GSA Bulletin Articles Published Ahead of Print in January

Boulder, Colo., USA: Sixteen articles were published online ahead of print for GSA Bulletin in January. Topics include insights from the Sawtooth metamorphic complex in Idaho, fingerprinting sand from ancient rivers, eroding Cascadia, and the Troodos ophiolite.

Three-dimensional geometry and growth of a basement-involved fault network developed during multiphase extension, Enderby Terrace, North West Shelf of Australia

Hongdan Deng; Ken McClay

Abstract: Basement fault reactivation, and the growth, interaction, and linkage with new fault segments are fundamentally three-dimensional and critical for understanding the evolution of fault network development in sedimentary basins. This paper analyzes the evolution of a complex, basement-involved extensional fault network on the Enderby Terrace on the eastern margin of the Dampier sub-basin, North West Shelf of Australia. A high-resolution, depth-converted, 3-D seismic reflection data volume is used to show that multiphase, oblique extensional reactivation of basement-involved faults controlled the development of the fault network in the overlying strata. Reactivation of the pre-existing faults initially led to the formation of overlying, en échelon Late Triassic−Middle Jurassic fault segments that, as WNW-directed rifting progressed on the margin, linked by breaching of relay zones to form two intersecting fault systems (F1 and F2−F4). Further reactivation in the latest Jurassic−Early Cretaceous (NNW-SSE extension) produced an additional set of en échelon fault arrays in the cover strata. The final fault network consists of main or principal faults and subordinate or splay faults, together with branch lines that link the various components. Our study shows that breaching of relay ramps and/or vertical linkages produces vertical and horizontal branch lines giving complex final fault geometries. We find that repeated activity of the basement-involved faults tends to form continuous and planar fault architectures that favor displacement transfer between the main constituent segments along strike and with depth.

View article:

Age and deposition of the Fort Crittenden Formation: A window into Late Cretaceous Laramide and Cenozoic tectonics in southeastern Arizona

Emilia A. Caylor; Barbara Carrapa; Kurt Sundell; Peter G. DeCelles; Joshua M. Smith

Abstract: The Upper Cretaceous Fort Crittenden Formation exposed in the Santa Rita and Huachuca Mountains of southeastern Arizona is a syntectonic deposit that has been associated with Laramide tectonic activity. However, the spatio-temporal relationships among Cretaceous sedimentation, magmatism, basement exhumation, and possible flat slab-related processes in the southern Laramide region remain poorly understood. Age controls for uplift and erosion of local topography and syntectonic deposition in response to deformation remain particularly poor. The Fort Crittenden Formation comprises 800−2500 m of locally derived fluvial to alluvial fan sedimentary rocks and records paleodrainage reorganization in response to active tectonics. Changes in sedimentary facies, provenance, and paleoflow suggest deposition in a tectonically partitioned intraforeland basin. New detrital zircon data constrain the timing of deposition of the Fort Crittenden Formation between ca. 86 Ma and ca. 76 Ma. The lack of depositional age zircons throughout the majority of the Fort Crittenden Formation is consistent with a magmatic lull in the Cordilleran arc between ca. 90 Ma and ca. 76 Ma. The overlying Salero Formation and Late Cretaceous intrusions are expressions of renewed magmatism in southeastern Arizona at ca. 75 Ma. New Lu-Hf data indicate that magmas evolved from contamination of old juvenile crust. When interpreted in conjunction with other regional data sets, our study indicates that the Laramide deformation front migrated eastward into southwestern New Mexico by 75 Ma. Thermal modeling of apatite fission track and (U-Th)/He data from granitic clasts are consistent with Late Cretaceous−Paleocene (ca. 76−55 Ma) heating related to magmatism and cooling and exhumation during the Eocene and Oligocene.

View article:

Mesozoic crustal melting and metamorphism in the U.S. Cordilleran hinterland: Insights from the Sawtooth metamorphic complex, central Idaho

Chong Ma; David A. Foster; Paul A. Mueller; Barbara L. Dutrow; Jeffery Marsh

Abstract: In this study, we present whole-rock geochemistry and Sm-Nd data; zircon trace element, U-Pb, and Lu-Hf data; titanite U-Pb dating; and structural analysis of igneous and metasedimentary rocks of the Sawtooth metamorphic complex that provide insight into regional metamorphism, partial melting, and crustal thickening in the Idaho batholith segment of the Cordilleran orogen. Four magmatic events are revealed: (1) pre-tectonic felsic magmatism at ca. 156 Ma, (2) syn-tectonic mafic and felsic magmatism between ca. 100 Ma and ca. 92 Ma, (3) felsic magmatism concurrent with late-stage deformation at ca. 89−84 Ma, and (4) post-tectonic felsic magmatism at ca. 77 Ma. The multiple generations of felsic magmatism include a variety of sedimentary- and igneous-derived granitoids distinguished by zircon trace element compositions (e.g., U/Ce versus Th and Ce/Sm versus Yb/Gd) and were sourced from progressively more evolved crustal components as shown by Lu-Hf and Sm-Nd isotopic data. U-Pb data of metamorphic zircons and titanites from high-grade metasedimentary rocks suggest that regional metamorphism occurred from ca. 100−93 Ma, which was characterized by granulite-facies partial melting and concurrent growth of metamorphic zircons and garnets. The episodic magmatism in the Sawtooth metamorphic complex records pervasive melt migration in a hot, mid-crustal setting at ca. 100‒92 Ma and additional magma ascent in a cool, upper-crustal setting at ca. 77 Ma. The uplift of the Sawtooth metamorphic complex from mid- to upper-crust was likely caused by underthrusting at lower crustal levels coupled with erosion and thinning of the upper crust. This work suggests that the crust of the Cordilleran hinterland in the Idaho batholith region underwent significant thickening from ca. 100‒84 Ma, and a crust of Andean-like thickness was probably achieved by ca. 84 Ma. By ca. 77 Ma, the central Idaho crust started to thin likely due to mid-crustal flow and surface erosion. The new data from the Sawtooth metamorphic complex are consistent with the two major magmatic flare-ups in the Late Jurassic and Late Cretaceous in the U.S. Cordilleran orogen.

View article:

Contribution of an Eastern Indochina-derived fragment to the formation of island arc systems in the Philippine Mobile Belt

Lin Gong; Pete Hollings; Yu Zhang; Jing Tian; Dengfeng Li ...

Abstract: The Philippine Mobile Belt is a complex plate boundary with multiple terranes in Southeast Asia, yet its early tectonic evolution is still not fully understood due to a scarcity of solid evidence. Here we report new whole rock geochemical, Sr-Nd isotopic, and zircon U-Pb-Hf isotopic data for Cretaceous-Miocene arc magmatic rocks from the Cebu and Bohol Islands, Philippine Mobile Belt. Bulk geochemical data display arc affinities with enriched large ion lithophile elements (e.g., Sr and Ba) and depleted high field strength elements (e.g., Nb, Ta, and Ti). The high positive εNd(t) (+4.6 to +9.1) values and low initial 87Sr/86Sr ratios (0.7032−0.7048) suggest that these igneous rocks were generated by partial melting of mantle wedge in an arc setting. U-Pb dating of zircons revealed Cretaceous (ca. 120−90 Ma), middle Eocene to early Oligocene (ca. 43−30 Ma), and middle Miocene (ca. 14 Ma) crystallization ages for the arc magmatism with abundant Permian-Triassic zircon xenocrysts clustering at ca. 250 Ma. The Permian-Triassic grains show dominantly negative εHf(t) values ranging from −16.2 to −6.6, which are similar to those of coeval rocks in Eastern Indochina. Combined with previous paleomagnetic studies, we propose that an Eastern Indochina-derived continental fragment was involved during the formation of arcs in the Cebu and Bohol Islands, which highlights the potential contribution of ancient continental materials in the formation of intra-oceanic arcs. This scenario does not support the previously proposed model that the Cretaceous arc in the Philippine Mobile Belt formed in the northern margin of the proto-Philippine Sea Plate and Australian margin.

View article:

Fingerprinting sand from Asian rivers to the deep central South China Sea since the Late Miocene

Licheng Cao; Tao Jiang; Jingke He

Abstract: The complex sedimentary processes from source to sink lead to a substantial fractionation of sediment size and composition. Relatively coarse-grained, continent-derived detritus is rarely transported and deposited in the deep ocean, and the terminus of this sediment routing system is poorly understood. Sandy turbidite deposits within the Upper Miocene−Pleistocene strata drilled in the deep central South China Sea during the International Ocean Discovery Program (IODP) Expedition 349 provide valuable samples for evaluating the evolution of sediment contributions from different Asian landmasses. This study reconstructs this ancient source-to-sink system based on an integration of heavy mineral and detrital zircon analyses (including U-Pb age, trace element, grain size and shape), obtained from IODP sites U1431 and U1432, as well as a zircon age-based mixture modeling of well-defined provenance end-members. The results show several provenance shifts that correspond to more complex and dynamic source-to-sink scenarios than previously envisaged. Certain source areas, like East Vietnam, present a different provenance signature than that of today. Multiple provenances have been mixed and diluted during sediment transport, exhibiting a large regional variability. We interpret that siliciclastic turbidite deposits in the central South China Sea were mainly derived from East Vietnam during the early Late Miocene and Pliocene, and the Pearl River Basin during the late Late Miocene and Pleistocene. Additional, but less significant, contributions from the Red and Mekong river basins and coastal Southeast China are also observed.

View article:

Metamorphic P-T-t evolution of amphibolite in the north Hengshan terrane, North China Craton: Insights into the late Paleoproterozoic tectonic processes from initial collision to final exhumation

Jiahui Qian; Changqing Yin; Shun Li; Jian Zhang

Abstract: Amphibolite retrograded from high-pressure (HP) mafic granulite can provide valuable insight into exhumation of deeply buried crust in orogenic belts. In the north Hengshan terrane of the North China Craton, amphibolite occurs as rims of HP mafic granulite block or as smaller homogeneous boudins representing retrograde products of the granulite. Three amphibolite samples were selected. The rocks are mainly composed of hornblende + plagioclase + quartz + biotite + ilmenite with or without garnet pseudomorph consisting of plagioclase + hornblende + ilmenite and symplectite of hornblende + plagioclase ± clinopyroxene. The pseudomorph-, symplectite-bearing sample experienced a post-peak isothermal decompression at >800 °C that was accompanied by breakdown of garnet and clinopyroxene. Isopleths of the maximum An in plagioclase and Ti in hornblende were used to constrain the Tmax stage of ca. 6 kbar/825−850 °C, which was followed by cooling and post-cooling decompression. For the sample showing an “equilibrated” mineral assemblage, a medium-temperature decompression from >6.8 kbar/685 °C to 3.6−4.8 kbar/640−660 °C was inferred. P-T evolution of the north Hengshan terrane is characterized by two discrete (post-peak and post-cooling) decompression processes. Zircon U-Pb dating of amphibolite yields a metamorphic age of 1868 ± 15 Ma, which is interpreted to record the timing of late amphibolization. Synthesized metamorphic P-T-t data in the Hengshan-Wutai area indicate a complicated tectonic evolution that includes an older collisional orogeny at ca. 1.95 Ga and a younger metamorphism at ca. 1.85 Ga. The post-cooling decompression path of the amphibolite may reflect final exhumation of the north Hengshan terrane through the late deformation/metamorphism of the Zhujiafang shear zone.

View article:

Protracted northward drifting of South China during the assembly of Gondwana: Constraints from the spatial-temporal provenance comparison of Neoproterozoic−Cambrian strata

Qiong Chen; Guochun Zhao; Min Sun

Abstract: Neoproterozoic to Paleozoic sedimentation shows systematic temporal-spatial variations within South China, which must be considered in reconstructing geological evolution of South China in response to global plate reorganization from the breakup of Rodinia to the assembly of Gondwana. We use >1000 new U-Pb and Hf isotopic data for detrital zircons from Neoproterozoic−Cambrian strata across the western (i.e., Longmenshan) and eastern (i.e., Wuyishan) margins of South China, coupled with compiled stratigraphic and magmatic information, to constrain change in provenance through time. First-order conclusions are as follows: (1) detrital zircons from the Neoproterozoic strata of the two margins were mainly sourced from the Panxi-Hannan arc and the Jiangnan orogen, signaling a rough self-sufficient sedimentary system; (2) newly identified Cambrian molasse-like sediments in the western margin, in which abundant detrital zircons are 550−500 Ma old with positive εHf(t) values, were mainly derived from the 580−500 Ma Cadomian arc belt along the Iran-Turkey margin; and (3) the Cambrian sediments in the eastern margin document more increased contributions from the Grenvillian-age provinces most possibly in Australia. Such spatial-temporal provenance variations signal the northward drifting of South China, from a position connecting with Iran-Turkey and northern India to that approaching Australia during the late Neoproterozoic−Cambrian period. We highlight that the activity of oblique oceanic-continental convergence accreted Asian terranes onto the northern margin of Gondwana, hence contributing to the ultimate Gondwana architecture under global plate reorganization.

View article:

The convexity of carbonate hilltops: 36Cl constraints on denudation and chemical weathering rates and implications for hillslope curvature

Matan Ben-Asher; Itai Haviv; Onn Crouvi; Joshua J. Roering; Ari Matmon

Abstract: Carbonate hillslopes are often soil mantled and display a classic convex morphology. In this study we examine controls on carbonate hillslope denudation and morphology using a modified regolith mass balance equation to account for chemical weathering and dust input—two fluxes that are commonly neglected in settings with silicate-dominated bedrock. We utilize seven study sites in the Eastern Mediterranean across a significant gradient in the mean annual rainfall and dust deposition flux. Combining cosmogenic 36Cl-derived hilltop denudation rates with an estimate of the regolith chemical depletion and the quantified fraction of dust in the regolith we predict hilltop curvature and compare our predictions with observations based on high-resolution airborne LiDAR (light detection and ranging). Denudation rates vary from 5 to 210 mm/k.y. and increase with mean annual rainfall. Less resistant carbonates (chalk) experience faster denudation rates relative to more resistant dolo-limestone and are less prone to chemical weathering. Soil production exhibits a humped dependency on soil thickness. The observed hilltop curvature varies as a function of rainfall and dust flux with a minimum at sub-humid sites. While trends in hilltop convexity are often solely attributed to variations in erosion rate, our results illustrate the additional effects of dust production and chemical depletion. Our mass balance model implies that drier sites in the south probably experienced a more intricate history of regolith production due to dust flux fluctuations. Thus, by incorporating dust flux and chemical weathering to the classic hillslope evolution model we are able to identify a complex relation between hilltop curvature, soil production, and climate.

View article:

Surface slip distributions and geometric complexity of intraplate reverse-faulting earthquakes

Haibin Yang; Mark Quigley; Tamarah King

Abstract: Earthquake ground surface ruptures provide insights into faulting mechanics and inform seismic hazard analyses. We analyze surface ruptures for 11 historical (1968−2018) moment magnitude (Mw) 4.7−6.6 reverse earthquakes in Australia using statistical techniques and compare their characteristics with magnetic, gravity, and stress trajectory data sets. Of the total combined (summative) length of all surface ruptures (∼148 km), 133 km (90%) to 145 km (98%) align with the geophysical structure in the host basement rocks. Surface rupture length (SRL ), maximum displacement (MD), and probability of surface rupture at a specified Mw are high compared with equivalent Mw earthquakes globally. This is attributed to (1) a steep cratonic crustal strength gradient at shallow depths, promoting shallow hypocenters (∼1−6 km) and limiting downdip rupture widths (∼1−8.5 km), and (2) favorably aligned crustal anisotropies (e.g., bedrock foliations, faults, fault intersections) that enhanced lateral rupture propagation and/or surface displacements. Combined (modeled and observed) MDs are in the middle third of the SRL with 68% probability and either the ≤33rd or ≥66th percentiles of SRL with 16% probability. MD occurrs proximate to or directly within zones of enhanced fault geometric complexity (as evidenced from surface ruptures) in 8 of 11 earthquakes (73%). MD is approximated by 3.3 ± 1.6 (1σ) × AD (average displacement). S-transform analyses indicates that high-frequency slip maxima also coincide with fault geometric complexities, consistent with stress amplifications and enhanced slip variability due to geometric and kinematic interactions with neighboring faults. Rupture slip taper angles exhibite large variations (−90% to +380% with respect to the mean value) toward rupture termini and are steepest where ruptures terminate at obliquely oriented magnetic lineaments and/or lithology changes. Incremental slip approximates AD between the 10th and 90th percentiles of the SRL. The average static stress drop of the studied earthquakes is 4.8 ± 2.8 MPa. A surface rupture classification scheme for cratonic stable regions is presented to describe the prevailing characteristics of intraplate earthquakes across diverse crustal structural-geophysical settings. New scaling relationships and suggestions for logic tree weights are provided to enhance probabilistic fault displacement hazard analyses for bedrock-dominated intraplate continental regions.

View article:

Eroding Cascadia—Sediment and solute transport and landscape denudation in western Oregon and northwestern California

Jim E. O’Connor; Joseph F. Mangano; Daniel R. Wise; Joshua R. Roering

Abstract: Riverine measurements of sediment and solute transport give empirical basin-scale estimates of bed-load, suspended-sediment, and silicate-solute fluxes for 100,000 km2 of northwestern California and western Oregon. This spatially explicit sediment budget shows the multifaceted control of geology and physiography on the rates and processes of fluvial denudation. Bed-load transport is greatest for steep basins, particularly in areas underlain by the accreted Klamath terrane. Bed-load flux commonly decreases downstream as clasts convert to suspended load by breakage and attrition, particularly for softer rock types. Suspended load correlates strongly with lithology, basin slope, precipitation, and wildfire disturbance. It is highest in steep regions of soft rocks, and our estimates suggest that much of the suspended load is derived from bed-load comminution. Dissolution, measured by basin-scale silicate-solute yield, constitutes a third of regional landscape denudation. Solute yield correlates with precipitation and is proportionally greatest in low-gradient and wet basins and for high parts of the Cascade Range, where undissected Quaternary volcanic rocks soak in 2−3 m of annual precipitation. Combined, these estimates provide basin-scale erosion rates ranging from ∼50 t ∙ km−2 ∙ yr −1 (approximately equivalent to 0.02 mm ∙ yr−1) for low-gradient basins such as the Willamette River to ∼500 t ∙ km −2 ∙ yr−1 (∼0.2 mm ∙ yr−1) for steep coastal drainages. The denudation rates determined here from modern measurements are less than those estimated by longer-term geologic assessments, suggesting episodic disturbances such as fire, flood, seismic shaking, and climate change significantly add to long-term landscape denudation.

View article:

Hydrothermal formation of iron-oxyhydroxide chimney mounds in a shallow semi-enclosed bay at Satsuma Iwo-Jima Island, Kagoshima, Japan

Shoichi Kiyokawa; Takashi Kuratomi; Tatsuhiko Hoshino; Shusaku Goto; Minoru Ikehara

Abstract: Hydrothermal iron-oxyhydroxide chimney mounds (iron mounds) have been discovered in a fishing port in Nagahama Bay, located on the southwest coast of Satsuma Iwo-Jima Island, south of Kyushu Island, Japan. In the fishing port, uncovered ∼1.0-m-high iron mounds in shallow waters formed under relatively calm conditions. Typically, the fishing port has orange-colored turbid waters that mix with outer ocean waters during high tide. Colloidal iron-oxyhydroxides form due to the oxidation of ferrous iron in hydrothermal waters (pH = 5.5; temperature = 55 °C) as they mix with seawater. The mounds are made of two types of material: hard, dark brown−orange, high-density material; and soft, brownish orange−yellow, low-density material. Computed tomography scans of the harder iron mound material revealed a cabbage-like structure consisting of micropipe structures with diameters of 2−5 mm. These micropipes have relatively hard walls made of iron oxyhydroxides (FeOH) and are identified as discharge pipes. Nucleic acid staining genetic sequencing and scanning electron microscope observations suggest that the mounds formed mainly from bacterial stalks with high concentrations of FeOH colloidal matter. In the harder parts of the mounds, these “fat stalks,” which contain oxyhydroxide colloidal aggregates, are entwined and concentrated. The softer material contains twisted stalk-like structures, which are coated with FeOH colloidal particles. Deoxyribonucleic acid (DNA) examination of the iron mounds revealed the presence of iron-oxidizing bacteria, especially at the mound surface. We estimate that the iron mounds accumulated at a rate of ∼1700 tons/1000 m2/yr. This is an order of magnitude higher than the rate of FeOH sedimentation via chemical precipitation of FeOH colloids within the fishing port. This suggests that biogenic activity, resulting in the production of entwined FeOH stalks, leads to the rapid accumulation of FeOH beds and that biogenic activity within the water mass rich in FeOH colloids is an efficient means of generating thick iron-rich sedimentary sequences. As such, we propose that some ancient iron formations may have also formed through the biogenic production of FeOH stalks rather than solely through chemical sedimentation in a water mass rich in FeOH colloids. It appears that these rapidly forming biogenic FeOH iron mounds, distributed over a wide area of ocean floor, are also relatively protected from erosion and diagenetic alteration (reduction). Previous studies have reported that ancient iron formations were commonly deposited in deeper environments via direct iron oxidation from the water column in a ferruginous ocean. However, there are several hydrothermal vent inflows preserved with FeOH that would have formed appropriate redox boundary conditions in an otherwise anoxic ocean. Under these conditions, iron mound mat-type sedimentary deposits might have formed and been well preserved and affected by early diagenesis where higher heat flow occurred in the Archean ocean. The FeOH mounds in Nagahama Bay provide an example of the iron formation sedimentary environment and important information for estimating the past depositional state of iron formations.

View article:

Textural and geochemical window into the IDDP-1 rhyolitic melt, Krafla, Iceland, and its reaction to drilling

E. Saubin; B. Kennedy; H. Tuffen; A.R.L. Nichols; M. Villeneuve ...

Abstract: The unexpected intersection of rhyolitic magma and retrieval of quenched glass particles at the Iceland Deep Drilling Project-1 geothermal well in 2009 at Krafla, Iceland, provide unprecedented opportunities to characterize the genesis, storage, and behavior of subsurface silicic magma. In this study, we analyzed the complete time series of glass particles retrieved after magma was intersected, in terms of distribution, chemistry, and vesicle textures. Detailed analysis of the particles revealed them to represent bimodal rhyolitic magma compositions and textures. Early-retrieved clear vesicular glass has higher SiO2, crystal, and vesicle contents than later-retrieved dense brown glass. The vesicle size and distribution of the brown glass also reveal several vesicle populations. The glass particles vary in δD from −120‰ to −80‰ and have dissolved water contents spanning 1.3−2 wt%, although the majority of glass particles exhibit a narrower range. Vesicular textures indicate that volatile overpressure release predominantly occurred prior to late-stage magma ascent, and we infer that vesiculation occurred in response to drilling-induced decompression. The textures and chemistry of the rhyolitic glasses are consistent with variable partial melting of host felsite. The drilling recovery sequence indicates that the clear magma (lower degree partial melt) overlays the brown magma (higher degree partial melt). The isotopes and water species support high temperature hydration of these partial melts by a mixed meteoric and magmatic composition fluid. The textural evidence for partial melting and lack of crystallization imply that magma production is ongoing, and the growing magma body thus has a high potential for geothermal energy extraction. In summary, transfer of heat and fluids into felsite triggered variable degrees of felsite partial melting and produced a hydrated rhyolite magma with chemical and textural heterogeneities that were then enhanced by drilling perturbations. Such partial melting could occur extensively in the crust above magma chambers, where complex intrusive systems can form and supply the heat and fluids required to re-melt the host rock. Our findings emphasize the need for higher resolution geophysical monitoring of restless calderas both for hazard assessment and geothermal prospecting. We also provide insight into how shallow silicic magma reacts to drilling, which could be key to future exploration of the use of magma bodies in geothermal energy.

View article:

Intracontinental extension and geodynamic evolution of the Paleoproterozoic Jiao-Liao-Ji belt, North China craton: Insights from coeval A-type granitic and mafic magmatism in eastern Liaoning Province

Jian-Hui Liu; Xiang-Jian Wang; Hui Chen

Abstract: To better understand the origin and tectonic evolution of the Paleoproterozoic Jiao-Liao-Ji belt, North China craton, which have long been debated, we have conducted whole-rock geochemical and zircon U-Pb and Hf isotope analyses on Paleoproterozoic monzo(syeno)granitic gneisses and metamafic rocks from eastern Liaoning Province. The results indicate that these monzo(syeno)granitic gneisses and metamafic rocks formed concurrently at ca. 2.2−2.1 Ga. The monzo(syeno)granitic gneisses show geochemical features of A-type granites and were derived from dehydration melting of Archean tonalite-trondhjemite-granodiorite (TTG) with minor involvement of coeval mantle-derived magma in an extensional setting. The metamafic rocks are geochemically similar to mid-ocean-ridge basalt; they were generated by partial melting of isotope-depleted asthenospheric mantle in an intracontinental extensional setting and experienced crustal and continental lithospheric mantle contamination and fractional crystallization. Based on this study and published geological evidence, we propose the following tectonic model for the origin and geodynamic evolution of the Jiao-Liao-Ji belt: (1) Generation of high-density eclogitized lower crust accompanied by formation of voluminous Archean TTGs in the Eastern block; (2) intracontinental extension driven by upwelling of an asthenospheric hotspot along the Jiao-Liao-Ji belt in period of 2.20−2.0 Ga; (3) convergent orogenic processes between the Longgang block and Nangrim block governed by gravity-driven subduction of high-density eclogitized lower crust at ca. 1.95−1.85 Ga; and (4) postorogenic extension caused by delamination of the orogenic root and subsequent hot asthenospheric upwelling. This tectonic model can well explain the origin and geodynamic evolution of the Paleoproterozoic Jiao-Liao-Ji belt.

View article:

High-Ca boninitic melt inclusions in lavas of the Troodos ophiolite and a reappraisal of genetic relationships between different lava types

Wen-Jun Hu; Mei-Fu Zhou; John Malpas; Zhong-Yuan Ren

Abstract: Boninites in many supra-subduction zone ophiolites are thought to mark the early stage of subduction. The Troodos ophiolite in Cyprus contains a variety of volcanic rocks including the lower pillow lavas and the boninite-bearing upper pillow lavas (UPL). Here we present major, trace element and Pb isotope data for melt inclusions in olivine from the UPL on the northern flank of the Troodos ophiolite. Melt inclusions hosted by high-Mg (Fo = 89.4−93.5) olivine grains have typical high-Ca boninitic compositions with high MgO (11.2−18.4 wt%) and SiO2 (51.2−55.0 wt%) and low TiO2 (0.2−0.6 wt%) contents, in line with the existence of boninites in the northern part of the ophiolite. The melt inclusions have trace element compositions similar to the Troodos boninitic lavas and can be regarded as representing the parental magma of the boninites. The boninitic magmas were derived from high-degrees of partial melting of a refractory source at 1400 °C and 1.5 GPa based on thermobarometers and phase equilibria. The melt inclusions have significantly variable Pb isotopic compositions (208Pb/ 206Pb = 2.059−2.122; 207Pb/206Pb = 0.828−0.862) and are enriched in fluid-mobile trace elements (e.g., U, Sr, and Pb), indicating that the refractory source was metasomatized by slab fluids. In comparison, recalculation of literature data suggests that the lower pillow lavas were derived from a less refractory and shallower source. We propose a subduction initiation model in which melting of two unique sources was needed to form the chemostratigraphy of lavas in the Troodos ophiolite.

View article:

Structural geometry and kinematics of thrust belts between the Dabashan and eastern Sichuan Basin, South China block: Constraints from (U-Th)/He dating and seismic data

Guo Lu; Chuanxin Li; Wenzheng Li; Shang Deng; Jianyong Zhang

Abstract: The relationship between the North and South China blocks, particularly their spatial-temporal framework and evolutionary history, has been widely debated. We conducted a comprehensive study of the structural geometry and kinematics of the thrust belts in the intersection zone between the Dabashan and eastern Sichuan Basin based on seismic data, drilling data, field investigation data, and zircon and apatite (U-Th)/He thermochronology data. These data sets provide important insights into the relationship between the North and South China blocks during the post-orogenic process. Due to the detachments, the strata of the eastern Sichuan Basin can be divided into four structural intervals characterized by duplex structures and fault-related fold structures. Balanced restoration indicates the vertical differences are represented by the shortening ratio of the upper structural interval of ∼6%, that of the middle ∼10%, and that of the lower ∼7%. Besides, the shortening ratios indicate an increasing tendency of the deformation intensity from the west to the east. The (U-Th)/He dating results suggest two age ranges, i.e., from 180 Ma to 130 Ma and from 50 Ma to 20 Ma, respectively. These age data sets also imply a younger tendency westward. The intersection zone may have experienced the following stages since the late Mesozoic era: (1) the stable continental sedimentation stage from the Late Triassic to the Late Jurassic epochs; (2) the continuous thrust stage from the Late Jurassic to the early Paleogene epochs, which is associated with the subduction of the Paleo-Pacific Ocean; and (3) the uplift and denudation stage as a whole caused by the uplift of the Tibetan Plateau since the Paleogene period.

View article:

Timing of closure of the Meso-Tethys Ocean: Constraints from remnants of a 141−135 Ma ocean island within the Bangong−Nujiang Suture Zone, Tibetan Plateau

Jian-Jun Fan; Yaoling Niu; Yi-Ming Liu; Yu-Jie Hao

Abstract: The relationship between the North and South China blocks, particularly their spatial-temporal framework and evolutionary history, has been widely debated. We conducted a comprehensive study of the structural geometry and kinematics of the thrust belts in the intersection zone between the Dabashan and eastern Sichuan Basin based on seismic data, drilling data, field investigation data, and zircon and apatite (U-Th)/He thermochronology data. These data sets provide important insights into the relationship between the North and South China blocks during the post-orogenic process. Due to the detachments, the strata of the eastern Sichuan Basin can be divided into four structural intervals characterized by duplex structures and fault-related fold structures. Balanced restoration indicates the vertical differences are represented by the shortening ratio of the upper structural interval of ∼6%, that of the middle ∼10%, and that of the lower ∼7%. Besides, the shortening ratios indicate an increasing tendency of the deformation intensity from the west to the east. The (U-Th)/He dating results suggest two age ranges, i.e., from 180 Ma to 130 Ma and from 50 Ma to 20 Ma, respectively. These age data sets also imply a younger tendency westward. The intersection zone may have experienced the following stages since the late Mesozoic era: (1) the stable continental sedimentation stage from the Late Triassic to the Late Jurassic epochs; (2) the continuous thrust stage from the Late Jurassic to the early Paleogene epochs, which is associated with the subduction of the Paleo-Pacific Ocean; and (3) the uplift and denudation stage as a whole caused by the uplift of the Tibetan Plateau since the Paleogene period.

View article:

GSA BULLETIN articles published ahead of print are online at Representatives of the media may obtain complimentary copies of articles by contacting Kea Giles. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to The Geological Society of America Bulletin in articles published. Non-media requests for articles may be directed to GSA Sales and Service,

# # #

For Immediate Release
29 January 2021
GSA Release No. 21-07

Kea Giles