"Geochemistry, Geophysics, Geosystems"

Nixon, C. G.; Schmitt, D. R.; Kofman, R.; Lofi, J.; Gulick, S. P. S.; Saustrup, S.; Christeson, G. L.; Kring, D. A.

doi: 10.1029/2021gc009959pmid: N/A

We conducted a vertical seismic profile (VSP) in the borehole of International Ocean Discovery Program/International Continental Scientific Drilling Program Expedition 364 Site M0077 to better understand the nature of the seismic reflectivity and the in situ seismic properties associated with the Chicxulub impact structure peak ring. Extraction of the up‐going wavefield from the VSP shows that a strong seismic reflection event imaged in seismic reflection data results from discontinuities in the elastic impedance Z (the product of density and wave speed) at the top and bottom of a zone of hydrothermally altered melt‐bearing polymict breccia (suevite) that are characterized by anomalously low Z. Below this strong carbonate/suevite reflection event, the upgoing seismic wavefield is chaotic, indicating high levels of scattering from the suevites and underlying melt rocks and shocked granitoids of the peak ring, in contrast to the clear coherent reflections throughout the overlying Cenozoic sediments. We extract shear wave speeds, which, together with those provided from the complementary sonic log and densities from core scanning, allowed determination of VP/VS and Poisson's ratio v. These values are anomalously high relative to comparable terrestrial lithologies. We also calculate a variety of damage parameters for the disrupted peak ring granitoids. These values may assist in linking seismic observations to shock levels that are necessary to calibrate current impact models and may also be useful in assessing levels of fracturing within major fault zones.

Finger, N.‐P.; Kaban, M. K.; Tesauro, M.; Mooney, W. D.; Thomas, M.

doi: 10.1029/2021gc010296pmid: N/A

Recently, the continually increasing availability of seismic data has allowed high‐resolution imaging of lithospheric structure beneath the African cratons. In this study, S‐wave seismic tomography is combined with high resolution satellite gravity data in an integrated approach to investigate the structure of the cratonic lithosphere of Africa. A new model for the Moho depth and data on the crustal density structure is employed along with global dynamic models to calculate residual topography and mantle gravity residuals. Corrections for thermal effects of an initially juvenile mantle are estimated based on S‐wave tomography and mineral physics. Joint inversion of the residuals yields necessary compositional adjustments that allow to recalculate the thermal effects. After several iterations, we obtain a consistent model of upper mantle temperature, thermal and compositional density variations, and Mg# as a measure of depletion, as well as an improved crustal density model. Our results show that thick and cold depleted lithosphere underlies West African, northern to central eastern Congo, and Zimbabwe Cratons. However, for most of these regions, the areal extent of their depleted lithosphere differs from the respective exposed Archean shields. Meanwhile, the lithosphere of Uganda, Tanzania, most of eastern and southern Congo, and the Kaapvaal Craton is thinner, warmer, and shows little or no depletion. Furthermore, the results allow to infer that the lithosphere of the exposed Archean shields of Congo and West African cratons was depleted before the single blocks were merged into their respective cratons.

Rodriguez Piceda, C.; Scheck‐Wenderoth, M.; Cacace, M.; Bott, J.; Strecker, M. R.

doi: 10.1029/2021gc010171pmid: N/A

We examined the relationship between the mechanical strength of the lithosphere and the distribution of seismicity within the overriding continental plate of the southern Central Andes (SCA, 29°–39°S), where the oceanic Nazca Plate changes its subduction angle between 33°S and 35°S, from subhorizontal in the north (<5°) to steep in the south (∼30°). We computed the long‐term lithospheric strength based on an existing 3D model describing variations in thickness, density, and temperature of the main geological units forming the lithosphere of the SCA and adjacent forearc and foreland regions. The comparison between our results and seismicity within the overriding plate (upper‐plate seismicity) shows that most of the events occur within the modeled brittle domain of the lithosphere. The depth where the deformation mode switches from brittle frictional to thermally activated ductile creep provides a conservative lower bound to the seismogenic zone in the overriding plate of the study area. We also found that the majority of upper‐plate earthquakes occurs within the realm of first‐order contrasts in integrated strength (12.7–13.3 log Pam in the Andean orogen vs. 13.5–13.9 log Pam in the forearc and the foreland). Specific conditions characterize the mechanically strong northern foreland of the Andes, where seismicity is likely explained by the effects of slab steepening.

Wang, Wei; Zhao, Bin; Qiao, Xuejun; Ding, Kaihua

doi: 10.1029/2021gc010216pmid: N/A

The North China Craton (NCC) is an important tectonic element in China, which is characterized by intense seismic activity and complex tectonic setting. Thus quantifying its deformation is essential for studying the tectonic processes and deformation mechanisms in this region. Here we use a combination of dense GPS data sets to analyze the crustal movement and present updated estimates of fault motions based on elastic block models. We uncover spatial variations in deformation patterns, indicating that different mechanisms may dominate the tectonic processes for different parts of the NCC. In the western part, the rifts around the Ordos block manifest strike‐slip and extensional deformation, and the strike‐slip motion is more significant than the extensional component; whereas in the east, the Tanlu fault experiences right‐lateral and shortening slips, consistent with the features from historical seismic activity and regional tectonics. Our analysis indicates that the deformation in the eastern NCC is principally influenced by the Pacific subduction, whereas the deformation in the western NCC is mainly related to the India‐Eurasia collision. Moreover, our estimate of motion along the Tanlu fault supports a relatively long seismic interval of ∼10 ka, comparable to those inferred from recent paleoseismic research on this fault and other slow‐slip faults in the NCC. Our study indicates that the faults in the NCC even with very slow motions may still have the potential to produce strong earthquakes. Thus seismic risks across these slow‐slip faults cannot be ignored and should not be determined solely based on the rate of fault motions.

Buffett, B. A.; Avery, M. S.; Davis, W.

doi: 10.1029/2021gc010239pmid: N/A

Observations of relative paleointensity reveal several forms of asymmetry in the time dependence of the virtual axial dipole moment (VADM). Slow decline of the VADM into a reversal is often followed by a more rapid rise back to a quasi‐steady state. Asymmetry is also observed in trends of VADM during times of stable polarity. Trends of increasing VADM over time intervals of a few 10s of kyr are more intense and less frequent than decreasing trends. We examine the origin of this behavior using stochastic models. The usual (Langevin) model can account for asymmetries during reversals, but it cannot reproduce the observed asymmetry in trends during stable polarity. Better agreement is achieved with a different class of stochastic models in which the dipole is generated by a series of impulsive events in time. The timing of each event occurs randomly as a Poisson process and the amplitude is also randomly distributed. Predicted trends replicate the observed asymmetry when the generation events are large and the recurrence time is long (typically longer than 3 kyr). Large and infrequent generation events argue against dipole generation by small‐scale turbulent flow. Instead, the observations favor a mechanism that relies on expulsion of poloidal magnetic field from the core.

Guo, Yangrui; Deng, Wenfeng; Wei, Gangjian

doi: 10.1029/2021gc010249pmid: N/A

Reliable temperature reconstruction by carbonate clumped isotope (Δ47) thermometry requires isotopic equilibrium during carbonate growth. However, many carbonate minerals grow at high rates and exhibit disequilibrium isotopic states. Carbonate Δ47 disequilibrium arising from kinetic isotope effects (KIEs) specific to carbonate growth still remains unclear, and requires experimental constraints. Here we present a series of rapid witherite (BaCO3) precipitation experiments intended to constrain bulk carbon, oxygen and clumped isotopic fractionation during unidirectional precipitation of BaCO3 from dissolved inorganic carbon solutions under various pH and temperature conditions. We found that rapid BaCO3 growth can lead to lower δ13C, δ18O, and Δ47 values in HCO3−‐dominated solutions, especially at low temperatures. Our experiments provide constraints on kinetic fractionation factors (KFFs) associated with the rapid carbonate growth and their temperature dependence. KFFs for δ13C and δ18O are broadly consistent with previous experimental estimates, although the Δ47 KFF for BaCO3 precipitation from HCO3−‐dominated solution is not consistent with an earlier theoretical estimate, necessitating a re‐evaluation of the current model of the KIE associated with carbonate growth. Our results clearly verify a preference for the CO32− pathway during carbonate precipitation, with important implications for new isotope fractionation models for natural carbonates. The fractionation relationship between Δ47 and δ18O found in this study allows more precise identification of KIEs associated with rapid carbonate growth.

Mueller, Megan A.; Licht, Alexis; Campbell, Clay; Ocakoğlu, Faruk; Akşit, Gui G.; Métais, Grégoire; Coster, Pauline M. C.; Beard, K. Christopher; Taylor, Michael H.

doi: 10.1029/2021gc010232pmid: N/A

Collision between the Pontides and Anatolide‐Tauride Block along the İzmir‐Ankara‐Erzincan suture in Anatolia has been variously estimated from the Late Cretaceous to Eocene. It remains unclear whether this age range results from a protracted, multi‐phase collision or differences between proxies of collision age and/or along strike diachroneity. Here, we leverage the Cretaceous‐Eocene evolution of the forearc‐to‐foreland Central Sakarya Basin system in western Anatolia to determine when and how collision progressed. New detrital zircon (DZ) and sandstone petrography results indicate that the volcanic arc was the main source of sediment to the forearc basin in the Late Cretaceous. The first appearance of Pontide basement‐aged DZs, in concert with exhumation of the accretionary prism and a decrease in regional convergence rates, indicates intercontinental collision initiated no later than 76 Ma. However, this first contractional phase does not produce advanced thick‐skinned deformation and basin partitioning until ca. 54 Ma. We propose three non‐exclusive and widely applicable mechanisms to reconcile the observed ∼20 Myr delay between initial intercontinental collision and thick‐skinned upper plate deformation: slab breakoff, relict basin closure north and south of the İAES, and underthrusting of progressively thicker passive margin lithosphere. These mechanisms highlight the links between upper plate deformation and plate coupling during continental collision.

Zhou, Xiaolong; Kuiper, Klaudia; Wijbrans, Jan; Vroon, Pieter

doi: 10.1029/2021gc009839pmid: N/A

The mechanism controlling the volcanic eruption frequency is yet poorly understood. The key to a better understanding this mechanism comes from integrating accurate geochronology with numerical models. In many silicic volcanic systems, the eruption frequency is studied for short timescales of <500 kyr. Here, we combine two published numerical models (Caricchi et al., 2014, https://doi.org/10.1038/ngeo2041; Degruyter & Huber, 2014, https://doi.org/10.1016/j.epsl.2014.06.047) to improve our understanding of the eruption frequency in a long‐lived (>3 Ma) felsic magmatic system, the Milos volcanic field. We use the published Milos geochronological data as input into the Caricchi et al. (2014, https://doi.org/10.1038/ngeo2041) model to derive average magma supply rates (Qav) and eruptible volumes. These two parameters are then used in the Degruyter and Huber (2014, https://doi.org/10.1016/j.epsl.2014.06.047) model to compute the chamber growth rate (Gmc) for the Milos magmatic system. The results of the Caricchi et al. (2014, https://doi.org/10.1038/ngeo2041) model indicate that the time intervals between magma pulses into the subvolcanic reservoir (ti) and Qav are the key parameters controlling the eruption frequency. During the time intervals of 1.48–1.04 and 0.97–0.63 Ma the ti is longer than 1000 years and the volcanic quiescence periods are longer than 350 kyr. Furthermore, these periods are characterized by low values for Qav (≤0.001 km3 · yr−1) and Gmc (<0.0001 km3 · yr−1). In contrast, during the time intervals of 2.0–1.5 and 0.60–0.06 Ma, the ti is shorter (<0.5 kyr) and the values for Qav (>0.001 km3 · yr−1) are higher corresponding to frequent eruptions.

Golos, E. M.; Fischer, K. M.

doi: 10.1029/2021gc010252pmid: N/A

The Colorado Plateau and its surroundings serve as an archetypal case to investigate the interaction of mantle melting processes and lithospheric structure. It has been hypothesized that widespread Cenozoic volcanism indicates the encroachment of the convective upwelling of asthenosphere toward the Plateau center. In this study, we generate a Common Conversion Point (CCP) stack of S‐to‐p (Sp) receiver functions to image the locations of lithospheric discontinuities in the southwestern United States. Our results are broadly similar to prior work, showing a strong and continuous Negative Velocity Gradient (NVG) consistent with the Lithosphere‐Asthenosphere Boundary (LAB) over much of the study area. However, with several methodological improvements, we are able to obtain more reliable NVG depth picks below the Colorado Plateau where the LAB becomes weaker, deeper, and broader. We compare the inferred topography of NVGs with the locations of volcanoes, and find that the majority of recent volcanoes are co‐located with lithosphere that is ∼80 km thick. This appears to be the critical depth at which partial melt from upwelling asthenosphere pooling at the base of (or within) the lithosphere may percolate to the surface. We compare our CCP profiles with magma equilibration conditions determined from petrologic analysis and find good agreement between the depth of NVGs and depth of magma equilibration. This analysis provides insight into the progression of magmatism and lithospheric loss toward the center of the Colorado Plateau, and demonstrates how small‐scale processes like melting influence lithosphere‐asthenosphere interactions that persist over large temporal and spatial scales.

Haase, K. M.; Schoenhofen, M. V.; Storch, B.; Beier, C.; Regelous, M.; Rubin, K. H.; Brandl, P. A.

doi: 10.1029/2021gc010066pmid: N/A

Abundant volcanic activity occurs in the back‐arc region of the northern Tofua island arc where the Northeast Lau Spreading Center (NELSC) propagates southwards into older crust causing the formation of numerous seamounts at the propagating rift tip. An off‐axis volcanic diagonal ridge (DR) occurs at the eastern flank of the NELSC, linking the large rear‐arc volcano Niuatahi with the NELSC. New geochemical data from the NELSC, the southern propagator seamounts, and DR reveal that the NELSC lavas are tholeiitic basalts whereas the rear‐arc volcanoes typically erupt lavas with boninitic composition. The sharp geochemical boundary probably reflects the viscosity contrast between off‐axis hydrous harzburgitic mantle and dry fertile mantle beneath the NELSC. The new data do not indicate an inflow of Samoa plume mantle into the NELSC, confirming previously published He isotope data. The NELSC magmas form by mixing of an enriched and a depleted Indian Ocean‐type upper mantle end‐member implying a highly heterogeneous upper mantle composition in this area. Most NELSC lavas are little affected by a slab component implying that melting is adiabatic beneath the spreading center. The DR lavas show the influence of a component from the subducted Louisville Seamount Chain, which was previously thought to be restricted to the nearby arc volcanoes Niuatoputapu and Tafahi. This signature is rarely detected along the NELSC implying little mixing of melts from the low‐viscosity hydrous portion of the mantle wedge beneath the rear‐arc volcanoes into the melting region of the dry mantle beneath the NELSC.