SEARCH · USGS Science

Results for “Lithosphere”

Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 379 records · Page 21Linked to original sources

USGS1991

Crustal extension in the Baikal rift zone

Crustal extension is evaluated based on the idea of mass and volume balance of material introduced into and removed from the initial volume of the crust. Extension in the Baikal rift increases southwestward from 0.9 km in the Chara depression to 19.3 km in the South Baikal depression. These values generally agree with the position of the Euler pole determined from seismic data (fault plane solutions). Average rotation velocity for the lithospheric plates separated by the rift zone is estimated to be 5.93 × 10 -4 rad/m.y. over about 30 m.y.

Tectonophysics↗

USGS1991

Tectonic development of passive continental margins of the southern and central Red Sea with a comparison to Wilkes Land, Antarctica

The continental margins of the southern and central Red Sea and most of Wilkes Land, Antarctica have bulk crustal configurations and detailed structures that are best explained by a prolonged history of magmatic expansion that followed a brief, but intense period of mechanical extension. Extension on the Red Sea margins was spatially confined to a rift that was 20–30 km in width. The rifting phase along the Arabian margin of the central and southern Red Sea occurred 25–32 Ma ago, primarily by detachment faulting at upper crustal levels and ductile uniform stretching at depth. Rifting was followed by an early magmatic phase during which the margin was invaded by dikes and plutons, primarily of gabbro and diorite, at 20–24 Ma, after the crust was mechanically thinned from 40 km to ≈ 20 km. We infer continued spreading after that in which broad shelves were formed by a process of magmatic expansion, because the offshore crust is only 8–15 km thick, including sediment, and seismic reflection data do not depict horst and graben or half graben structures from which mechanical extension might be inferred. The Wilkes Land margin is similar to the Arabian example. The margin is about 150 km in width, the amount of upper crustal extension is too low to explain the change in sub-sediment crustal thickness from ≈ 35 km on the mainland to < 10 km beneath the margin and reflectors in the deepest seismic sequence are nearly flat lying. Our model requires large volumes of melt in the early stages of continental rifting. The voluminous melt might be partly a product of nearby hot spots, such as Afar and partly the result of an initial period of partial fusion in the deep continental lithosphere under lower temperatures than ordinarily required by dry solidus conditions.

Tectonophysics↗

USGS1993

Seismicity of the Adriatic microplate

The Adriatic microplate was previously considered to be a unique block, tectonically active only along its margins. The seismic sequences that took place in the basin from 1986 to 1990 give new information about the geodynamics of this area. Three subsets of well recorded events were relocated by the joint hypocentre determination technique. On the whole, this seismic activity was concentrated in a belt crossing the southern Adriatic sea around latitude 42°, in connection with regional E-W fault systems. Some features of this seismicity, similar to those observed in other well known active margins of the Adriatic plate, support a model of a southern Adriatic lithospheric block, detached from the Northern one. Other geophysical information provides evidence of a transitional zone at the same latitude.

Tectonophysics↗

USGS1994

The KRISP 90 seismic experiment-a technical review

On the basis of a preliminary experiment in 1985 (KRISP 85), a seismic refraction/wide-angle reflection survey and a teleseismic tomography experiment were jointly undertaken to study the lithospheric structure of the Kenya rift down to depths of greater than 200 km. This report serves as an introduction to a series of subsequent papers and will focus on the technical description of the seismic surveys of the main KRISP 90 effort. The seismic refraction/wide-angle reflection survey was carried out in a 4-week period in January and February 1990. It consisted of three profiles: one extending along the rift valley from Lake Turkana to Lake Magadi, one crossing the rift at Lake Baringo, and one located on the eastern flank of the rift proper. A total of 206 mobile vertical-component seismographs, with an average station interval of about 2 km, recorded the energy of underwater and borehole explosions to distances of up to about 550 km. During the teleseismic survey an array of 65 seismographs was deployed to record teleseismic, regional and local events for a period of about 7 months from October 1989 to April 1990. The elliptical array spanned the central portion of the rift, with Nakuru at its center, and covered an area about 300 × 200 km, with an average station spacing of 10–30 km.

Tectonophysics↗

USGS1974

Bimodal tholeiitic-dacitic magmatism and the Early Precambrian crust

Interlayered plagioclase-quartz gneisses and amphibolites from 2.7 to more than 3.6 b.y. old form much of the basement underlying Precambrian greenstone belts of the world; they are especially well-developed and preserved in the Transvaal and Rhodesian cratons. We postulate that these basement rocks are largely a metamorphosed, volcanic, bimodal suite of tholeiite and high-silica low-potash dacite—compositionally similar to the 1.8-b.y.-old Twilight Gneiss — and partly intrusive equivalents injected into the lower parts of such volcanic piles. We speculate that magmatism in the Early Precambrian involved higher heat flow and more hydrous conditions than in the Phanerozoic. Specifically, we suggest that the early degassing of the Earth produced a basaltic crust and pyrolitic upper mantle that contained much amphibole, serpentine, and other hydrous minerals. Dehydration of the lower parts of a downgoing slab of such hydrous crust and upper mantle would release sufficient water to prohibit formation of andesitic liquid in the upper part of the slab. Instead, a dacitic liquid and a residuum of amphibole and other silica-poor phases would form, according to Green and Ringwood's experimental results. Higher temperatures farther down the slab would cause total melting of basalt and generation of the tholeiitic member of the suite. This type of magma generation and volcanism persisted until the early hydrous lithosphere was consumed. An implication of this hypothesis is that about half the present volume of the oceans formed before about 2.6 b.y. ago.

Precambrian Research↗

USGS1988

Precambrian ophiolites of Arabia: Geologic settings, U Pb geochronology, Pb-isotope characteristics, and implications for continental accretion

Disrupted ophiolites occur in linear belts up to 900 km long between microplates that collided during the late Proterozoic to form the Arabian Shield. UPb zircon ages and Pb-isotope data from these ophiolitic rocks help constrain the history of accretion of the Arabian Shield and thereby contribute to the definition of its microplates and terranes. Terranes of the central and western Arabian Shield are generally thought to represent intraoceanic island arcs that range in age from about 900 to 640 Ma; however, a region of the eastern Arabian Shield contains rocks of Early Proterozoic age and may represent an exotic continental fragment entrained between the arc complexes. Ophiolites of the Yanbu suture (northwestern shield), dated by UPb (zircon) and SmNd (mineral isochron) methods, yield model ages of 740–780 Ma. These are among the oldest well-dated rocks in the northwestern Arabian Shield. Ages from the Jabal al Wask complex overlap with ages of adjacent arc rocks. This overlap in age supports geologic and geochemical evidence that the Wask complex represents a fragment of back-arc oceanic lithosphere formed during arc magmatism. Older ages of about 780 Ma for gabbro from the Jabal Ess ophiolite suggest that the ophiolite is either a fragment of fore-arc oceanic crust or oceanic basement on which an arc was built. Gabbro samples from ophiolites of the Bir Umq suture (west-central Arabian Shield) yield zircons with ages of 820–870 Ma and $ ̆ 1250 Ma "> 1250Ma . The 820–870 Ma dates overlap with ages of the oldest nearby arc rocks; this favors an intra-arc or near-arc paleotectonic setting. The older zircons suggest that middle or early Proterozoic crustal material, possibly derived from the Mozambique belt of Africa, was present during back- or intra-arc magmatism. Plagiogranite from the Bir Tuluhah ophiolitic complex at the northern end of the 900 km-long Nabitah mobile belt was dated by the zircon U Pb method at ∼ 830 Ma. This date is in the range of the oldest dated arc rocks along the northern and central parts of the Nabitah suture, but is ∼ 100 Ma older than the oldest arc plutons (tonalites) associated with the southern part of the belt. These age relations suggest that the northern part of the Nabitah belt contains an extension of the Bir Umq suture that was transposed parallel to the Nabitah trend during collision of the arc terranes of the northwest Arabian Shield with the Afif plate to the east. Feldspar lead-isotope data from the ophiolites are of three types: (1) lead from the ophiolitic rocks and arc tonalites of the northwestern Shield and ophiolitic rocks of the Nabitah suture is similar to lead in modern mid-ocean ridge basalt, (2) anomalous radiogenic data from the Thurwah ophiolite are from rocks that contain zircons from pre-late Proterozoic continental crust, and (3) feldspar from the Urd ophiolite shows retarded uranogenic lead growth and is related either to an anomalous and perhaps primitive oceanic mantle source, or in an unknown manner to ancient continental mantle or lower crust of the eastern Arabian Shield.

Precambrian Research↗

USGS1990

Epithermal gold-siver deposits in the western United States: time-space products of evolving plutonic, volcanic and tectonic environments

The western United States has been the locus of considerable subaerial volcanic and plutonic igneous activity since the mid-Mesozoic. After the destruction of the Jurassic-Cretaceous magmatic arc-trench system, subduction was re-established in the Late Mesozoic with low-angle underthrusting of the oceanic plate beneath western North America. This resulted in crustal shortening during the Late Cretaceous to Early Tertiary and removal of the mantle lithosphere west of the Rocky Mountains. Commencing in the Eocene, flat subduction ceased, the volcanic arc began to re-establish itself along the continental margin, and the hingeline along the steepening subducting plate migrated from east to west. The crust east of the migrating hingeline was exposed to hot asthenosphere, and widespread tectonics and volcanic activity resulted. Hydrothermal activity accompanied the volcanism resulting in numerous epithermal gold-silver deposits. The temporal and spatial distributions of epithermal deposits in the region are therefore systematic and can be subdivided into discrete time intervals which are related to widespread changes in magmatic activity. Time intervals selected for discussion are Pre-Cenozoic, 66-55 Ma, 54-43 Ma, 42-34 Ma, 33-24 Ma, 23-17 Ma, and <17 Ma. Many of these intervals contain both sedimentary-rock and two varieties of volcanic-rock hosted deposits (adularia-sericite and alunite-kaolinite ± pyrophyllite). Continental rifting is important to the formation of deposits, and, within any given region, it is at the initiation of deep rifting that alunite-kaolinite ± pyrophyllite type epithermal deposits are formed. Adularia-sericite type deposits are most common, being related to all compositions and styles of volcanic activity. Therefore, the volcano-tectonic context of the western United States provides a unified framework in which to understand and explore for epithermal type deposits.

Journal of Geochemical Exploration↗

USGS2004

A combined basalt and peridotite perspective on 14 million years of melt generation at the Atlantis Bank segment of the Southwest Indian Ridge: Evidence for temporal changes in mantle dynamics?

Little is known about temporal variations in melt generation and extraction at midocean ridges largely due to the paucity of sampling along flow lines. Here we present new whole-rock major and trace element data, and mineral and glass major element data, for 71 basaltic samples (lavas and dykes) and 23 peridotites from the same ridge segment (the Atlantis Bank segment of the Southwest Indian Ridge). These samples span an age range of almost 14 My and, in combination with the large amount of published data from this area, allow temporal variations in melting processes to be investigated. Basalts show systematic changes in incompatible trace element ratios with the older samples (from ???8-14 Ma) having more depleted incompatible trace element ratios than the younger ones. There is, however, no corresponding change in peridotite compositions. Peridotites come from the top of the melting column, where the extent of melting is highest, suggesting that the maximum degree of melting did not change over this interval of time. New and published Nd isotopic ratios of basalts, dykes and gabbros from this segment suggest that the average source composition has been approximately constant over this time interval. These data are most readily explained by a model in which the average source composition and temperature have not changed over the last 14 My, but the dynamics of mantle flow (active-to-passive) or melt extraction (less-to-more efficient extraction from the 'wings' of the melting column) has changed significantly. This hypothesised change in mantle dynamics occurs at roughly the same time as a change from a period of detachment faulting to 'normal' crustal accretion. We speculate that active mantle flow may impart sufficient shear stress on the base of the lithosphere to rotate the regional stress field and promote the formation of low angle normal faults. ?? 2004 Elsevier B.V. All rights reserved.

Chemical Geology↗

USGS2006

Geodetic observations of post-seismic transients in the context of the earthquake deformation cycle

Satellite geodetic techniques that can measure displacements with millimeter-level accuracy reveal transient signals in the deformation fields produced by both moderate and large earthquakes. These post-seismic signals exhibit characteristic time scales ranging from weeks to decades and distance scales from hundreds of meters to hundreds of kilometers. By considering them in the context of the earthquake deformation cycle, we can test hypotheses about the processes driving them and constrain the rheology of the lithosphere. We discuss three broad categories of mechanism: afterslip in the plane of the co-seismic rupture (analogous to a rubber eraser), fluid flow in the fault zone (analogous to a water-laden sponge), and ductile flow in a weak substrate (analogous to a pot of honey).

Comptes Rendus - Geoscience↗

USGS2024

Influence of inherited structure on flexural extension in foreland basin systems: Evidence from the northern Arkoma basin and southern Ozark dome, USA

Extensional faults are key components of foreland basin systems. They form within the upper crust in response to flexure of the lithosphere and accommodate subsidence within the foredeep and forebulge depozones. Such faults are excellent proxies for orogenic system evolution and control the distribution of natural resources and hazards. However, the spatiotemporal evolution of flexural extension has not been documented previously at a regional scale, thereby limiting our understanding of underlying geodynamic controls. Here, we resolve late Paleozoic flexural extension in the northern Arkoma basin and southern Ozark dome, USA. We synthesize a large database of previous mapping, existing research, subsurface data, and geophysical data into 3D geologic and 2D kinematic models. Mesh surfaces representing several key horizons from the Carboniferous Period (ca. 335-306 Ma) were constructed. These surfaces were built from oil and gas well tops (n = ∼10,000) and surface geologic map contacts using an advanced kriging method. The mesh surfaces are offset by a complex 3D fault network, allowing detailed analysis of along-strike and down-dip variations in fault displacement. Analysis of the 3D model reveals a regular and repeated fault segmentation pattern wherein E -W striking, left- and foreland-stepping en échelon normal faults are segmented by inherited NE striking basement faults. Maximum vertical separation along the E -W normal faults is generally focused between the inherited NE-trending faults. This suggests that the inherited basement faults delocalized extensional strain during late Paleozoic normal faulting. Maximum vertical separation and fault localization may correlate to areas with high-amplitude positive magnetic anomalies interpreted as Mesoproterozoic granitic rocks. Speculative covariance of magnetic anomalies and fault displacements implies that the relatively strong basement granite concentrated stress, leading to localized faulting within the relatively thin sedimentary cover. Lastly, we show that flexural extension migrated southeast to northwest from the Chesterian-Morrowan (ca. 335-319 Ma) to the Desmoinesian (ca. 306 Ma). The migratory flexural extension may be explained by diachronous loading during Pangean assembly, or by synchronous loading but variable load compensation due to inherent factors.

Arkansas, Oklahoma↗

USGS2005

The South China sea margins: Implications for rifting contrasts

Implications regarding spatially complex continental rifting, crustal extension, and the subsequent evolution to seafloor spreading are re-examined for the northern and southern-rifted margins of the South China Sea. Previous seismic studies have shown dramatic differences in the present-day crustal thicknesses as the manifestations of the strain experienced during the rifting of the margin of south China. Although the total crustal extension is presumed to be the same along the margin and adjacent ocean basin, the amount of continental crustal extension that occurred is much less along the east and central segments of the margin than along the western segment. This difference was accommodated by the early formation of oceanic crust (creating the present-day South China Sea basin) adjacent to the eastern margin segment while continued extension of continental crust was sustained to the west. Using the observed cross-sectional areas of extended continental crust derived from deep penetration seismics, two end-member models of varying rift zone widths and varying initial crustal thicknesses are qualitatively examined for three transects. Each model implies a time difference in the initiation of seafloor spreading inferred for different segments along the margin. The two models examined predict that the oceanic crust of the South China Sea basin toward the west did not begin forming until sometime between 6-12 my after its initial formation (???32 Ma) toward the east. These results are compatible with crustal age interpretations of marine magnetic anomalies. Assuming rifting symmetry with conjugate margin segments now residing along the southern portions of the South China Sea basin implies that the total width of the zone of rifting in the west was greater than in the east by about a factor of two. We suggest the most likely causes of the rifting differences were east-west variations in the rheology of the pre-rift crust and associated east-west variations in the thermal structure of the pre-rift lithosphere. The calculated widths of rifted continental crust for the northern and southern margins, when combined with the differential widths of seafloor generated during the seafloor spreading phase, indicate the total crustal extension that occurred is about 1100 km and is remarkably consistent to within ???10% for all three (eastern, central, western) segments examined. ?? 2005 Elsevier B.V. All rights reserved.

Earth and Planetary Science Letters↗

USGS2006

Temporal variations in slip rate of the White Mountain Fault Zone, Eastern California

The evolution of fault slip through time may yield insight into the geodynamics of deforming lithosphere. Precise determination of temporal variations in fault slip is often hindered, however, by a dearth of markers of varying age from which to reconstruct fault slip. Here we determine slip rates across the White Mountain Fault Zone over the past ca. 0.8??Ma from displaced alluvial deposits preserved along the flank of the White Mountains. Displacement histories inferred from deposits containing the ??? 760??ka Bishop Tuff contrast strongly with those inferred from Late Pleistocene alluvial fans dated by cosmogenic 36Cl, indicating that the fault has experienced significant temporal variations in slip rate. Oblique-slip rates over the past ??? 760??ky are determined to have been > 0.9??m/ky parallel to a net slip vector plunging shallowly (< 20??) toward 340??-350??. Displacement of markers in Late Pleistocene alluvial deposits, in contrast, yield slip rates between 0.3 and 0.4??m/ky parallel to a vector plunging somewhat more steeply (20??-40??) toward 320??-340??. These variations in slip rate appear to have operated over timescales > ca. 70??ky, yet occurred on a well-established fault system. Moreover, the timing and magnitude of slip rate variation mimics behavior documented for the northern Fish Lake Valley fault zone. Together, our data provide evidence for coordinated slip rate variations across the Eastern California Shear Zone during the Pleistocene. ?? 2006 Elsevier B.V. All rights reserved.

Earth and Planetary Science Letters↗

USGS2016

Large along-strike variations in the onset of Subandean exhumation: Implications for Central Andean orogenic growth

Plate tectonics drives mountain building in general, but the space-time pattern and style of deformation is influenced by how climate, geodynamics, and basement structure modify the orogenic wedge. Growth of the Subandean thrust belt, which lies at the boundary between the arid, high-elevation Central Andean Plateau and its humid, low-elevation eastern foreland, figures prominently into debates of orogenic wedge evolution. We integrate new apatite and zircon (U-Th)/He thermochronometer data with previously published apatite fission-track data from samples collected along four Subandean structural cross-sections in Bolivia between 15° and 20°S. We interpret cooling ages vs. structural depth to indicate the onset of Subandean exhumation and signify the forward propagation of deformation. We find that Subandean growth is diachronous south (11 ± 3 Ma) vs. north (6 ± 2 Ma) of the Bolivian orocline and that Subandean exhumation magnitudes vary by more than a factor of two. Similar north-south contrasts are present in foreland deposition, hinterland erosion, and paleoclimate; these observations both corroborate diachronous orogenic growth and illuminate potential propagation mechanisms. Of particular interest is an abrupt shift to cooler, more arid conditions in the Altiplano hinterland that is diachronous in southern Bolivia (16-13 Ma) vs. northern Bolivia (10-7 Ma) and precedes the timing of Subandean propagation in each region. Others have interpreted the paleoclimate shift to reflect either rapid surface uplift due to lithosphere removal or an abrupt change in climate dynamics once orographic threshold elevations were exceeded. These mechanisms are not mutually exclusive and both would drive forward propagation of the orogenic wedge by augmenting the hinterland backstop, either through surface uplift or spatially variable erosion. In summary, we suggest that diachronous Subandean exhumation was driven by piecemeal hinterland uplift, orography, and the outward propagation of deformation.

Andes↗

USGS2018

Rapid late Miocene surface uplift of the Central Anatolian Plateau margin

The Central Anatolian Plateau (CAP), Turkey, is bordered to its south by a steep mountain belt that emerged ∼8–7 Ma ago from the Mediterranean Sea. Knowledge of the onset, duration and rate of surface uplift and orographic barrier formation along the plateau margin is crucial for understanding the geodynamic drivers of plateau uplift. We present a new comprehensive data set that includes 12 40 Ar/ 39 Ar ages and lacustrine carbonate δ 18 "> δ18 O data ( n = 637 "> n=637 ) from 13 sections in upper Oligocene to Pliocene continental basins of the CAP interior. We aim at documenting the development of a rain shadow and therefore the surface uplift history of the CAP and its southern margin (Tauride Mts.). In the rain shadow of the Tauride Mts. we observe a gradual 3.9‰ decrease of δ 18 "> δ18 O values of lacustrine carbonate between ∼11 and 5 Ma that we interpret to originate from a similar change in δ 18 "> δ18 O values of precipitation owing to the late Miocene development of an orographic barrier. Our stable isotope paleoaltimetry data show that by 5 Ma the southern CAP margin had reached similar-to-present elevations of ∼2 km. Surface uplift was coeval with ignimbritic magmatism, forearc shortening and distributed compression. We suggest that the removal of lithospheric mantle below Anatolia led to surface uplift of the CAP interior, which was followed by surface uplift of the southern CAP margin due to crustal thickening as a result of northward subduction of the African plate below central Anatolia.

Earth and Planetary Science Letters↗

USGS2024

Revised timing of rapid exhumation in the West Qinling: Implications for geodynamics of Oligocene-Miocene Tibetan plateau outward expansion

Two contrasting age models for initial mountain building in the northeastern (NE) Tibetan Plateau (Paleocene-early Eocene versus late Oligocene-early Miocene) have led to the debate on how the deformed continental lithosphere absorbs plate convergence in general. The initial compressional deformation in the West Qinling (WQL) of the NE Tibetan Plateau figures prominently in this ongoing debate. Here, apatite (U-Th)/He (AHe) thermochronology combined with geomorphological analysis are used to refine the onset of compressional deformation in the WQL. New AHe ages from two vertical transects and an updated reconstruction of an obliquely-tilted erosion surface document the accelerated exhumation in the northern WQL at 23-22 Ma, interpreted as the onset of north-vergent thrusting. The AHe results, together with sedimentary records in the intermontane and foreland basins, suggest that the entire WQL began experiencing compressional deformation in the late Oligocene-early Miocene. When integrated with previous studies, our findings show that the northern plateau boundary has not remained stationary since the collision, but has instead experienced ∼750 km of outward expansion during the late Oligocene to middle Miocene. This phase of rapid plateau growth is coeval with the ∼30–50 % reduction of the India-Eurasia convergence rate, which suggests that the increased gravitational potential energy of orogenic belts played a key role in plate motion changes.

Tibetan plateau↗

USGS2026

Patterns of rift basin development and the fidelity of the subsidence record: Insights through Bayesian modeling of rapid tectonic subsidence in a Rio Grande rift basin, Socorro, NM, U.S.A

Characterizing tectonic subsidence rates within depositional sequences provides direct insight into the driving mechanism(s) of accommodation in a basin. However, the temporal resolution of this record is often stymied by a lack of high-precision and high-resolution ages, which enable a more complete description of basin subsidence drivers. We explore the effect of high-precision and high-resolution ages in modeling accommodation for the Miocene La Jencia Basin of the central Rio Grande rift (RGR) and interpret driving mechanism behavior from these models (e.g., lithospheric thinning). We present a new geochronologic dataset of both laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) and chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) data and use these ages in Bayesian accommodation modeling. Models constrained by high-precision and high-accuracy TIMS ages yield peak tectonic subsidence rates exceeding 220 m/Myr, and an average Miocene subsidence of ∼120 m/Myr. While timing and magnitude vary, all models suggest two pulses of rapid Miocene tectonic subsidence, which we interpret to reflect basin-bounding fault movement. Prior to peak subsidence, there was an initial period of fault linkage and organization that occurred over <1–3 Myr that produced the basin-bounding La Jencia-Cerro Colorado fault zone. A comparison of published tectonic subsidence rates to those modeled here shows that while tectonic subsidence during continental rifting is highly variable, the La Jencia Basin rates appear relatively high. However, the significant difference between peak and average La Jencia Basin rates modeled here highlights the potential for underestimation of many records of tectonic subsidence due to a lack of high-precision and high-resolution age constraints. Furthermore, age data and modeling results presented here document fault movement and consequent rates of tectonic subsidence that lower-resolution data would not, providing a high-fidelity case study of continental rift basin development.

New Mexico↗

USGS2016

Isotopic constraints on the genesis and evolution of basanitic lavas at Haleakala, Island of Maui, Hawaii

To understand the dynamics of solid mantle upwelling and melting in the Hawaiian plume, we present new major and trace element data, Nd, Sr, Hf, and Pb isotopic compositions, and 238 U– 230 Th– 226 Ra and 235 U– 231 Pa– 227 Ac activities for 13 Haleakala Crater nepheline normative basanites with ages ranging from ∼900 to 4100 yr B.P. These basanites of the Hana Volcanics exhibit an enrichment in incompatible trace elements and a more depleted isotopic signature than similarly aged Hawaiian shield lavas from Kilauea and Mauna Loa. Here we posit that as the Pacific lithosphere beneath the active shield volcanoes moves away from the center of the Hawaiian plume, increased incorporation of an intrinsic depleted component with relatively low 206 Pb/ 204 Pb produces the source of the basanites of the Hana Volcanics. Haleakala Crater basanites have average ( 230 Th/ 238 U) of 1.23 ( n = 13), average age-corrected ( 226 Ra/ 230 Th) of 1.25 ( n = 13), and average ( 231 Pa/ 235 U) of 1.67 ( n = 4), significantly higher than Kilauea and Mauna Loa tholeiites. U-series modeling shows that solid mantle upwelling velocity for Haleakala Crater basanites ranges from ∼0.7 to 1.0 cm/yr, compared to ∼10 to 20 cm/yr for tholeiites and ∼1 to 2 cm/yr for alkali basalts. These modeling results indicate that solid mantle upwelling rates and porosity of the melting zone are lower for Hana Volcanics basanites than for shield-stage tholeiites from Kilauea and Mauna Loa and alkali basalts from Hualalai. The melting rate, which is directly proportional to both the solid mantle upwelling rate and the degree of melting, is therefore greatest in the center of the Hawaiian plume and lower on its periphery. Our results indicate that solid mantle upwelling velocity is at least 10 times higher at the center of the plume than at its periphery under Haleakala.

Hawaii↗

USGS2016

Geologic and geochemical insights into the formation of the Taiyangshan porphyry copper–molybdenum deposit, Western Qinling Orogenic Belt, China

Taiyangshan is a poorly studied copper–molybdenum deposit located in the Triassic Western Qinling collisional belt of northwest China. The intrusions exposed in the vicinity of the Taiyangshan deposit record episodic magmatism over 20–30 million years. Pre-mineralization quartz diorite porphyries, which host some of the deposit, were emplaced at 226.6 ± 6.2 Ma. Syn-collisional monzonite and quartz monzonite porphyries, which also host mineralization, were emplaced at 218.0 ± 6.1 Ma and 215.0 ± 5.8 Ma, respectively. Mineralization occurred during the transition from a syn-collisional to a post-collisional setting at ca. 208 Ma. A barren post-mineralization granite porphyry marked the end of post-collisional magmatism at 200.7 ± 5.1 Ma. The ore-bearing monzonite and quartz monzonite porphyries have a ε Hf (t) range from − 2.0 to + 12.5, which is much more variable than that of the slightly older quartz diorite porphyries, with T DM2 of 1.15–1.23 Ga corresponding to the positive ε Hf (t) values and T DM1 of 0.62–0.90 Ga corresponding to the negative ε Hf (t) values. Molybdenite in the Taiyangshan deposit with 27.70 to 38.43 ppm Re suggests metal sourced from a mantle–crust mixture or from mafic and ultramafic rocks in the lower crust. The δ 34 S values obtained for pyrite, chalcopyrite, and molybdenite from the deposit range from + 1.3&permil; to + 4.0&permil;, + 0.2&permil; to + 1.1&permil;, and + 5.3&permil; to + 5.9&permil;, respectively, suggesting a magmatic source for the sulfur. Calculated δ 18 O fluid values for magmatic K-feldspar from porphyries (+ 13.3&permil;), hydrothermal K-feldspar from stockwork veins related to potassic alteration (+ 11.6&permil;), and hydrothermal sericite from quartz–pyrite veins (+ 8.6 to + 10.6&permil;) indicate the Taiyangshan deposit formed dominantly from magmatic water. Hydrogen isotope values for hydrothermal sericite ranging from − 85 to − 50&permil; may indicate that magma degassing progressively depleted residual liquid in deuterium during the life of the magmatic–hydrothermal system. Alternatively, δD variability may have been caused by a minor amount of mixing with meteoric waters. We propose that the ore-related magma was derived from partial melting of the ancient Mesoproterozoic to Neoproterozoic middle to lower continental crust. This crust was likely metasomatized during earlier subduction, and the crustal magmas may have been contaminated with lithospheric mantle derived magma triggered by MASH (e.g., melting, assimilation, storage, and homogenization) processes during collisional orogeny. In addition, a significant proportion of the metals and sulfur supplied from mafic magma were simultaneously incorporated into the resultant hybrid magmas.

Gondwana Research↗