the Gyeongsang basin are high-K to shoshonitic basaltic trachyandesites. The Early Cretaceous basaltic rocks studied mostly fall within the field of within-plate basalts on the Zr/Y-Zr and Nb-Zr-Y tectonic discrimination diagrams. On a Sr-Nd isotope correlation diagram, basaltic rocks from the North China block (NCB) and the continent-continent collision zone (CZ) between the North and South China blocks plot into the enriched lower right quadrant along the extension of the mantle array. The initial <TEX>$^{87}Sr/^{86}Sr$</TEX> ratios of basaltic rocks from the South China block (SCB) are indistinguishable from those of the NCB and CZ basaltic rocks, but their <TEX>${\varepsilon}_{Nd}$</TEX> (t) values are relatively more elevated, plotting in right side of the mantle array. Basaltic rocks from the NCB and CZ are characterized by low <TEX>$^{206}Pb/^{204}Pb(t)$</TEX> ratios, lying to the left of the Geochron on the <TEX>$^{207}Pb/^{204}Pb(t)$</TEX> vs. <TEX>$^{206}Pb/^{204}Pb(t)$</TEX> correlation. Meanwhile, the SCB basaltic rocks have relatively radiogenic Pb isotopic compositions compared with those of the NCB and CZ basaltic rocks. Basaltic rocks from the Hayang Group plot within the field of the NCB basaltic rocks in Sr-Nd and Pb-Pb isotope spaces. Metasomatically enriched subcontinental lithospheric mantle (SCLM) is likely to have been the dominant source for the early Cretaceous magmatism. Asthenospheric upwelling under an early Cretaceous extensional tectonic setting in eastern China and the Korean Peninsula might be a heat source for melting of the enriched SCLM. Metasomatic agents proposed include partial melts of lower continental crust delaminated and foundered into the mantle or subducted Yangtze continental crust, or fluid/melt derived from the subducted paleo-Pacific plate.

Crustal stretching in the Scandinavian Caledonides as revealed by deep seismic data

The normalization of overthickened orogenic crust after a continent-continent collision typically involves several different processes, including erosion, plate divergence, gravity-driven collapse of the orogenic wedge, and viscous flow of the lower crust. As a contribution to this discussion, we here utilize reprocessed deep seismic data to image the Scandinavian part of the major Caledonian continent-continent collision zone where we identify a double set of extensional shear zones: one in the upper to middle crust (Hardangerfjord shear zone) and a major oppositely dipping Moho-offsetting shear zone in the lower crust and upper mantle. The latter shear zone may have a true displacement close to 50 km and appears to offset the Moho vertically by ~10 km, extending downward beyond the 50 km depth limit of the seismic data. While the upper structure offsets the basal Caledonian thrust, the Moho shear zone is more difficult to date. However, they are kinematically consistent and located at the transition zone between thick- and thin-skinned post-collisional tectonics, and we suggest that they may represent a crustal “pinch” or zipper-like structure that separated viscously flowing Caledonian lower crust from cooler rigid basement representative for the rest of the Baltic Shield. Later extension created the North Sea rift, but the general rift axis and the associated riftrelated thinning developed in a narrower zone that runs oblique to the Devonian trend, hence the two generations of extension and thinning can be distinguished at the crustal scale.

A new UHP metamorphic complex in the 1.8 Ga Nagssugtoqidian Orogen of West Greenland

The Nagssugtoqidian Orogen is a ca. 1.8 Ga belt of east-west trending, highly deformed rocks that bisects central Greenland. Although a variety of data have suggested this belt marks the location of a continent-continent collision zone, evidence of subduction has been lacking. We report here mineralogical evidence from four samples within a well-defined lithologic unit of metabasic and metasedimentary ocean floor rocks of a previously unrecognized UHP metamorphic episode. The UHP episode is recorded by remnants of orthopyroxene exsolved from majoritic garnet, graphitized diamond, exsolution of rutile from garnet and pyroxenes, exsolution of magnetite from olivine, and complex exsolution textures in ortho- and clinopyroxenes (including omphacite). Associated with these mineralogical features is an unusual occurrence of quartz needles in Mn-rich fayalite. From textural characteristics, we infer that the quartz needles exsolved from the fayalite. To our knowledge, olivine with exsolved silica has not been reported. We note, however, that experimental studies have shown that b-spinel can incorporate excess silica. We therefore speculate these quartz needles may be silica that exsolved from Mn-rich ahrensite, the Fe analog of ringwoodite, upon decompression and inversion to fayalite. If correct, this occurrence would be the first reported sample of naturally occurring olivine (fayalite) that inverted from ahrensite. Corroborating an early UHP history are reaction relationships that delineate a path through high-pressure and high-temperature conditions during decompression. P-T conditions inferred for the UHP episode are ~7 GPa at ~975 °C. The unusually low T for this UHP system at ~1.8 Ga may reflect either very rapid subduction rates at that time, or unexpectedly cool mantle conditions. Preservation of the UHP assemblages probably is due, in large part, to the exceptionally low a H2O during decompression and cooling. These UHP rocks establish that the location of the subduction and suture zones that must have existed prior to and during the collision of continents was along what is now the northern edge of the Nordre Strømfjord shear zone.

Upper mantleSwave velocity structure of the East Anatolian-Caucasus region

Geodynamic processes occurring in the upper mantle such as slab break off and lithosphere delamination often result in high rates of lithospheric deformation and rapid tectonic uplift of large areas. The continent-continent collision zone between Arabia and Eurasia has been widely studied in this context, but several different viable geodynamic models exist to explain the uplift and deformation of the Anatolian Plateau and the Caucasus Mountains. We have imaged the uppermost mantle shear wave velocity structure of the East Anatolian-Caucasus region using surface wave tomography to better understand the regional tectonic activity since the onset of the collision between the Arabian and Eurasian Plates. Furthermore, we used our tomographic models to better understand the processes, which are responsible for the formation of the 2 km high plateau and the widespread volcanism in eastern Turkey, as well as reactivation of deformation and deep seismicity in the eastern Greater Caucasus. Our model of regional upper mantle shear wave velocity structure supports subduction of the northern and southern branches of Neo-Tethys lithosphere between Eurasia and Gondwana and suggests a possible underthrusting of the Kura Basin lithosphere beneath the Greater Caucasus.

Integrated geophysical evidence for a new style of continent-continent collision beneath the western Kunlun in the northwestern China

Along the western Kunlun-Tarim-Tianshan geoscience transect in the northwestern China, an integrated geophysical investigation was carried out. Owing to the abominable natural conditions there, the sounding profile could not cross the whole transect, consequentially, a variety of velocity structures in the transverse and vertical orientations beneath the whole transect were not obtained, such as the case within the western Kunlun orogenic belt. To supply a gap of deep seismic soundings within the western Kunlun orogenic belt, we used the Bouguer gravity anomaly data and the relationship between the compressive wave and the density to obtain the density structure of the crust beneath the western Kunlun and the southern Tarim basin by a forward fitting of gravity anomalies within the two-dimensional polygonal model of uniform medium. The crust of the Tarim basin with a rigid basement was like an asymmetrical arc, whose surface feature was the Bachu uplift in the middle of the Tarim basin. Beneath the conjoint area between the Tarim basin and the western Kunlun belt, there was a V-shape structure located just up to the top of the uplifted Moho. The multi-seismological structures jointly revealed that the face-to-face continent-continent collision beneath the western Kunlun is a new structural style within the continent-continent collision zone, which is a real model proved by the numerical modeling.

Paleoproterozoic evolution of Fennoscandia and Greenland

The Paleoproterozoic evolution of Fennoscandia and Greenland can be divided into major rifting and orogenic stages. The Paleoproterozoic rifting of Fennoscandia started with 2.505‐2.1 Ga, multiphase, southwest-prograding, intraplate rifting. Both Fennoscandia and Greenland experienced 2.1‐ 2.04 Ga drifting and separation of their Archean cratons by newly-formed oceans. The main Paleoproterozoic orogenic evolution of Fennoscandia resulted in the Lapland-Kola orogen (1.94‐1.86 Ga) and the composite Svecofennian orogen (1.92‐1.79 Ga). The Paleoproterozoic orogens in Greenland, from north to south, are the Inglefield mobile belt (1.95‐1.92 Ga), the Rinkian fold belt/Nagssugtoqidian orogen (1.88‐1.83 Ga) and the Ketilidian orogen (c. 1.8 Ga). The Lapland-Kola orogen, Inglefield mobile belt and the Rinkian fold belt/Nagssugtoqidian orogen are continent-continent collision zones with limited formation of new Paleoproterozoic crust, whereas the Ketilidian orogen displays a convergent plate-tectonic system, without subsequent collision. The composite Svecofennian orogen is responsible for the main Paleoproterozoic crustal growth of Fennoscandia.

Lithospheric structure of the continent-continent collision zone: eastern Turkey

We infer the lithospheric structure in eastern Turkey using teleseismic and regional events recorded by 29 broad-band stations from the Eastern Turkey Seismic Experiment (ETSE). We combine the surface wave group velocities (Rayleigh and Love) with telesesimic receiver functions to jointly invert for the S-wave velocity structure, Moho depth and mantle-lid (lithospheric mantle) thickness. We also estimated the transverse anisotropy due to Love and Rayleigh velocity discrepancies. We found anomalously low shear wave velocities underneath the Anatolian Plateau. Average crustal thickness is 36 km in the Arabian Plate, 44 km in Anatolian Block and 48 km in the Anatolian Plateau. We observe very low shear wave velocities at the crustal portion (30–38 km) of the northeastern part of the Anatolian Plateau. The lithospheric mantle thickness is either not thick enough to resolve it or it is completely removed underneath the Anatolian Plateau. The shear velocities and anisotropy down to 100 km depth suggest that the average lithosphere—asthenosphere boundary in the Arabian Plate is about 90 and 70 km in Anatolian block. Adding the surface waves to the receiver functions is necessary to constrain the trade-off between velocity and the thickness. We find slower velocities than with the receiver function data alone. The study reveals three different lithospheric structures in eastern Turkey: the Anatolian plateau (east of Karliova Triple Junction), the Anatolian block and the northernmost portion of the Arabian plate. The boundary of lithospheric structure differences coincides with the major tectonic boundaries.

Mesozoic monazite in Neoproterozoic metasediments: Evidence for low-grade metamorphism of Sinian sediments during Triassic continental collision, Liaodong Peninsula, NE China

Sericite phyllite from the Sinian Shisanlitai Formation, Dalian area, Liaodong Peninsula, NE China, contains an assemblage of newly-formed lower-greenschist facies minerals (sericite, chlorite, Fe minerals and Ti minerals) plus aggregates of fine-grained monazite. The texture of the monazite, its mineral inclusions, and its close association with Fe oxide minerals show that it is not detrital or diagenetic, but a product of the low-grade metamorphism. SHRIMP U-Th-Pb dating of the monazite at 217 ± 15 Ma shows that the metamorphism, and associated regional deformation and fluid flow, occurred in the Late Triassic, coeval with the waning stages of the Dabie-Sulu orogeny. The Dabie-Sulu tectonothermal event has produced both deformation and metamorphism in rocks of the eastern North China Block at least up to ∼200 km north of the main continent-continent collision zone.

Metamorphic petrology and geochemistry of the Sarkadkeresztúr Basement-High with special regard to orthogneiss (Tisza Mega-unit, SE Hungary)

Core samples of metamorphic rocks from the Sarkadkeresztur Basement High (Bekes-Codru Unit, Tisza Mega-unit) were examined, consisting mainly of two-feldspar augengneiss, garnet-andalusite-staurolite-bearing micaschist and paragneiss. The protolith of the plagioclase-microcline-augengneiss was found to be S-type granite (syenogranite) of unknown age. According to the major element analyses the protoliths of the studied metagranitoids of this basement can be classified as a syenogranite of metaluminous and peraluminous character displaying subalkaline affinity and a calcic granitic composition. The REE patterns are characterized by a slight enrichment of the light REEs and negative Eu and Yb anomalies. On the basis of discrimination diagrams for major and trace elements, the studied rocks were formed in a syn-collisional (continent-continent collision zone) tectonic environment. The low-pressure amphibolite facies metamorphism, which affected this basement, can be assigned to the Variscan.

Study of crust-mantle transitional zone along the northeast margin of Qinghai-Xizang plateau

This study deals with complexity, frequency spectrum and velocity model of the crust-mantle transitional zone in different tectonic units along the northeast margin of Qinghai-Xizang plateau, based on PmP waveform data from two deep seismic sounding profiles passing through the area. It reveals that Moho has stable tectonic features in Ordos and Lingzhong basins, where crust and mantle are coupled as first-order discontinuity. Moho shows obvious signs of activity in Haiyuan seismic region and in the contact zone between Bayanhar block and Qaidam block. Crust and mantle in these two areas are coupled as complicated crust-mantle transitional zone consisting of multiple laminae with alternate high and low velocities, totaling 20 km in thickness. The difference between Moho of different tectonic units reflects heterogeneity of the coupled crust-mantle zone; the difference between fine structures of Haiyuan seismic region and Maqin fault zone reflects different deep material composition of the two continent-continent collision zones and the interaction between blocks.

Application of Wavelet Multi‐Resolution in the Study of Crust‐Mantle Heterogeneous Scale

AbstractUsing “zoom” multi‐resolution of wavelets, we developed a varying scale phase analysis method for complicated waveforms of crust‐mantle transitional zone (CMTZ), and integrated the method with the study of heterogeneous scale of CMTZ, thus transforming the conventional phase identification from “static” into “dynamic”. Relevant computer programming has been completed. With reference to Pm waveform of CMTZ along Maqin‐Lanzhou‐Jingbian profile, two continent‐continent collision zones were revealed with the complexity coefficient method. The two zones correspond to the Maqin fault and the M=8.5 Haiyuan earthquake region, respectively. The multi‐layer Pm phase under the Maqin fault and Haiyuan earthquake was obtained with the varying scale phase analysis method. The result shows that CMTZ under Haiyuan earthquake is about 20km thick, with distinguishable layers. It consists at least of 3 layers of high and low velocity alternatively, whereas CMTZ corresponding to the Maqin fault is composed of 2 thin layers. This difference in stratification and thickness shows that the contact zone between Ordos block and Qilian block differs obviously from the contact zone between Bayankar block and Chaidamu block in the present distinguishable heterogeneous scale of CMTZ, because the continent–continent collisions happened at different times and the continents interacted in different ways.

Scales of structure in the lithosphere — images of processes

Advances in Deep-Seismic Sounding observations of the lithosphere on continents allow to identify various scales of heterogeneities in the crust-mantle transition. In this paper a fresh look is taken at the dynamic properties of seismic waves propagating through this zone ranging from the lower crust well into the lithospheric mantle as observed in near-vertical reflection and wide-angle refraction experiments. The abrupt termination of near-vertical reflections at the Moho and the coincident presence of a strong coda, accompanying the supercritical PMP reflection phase in record sections of continental seismic wide-angle refraction experiments, reveal a new property of the crust-mantle boundary. The Moho forms a sandwiched mix of crust-mantle material between lower crust and topmost mantle in combination with a step in mean velocity, a significant change in scale of the structural dimensions and of velocity variance. This view is strongly supported by the recognition of the subcrustal lithosphere as a high-frequency (5–10 Hz) scattering wave-guide for long-distance propagation of the Pn phase over more than 3000 km, detected during seismic experiments in Russia using Peaceful Nuclear Explosions. Evidence is brought forward that this high-frequency wave-guide is likely to be present also in the Phanerozoic upper mantle of western Europe.It is furthermore hypothesized that the global propagation of teleseismic Sn or corresponding Pn waves observed on continents and oceans combined with the recording of strong PMP-codas for wave paths on continents points to a world-wide change of scale of structure and velocities near the Moho. A change in scale of heterogeneities under almost all physical and chemical conditions indicates possibly a differential horizontal movement or decoupling of crust and upper mantle at the level of the Moho. The global presence of the scattering Pn wave-guide with a preferred horizontal elongation is hypothetically described as a seismic image of processes in the lithospheric mantle. Locations of present-day tectonic activity where horizontal decoupling between crust and mantle are likely to occur (e.g., subduction at continent-continent collision zones, change of tectonic regimes and continental transform faults) are discussed.

Paleogeothermal gradient and deformation style, Pacific front of the Southern Alps Orogen: Constraints from fission track thermochronology

The application of apatite fission track (FT) thermochronology to a suite of samples from different elevations within the exhumed apatite partial annealing zone, in Rakaia valley region of the Southern Alps (New Zealand), establishes a pre-cooling/denudation (latest Miocene-early Pliocene), upper crustal, geothermal gradient value of 20 ± 3°C/km within indurated Triassic sandstone. This is comparable to the equivalent modern value of 23 ± 6°C/km in basement outside the oblique continent-continent collision zone; that is, where isotherms have not been perturbed by late Cenozoic advection resulting from crustal thickening, rock uplift and denudation. The normalisation of the maximum paleotemperature estimates for sample sites at various elevations to those expected at a standard elevation (1000 m), shows that the exhumed late Cenozoic partial annealing zone is disrupted by reverse faults striking parallel (throws of 1000–2400 m) and orthogonal (throw of 500–1000 m) to the orogen, and by an open symmetrical dome (dimensions: 20 km long and 13 km wide) that conforms to the shape of the Mt Hutt Range. An effect of this deformation is repetition of the exhumed partial annealing zone across the eastern margin of the Southern Alps. Interpretation of existing geological maps suggests that the basement folding quantified for the Mt Hutt Range from FT data, is widespread in Mid and South Canterbury. A triangular zone along the eastern range front, which increases in width along the orogen to the southwest, comprises numerous antiformal reverse faulted basement ranges flanked by outliers of mid-Cenozoic marine cover rocks that were formerly continuous across the region. This basement folding and reverse faulting represents propagation of the orogen back across the incoming Pacific plate, as predicted by recent numerical and geodynamical models.

A lithospheric cross-section through the Swiss Alps-II. Constraints on the mechanical structure of a continent-continent collision zone

SUMMARY The calculation of strength profiles along the European Geotraverse (EGT) through the Swiss Alps yields constraints on the large-scale vertical and lateral mechanical structure through the Alpine continent-continent collision zone. Strength profiles are evaluated for different assumptions on petrological stratification and strain rate and are based on temperature-depth profiles derived from transient thermo-kinematic modelling of the Neoalpine orogeny. The main contribution to the total strength results from the mantle lithosphere, which is strongly controlled by temperature. In contrast, the crustal contribution is mainly determined by variations in petrological stratification. A direct correlation between surface heat flow and the total strength of the crust, the mantle lithosphere and the entire lithosphere (crust and mantle lithosphere) is not observed. Our results demonstrate that in tectonically active areas a transient thermal model, along with detailed knowledge of the deep structure and petrology, is necessary to evaluate lithospheric strength envelopes. Inside the collision zone, strain rate has a strong control on the bottom of the mechanically strong crust, whereas outside the collision zone the effect is less significant. The cut-off depth of seismicity along the profile, which correlates largely with the bottom of the mechanically strong crust, deviates from the 300-400°C isotherm. The inferred effective elastic thickness for the Molasse Basin north of the Alps is in agreement with flexural modelling results, whereas for the Southern Alps the predictions deviate.

Petrotectonics of Sulu-Dabie metamorphic belt,Central and East China

The Sulu-Dabieshan metamorphic belt separates the North China and Yangtze cratons. This metamorphic belt is further divided into three petrological-tectonic units which are respectively equivalent to a root zone of an Andean-type magmatic arc (northern Dabie terrane), continental collision suture zone (southern Dabie terrane) and metamorphosed cover sediments of the Yangtze continental margin (Susong metamorphic complex). They are regularly located in the space from north to south in Dabieshan and roughly from northwest to southeast in Sulu region in spite of being strongly reconstructed by the Tan-Lu Fault. The occurrence of the Mesozoic granites and alkali dike swarm are also regular in the space. They occur in the Dabie area, especially in the northern Dabie terrane. These facts support a suggestion that there was subduction of the Yangtze Plate under the North China Plate or obduction of the North China Plate over the Yangtze Plate. Up to now, no evident remnant of old occean crust has been identified. Ultrahigh pressure metamorphic (UHPM) rock-bearing southern terrane is possibly a root of continent-continent collision zone representing a suture between the North China and Yangtze Plates. The fact that UHPM lenses were tectonically emplaced into not-high pressure metamorphic host gneisses indicates deeply subducted rocks uplifted to middle-upper crust after continental collision. The UHPM rocks and related rocks occurring on the surface at the moment suffered at least two stages of uplifting and exhumation.