Changes In Crustal Thickness Research Articles

Insight into intracontinental deformation and crustal reworking of ancient continents implied by crustal structure imaging of the Yangtze Craton

The crustal imprints from multistage tectonic activities in cratons offer valuable insights into continental evolution. Utilizing seismic data from two densely deployed, nearly perpendicular linear arrays, and a newly developed stepwise joint inversion of depth-domain receiver function and surface wave dispersion, we constructed a detailed crustal layering model for the Yangtze Craton. Our analysis revealed elongated double velocity reversal zones as salient features of the crust, which likely record ancient crustal reworking and juvenile crustal growth associated with Neoproterozoic rift-related magmatic processes. The interlayering of low- and high-velocity structures may contribute to the enduring stability of the Yangtze Craton. Additionally, superimposed layers separated by east-dipping interfaces and abrupt changes in crustal thickness in the boundary belts surrounding the Yangtze Craton document the crust's structural responses to intracontinental deformation during continent assembly.

Lithospheric Structure in the Northern Appalachian Mountains: A Detailed Examination of the Abrupt Change in Crustal Thickness in Northwestern Massachusetts

AbstractPrevious geophysical studies in the New England Appalachians identified a ∼15 km offset in crustal thickness near the surface boundary between Laurentia and the accreted terranes. Here, we investigate crustal structure using data from a denser array: New England Seismic Transects experiment, which deployed stations spaced ∼10 km apart across the Laurentia‐Moretown terrane suture in northwestern Massachusetts. We used receiver function (RF) analysis to detect P to SV converted waves and identified multiple interfaces beneath the transect. We also implemented a harmonic decomposition analysis to identify features at or near the Moho with dipping and/or anisotropic character. Beneath the Laurentian margin, the Ps converted phase from the Moho arrives almost 5.5 s after the initial P wave, whereas beneath the Appalachian terranes, the pulse arrives at 3.5 s, corresponding to ∼48 and ∼31 km depth, respectively. The character of the RF traces beneath stations in the middle of our array suggests a complex transitional zone with dipping and/or anisotropic boundaries extending at least ∼30 km. This extension is measured in our profiles and perpendicular to the suture. We propose one possible crustal geometry model that is consistent with our observations and results from previous studies.

Neoproterozoic tectonic shift along the western margin of the Yangtze block, South China craton: Implications for its paleogeographic position during the Rodinia−Gondwana transition

Neoproterozoic igneous rocks in the Panxi region from the western margin of the Yangtze block, South China craton, record major changes in tectonic evolution and crustal thickness, and provide important constraints on the paleogeographic position and climate changes of the block in Rodinia and Gondwana reconstructions. Our new dating results show that the Pengguan complex from the Panxi region comprises ca. 743 Ma adakitic granodiorites and ca. 741 Ma A-type granites. The former is plagioclase-rich and has relatively low SiO2 content (65.22−67.36 wt%), high Mg# values (44.40−52.33), Sr/Y (23.22−61.94) and (La/Yb)N ratios (normalized to chondritic values; 13.41−24.00), with positive whole-rock εNd(t) values (+1.17 to +5.15), and high zircon εHf(t) (+5.53 to +8.83) and low zircon δ18O (4.21‰ to 5.51‰) values, indicating derivation from the partial melting of thickened juvenile mafic lower crust. By contrast, the granites are rich in K-feldspar and have high SiO2 contents (78.71−84.62 wt%), 10,000 × Ga/Al ratios (>2.6), and Fe/Mg ratios, with negative εNd(t) values (−1.47 to −0.02), high zircon εHf(t) (+2.94 to +9.77), and high saturation temperatures (828−920 °C). These features indicate fractional crystallization and partial melting of continental crust in a low-pressure, high-temperature, extensional environment. Integrated with the zircon Eu anomalies (Eu/Eu*) data and Hf-O isotopic data on pre-740 Ma magmatism in the Panxi region, we find that a rapid increase of crustal thickening (820−740 Ma) was synchronous with a gradual decrease of zircon δ18O and an increase of zircon εHf(t) values. These variations indicate an increase in the degree of hybridization between mantle sources and subducting slab-derived materials, thus generating a great number of low-δ18O melts, and continuous crustal thickening through the underplating of juvenile magmas. In addition, thinning of the crust since ca. 740 Ma, along with the gradual increase in zircon δ18O and εHf(t), suggests a shift from regional compression to extension. This documented shift, together with regional data from along the western margin of the Yangtze block, indicates a change from the well-developed and long-established Panxi convergent plate margin arc system (at least ca. 870−750 Ma) to passive-margin development and Cryogenian (720−635 Ma) glaciation. The long-lived subduction zone requires a peripheral location of South China that was linked to northern India during the Rodinia breakup. The subsequent and contemporaneous passive-margin development of the Yangtze block and India, along with the similar tectonic affinities of the Cathaysia block with India, indicate that South China lay outboard of the northern Gondwana margin during the assembly of this supercontinent.

Timing of the deformation in the Gondwanide orogeny: A structural and geochemical shift from syn- to post-tectonic magmatism recorded in the Permian Mamil Choique granitoids, Patagonia

The Sierra de Mamil Choique calc-alkaline granite batholith, covering an area of 320 km2, serves as the key reference for Late Paleozoic magmatism in central-west Patagonia. It comprises I-type weakly to mildly peraluminous tonalites to granites (59.6–75.7%SiO2) displaying various deformational features. This paper presents a micro- and mesostructural study alongside temperature-pressure (T-P) constraints coupled with U-Pb zircon crystallization ages, as well as Ar-Ar and K-Ar mica ages, and mineral and whole-rock geochemistry. The older units (288 ± 1 Ma Cerro Mojón Monzogranite and 281 ± 2 Ma Huenchuquil Granodiorite), exhibit syn-kinematic banding and strong NW-SE foliation showing a transition from magmatic-sub magmatic to high-T subsolidus deformational features, such as parallel alignment of magmatic minerals, sub magmatic fractures, melt pockets, and chessboard subgrains in quartz. Quartz recrystallization by grain boundary migration is also observed. These microstructures developed concurrently with the regional D3 event that affected the Devonian metamorphic host. Their crystallization started at 8 Kbar and ∼ 790 °C within a thickened crust (LaN/YbN = 13–15, average). In contrast, the younger units (278 ± 2 Ma; Nahuelfil and Antinao Monzogranites) exhibit mainly magmatic deformation and display a NE-SW parallel alignment of mostly subhedral K-feldspar. A D4 deformation younger than 278 Ma, with a sigma 1 NW-SE (in plain view- horizontal) would have controlled the emplacement of Nahuelfil and Antinao Monzogranites. Younger zircon ages (ca 265 Ma) in the 278 Ma monzogranites would result from resetting due to the magmatic-hydrothermal alteration associated with the later magmatic pulse of leucogranites and pegmatites of the 267 ± 8 Ma (Rb-Sr WR isochron) La Pintada Leucogranites. These leucogranites were emplaced in an already thinner crust (LaN/YbN = 2.6 average). The latest magmatic activity is represented by two groups of pegmatitic bodies one from 265 ± 6 to 257 ± 3 Ma and a younger one of ca. 252–251 Ma (Ar-Ar and K-Ar muscovite cooling ages). All units share a common mafic source, but the younger units crystallized from melts at lower pressure and temperature (748–725 °C). The magmatism, involving crustal recycling, occurred at an active margin during a stage of thickened crust from 290 to 280 Ma, followed by gradual thinning after 280 Ma. This change in crustal thickness fits models proposing a continuous Permian subduction with a variable dip angle of the subducted slab along the southwestern margin of Gondwana.

Regional trends and petrologic factors inhibit global interpretations of zircon trace element compositions

The trace element composition of zircon reveals information about the melt that they are derived from, as such, detrital zircon trace element compositions can be used to interrogate melt compositions, and thus the evolution of the continental crust in time and space. Here, we present a global database of detrital zircon compositions and use it to test whether average global trends for five common petrogenetic proxies truly represent secular changes in continental evolution. We demonstrate that the secular trend is broadly comparable across continental regions for Ti-in-zircon temperatures, but for other trace element ratios interrogated, secular trends are highly variable between continental regions. Because trace element ratios result from multiple petrologic variables, we argue that these petrogenetic proxies can be overinterpreted if projected to global geologic processes. In particular, we caution against the interpretation of crustal thickness from trace elements in zircon, and we argue that our results negate current hypotheses concerning secular changes in crustal thickness.

Subduction history of the Paleo-Asian oceanic slab along the northern margin of the North China Craton: A case study of zircons and cherts from central Jilin Province, NE China

This paper presents U-Pb ages and Hf isotope data for detrital zircons and whole-rock geochemical data for siliceous-clastic sedimentary rocks of the Shitoukoumen Formation, central Jilin Province, NE China. The new data combined with published data for the northern North China Craton enable us to constrain the temporal changes in crustal thickness along the Solonker−Xar Moron−Changchun−Yanji suture belt (SXCYB). The Shitoukoumen Formation contains ophiolitic lenses, radiolarian-bearing cherts, and volcanic-sedimentary units of various ages. A radiolarian-bearing volcanic-sedimentary unit exhibits a Bouma sequence, in which detrital zircons from a rhyolitic tuff and siliceous-clastic sedimentary rocks yielded youngest ages of 278−273 Ma, which constrain the depositional age of the unit. The geochemical compositions of the cherts suggest the pelagic deposits contain some continent-derived clastic material and may be near-trench gravity flow deposits. The dominantly depleted zircon εHf(t) values of the sedimentary rocks in the Shitoukoumen Formation record extensive crustal growth during the Paleozoic. The variations in the crustal thicknesses during the Paleozoic to early Mesozoic record a tectonic transition from subduction initiation, mature continental arc, and finally to Paleo-Asian ocean closure. The thickest crust (∼58 km) at ca. 245 Ma might have been related to an orogenic peak during the final closure of the Paleo-Asian Ocean along the SXCYB.

Inherited zircons in Early Cretaceous dyke: Implication of growth and reworking of continental crust in the Jiaodong Peninsula

Inherited zircons from Early Cretaceous dykes across the Jiaodong Peninsula provide a means of exploring magmatic episodes. Negative Eu anomalies, high Th and U contents, and high U/Yb ratios confirmed a zircon source of ancient continental crust. Differences in mineralogy, chronology, geochemistry, and Hf isotopes showed that these inherited zircons were not brought in by the Triassic Yangtze Craton collision. Zircons within 2938–2800 Ma confirmed that the ancient crust in the Jiaobei Terrane originated from the Mesoarchean crustal growth. This crustal growth continued until the Early Paleoproterozoic and was accompanied by crustal transformation activities in the later period. After the collision of the Jiao–Liao–Ji Belt, the Jiaodong Peninsula entered a typical craton period until it was reactivated at the end of the Neoproterozoic, which was characterised by the Archean crustal transformation. During the Palaeozoic, smooth crustal thickness change with limited addition of oceanic crust material indicated the Qinling–Dabie–Sulu Orogenic Belt as a distal island arc system that slowed the impact of Paleo-Tethys subduction. There was no record of positive-εHf(t) zircons in the Jiaodong Peninsula during the Mesozoic, but the crust thickened rapidly. This confirmed that the Paleo-Pacific Plate may have directly subducted beneath the lithospheric mantle and had little contact with the lower crust. This subduction pattern released little hydrous fluid but much heat, which prompted the remodelling of the lower crust in the Mesozoic. Furthermore, the large-scale lithospheric thinning event around 120 Ma was identified as a result of the delamination of the lithospheric mantle together with lower crust.

Geochemical evidence for evolving Proterozoic crustal thickness and orogenic styles in southwestern Laurentia

It has long been challenging for researchers to track the crustal thickness and mode(s) of crustal modification in ancient convergent margins, limiting evaluation of the tectonic styles and processes that modify continental crust during orogenesis. We present trace element igneous geochemical crustal thickness proxies that quantitatively track the crustal thickness evolution of the long-lived Proterozoic active margin in the southwestern U.S.A. We integrate these results with geobarometric data to constrain the mode of crustal modification. The data indicate a complex record of crustal thickness change in space and time and evolving orogenic styles. Geochemical proxies at 1.84–1.72 Ga are consistent with 20–40 km thick magmatic arcs that were locally thickened to ∼50 km during ∼1.75 Ga tectonism. During the Yavapai orogeny, 1.72–1.69 Ga, a ∼200-km-wide belt of 50-60 km thick crust extended from southern California to northern Colorado and was rapidly thinned and exhumed by ∼1.68 Ga. Crustal thickening and thinning during the Yavapai orogeny largely occurred by shortening and exhumation, respectively, in the upper 25 km of the crust. Subsequent 1.68–1.60 Ga tectonism involved crustal growth, local crustal thickening, and low-P, high-T metamorphism, consistent with extensional accretionary orogenesis. The 1.47–1.37 Ga Picuris orogeny was associated with 50–60 km thick crust across much of the Southwest and involved crustal shortening with ∼10 km of magmatic underplating. Advective heat from the emplacement of ferroan granites in the mid-crust likely contributed to elevated geothermal gradients and rheologically weakened the crust. Our results suggest evolving orogenic styles in the Southwest from 1.75–1.69 Ga short-lived crustal thickening associated with terrane accretion to 1.69–1.60 Ga largely extensional accretionary orogenesis, and regional, long-lived crustal thickening at 1.47–1.37 Ga involving extensive basaltic underplating. Contrasting with some recent hypotheses, our data document a complex middle Proterozoic record for the Southwest that was not orogenically quiescent or tectonically stagnant but involved complex mountain building styles.

Variations in Volcanism and Tectonics Along the Hotspot‐Influenced Reykjanes Ridge

AbstractMapping and sampling four sections of the slow‐spreading Reykjanes Ridge provide insight into how tectonic and volcanic activity varies with distance from the Iceland plume. The studied areas are characterized by significant variations in water depth, lava chemistry, crustal thickness, thermal structure, and ridge morphology. For each study area, fault pattern and dimension, tectonic strain, seamount morphology, and density are inferred from 15 m‐resolution bathymetry. These observations are combined with geochemical analysis from glass samples and sediment thickness estimations along Remotely Operated Vehicle‐dive videos. They reveal that (a) tectonic and volcanic activity along the Reykjanes Ridge, do not systematically vary with distance from the plume center. (b) The tectonic geometry appears directly related to the deepening of the brittle/ductile transition and the maximum change in tectonic strain related to the rapid change in crustal thickness and the transition between axial‐high and axial valley (∼59.5°N). (c) Across‐axis variations in the fault density and sediment thickness provide similar widths for the neo‐volcanic zone except in regions of increased seamount emplacement. (d) The variations in seamount density (especially strong for flat‐topped seamounts) are not related to the distance from the plume but appear to be correlated with the interaction between the V‐shape ridges (VSR) flanking the ridge and the ridge axis. These observations are more compatible with the buoyant upwelling melting instability hypothesis for VSR formation and suggest that buoyant melting instabilities create many small magma batches which by‐pass the normal subaxial magmatic plumbing system, erupting over a wider‐than‐normal area.

Geochemical Evidence for Diachronous Uplift and Synchronous Collapse of the High Elevation Variscan Hinterland

AbstractCompeting end‐member models for the late Paleozoic Variscan orogeny (ca. 360‐290 Ma) alternatively suggest moderate 2–3 km elevations underlain by relatively thin crust (<50 km) or a thick crust (>55 km) that supported high 4–5 km elevations. We tested these models and quantified the crustal thickness and elevation evolution of the Variscan orogeny using igneous trace element geochemical proxies and geochronologic data. These data suggest that thick crust (55–70 km) capable of supporting 3–5 km elevations developed diachronously from east to west between ca. 350 and 315 Ma. Crustal thinning occurred from ca. 315 to 290 Ma across the orogen. Crustal thickness and elevation changes at ca. 340‐325 and 315‐290 Ma correspond with increases in silicate weathering recorded by Sr and Li isotopes, consistent with models in which silicate weathering of the Variscan orogen contributed to global cooling associated with the late Paleozoic ice age.

Changes in crustal thickness of Zhongdian block, eastern Tibetan Plateau, and implications for formation and preservation of porphyry ore deposits

The eastern Tibetan Plateau has the highest topographic relief on Earth and is studded with abundant mineral reserves. However, the orogenic evolution and its link to mineral resources are unclear. The Zhongdian block, as the convergent center of several blocks during the final closure of the Paleotethys, is one of the most important regions for exploring the evolution of the eastern Tibetan Plateau. The geochemical parameters of intermediate-composition magmatic rocks can reflect mineral assemblages in the lower crust and can be used to constrain crustal thickness. Here, we show that the whole rock Sr/Y and (La/Yb) N and zircon Eu/Eu* values for the Late Triassic and Late Cretaceous intermediate to silicic magmatic rocks reveal two changes in crustal thickness of the Zhongdian block. The crustal thickness of the Zhongdian block increased to >70 km during ca. 230 to 210 Ma, while it decreased from >70 km to ~55 km during ca. 90 to 80 Ma. The two changes in crustal thickness could have benefited the formation of Late Triassic subduction-related porphyry copper deposits and Late Cretaceous collision-related porphyry molybdenum deposits. By quantitatively constraining the paleoelevation of the Zhongdian block, we interpret the existence of a paleoplateau (~5500 m in elevation) during the Early Jurassic–Early Cretaceous and a relict peneplain (~3500 m) during the Late Cretaceous to the present day. This is supported by the absence of Jurassic–Cretaceous strata and the decrease in the erosion rate during the Late Cretaceous. The slowing of the erosion rate caused by the decrease in elevation during the Late Cretaceous may be key to the preservation of these Mesozoic porphyry ore deposits. Quantifying crustal thickness is of great significance for understanding how orogenesis impacts elevation, magmatism, and mineralization in the eastern Tibetan Plateau. • Crustal thickness increased to >70 km in the Zhongdian block during the Late Triassic. • Crustal thickness decreased to ~55 km during the Late Cretaceous. • The two changes in crustal thickness favored the formation of porphyry ore deposits. • The decrease in elevation may have been key to the preservation of these deposits.

Breakup mechanism of the northern South China Sea: Evidence from the deep crustal structure across the continent-ocean transition

To improve constraints on breakup processes that eventually lead to seafloor spreading and the subsequent formation of new oceanic crust in the northern margin of the South China Sea (SCS), we collect a 172-km-long deep-penetration wide-angle seismic data OBS2018-L5. In the continental domain, the results present a gradually thinning crust with thickness ranging from ∼17 km to ∼8 km, with a slight thickening crust at the Outer Margin High. The attenuated continental crust is fairly rough and highly offset by distinctly different scale faults system. In the oceanic domain, the top of the basement is highly reflective with weak faulting and relatively constant crust thickness ranging from 5 km to 8 km (average 6 km). The continent-ocean boundary is characterized by an abrupt change in crustal thickness accompanied by a specific boundary between mostly positive gravity anomalies in the oceanic basin and grossly negative anomalies in the thinned continental crust, and between low amplitude, out-of-shape magnetic anomalies in the thinned continental domain and high amplitude, linear anomalies in the oceanic domain. A 0–5 km thick high-velocity layer (7.0–7.5 km/s) is present in the distal margin, which might represent mafic magma underplating. We also identify other magmatic features such as distinct buried volcanic edifices overprinting the original velocity structure. We propose that three mechanisms control the extension and breakup of the SCS: (1) the rapid transition from continental to oceanic crust, (2) elevated potential mantle temperature, and (3) the ductile lower crustal necking. Their interactions lead to the intense thinning of the continental lithosphere, decompression melting of the enriched asthenosphere followed by fast upwelling of mafic melt, relatively strong volcanic activities during the final stage of syn-rift and breakup, and finally generating a margin with intermediately rich magma, referred to as in intermediate-type margin.

Variation of crustal thickness in central west Junggar orogenic belt: insight into its Late Palaeozoic tectonic evolution

ABSTRACT The variation of crustal thickness of an orogenic belt through time will provide critical insights into the switch of tectonic setting. The geochemistry of subduction and collision-related magmatism is indicative of the crustal thickness. Late Palaeozoic magmatic products are prevalent in central west Junggar orogenic belt (C-WJOB), and can be employed to determine the temporal changes in crustal thickness as well as the tectonic evolution. Here, we constrain the crustal thickness and discuss the tectonic evolution of C-WJOB through by integrating Sr/Y and La/Yb ratios, zircon U-Pb ages and Hf isotopic compositions of the Late Palaeozoic igneous rocks. Our results show that the crust of C-WJOB was much thinner (20–25 km) than normal continental crust (average at 33 km) in the Early Carboniferous, and began to thicken from 350 Ma to 330 Ma at a slow rate. Crustal thickening was caused by the addition of subduction-related magma sourced from depleted-mantle but with contributions of subducted sediments. The crust started to thicken significantly at ~330 Ma and reached to a maximum thickness of ~40 km between 322 Ma and 310 Ma. The timing of maximum thickness corresponded with the emplacement of calc-alkaline to high-K calc-alkaline I-type dioritic magmas as well as adakitic magma. These magmas were generated by melting of both depleted mantle and juvenile lower crust, which was probably triggered by underplated mantle-derived basaltic magma. Lastly, crustal thinning happened between 310 Ma and 295 Ma due to regional post-collisional extension, although voluminous granitic magmatism occurred. The granitic magmas were formed by remelting of the juvenile lower crust, with a few contribution of mantle material. This study on the variation of crustal thickness in the Late Palaeozoic C-WJOB provides insight into how integrated geochemical and isotopic data can elucidate the complex histories of orogenic belts.

Seismicity patterns along the northern Manila Trench reflect crustal properties of the subducting plate

We examine the seismicity recorded by two temporary ocean bottom seismometer arrays deployed along the northeastern Manila Trench, covering the outer rise and part of the accretionary wedge. A total of 1437 events were determined during the 44-day recording period, showing a relatively more dynamic seismic activity than previously thought. Most events occurred in the subducting plate, oceanward of the trench, at depths between 25 and 50 km and revealed normal-fault type mechanisms. The relatively deeper focal depth and the lack of well-developed outer rise bulge near the seismic cluster suggest an untypical pattern of outer rise earthquakes. Seismicity with a high occurrence frequency and small magnitude is typical of earthquakes swarm, which is generally thought to be driven by fluid pressure variations. We suggest the infiltration of seawater through fractures into the rifted South China Sea margin is the main driver of the earthquakes detected during our deployments. Because earthquakes are also more abundant in the area where a significant change of crustal thickness is documented, we suggest the inherited crustal properties and tectonic structures strongly control the seismic behavior along the Manila subduction system.

Enigmatic Mid‐Proterozoic Orogens: Hot, Thin, and Low

AbstractSince the Archean, secular change in orogenic style is demonstrated through evolution of metamorphic conditions and geochemical proxies. Linked to orogenic style is the amount of crustal thickening and elevation, whereas orogenic vigor is related to the supercontinent cycle. An array of Proterozoic orogens spanned the assembly of supercontinents Columbia and Rodinia, but the vigor of orogenesis is debated, with proposals for both Mesoproterozoic quiescence and climax. We show mid‐Proterozoic orogenesis occurred globally and was broadly continuous; furthermore, orogens exhibit elevated metamorphic thermobaric ratios with large volumes of high‐temperature felsic magmatic rocks. These features reflect higher mantle heat flux leading to increased mid‐crustal flow and lower elevation. In this context, proposals that geochemical proxies for crustal thickness record orogenic quiescence are inconsistent with the geological record. Alternatively, secular change in crustal thickness is attributed to orogenic style, namely the prevalence of hot, thin, and low orogens in the mid‐Proterozoic.

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

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

Trace-element and Sr and Nd isotopic geochemistry of Cretaceous bentonites in Wyoming and South Dakota tracks magmatic processes during eastward migration of Farallon arc plutons

Abstract Bentonite beds, which are clay deposits produced by the submarine alteration of volcanic tephra, preserve millions of years of volcanic products linked to magmatic systems for which records are otherwise lost through erosion and alteration. Cretaceous strata from the Bighorn Basin, Wyoming, and southwestern South Dakota contain bentonites that originated from arc magmatism produced by subduction of the Farallon plate. We analyzed the bulk major- and trace-element geochemistry, and the 87Sr/86Sr (n = 87) and 143Nd/144Nd (n = 26) isotopic compositions of individual bentonite beds from these areas spanning 40 m.y. of volcanism to recover signals of magmatic processes and to attempt to trace bentonite geochemical and isotopic signatures to contemporaneous Cordilleran plutonic rocks. Using multiple immobile elements (e.g., Zr, TiO2, Nb, Ta, and rare earth elements), distinct temporal trends show variations in the effects of mineral fractionation and changes in crustal thickness. Bentonite Sr and Nd isotopic compositions allow ash beds to be correlated with specific batholithic complexes in Idaho and western Montana. With this data set, we observed the following: (1) The volcanic arc migrated across the 0.706 isopleth between 115 and 105 Ma; (2) between 105 and 95 Ma, magmatism stalled in central Idaho and was supported through significant MASH (mixing-assimilation-storage-homogenization) processing; (3) by 85 Ma, a shallowing subduction angle resulted in the eastward migration of the volcanic front into western Montana while volcanism in Idaho diminished; and (4) around 75 Ma, evidence of Idaho volcanism is lost. Montana plutonism continued with significant assimilation of radiogenic basement and regional centers of local magma emplacement (i.e., Pioneer batholith).

Integrated geophysical characterization of crustal domains in the eastern Black Sea

Abstract Rifting may lead ultimately to continental breakup, but the identification and characterization of the resulting crustal distribution remains challenging. Also, spatial and temporal changes in breakup magmatism may affect the geophysical character of the newly formed oceanic crust, resulting in contrasting interpretations of crustal composition and distribution. In the Eastern Black Sea Basin (EBSB), the evolution from rifting to breakup has been long debated, with several interpretations for the distribution of stretched continental and oceanic crust. We interpret basement morphological variations from long-offset seismic reflection profiles, highlighting a northwest-to-southeast transition from faulted and tilted continental blocks to a rough and then smoother basement. We model magnetic anomalies to further constrain the various basement domains, and infer the presence of a weakly magnetized, stretched continental crust in the northwest, and a 0.4–3.8 A/m layer coinciding with the smooth basement in the central and southeastern area. We conclude that the EBSB oceanic crust extends farther to the northwest than was suggested previously from an abrupt change in crustal thickness and lower-crustal velocity. The apparent discrepancy between these different types of geophysical evidence may result from changes in magma supply during breakup, affecting the thickness and velocity structure of the resulting oceanic crust.

A zircon petrochronologic view on granitoids and continental evolution

Temporal trends in granitoid chemistry and thermometry constrain major global changes in magmatism, tectonism or crustal thickness in the continents. Our study relies on zircon geochronology and trace element geochemistry on four new detrital rocks (two modern sediments and two Archean metasedimentary rocks) and a global compilation of published single zircon detrital chronology and trace chemistry data acquired on 5587 individual grains. Zircons of all ages from 4.4 Ga to present exist in this archive. Ti-in-zircon thermometry indicates that more than 98% of the grains with concordant U-Pb ages formed at temperatures exceeding 650°C. The great majority of these zircons formed in the 650–850°C range consistent with growth in intermediate to silicic magmas. Magmatic temperatures increased over time for the first 1.2 Ga of Earth's history after which they stayed constant before decreasing during the more recent past. U/Th<5 values in the overwhelming majority of grains are consistent with a magmatic origin. La/Yb, Sm/Yb and Eu/Eu* values are relatively constant throughout the history of the Earth suggesting that most granitoids formed at, or evolved from magmatic reservoirs located at depths of 35–45 km in the presence of amphibole, garnet and limited plagioclase. Such reservoirs are common today in hot deep crustal environments beneath some of the thicker island arcs and all continental arcs along subduction zones. Processes other than modern day style subduction may have contributed to the formation of granitoids in the early Earth but temperatures, depths and the presence of water arbitrated by the presence of amphibole were similar. These results also suggest that the thickness of continental crust in areas that produced granitoids was similar to today's global average throughout the 4.4 Ga time period covered by the zircon archive. There is no correlation between zircon chemistry over time and the assembly of supercontinents.

Regional crustal and lithospheric thickness model for Alaska, the Chukchi shelf, and the inner and outer bering shelves

SUMMARYThis study presents for the first time an integrated image of the crust and lithospheric mantle of Alaska and its adjacent western shelves of the Chukchi and Bering seas based on joint modelling of potential field data constrained by thermal analysis and seismic data. We also perform 3-D forward modelling and inversion of Bouguer anomalies to analyse density heterogeneities at the crustal level. The obtained crustal model shows northwest-directed long wavelength thickening (32–36 km), with additional localized trends of thicker crust in the Brooks Range (40 km) and in the Alaska and St Elias ranges (50 km). Offshore, 28–30-km-thick crust is predicted near the Bearing slope break and 36–38 km in the northern Chukchi Shelf. In interior Alaska, the crustal thickness changes abruptly across the Denali fault, from 34–36 to the north to above 30 km to the south. This sharp crustal thickness gradient agrees with the presence of a crustal tectonic buttress guiding block motion west and south towards the subduction zone. The average crustal density is 2810 kg m−3. The denser crust, up to 2910 kg m−3, is found south of the Denali Fault likely related to the oceanic nature of the Wrangellia Composite Terrane rocks. Offshore, less dense crust (&lt;2800 kg m−3) is found along the sedimentary basins of the Chukchi and Beaufort shelves. At LAB levels, there is a regional SE–NW trend that coincides with the current Pacific Plate motion, with a lithospheric root underneath the Brooks Range, Northern Slope, and Chuckchi Sea, that may correspond to a relic of the Chukotka-Artic Alaska microplate. The obtained lithospheric root (above 180 km) agrees with the presence of a boundary of cold, strong lithosphere that deflects the strain towards the South. South of the Denali Fault the LAB topography is quite complex. East of 150°W, below Wrangellia and the eastern side of Chugach terranes, the LAB is much shallower than it is west of this meridian. The NW trending limit separating thinner lithosphere in the east and thicker in the west agrees with the two-tiered slab shape of the subducting Pacific Plate.