Lithospheric Drip Research Articles

Seismicity in the central and southeastern United States due to upper mantle heterogeneities

SUMMARYSources of stress responsible for earthquakes occurring in the Central and Eastern United States (CEUS) include not only far-field plate boundary forces but also various local contributions. In this study, we model stress fields due to heterogeneities in the upper mantle beneath the CEUS including a high-velocity feature identified as a lithospheric drip in a recent regional P-wave tomography study. We calculate velocity and stress distributions from numerical models for instantaneous 3-D mantle flow. Our models are driven by the heterogeneous density distribution based on a temperature field converted from the tomography study. The temperature field is utilized in a composite rheology, assumed for the numerical models. We compute several geodynamic quantities with our numerical models: dynamic topography, rate of dynamic topography, gravitational potential energy (GPE), differential stress, and Coulomb stress. We find that the GPE, representative of the density anomalies in the lithosphere, is an important factor for understanding the seismicity of the CEUS. When only the upper mantle heterogeneities are included in a model, differential and Coulomb stress for the observed fault geometries in the CEUS seismic zones acts as a good indicator to predict the seismicity distribution. Our modelling results suggest that the upper mantle heterogeneities and structure below the CEUS have stress concentration effects and are likely to promote earthquake generation at preexisting faults in the region’s seismic zones. Our results imply that the mantle flow due to the upper-mantle heterogeneities can cause stress perturbations, which could help explain the intraplate seismicity in this region.

Lithosphere Destabilization and Small‐Scale Convection Constrained From Geophysical Data and Analogical Models

AbstractThe destabilization of oceanic lithosphere by small scale convection at its base is important for providing a holistic picture of mantle/lithosphere coupling. We use three highly resolved tomography models to characterize the base of the oceanic lithosphere in the Pacific Ocean. Regions associated with abnormally thick lithosphere are associated with seafloor older than 100 Ma and are elongated parallel to the direction of present‐day Pacific plate motion. They are correlated with bathymetric lows and negative geoid anomalies (for l = 10–39 and l = 14–39), which can be accounted for by dynamic topography. They do not correlate with volcanic features. We interpret these regions of thickened lithosphere as evidence for sites of lithospheric instabilities where denser lithosphere detaches and sinks into the underlying mantle. To understand the phenomena at the origin of these lithospheric “drips,” we performed laboratory experiments. Fluids with different properties are heated from one side to generate a large‐scale convection and cooled from the top. This configuration results in the generation of small‐scale convection at the base of the upper cold thermal boundary layer. The experimental results show the existence of two possible structures: instabilities organized into longitudinal rolls, aligned in the direction of the large‐scale motion and 3D, time‐dependent cold plumes that drip from the base of the lithosphere and are sheared away by the large‐scale flow. The 3D plume morphology is similar to what we observe in tomography models. This provides insights into the phenomenology at the origin of the lithospheric drips observed in the geophysical data.

Tracking decratonization process along a cratonic edge through late Permian to late Triassic magmatic flare-up in northwestern Liaoning, North China Craton

Resolving magmatic tempos of multiple scales along convergent plate margins presents a pivotal task in any regional tectonic and supercontinental reconstructions, as is of paramount importance for the northern North China Craton (NCC) that converged with the Central Asian Orogenic belt (CAOB) during the Paleozoic to early Mesozoic. This study applies the integrated zircon U–Pb dating and geochemical tracing to late Permian to late Triassic intrusive flare-up in northwestern Liaoning, with three episodes of granitic suites deciphered. Their contrasting elemental and isotopic features called for three distinct crustal end-members involved in their formation: ancient amphibolitic protolith, Paleozoic-accreted mafic meta-igneous ingredients and relaminated juvenile intermediate arc plutons. Sequential crustal anatexis of these multiply juxtaposed protoliths in a felsic hot zone tends to yield successive parental magmas for forming three granite suites. While the late Permian (260–250 Ma) granites, with their felsic adakitic affinity and variably evolved isotopic compositions (87Sr/86Sri = 0.70637 to 0.70659, εNd(t) = −16.7 to −10.2, zircon εHf(t) = −18.1 to +1.1), are consistent with partial melts of major ancient crustal materials and minor newly-underplated mafic ingredients, the middle Triassic (242–240 Ma) granites show a more juvenile isotope signature (εNd(t) = −11.8 to −4.7, zircon εHf(t) = −14.7 to +4.1) and attest to distinctively higher input from newly-underplated mafic lower crust in their formation. By contrast, the late Triassic (227–223 Ma) granites exhibit an A-type magmatic affinity. Their radiogenic whole-rock Nd (εNd(t) = −3.7 to +1.1) and zircon Hf (εHf(t) = +0.7 to +12.0) isotopic values suggest magma derivation from high-temperature fusion of dominantly relaminated juvenile charnockitic protolith. What follows is one phase of shoshonitic mafic dykes that fingerprint a slab-melt metasomatized asthenosphere source with depleted isotopic compositions (87Sr/86Sri = 0.70374 to 0.70388, εNd(t) = +3.5 to +4.1). Synthesizing these events with the previously recognized ones leads to a first-order duration of ca. 260–218 Ma for the flare-up and its further differentiation into three secondary pulses including ca. 260–250 Ma appinite and granite, ca. 242–238 Ma ferroan granites and ca. 227–218 Ma A-type granites and mafic dykes. While such primary duration is typical of post- collisional/orogenic magmatic tempos of convergent continental margins, three secondary pulses encapsulate successive geodynamic processes from crustal thickening through orogenic collapse to lithospheric dripping in the aftermath of the Paleo-Asian oceanic closure. Featuring a coupled scenario of lithospheric thinning and crustal growth, the northwestern Liaoning case of cratonic-edge destruction serves as a prelude but also a contrast to the late Mesozoic large-scale decratonization across the eastern NCC possibly due to oceanic subduction of the paleo-Pacific plate.

Calibrating the Yield Strength of Archean Lithosphere Based on the Volume of Tonalite-Trondhjemite-Granodiorite Crust

Lithospheric yield stress is a key parameter in controlling tectonic processes. Using high resolution, 2D numerical modelling, we calculate the yield stress for a range of conditions appropriate to the early-to-mid Archean Earth, including hotter mantle potential temperatures and Moho temperatures. We then evaluate its effect on generating tonalite-trondhjemite-granodiorite (TTG) crust and benchmark the results against the preserved igneous rock record. Our results indicate that lithospheric yield stress values lower than the present-day values (i.e. ≤ 100 MPa) can generate TTG volumes similar to those preserved in the rock record. The models further highlight the dominance of lithospheric dripping in producing the Archean TTGs. Large volumes of TTG melts form within the thin, tail portions of the drips as these regions are more efficiently heated by the enclosing hotter mantle. In contrast, only limited melting occurs within the thickened parts of lithosphere as they are significantly weak and cannot sustain crustal thickening for long time periods, resulting in its removal via dripping. This study, therefore, reaffirms the dominance of non-plate tectonic mechanisms in producing TTGs under the conditions that operated on the hotter Archean Earth.

Evolution of the Oceanic Lithosphere in the Equatorial Atlantic From Rayleigh Wave Tomography, Evidence for Small‐Scale Convection From the PI‐LAB Experiment

AbstractThe oceanic lithosphere is a primary component of the plate tectonic system, yet its evolution and its asthenospheric interaction have rarely been quantified by in situ imaging at slow spreading systems. We use Rayleigh wave tomography from noise and teleseismic surface waves to image the shear wave velocity structure of the oceanic lithosphere‐asthenosphere system from 0 to 80 My at the equatorial Mid‐Atlantic Ridge using data from the Passive Imaging of the Lithosphere‐Asthenosphere Boundary (PI‐LAB) experiment. We observe fast lithosphere (VSV > 4.4 km/s) that thickens from 20–30 km near the ridge axis to ~70 km at seafloor >60 My. We observe several punctuated slow velocity anomalies (VSV < 4.1 km/s) in the asthenosphere between 50 and 150 km depth, not necessarily focused beneath the ridge axis. Some of the slow velocity regions are located within 100 km of the ridge axis, but other slow velocity regions are observed at distances > 400 km from the ridge. We observe a high velocity lithospheric downwelling drip beneath 30 My seafloor that extends to 80–130 km depth. The asthenospheric slow velocities likely require partial melt. Although melt is present off axis, the lack of off‐axis volcanism suggests the lithosphere acts as a permeability boundary for deeper melts. The punctuated and off‐axis character of the asthenospheric anomalies and lithospheric drip suggests small‐scale convection is active at a range of seafloor ages. Small‐scale convection and/or more complex mantle flow may be aided by the presence of large offset fracture zones and/or the presence of melt and its associated low‐viscosities and enhanced buoyancies.

Corona structures driven by plume–lithosphere interactions and evidence for ongoing plume activity on Venus

In the absence of global plate tectonics, mantle convection and plume–lithosphere interaction are the main drivers of surface deformation on Venus. Among documented tectonic structures, circular volcano-tectonic features known as coronae may be the clearest surface manifestations of mantle plumes and hold clues to the global Venusian tectonic regime. Yet, the exact processes underlying coronae formation and the reasons for their diverse morphologies remain controversial. Here we use three-dimensional thermomechanical numerical simulations of impingement of a thermal mantle plume on the Venusian lithosphere to assess the origin and diversity of large Venusian coronae. The ability of the mantle plume to penetrate into the Venusian lithosphere results in four main outcomes: lithospheric dripping, short-lived subduction, embedded plume and plume underplating. During the first three scenarios, plume penetration and spreading induce crustal thickness variations that eventually lead to a final topographic isostasy-driven topographic inversion from circular trenches surrounding elevated interiors to raised rims surrounding inner depressions, as observed on many Venusian coronae. Different corona structures may represent not only different styles of plume–lithosphere interactions but also different stages in evolution. A morphological analysis of large existing coronae leads to the conclusion that at least 37 large coronae (including the largest Artemis corona) are active, providing evidence for widespread ongoing plume activity on Venus. Thermomechanical modelling shows that the formation and diverse morphologies of coronae on Venus can be explained by interactions between the lithosphere and impinging mantle plumes. Some corona structures are consistent with ongoing plume activity.

Generation of the 105–100 Ma Dagze volcanic rocks in the north Lhasa Terrane by lower crustal melting at different temperature and depth: Implications for tectonic transition

AbstractSuccessively erupted intermediate-felsic rocks with variations in their geochemical compositions indicate physical changes in lower-crust conditions, and the variations can provide important insights into the regional tectonic setting. What triggered the late Early Cretaceous tectonic transition of the central-north Lhasa Terrane remains controversial, hindering the understanding of the mechanisms behind the formation of the central Tibetan Plateau. The sodic Dagze volcanic rocks in the north Lhasa Terrane are characterized by high contents of SiO2 and Na2O, low contents of MgO, Fe2O3, and K2O, and low values of Mg#. However, the trace element compositions of the whole-rocks and their zircons allow the rocks to be divided into two groups. The Group I rocks (ca. 105 Ma) have higher contents of Sr and Ba, higher Sr/Y and La/Yb ratios, and lower contents of Y, Yb, Ti, and Zr than Group II rocks (ca. 100 Ma). Besides, the zircons from Group I rocks have higher values of Yb/Gd and U/Yb, lower values of Th/U, and lower Ti contents than the zircons from Group II rocks. However, the rocks of both groups have identical depleted whole-rock Sr-Nd and zircon Hf isotope values. The geochemical data indicate that rocks of both groups were generated by partial melting of a juvenile lower crust, but the differences in the two groups reflect a transition from deep-cold melting to relatively shallower-hotter melting in the period from ca. 105 to 100 Ma. This transition was synchronous with the rapid cooling of granitoids, topographic uplift, and the shutdown of magmatism in the central-north Lhasa Terrane, and followed by sedimentation and the resumption of magmatism in the south Lhasa Terrane. The above observations collectively indicate that the central-north Lhasa Terrane was under an extensional setting in late Early Cretaceous, and we tentatively suggest that it was in response to lithospheric drip during roll-back of the northward-subducting Neo-Tethyan oceanic plate.

Reconciling thermal regimes and tectonics of the early Earth

Abstract Thermomechanical models of mantle convection and melting in an inferred hotter Archean Earth show the emergence of pressure-temperature (P-T) regimes that resemble present-day plate tectonic environments yet developed within a non–plate tectonics regime. The models’ P-T gradients are compatible with those inferred from evolving tonalite-trondhjemite-granodiorite series rocks and the paired metamorphic belt record, supporting the feasibility of divergent and convergent tectonics within a mobilized, yet laterally continuous, lithospheric lid. “Hot” P-T gradients of 10–20 °C km–1 form along asymmetric lithospheric drips, then migrate to areas of deep lithospheric downwelling within ∼300–500 m.y., where they are overprinted by high-pressure warm and, later, cold geothermal signatures, up to ∼8 °C km–1. Comparisons with the crustal production and reworking record suggest that this regime emerged in the Hadean.

Olivine-hosted melt inclusions in Pliocene–Quaternary lavas from the Qorveh–Bijar volcanic belt, western Iran: implications for source lithology and cooling history

ABSTRACT Silicate melt inclusions (SMIs) are small droplets of magma that become trapped in minerals during crystal growth. SMIs in olivine crystals can provide critical information on the range of melt compositions and processes that occur during melt generation, evolution, transport, and eruption. The Pliocene–Quaternary volcanic rocks in the Qorveh–Bijar volcanic belt of western Iran show porphyritic and microlithic textures, with olivine and clinopyroxene being the dominant minerals. Magnesian olivines in these volcanic rocks contain primary SMIs. The composition and characteristic of olivine-hosted SMI of these rocks are investigated to constrain the source lithology for mafic volcanism. Bulk compositions of the SMIs overlap those of their host rocks and extend to higher CaO/Al2O3 values. The estimated entrapment pressures and temperatures of the studied SMIs are 9.1–10.3 kbar and 1220–1355°C. The calculated mafic parental melt contains 42.36 wt.% SiO2, low total alkalis (3.22 wt.%), and high MgO (16.1 wt.%). Exploratory calculations using pMELTS show that this parental composition underwent variable degrees of fractional crystallization, as reflected by the variable compositions of the SMIs. Several lines of evidence including pyroxene xenocrysts and high FeO/MnO, FC3MS (FeO/CaO – 3*MgO/SiO2), and Zn/Fe ratios (14–21), suggest that a metasomatized pyroxenitic source contributed to the genesis of the parental melt. Amphibole in the SMIs indicates a high volatile content in the parental melt, which we conclude was generated from a metasomatized lithospheric mantle source. The pyroxenite source also contained garnet. Our geochemical results lead us to propose a new petrogenetic model. Specifically, we infer that a dense and unstable portion of the lithosphere underwent localized laminar detachment and downward flow, i.e. lithospheric drip. This drip underwent volatile-enhanced partial melting during descent through the underlying hot asthenosphere and generated the studied volcanic rocks.

Early Cretaceous gabbro–granite complex from central Inner Mongolia: Insights into initial rifting and crust–mantle interaction in the northern China–Mongolia basin–range tract

Coeval high-K calc-alkaline to shoshonitic bimodal igneous suites hold a key to monitoring important geodynamic processes and crust–mantle interactions within post-collisional to within-plate extensional settings. Here we report a unique early Cretaceous gabbro–granite complex from the poorly exposed Erenhot basin region of central Inner Mongolia, eastern Central Asian Orogenic Belt (CAOB). The mafic suite includes small syenogabbro to monzodiorite stocks, while the felsic suite mainly comprises syenogranites with minor mafic to intermediate microgranular enclaves. The former exhibits a transitional character from alkali-calcic to alkalic, notable enrichment in large ion lithophile elements (LILEs) and weak to moderate depletion in high field strength elements (HFSEs), and possesses low initial 87Sr/86Sr ratios (0.70486–0.70537), positive εNd(t) (+2.1 − +2.6) and zircon εHf(t) (+2.3 − +9.0) values. These features suggest its derivation from low-degree melting of amphibole-bearing lherzolites in the lithospheric mantle that had been metasomatized by subducting slab-derived fluids. The host granites are magnesian, alkali-calcic and peraluminous. They show typical trace elemental features of arc-related felsic melts, with initial 87Sr/86Sr ratios of 0.70499 to 0.70593, εNd(t) of −0.1 to +1.4, zircon εHf(t) of +4.8 to +8.3 and δ18O values from +4.53 to +6.60‰. These elemental and isotopic traits are consistent with partial melts of a mixed protolith composed of newly-underplated basaltic materials and extant older lower crust. The monzonitic enclaves are alkalic, highly enriched in LILEs and depleted in HFSEs, with variable whole-rock SrNd (ISr(t) = 0.70216–0.704678, εNd(t) = +0.2 − +1.8) and zircon Hf (εHf(t) = +0.9 − +7.1) isotopic values. These transitional compositions are compatible with hybridization processes between mantle-derived mafic magma and crust-derived felsic magma. Given its temporal coincidence with initial rifting in the regional basin formation and the closure of Mongol–Okhotsk ocean, Early Cretaceous Erenhot gabbro–granite complex could not only serve as spatial markers for relating magmatic underplating with basin formation, but also catch a unique snapshot into crust–mantle interactions within a post-orogenic extensional regime probably underpinned by lithospheric dripping and gravitational collapse after the Mongol–Okhotsk suturing.

Drip tectonics and the enigmatic uplift of the Central Anatolian Plateau

Lithospheric drips have been interpreted for various regions around the globe to account for the recycling of the continental lithosphere and rapid plateau uplift. However, the validity of such hypothesis is not well documented in the context of geological, geophysical and petrological observations that are tested against geodynamical models. Here we propose that the folding of the Central Anatolian (Kırşehir) arc led to thickening of the lithosphere and onset of “dripping” of the arc root. Our geodynamic model explains the seismic data showing missing lithosphere and a remnant structure characteristic of a dripping arc root, as well as enigmatic >1 km uplift over the entire plateau, Cappadocia and Galatia volcanism at the southern and northern plateau margins since ~10 Ma, respectively. Models show that arc root removal yields initial surface subsidence that inverts >1 km of uplift as the vertical loading and crustal deformation change during drip evolution.

Petrogenesis of the Middle Triassic Erenhot granitoid batholith in central Inner Mongolia (northern China) with tectonic implication for the Triassic Mo mineralization in the eastern Central Asian Orogenic Belt

Voluminous post-orogenic magmatism holds a key to monitoring important geodynamic processes and crustal evolution within an extensional accretionary orogen. The Middle Triassic Erenhot granitoid batholith from central Inner Mongolia, northern China, is tectonically located in the eastern Central Asian Orogenic Belt. This batholith consists of monzogranite, syenogranite, alkali-feldspar granite and albite granite with minor mafic to felsic enclaves and dykes. The formation of this composite batholith involved distinct mafic and felsic magma end-members, concomitant episodic mafic-felsic interaction and intra-felsic magma evolution. Mafic (diabasic) and high-Mg intermediate (dioritic) dykes show enrichment in large ion lithophile elements, depletion in high field strength elements and heterogeneous isotopic compositions, indicative of a metasomatized lithospheric mantle source inherited from prior oceanic slab subduction. The monzogranites and aplites have arc-like elemental features and variable isotopic compositions (ISr(t) = 0.706132, εNd(t) = −0.4, zircon εHf(t) = 2.8 to 7.4, zircon δ18O = 5.32 to 6.74‰), clearly suggesting that the felsic magma end-member of a dual heritage was derived from dominant newly-underplated mafic crust and subordinate ancient lower crust. Hybridization processes between mantle-derived mafic magma and crust-derived felsic magma result in the formation of monzonitic enclaves (ISr(t) = 0.704850, εNd(t) = −0.1) with an isotopic resemblance to the host granites. Fractional crystallization of the monzogranitic magmas endows the differentiated syenogranites and alkali-feldspar granites with highly evolved elemental characteristics. We further deduce that the magmatic immiscibility between K-rich and Na-rich residual melts in a deep reservoir produces albite granites with distinct deformation imprints, variable K2O contents and wide range of crystallization temperatures. Combining with coeval regional igneous episodes as well as concomitant sedimentary and structural events in the northern China-Mongolia tract, the Triassic Erenhot composite batholith can be considered as post-orogenic magmatic products possibly within a coupled geodynamic scenario between lithospheric dripping and concurrent gravitational collapse. Therefore, the documentation of this large-scale Triassic post-orogenic magmatism could not only lead to the characterization of the eventual closure of the Paleo-Asian Ocean, but also capture a snapshot into the tectonic background for the widespread porphyry-type Mo mineralization in the eastern Central Asian Orogenic Belt during early Mesozoic.

Evolution of the East African rift: Drip magmatism, lithospheric thinning and mafic volcanism

The origin of the Ethiopian-Yemeni Oligocene flood basalt province is widely interpreted as representing mafic volcanism associated with the Afar mantle plume head, with minor contributions from the lithospheric mantle. We reinterpret the geochemical compositions of primitive Oligocene basalts and picrites as requiring a far more significant contribution from the metasomatized subcontinental lithospheric mantle than has been recognized previously. This region displays the fingerprints of mantle plume and lithospheric drip magmatism as predicted from numerical models. Metasomatized mantle lithosphere is not dynamically stable, and heating above the upwelling Afar plume caused metasomatized lithosphere with a significant pyroxenite component to drip into the asthenosphere and melt. This process generated the HT2 lavas observed today in restricted portions of Ethiopia and Yemen now separated by the Red Sea, suggesting a fundamental link between drip magmatism and the onset of rifting. Coeval HT1 and LT lavas, in contrast, were not generated by drip melting but instead originated from shallower, dominantly anhydrous peridotite. Looking more broadly across the East African Rift System in time and space, geochemical data support small volume volcanic events in Turkana (N. Kenya), Chyulu Hills (S. Kenya) and the Virunga province (Western Rift) to be derived ultimately from drip melting. The removal of the gravitationally unstable, metasomatized portion of the subcontinental lithospheric mantle via dripping is correlated in each case with periods of rapid uplift. The combined influence of thermo-mechanically thinned lithosphere and the Afar plume together thus controlled the locus of continental rift initiation between Africa and Arabia and provide dynamic support for the Ethiopian plateau.

Receiver Function and Geometric Tomography along the Monterey Microplate to Test Slab Delamination or Lithospheric Drip Models of the Isabella Anomaly, California

Abstract Two hypotheses were tested for the origin of the Isabella tomographic anomaly, which has been interpreted as either a lithospheric drip (Jones et al. , 1994; Ducea and Saleeby, 1998) or a remnant of the Farallon plate, possibly attached to the Monterey microplate (Wang et al. , 2013). P ‐wave receiver functions and travel‐time residuals based on teleseismic events recorded by 41 stations were used to construct simple geometric tomography models of the Isabella anomaly and to test whether the Monterey microplate, or a similar fragment of the Farallon slab, can be connected with the location of the anomaly. The travel‐time residual pattern was modeled with a slab geometry composed of a high‐velocity rectangular block, and a lithospheric drip was modeled as a dipping cylindrical anomaly. Both gave statistically similar fits, but the cylindrical block appears less well constrained than the rectangular block. The top surface of the best‐fit rectangular block was located at 50 km below the Great Valley and dips 65° northeast toward the Sierra Nevada, with 100 km thickness. West of the Isabella anomaly, receiver‐function P ‐to‐ S converted phases from the top and bottom surfaces of the oceanic crust of the Monterey microplate were modeled as late‐arriving negative–positive dipole signals. Such dipoles are observed at times that place their origin at 10 km below the Moho at the Coast Ranges. These converted phases from the oceanic crust could also be traced from the Coast Ranges to the Great Valley, where the top surface of the slab model is located. The combined result of the receiver functions and a geometric tomography model is consistent with the fossil slab hypothesis, which initiated as a flat subduction system that later went through a delamination process. Online Material: Figures of receiver functions, sorted by back azimuth and ray parameter at 31 stations.

Plate tectonics on the Earth triggered by plume-induced subduction initiation.

Scientific theories of how subduction and plate tectonics began on Earth--and what the tectonic structure of Earth was before this--remain enigmatic and contentious. Understanding viable scenarios for the onset of subduction and plate tectonics is hampered by the fact that subduction initiation processes must have been markedly different before the onset of global plate tectonics because most present-day subduction initiation mechanisms require acting plate forces and existing zones of lithospheric weakness, which are both consequences of plate tectonics. However, plume-induced subduction initiation could have started the first subduction zone without the help of plate tectonics. Here, we test this mechanism using high-resolution three-dimensional numerical thermomechanical modelling. We demonstrate that three key physical factors combine to trigger self-sustained subduction: (1) a strong, negatively buoyant oceanic lithosphere; (2) focused magmatic weakening and thinning of lithosphere above the plume; and (3) lubrication of the slab interface by hydrated crust. We also show that plume-induced subduction could only have been feasible in the hotter early Earth for old oceanic plates. In contrast, younger plates favoured episodic lithospheric drips rather than self-sustained subduction and global plate tectonics.

Late Miocene upper-crustal deformation within the interior of the southern Puna Plateau, central Andes

The origin and evolution of the central Andes, a noncollisional orogenic system, have been hypothesized to evolve as a result of several dynamic processes, including formation of an eastward-propagating orogenic wedge, segmentation into rhomb-shaped basins as a result of N-S gradients in crustal shortening, reactivation of inherited deep structures, and lithospheric foundering. How these proposed processes dominate the orogen spatially and temporally is uncertain; however, constraining the timing of upper-crustal deformation is critical for investigating these models. We document the formation and deformation of the Pasto Ventura basin (NW Argentina) in the southern Puna Plateau. Through field mapping, deformation analysis, secondary ion mass spectrometry U-Pb dating of zircon from interbedded volcanic ashes, and Ar/Ar geochronology of volcanics, we show that major basin formation started ca. 11.7-10.5 Ma and continued until at least ca. 7.8 Ma. The basin underwent syndepositional faulting and folding from ca. 10 to 8 Ma. Contractional deformation in the Pasto Ventura basin ended between ca. 7.3 and 4 Ma, based on the onset of regional horizontal extension. Data from the Pasto Ventura region allow us to bridge existing data and complete a regional compilation of upper-crustal deformation for the Puna Plateau. Our analysis shows that late Miocene formation and deformation of the Pasto Ventura basin represent an important out-of-sequence contractional event in the southern Puna Plateau. While a number of geodynamic processes likely shape the evolution of the southern Puna, multidisciplinary data sets, including deformation in the Pasto Ventura basin studied here, highlight the role of the formation and detachment of a late Miocene lithospheric drip in causing the upper-crustal deformation on the southern Puna Plateau since the mid-late Miocene.

The Pliocene–Quaternary Buffalo Valley volcanic field, Nevada: Post-extension, intraplate magmatism in the north-central Great Basin, USA

The Buffalo Valley volcanic field consists of Pliocene through Quaternary lava flows and spatter cones located south of Battle Mountain and adjacent to the Fish Creek Mountains, north-central Nevada. The volcanic rocks are split into two groups by age and geochemistry. The Pliocene sequence (4.02 to 2.75Ma) consists of olivine- and plagioclase-bearing alkali basaltic lava flows with minor pyroclastic deposits, found primarily along the south flank of Battle Mountain and also at the north end of the Fish Creek Mountains and within the Fish Creek Mountains caldera. The Quaternary series (1.99 to 1.14Ma) includes nearly a dozen trachybasaltic spatter cones with short lava flows erupted along the northwest flank of the Fish Creek Mountains. Normalized rare earth element and incompatible element plots for both groups are light rare earth and Nb–Ta enriched, resembling alkali basalts from ocean islands, but the Quaternary lavas are more light rare earth element-enriched and cross the Pliocene basalt patterns at Eu. Radiogenic and stable isotope ratios are consistent with an asthenospheric mantle source, and the rare earth element patterns indicate a shift from melting in the spinel to garnet peridotite field with time. Basaltic rocks from other intraplate fields in the Great Basin, including the Lunar Crater and Cima fields, only include lavas that originated at depth in the garnet peridotite field. Buffalo Valley is located at the margin of a proposed lithospheric drip (delamination) and within a zone of lithospheric thinning that extends across northern Nevada, both of which may control where melting in the asthenosphere may occur. The proximity to the edge of Precambrian–Phanerozoic lithosphere boundary may also be a factor in melt generation.

Stalled slab dynamics

Recent seismic imaging of the mantle beneath western North America reveals complexities interpreted as structures ranging from plumes to lithospheric drips and slab fragments. A prominent high-velocity curtain beneath Idaho has been interpreted as a remnant of the subducted Farallon plate left dangling within the upper mantle since >40 Ma. Consequently, using numerical models, we explore the rheological, chemical, geometrical, and dynamic conditions under which a slab fragment might persist in the mantle for tens of millions of years. With thermal buoyancy alone, stalled slabs extending to 500 km depth tend to detach and sink vertically within ∼17 m.y. for the slab age and rheologic conditions explored here, and shorter slabs 40 m.y. An increase in upper- to lower-mantle viscosity contrast (1.4× to 100×) can slow sinking velocities and extend slab dangling time by up to 5 m.y. Dynamic effects such as those arising from active nearby subduction only slightly delay or do not affect stalled slab detachment timing but do affect the geometry of the slabs as they respond to suction pressures within the wedge. Overall, a combination of buoyant, viscous, geometric, and dynamic factors may allow cases of extended slab stalling, and conditions we explore here within realistic ranges can so far account for a delay of up to 28 m.y.

Along-strike translation of a fossil slab

The Isabella anomaly is a high seismic velocity anomaly beneath the southern Central Valley of California that has previously been interpreted to represent a lithospheric drip or delaminated Sierra Nevada root. However, recent work suggests that the anomaly is a remnant slab, left over from Cenozoic subduction, attached to the Pacific plate underneath the edge of the North American plate. This hypothesis requires the slab to translate hundreds of kilometers along-strike while remaining intact and attached to the Pacific plate despite drag from the surrounding asthenosphere and overriding lithosphere. We use 3-D finite element models to simulate this scenario, and calculate the viscosity ratio between the slab and the surrounding asthenosphere required for the slab to translate undeformed. The required viscosity ratio increases with increasing downdip extent of the slab, and decreases with increasing slab dip; for geometries approximating that of our proposed slab, the required ratio ranges from 102 to 104. Given the thermal and petrologic history of the slab, mantle flow laws predict viscosity contrasts greater than or equal to these requirements. As such, we conclude that along-strike translation of a remnant slab is feasible, and serves as a possible explanation of the Isabella anomaly. The significance of this finding extends beyond our general understanding of subduction dynamics, in that the presence of such a slab could have implications for the water budget of the San Andreas Fault and its role in aseismic slip.

Contrasting Triassic ferroan granitoids from northwestern Liaoning, North China: Magmatic monitor of Mesozoic decratonization and a craton–orogen boundary

The diversity exhibited by ferroan granitoids testifies to multiple distinct crustal and mantle processes generally within extensional environments and thus holds the key to monitoring important geodynamic and crustal evolutionary processes and calibrating terrane tectonic affinity. This study documents three Middle to Late Triassic ferroan granitoid suites from northwestern Liaoning, North China, and proposes distinct petrogenetic scenarios for each. The Middle Triassic (ca. 238Ma) Ping'andi (PAD) granites are mainly calc-alkali and peraluminous. Their radiogenic whole-rock Nd and zircon Hf isotopic signatures argue for an origin consistent with partial melting of a juvenile quartzofeldspathic crust formed by prior mantle-derived magmatic underplating and their differentiations. By contrast, the Late Triassic (ca. 220Ma) Dashaoleng (DSL) and Sijiazi (SJZ) suites show an evolved character from alkali-calcic to alkali and from metaluminous to peraluminous. Their variably evolved whole-rock Sr–Nd and mixed zircon Hf isotopic compositions suggest that both suites were formed by variable mixing between depleted mantle-derived mafic magma and old crust-derived felsic magma, with distinctively higher input from juvenile components in the DSL suite. These contrasting ferroan granitoid suites not only provide a spatial marker for monitoring juxtaposition of the North China Craton (NCC) and the Central Asian Orogenic Belt (CAOB) along a lithospheric-scale boundary fault in the region, but also present a temporal snapshot that records a southwardly-progressing crustal growth scenario possibly in response to lithospheric dripping within a post-orogenic extensional regime. The cratonic scale synthesis further indicates that diachronous decratonization pattern of the NCC might record episodic response of the craton to evolved plate tectonic processes with two contrasting Phanerozoic orogenic systems.