Increase In Crustal Thickness Research Articles

Cenozoic temporal variation of crustal thickness in the Urumieh-Dokhtar and Alborz magmatic belts, Iran

We present regional variations of whole-rock Sr/Y and (La/Yb)N ratios of magmatic rocks along the Cenozoic Urumieh-Dokhtar and Alborz magmatic belts, Iran. Both the magmatic belts are located at the north of the main Zagros Neo-Tethyan suture. The Urumieh-Dokhtar magmatic belt (UDMB), which trends NW-SE for 1000 km across Iran, was characterized by the intensive volcanism and plutonism, and defined the magmatic front (MF) of the Zagros orogenic belt. The Alborz magmatic belt (AMB) is situated to the north, and characterized by less intense magmatic activity. The Alborz magmatic belt was formed behind it in the rear-arc (RA) domain. A striking feature of the both magmatic belts is the transition from normal calc-alkaline arc magmatism during the Eocene–Oligocene to adakite-like calc-alkaline magmatism during the Middle to Late Miocene–Pliocene. The late-Cenozoic magmatism of the UDMB and AMB shows higher Sr/Y and (La/Yb)N. However, it should be noted that crustal thickening event is intensive in the UDMB than AMB during Late Cenozoic. Using the composition of the Lale-Zar zircons from the SE UDMB we determined the oxygen fugacity (fO2) during zircon crystallization to be between FMQ (fayalite–magnetite–quartz buffer) -0.69 to +2.41, whereas those of the Hashroud-Teckmdash-Gormolla zircons from NW AMB range from −1.22 to +5.99. The fO2 estimates suggest relatively more oxidized conditions for the Late Cenozoic igneous rocks of the AMB. Compiled data from the UDMB and AMB intrusions show an increase in average zircon crystallization temperatures with decreasing age. These outcomes have been interpreted in terms of variation of the crustal thickness, from 30 to 35 km during Eocene-Oligocene to 40–55 km during the middle-late Miocene. We propose the increase in crustal thickness is associated with the collision between the Arabian plate and Iran and subsequent convergence during the middle-late Miocene.

Structure of the crust and uppermost mantle beneath the Sicily channel from ambient noise and earthquake tomography

• In this work, we study the crust and the uppermost mantle structure beneath the Sicily Channel, by applying the ambient noise and earthquake tomography method. After computing cross-correlation of the continuous ambient noise signals and processing the earthquake data, we extracted 104 group velocity and 68 phase velocity dispersion curves corresponding to the fundamental mode of the Rayleigh waves. We computed the average velocity of those dispersion curves to obtain tomographic maps at periods ranging from 5 s to 40 s for the group velocities and from 10 s to 70 for the phase velocities. We inverted group and phase speeds to get the shear-wave velocity structure from the surface down to 100 km depth with a lateral resolution of about 200 km. The resulted velocity models reveal a thin crust with thickness value of 15 km beneath the southern part of the Tyrrhenian basin and a thickness value of 20 km beneath Mount Etna. The obtained thickness values are well correlated with the reported extension of the Tyrrhenian lithosphere due to the past earthquake tomography subduction and rollback of the Ionian slab beneath the Calabrian Arc. The crustal thickness increases and reaches values between 28 and 30 km beneath the Tunisian coasts and Sicily Channel. The S-wave models reveal also the presence of high velocity body beneath the island of Sicily. This finding can be interpreted as the presence of the Ionian slab subducting beneath the Calabrian Arc. Another high velocity body is observed beneath the southern part of the Tyrrhenian basin, it might be interpreted as the presence of fragments of the African continental lithosphere beneath the Tyrrhenian basin.

Crustal disequilibrium of the central Pontides (northern Turkey) due to oroclinal bending revealed by gravity modelling

The Turkish Pontide orogenic belt belongs to the former Eurasian margin, which is the result of two major successive processes of the closure of the Tethyan oceans: subduction followed by collision. This almost E-W trending large mountain range shows an arc-shaped geometric structure in the northern Turkey. According to previous studies based on geological and paleomagnetic data, oroclinal deformation was proposed as a result of the geodynamic evolution due to the Africa-Eurasia convergence in the region. The aim of study is to determine how this important evolution is expressed in the lithosphere beneath the region by gravity modelling.A Bouguer gravity anomaly map was constructed and from this a gravity model is presented that reveals the general crustal structure underneath the northern Turkey. The results present a significant crustal thickness change that occurs in the N-S and as well as in the E-W directions. From S to N, a significant sharp crustal thickness decrease (~20 km) was found, while from E to W the crustal thickness beneath the Pontides increases by approximately 7 km.While an average thickness of 37 km was calculated in the East, it reached a maximum value of ~44 km in the West. Although the reason for the increase in crustal thickness from north to south is logical in terms of the geodynamic evolution, the increase from east to west is probably caused by the nature of the continental collision. The most plausible explanation is an (oblique) NW indentation of the Kırşehir Block into the Pontides.

Linking metamorphism, magma generation, and synorogenic sedimentation to crustal thickening during Southern Appalachian mountain building, USA

Abstract The nature of metamorphism, magma compositions, the spatial distribution of plutons, and foreland sediments reflect, in part, the character and thickness of continental crust. We utilized metamorphic pressure-temperature-time (P-T-t) paths, garnet Sm-Nd ages, zircon U-Pb ages, and pluton compositions to estimate paleocrustal thickness and temporal changes in crustal magma sources in the Blue Ridge of the southernmost Appalachians. Garnet Sm-Nd ages for amphibolite-facies metamorphic rocks range from 331 ± 4 to 320 ± 3 Ma. Low- and high-Sr/Y plutons that intruded these metamorphic rocks have zircon U-Pb ages of 390 ± 1 to 365 ± 1 Ma and 349 ± 2 to 335 ± 1 Ma, respectively. Therefore, garnet growth began during regional metamorphism synchronous with or shortly after intrusion of the youngest high-Sr/Y trondhjemite plutons. Phase diagram sections and thermobarometry indicate that garnet growth initiated at ∼5.8 kbar and 540 °C and grew during temperature increases of 60–100 °C and pressure increases of 2–3 kbar. The older, low-Sr/Y magmas are inferred to have been sourced in the crust at depths <∼30 km, insufficient for garnet to be stable. However, the younger, high-Sr/Y magmas are inferred to have been sourced at >30 km depths where garnet was stable. Hafnium isotopic compositions for all the plutons, but one, exhibit a range from negative initial εHf(i) to weakly positive initial εHf(i), indicating incomplete mixing of dominantly crustal sources. Our data require minimum crustal thicknesses of ∼33 km at 331 Ma; however, Alleghanian crustal thicknesses must have locally reached 39 km, based on crustal reconstruction adding the Alleghanian thrust sheet beneath the eastern Blue Ridge. We infer the presence of hot, tectonically thickened crust during intrusion of the early Alleghanian high-Sr/Y plutons and conclude that garnet growth and plutonism reflect a progressive increase in crustal thickness and depth of magma generation. The crustal thickening was synchronous with deposition of Mississippian to early Pennsylvanian sediments in the foreland basin of the Appalachian orogen between 350 and 320 Ma. This crustal thickening may have preceded emplacement of the Alleghanian thrust sheets onto the North American craton.

Westward-younging high-Mg adakitic magmatism in central Tibet: Record of a westward-migrating lithospheric foundering beneath the Lhasa–Qiangtang collision zone during the Late Cretaceous

Magmatic records with high Mg and low Y signatures provide important insights into the deep mantle dynamics that dictate the evolution of ancient orogenic belts. This paper reports the presence of felsic intrusive dikes with high Mg and low Y signatures intruding the Baingoin Batholith (central Tibet) at ca. 94 Ma. These dikes show adakitic signatures, including high Sr and low Y and Yb contents and high Sr/Y and (La/Yb)N ratios. The adakitic dacitic dikes can be grouped into high Mg (Mg# = 48–52) with positive zircon εHf(t) (8.3–12.4) and whole-rock εNd(t) (0.27) and low Mg (Mg# = 37–45) with positive zircon εHf(t) (7.2–13.5) varieties. These distinct geochemical features, along with geological observations, show that the Baingoin high-Mg adakitic dacitic dikes were most likely derived from the partial melting of a foundering juvenile lower crust and subsequent melt–mantle reaction, whereas the low-Mg adakitic dacitic dikes can be interpreted as resulting from the partial melting of a thickened lower crust that did not delaminate. The generation of these adakitic rhyolitic dikes can be attributed to hornblende-dominated fractional crystallization of the adakitic dacitic melts. A decrease in Y and heavy rare earth element contents of magmatic records from 120–100 Ma to 95–80 Ma within the Lhasa–Qiangtang collision zone suggest an increase in crustal thickness with time. The volumetrically minor westward-migrating high-Mg adakitic magmatism can readily be interpreted as the consequences of the westward-migrating foundering of the lithosphere triggered by Rayleigh-Taylor instabilities along the Lhasa–Qiangtang collision zone during the period 94–80 Ma.

Southern Central Andes Neogene magmatism over the Pampean Flat Slab: implications on crustal and slab melts contribution to magma generation in Precordillera, Western Argentina

A Miocene to Pliocene (13 to 4.6 Ma) mostly pyroclastic sequence is exposed along the Iglesia Valley, to the east of the former main volcanic arc. This area is a transitional region between Cordillera Frontal and Precordillera, over the f lat slab segment of the Southern Central Andes, at 29º30’ S to 30º00’ S. New radiometric ages, geochemical data, petrography and field relationships are evaluated to establish differences and similarities between Miocene arcrelated sequences across the main arc and its expansion towards a back arc position, in western Precordillera. Analyzed rocks have a magmatic arc signature partially like the former main volcanic arc to the west. The Iglesia Valley rocks are LREE-enriched (La/Sm: 3.7-6.5) with respect to HREE (Sm/Yb: 2.2-6.0) and define patterns with a pronounced slope. Sm/Yb ratios generally increase with time, as pressures increase, with retention of HREE in residual mineralogy, particularly garnet at Sm/Yb>4. Volcanic activity in Cordillera Frontal and the volcanic-volcaniclastic expression in Precordillera show a continuous increase in the La/Yb ratio with decreasing age. Variations in the residual mineralphase equilibrating with magmas would be related to the progressive increase in crustal thickness due to the tectonic compressive regime resulting from shallow subduction since Middle Miocene. The data presented suggest that the arc magmatic activity during the Miocene was expanded notably to the East in relation to the location of the main arc at Valle del Cura, in Cordillera Frontal. The extensive amplitude of the volcanic arc activity is indicative of the slab gradual f lattening. Particularly, the mantle-derived magmas from Lomas del Campanario Formation (Western Precordillera) are enriched by subduction related f luids but also by crustal components. It is interpreted that the cause of the geochemical differences between the back arc position rocks and the main arc lay in the heterogeneous composition of the underlying continental crust involved in both locations. Presence of volcanic rocks with adakitic geochemical affinity probably ref lect astenospheric-derived melts that interacted through a heterogeneous and thickened crust toward the surface.

Earth's Continental Lithosphere Through Time

The record of the continental lithosphere is patchy and incomplete; no known rock is older than 4.02 Ga, and less than 5% of the rocks preserved are older than 3 Ga. In addition, there is no recognizable mantle lithosphere from before 3 Ga. We infer that there was lithosphere before 3 Ga and that ∼3 Ga marks the stabilization of blocks of continental lithosphere that have since survived. This was linked to plate tectonics emerging as the dominant tectonic regime in response to thermal cooling, the development of a more rigid lithosphere, and the recycling of water, which may in turn have facilitated plate tectonics. A number of models, using different approaches, suggest that at 3 Ga the volume of continental crust was ∼70% of its present-day volume and that this may be a minimum value. The continental crust before 3 Ga was on average more mafic than that generated subsequently, and this pre-3 Ga mafic new crust had fractionated Lu/Hf and Sm/Nd ratios as inferred for the sources of tonalite-trondhjemite-granodiorite and later granites. The more intermediate composition of new crust generated since 3 Ga is indicated by its higher Rb/Sr ratios. This change in composition was associated with an increase in crustal thickness, which resulted in more emergent crust available for weathering and erosion. This in turn led to an increase in the Sr isotope ratios of seawater and in the drawdown of CO2. Since 3 Ga, the preserved record of the continental crust is marked by global cycles of peaks and troughs of U-Pb crystallization ages, with the peaks of ages appearing to match periods of supercontinent assembly. There is increasing evidence that the peaks of ages represent enhanced preservation of magmatic rocks in periods leading up to and including continental collision in the assembly of supercontinents. These are times of increased crustal growth because more of the crust that is generated is retained within the crust. The rates of generation of continental crust and mantle lithosphere may have remained relatively constant at least since 3 Ga, yet the rates of destruction of continental crust have changed with time. Only relatively small volumes of rock are preserved from before 3 Ga, and so it remains difficult to establish which of these are representative of global processes and the extent to which the rock record before 3 Ga is distorted by particular biases.

Seismic crustal structure of the North China Craton and surrounding area: Synthesis and analysis

AbstractWe present a new digital model (NCcrust) of the seismic crustal structure of the Neoarchean North China Craton (NCC) and its surrounding Paleozoic‐Mesozoic orogenic belts (30°–45°N, 100°–130°E). All available seismic profiles, complemented by receiver function interpretations of crustal thickness, are used to constrain a new comprehensive crustal model NCcrust. The model, presented on a 0.25° × 0.25°grid, includes the Moho depth and the internal structure (thickness and velocity) of the crust specified for four layers (the sedimentary cover, upper, middle, and lower crust) and the Pn velocity in the uppermost mantle. The crust is thin (30–32 km) in the east, while the Moho depth in the western part of the NCC is 38–44 km. The Moho depth of the Sulu‐Dabie‐Qinling‐Qilian orogenic belt ranges from 31 km to 51 km, with a general westward increase in crustal thickness. The sedimentary cover is 2–5 km thick in most of the region, and typical thicknesses of the upper crust, middle crust, and lower crust are 16–24 km, 6–24 km, and 0–6 km, respectively. We document a general trend of westward increase in the thickness of all crustal layers of the crystalline basement and as a consequence, the depth of the Moho. There is no systematic regional pattern in the average crustal Vp velocity and the Pn velocity. We examine correlation between the Moho depth and topography for seven tectonic provinces in the North China Craton and speculate on mechanisms of isostatic compensation.

Reinterpretation of adcoh and cocorp seismic reflection data with constraints from detailed forward modeling of potential field data — Implications for Laurentia-Peri-Gondwana suture

To better constrain the structure of the Laurentian – Peri-Gondwana suture zone, maps and a 2-dimensional regional cross-section model constrained by seismic data and surface geology have been developed by forward and inverse modeling the aeromagnetic and gravity fields. The Central Piedmont Suture (CPS), the boundary between the Laurentian Inner Piedmont and the Peri-Gondwanan Carolina terrane is a low-angle thrust fault (~30°) ramping up from an Alleghanian mid-crustal detachment at depths of about 12km. ADCOH and COCORP seismic data image anticlinal structures in the footwalls of the Hayesville thrust and the CPS, above the Alleghanian decollement. The footwall rocks have previously been interpreted as Paleozoic shelf strata on the basis of sub-horizontal seismic reflectors; however, the high densities required to fit the observed gravity anomaly suggest that the folded footwall reflectors may need to be reinterpreted as horse blocks or duplex structures of Grenvillian basement. The Appalachian paired gravity anomaly can be explained by an increase in crustal thickness and a decrease in upper crustal density moving northwestward from the Carolina Terrane toward the Appalachian core. A change in lower crustal density is not required, so that Grenville basement rocks may extend farther to the southeast than previously thought. The 5 to 10km of Alleghanian uplift and exhumation predicted by P-T crystallization data compiled in this paper can be easily accommodated by thrusting on four major low-angle thrust systems: Great Smoky Mountain Thrust (GSMT), Hayesville, Brevard, and CPS. Unroofing of metamorphic core complexes by normal faulting may therefore not be required to explain the observed exhumation. Alleghanian collision along the southeastern Appalachian margin was predominately orthogonal to strike consistent with the previous reconstructions that call for the counter-clockwise rotation of Gondwanan West Africa, creating head-on collision in the southern Appalachians and at least 370km of shortening.

Effects of crustal thickness on magmatic differentiation in subduction zone volcanism: A global study

The majority of arc magmas are highly evolved due to differentiation within the lithosphere or crust. Some studies have suggested a relationship between crustal thickness and magmatic differentiation, but the exact nature of this relationship is unclear. Here, we examine the interplay of crustal thickness and magmatic differentiation using a global geochemical dataset compiled from active volcanic arcs and elevation as a proxy for crustal thickness. With increasing crustal thickness, average arc magma compositions become more silicic (andesitic) and enriched in incompatible elements, indicating that on average, arc magmas in thick crust are more evolved, which can be easily explained by the longer transit and cooling times of magmas traversing thick arc lithosphere and crust. As crustal thickness increases, arc magmas show higher degrees of iron depletion at a given MgO content, indicating that arc magmas saturate earlier in magnetite when traversing thick crust. This suggests that differentiation within thick crust occurs under more oxidizing conditions and that the origin of oxidation is due to intracrustal processes (contamination or recharge) or the role of thick crust in modulating melting degree in the mantle wedge. We also show that although arc magmas are on average more silicic in thick crust, the most silicic magmas (>70 wt.% SiO2) are paradoxically found in thin crust settings, where average compositions are low in silica (basaltic). We suggest that extreme residual magmas, such as those exceeding 70 wt.% SiO2, are preferentially extracted from shallow crustal magma bodies than from deep-seated magma bodies, the latter more commonly found in regions of thick crust. We suggest that this may be because the convective lifespan of crustal magma bodies is limited by conductive cooling through the overlying crustal lid and that magma bodies in thick crust cool more slowly than in thin crust. When the crust is thin, cooling is rapid, preventing residual magmas from being extracted; in the rare case that residual magmas can be extracted, they represent the very last melt fractions, which are highly silicic. When the crust is thick, cooling is slow, so intermediate melt fractions can readily segregate and erupt to the surface, where they cool and crystallize before highly silicic residual melts can be generated.

Aleutian basin oceanic crust

We present two-dimensional P-wave velocity structure along two wide-angle ocean bottom seismometer profiles from the Aleutian basin in the Bering Sea. The basement here is commonly considered to be trapped oceanic crust, yet there is a change in orientation of magnetic lineations and gravity features within the basin that might reflect later processes. Line 1 extends ∼225 km from southwest to northeast, while Line 2 extends ∼225 km from northwest to southeast and crosses the observed change in magnetic lineation orientation. Velocities of the sediment layer increase from 2.0 km/s at the seafloor to 3.0–3.4 km/s just above basement, crustal velocities increase from 5.1–5.6 km/s at the top of basement to 7.0–7.1 km/s at the base of the crust, and upper mantle velocities are 8.1–8.2 km/s. Average sediment thickness is 3.8–3.9 km for both profiles. Crustal thickness varies from 6.2 to 9.6 km, with average thickness of 7.2 km on Line 1 and 8.8 km on Line 2. There is no clear change in crustal structure associated with a change in orientation of magnetic lineations and gravity features. The velocity structure is consistent with that of normal or thickened oceanic crust. The observed increase in crustal thickness from west to east is interpreted as reflecting an increase in melt supply during crustal formation.

Crustal thickening and uplift of the Tibetan Plateau inferred from receiver function analysis

The India–Eurasia collision, crustal thickening and northward convergence leading to uplift of the Tibetan Plateau have been topics of multidisciplinary studies. Here we employ the H–k stacking technique and depth domain receiver function to constrain the crustal thickness and P- and S-wave velocity ratio as well as the upper mantle discontinuity. Our results identify the signature of slab break-off and lower crustal delamination in southern Tibet, which might have induced the mantle upwelling and crustal melting. The convergence of the Indian plate inducing northward crustal flow might be a major factor that contributes to the uplift and crustal thickening of Tibet. We also evaluate the E–W trending cumulative compression as another dominant factor that contributes to the increase in crustal thickness in the northern Tibet.

Crustal shear-wave velocity structure beneath northeast India from teleseismic receiver function analysis

We investigated the seismic shear-wave velocity structure of the crust beneath nine broadband seismological stations of the Shillong–Mikir plateau and its adjoining region using teleseismic P-wave receiver function analysis. The inverted shear wave velocity models show ∼34–38km thick crust beneath the Shillong Plateau which increases to ∼37–38km beneath the Brahmaputra valley and ∼46–48km beneath the Himalayan foredeep region. The gradual increase of crustal thickness from the Shillong Plateau to Himalayan foredeep region is consistent with the underthrusting of Indian Plate beyond the surface collision boundary. A strong azimuthal variation is observed beneath SHL station. The modeling of receiver functions of teleseismic earthquakes arriving the SHL station from NE backazimuth (BAZ) shows a high velocity zone within depth range 2–8km along with a low velocity zone within ∼8–13km. In contrast, inversion of receiver functions from SE BAZ shows high velocity zone in the upper crust within depth range ∼10–18km and low velocity zone within ∼18–36km. The critical examination of ray piercing points at the depth of Moho shows that the rays from SE BAZ pierce mostly the southeast part of the plateau near Dauki fault zone. This observation suggests the effect of underthrusting Bengal sediments and the underlying oceanic crust in the south of the plateau facilitated by the EW-NE striking Dauki fault dipping 300 toward northwest.

Petrogenesis of Miocene Volcanic arc rocks over the Chilean- Pampean flat-slab segment of the Centr

Miocene arc volcanism is manifested widely in the Valle del Cura-El Indio belt region (29°30´–30° South latitude), in the southern Central Andes of Argentina and Chile. The magmas that fed this volcanism are well represented by the Cerro de las Tortolas Formation, which is divided into two volcanic episodes based on petrographic, chemical and age differences: an older basaltic-andesitic event (16–14Ma) and a younger andesitic to dacitic (13–10Ma) one. Representative plagioclase, orthopyroxene, clinopyroxene and amphibole phenocrysts from rock samples already characterized from geochemical and isotopic viewpoints were selected for electron microprobe determination of mineral chemistry. Results indicate an overall homogeneous composition for each of the mineral phases. Equilibrium temperatures were estimated through two-pyroxenes, amphibole-plagioclase and amphibole geothermometers, which show a consistent temperature range between 970 to 850°C. Equilibrium pressure calculated using amphibole composition for volcanic suites produced the most comprehensive results for pressure equilibrium conditions, with results close to 4kb. Changes in the residual mineral assemblages and variations in isotopic signatures indicate that primary magmas were equilibrated at the lower crust with a gradual increase of crustal thickness. These melts evolved towards intermediate magma chambers, where crystallization of phenocrysts occurred at the same temperature and pressure conditions, hence, no increase in depth of intermediate magma chambers is registered although the increase of crustal thickness registered from Lower to Middle Miocene times.

Late Paleozoic–Mesozoic tectonic evolution of the Trans-Altai and South Gobi Zones in southern Mongolia based on structural and geochronological data

The Trans-Altai Zone in southern Mongolia is characterized by thrusting of greenschist-facies Silurian oceanic rocks over Devonian and Lower Carboniferous volcano-sedimentary sequences, by E–W directed folding affecting the early Carboniferous volcanic rocks, and by the development of N–S trending magmatic fabrics in the Devonian–Carboniferous arc plutons. This structural pattern is interpreted as the result of early Carboniferous thick-skinned E–W directed nappe stacking of oceanic crust associated with syn-compressional emplacement of a magmatic arc. The southernmost South Gobi Zone represents a Proterozoic continental domain affected by shallow crustal greenschist-facies detachments of Ordovician and Devonian cover sequences from the Proterozoic substratum, whereas supracrustal Carboniferous volcanic rocks and Permian sediments were folded into N–S upright folds. This structural pattern implies E–W directed thin-skinned tectonics operating from the late Carboniferous to the Permian, as demonstrated by K–Ar ages ranging from ~320Ma to 257Ma for clay fractions separated from a variety of rock types. Moreover, the geographical distribution of granitoids combined with their geochemistry and SHRIMP U–Pb zircon ages form distinct groups of Carboniferous and Permian age that record typical processes of magma generation and increase in crustal thickness. The field observations combined with clay ages, the geochemical characteristics of the granitoids and their ages imply that the E–W trending zone affected by tectonism migrated southwards, leaving the Trans Altai Zone inactive during the late Carboniferous and Permian, suggesting that the two units were tectonically amalgamated along a major E–W trending strike slip fault zone. This event was related to late Carboniferous subduction that was responsible for the vast volume of granitoid magma emplaced at 300–305Ma in the South Gobi and at 307–308Ma in the Trans-Altai Zones. The formation and growth of the crust was initially due only to subduction and accretion processes. During the post-collisional period from 305 to 290Ma the addition of heat to the crust led to the generation of (per-) alkaline melts. Once amalgamated, these two different crustal domains were affected by N–S compression during the Triassic and early Jurassic (185–173Ma), resulting in E–W refolding of early thrusts and folds and major shortening of both tectonic zones.

The Moho: Boundary above upper mantle peridotites or lower crustal eclogites? A global review and new interpretations for passive margins

Abstract We have performed a global study of 2D crustal scale wide-angle profiles across passive margins, with regard to local elevations in the Moho which could possibly be interpreted as indicative of lower crustal eclogites. A total of 16 candidates have been found, mainly in the North Atlantic and around Australia. These cases make up c. 6% only of the total profile length studied, confirming the interpretation of the Moho generally representing the top of the mantle. The interpreted candidates for lower crustal eclogites indicate that there may be a link between eclogite bodies and continental suture zones in the Barents Sea, off mid-Norway, in the Newfoundland Appalachians and in the Yilgarn Craton, western Australia. It is also possible that there is a genetic link between the formation of Caledonian eclogites and the Jan Mayen Fracture Zone, which is the only major fracture zone in the North Atlantic. Several of the inferred lower crustal eclogite bodies are located close to lines of major changes in strain, such as coastlines and shelf edges, indicating that lower crustal eclogite bodies may be important in guiding the evolution of basin architecture. Interpreting the Moho beneath the Bedout High on the northwest shelf of Australia as the top of a body of lower crustal eclogites, may imply that the northern termination of the Lambert Shelf represents a paleo suture zone and that the western termination of the Broome Platform acted as a major transfer zone. The significant increase in crustal thickness implied by the eclogite model has important implications for estimates of stretching history, subsidence and hydrocarbon maturation modelling.

Petrology of the Miocene igneous rocks in the Altar region, main Cordillera of San Juan, Argentina. A geodynamic model within the context of the Andean flat-slab segment and metallogenesis

The Altar porphyry Cu–(Au–Mo) deposit (31° 29′ S, 70° 28′ W) is located in the Andean Main Cordillera of San Juan Province (Argentina), in the southern portion of the flat-slab segment (28–33°S), 25 km north of the world-class porphyry Cu–Mo deposits of Los Pelambres and El Pachón. Igneous rocks in the area have been grouped into the Early Miocene Lower Volcanic Complex -composed of intercalations of lava flows and thin volcaniclastic units that grade upwards to a thick massive tuff- and the Middle-Late Miocene Upper Subvolcanic Suite that consists of a series of porphyritic stocks and dikes and magmatic and hydrothermal breccias. The Lower Volcanic Complex represents an Early Miocene arc (20.8 Ma ± 0.3 Ma; U–Pb age) erupted over a steep subduction zone. Their magmas equilibrated with an assemblage consisting of plagioclase- and pyroxene-dominated mineral residues, and experienced fractional crystallization and crustal contamination procesess. Their radiogenic signatures are interpreted to indicate conditions of relatively thickened continental crust in Altar during the Early Miocene, compared to the south and west. The Upper Subvolcanic Suite represents the development of a Middle-Late Miocene arc (11.75 ± 0.24 Ma, 10.35 ± 0.32 Ma; U-Pb ages) emplaced over a shallow subduction zone. A magmatic gap in Altar area betwen the Lower Volcanic Complex and Upper Subvolcanic Suite correlates with documented higher rates of compression in this period, that may have favored the storage of the USS magmas in cameras within the crust. Magmas of the Upper Subvolcanic Suite require a hornblende-bearing residual mineral assemblage that is interpreted to reflect their higher water contents. The relatively uniform radiogenic isotope compositions of the Upper Subvolcanic Suite magmas suggest a homogeneously mixed crust-mantle contribution in the source region. They have similar REE signatures as other fertile intrusives of the flat-slab. The differences observed in their isotopic signatures reflect an increase in the amount of crustal components incorporated into magmas from south (El Teniente) to north (El Indio) which correlate with an increase of crustal thickness. U–Pb ages of Altar rocks confirm a temporal connection between the ridge arrival and the magmatism associated with mineralization in this zone of the flat-slab segment. We argue that since the Middle-Late Miocene, mantle and lower crust may have been hydratated by fluids from the slab and from the Juan Fernández Ridge at the latitude of Altar, which favored the generation of the Middle-Late Miocene magmas. We also suggest that at this latitude the collision ridge-trench at ∼11 Ma and the subduction of the Juan Fernández Ridge beneath Altar region at ∼11–10 Ma may have promoted changes in the tectonic stress regime, allowing the USS magmas to rise to shallower levels in the crust. This may explain the location of a cluster of contemporaneous giant porphyry Cu–Mo deposits including El Pachón-Los Pelambres, the Altar Cu (Mo–Au) deposit, and other nearby recently discovered Cu prospects such as Piuquenes, La Coipa, Rincones de Araya and Los Azules.

Nature of the Moho beneath the Scottish Highlands from a receiver function perspective

The geology of Scotland documents a protracted geological history from Precambrian basement formation through the Caledonian Orogeny and to the Tertiary opening of the Atlantic. A temporary deployment of 22 closely spaced broadband seismic stations in Scotland traverses many of the major terrane boundaries in the region and provides a unique opportunity to place spatial and temporal constraints on variations in crustal structure. With teleseismic P-wave receiver functions, we use the H − κ method to determine variations in bulk crustal parameters: crustal thickness ( H) and V P/ V S ratio. Mean crustal thickness is ~ 28 km, varying from ~ 23 km in the NE highlands and increasing to > 30 km near the Highland Boundary Faultin the southern part of the study area. Mean V P/ V S values of ~ 1.76 show no significant variation across the study area. An abrupt increase in crustal thickness of ~ 4.5 km NW across the Moine Thrust is not easily linked to Tertiary–Recent tectonic activity. Instead it appears that Scotland's crust has retained features for hundreds of millions of years since it was first formed, despite abundant volcanic activity in Tertiary times. Our work favours the view that the Moho is a compositional boundary rather than a mineralogical one defined by a reaction in P– T space.

New Insights Into the Lithospheric Structure of Southern Norway

Topography can result from a balance, called isostasy, between the overlying weight of the crust and the buoyancy of the mantle beneath it. The principle of isostasy was put forward almost two centuries ago as a method of explaining variations in mountain heights, and it does a good job of explaining most of the first‐order variations in the Earth's topography.For example, when the crust is in local isostatic balance, elevation increases can be compensated for by an increase in crustal thickness, which in mountainous areas is in the form of a crustal root. Called Airy isostasy, compensation through crustal roots involves a correlation between surface topography and crustal thickness. However, topography may not be solely compensated for by crustal thickness, as other mechanisms such as flexural rigidity, low mantle densities, and dynamic topography can also be in effect.

Moho undulations beneath Tibet from GRACE-integrated gravity data

SUMMARY Knowledge of the variation of crustal thickness is essential in many applications, such as forward dynamic modelling, numerical heat flow calculations, seismologic applications and geohistory reconstructions. We present a 3-D model of the Moho undulations over the entire Tibetan plateau derived from gravity inversion. The gravity field has been obtained by using the Gravity Recovery and Climate Experiment (GRACE) potential field development which has been integrated with terrestrial data, and is presently the best available in the studied area. For the effective use of the global geopotential model that has no height information of observation stations, upward continuation is applied. The Moho model is characterized by a sequence of troughs and ridges with a semi-regular pattern, which could reflect the continent‐continent collision between the Indian and Eurasian plates. The three deep Moho belts (troughs) and shallow Moho belts (ridges) between them are clearly found to have an E‐W directional trend parallel to the border of the plateau and tectonic lines, while variation of the directionality is observed in central to southeast Tibet. To describe the distinctive shape of the Moho troughs beneath Tibet, we introduce the term, ‘Moho ranges’. The most interesting aspects of the Moho ranges are (1) that they run in parallel with the border and tectonic sutures of the plateau, (2) that the distances between ranges are found at regular distances of about 330 km except in northeast Tibet and (3) that the splitting of the ranges into two branches is found as the distance between them is increasing. From our study, we conclude that the distinctive undulations of the Tibetan Moho have been formed by buckling in a compressional environment, superimposed on the regional increase in crustal thickness. According to our analysis, the GRACE satelliteonly data turns out to have good enough resolution for being used to determine the very deep Moho beneath Tibet. Our Moho model is the first one that covers the entire plateau.