Southernmost Segment Research Articles

Double-Vergent Plate Boundary Faults and Triggered Coseismic Rupture of the 2022 Chihshang Doublet Earthquake Occurred in Eastern Taiwan

Abstract The earthquake doublet of Mw 6.5 and 6.9 occurred along the west-dipping Central Range fault (CRF) adjacent to the east-dipping creep segment of the Longitudinal Valley fault in eastern Taiwan on 17–18 September 2022. The faulting model derived from the Interferometric Synthetic Aperture Radar and Global Positioning System observation suggests that the west-dipping CRF rupture is responsible for the two mainshocks. Meanwhile, the two major earthquakes resulted in ∼100 km of fault slip along the double-convergent plate boundary faults near the Longitudinal Valley in eastern Taiwan. The 2022 Chihshang earthquake sequence filled the seismic gap of the CRF located between the 2006 ML 6.1 Taitung earthquake and the 2013 ML 6.1 Ruisui earthquake. Finally, the 2022 Chihshang earthquake sequence increased the Coulomb failure stress in the southernmost segment of the CRF, which may cause a higher seismic potential in the future.

Segmentation of the Tashkurgan normal fault in the eastern Pamir: Insights from geomorphology and thermochronology and implications for fault-slip transfer

At the northwestern end of the India–Asia collision zone, in the eastern Pamir interior, the Kongur Shan extensional system extends for ∼250 km as a composite system of normal faults. As the southernmost segment of the extensional system, the Tashkurgan fault can be divided into a northern and southern segment by the intersection of the Tahman and Tashkurgan faults. To evaluate the tectonic activity along the Tashkurgan fault and explore the extensional characteristics of the southern Kongur Shan extensional system, we integrate geomorphologic analysis with thermochronologic data and modeling. Geomorphic indices, including local relief, normalized channel steepness index (ksn), and river χ-plots, show that the southern Tashkurgan fault is more active than the northern Tashkurgan fault. On the footwall of the northern Tashkurgan fault, most thermochronologic data, which include biotite 40Ar/39Ar (Arbt), zircon (U-Th)/He (ZHe), apatite fission track (AFT), and apatite (U-Th-Sm)/He (AHe) dates, record emplacement-related cooling, and only a few AHe dates possibly record exhumation of <2 km since ∼7 Ma ago. On the footwall of the southern Tashkurgan fault, almost all of the ZHe, AFT, and AHe dates record significant late Miocene to present exhumation; thus, we perform three-dimensional thermokinematic modeling for the normal slip along this fault segment. Modeling results indicate a probably constant slip rate on the southern Tashkurgan fault at a value of 1.4–1.5 mm/a, which corresponds to 6–7 km of both horizontal extension and footwall uplift since 6.5 Ma ago. All together, the geomorphic and thermochronologic evidences imply that the southern Tashkurgan fault is responsible for the ∼E–W extension along the southernmost portion of the Kongur Shan extensional system, while the northern fault segment plays a minor role in the extension. This result, combined with the previously published magnitude of extension along the southern Kongur Shan fault, indicates that the extension at any site along the southern Kongur Shan extensional system can be primarily attributed to normal faulting along a single border fault, which is a typical characteristic of an interbasin transfer zone with a transfer fault.

Diverse Late Triassic arc magmatism and porphyry Cu deposits in the southeastern Tibetan Plateau: Related to slab tear along a subducting passive margin?

The subduction of the ocean–continent transition (OCT) zone on a passive margin has received limited attention, despite its significant impact on tectono-magmatic processes. This study focuses on the Late Triassic basalts and andesites in the southeastern Tibetan Plateau, where the Yushu-Yidun Arc belt intersects the Longmen Shan Fault system. The goal is to understand the interplay between these linear systems. The basalts exhibit trace element and isotopic signatures resembling those of within-plate enriched mid-ocean ridge basalts or ocean island basalts (EOBs), such as positive NbTa anomalies, high Nb/Th (7.72 to 10.3) and εNd(t) (+2.23 to +6.34) values, and low La/Sm values (2.69 to 4.05), suggesting an asthenospheric mantle source. The contemporaneous andesites display typical arc-like geochemical characteristics (e.g., enrichment in Ba, Th, and U but depletion in Nb, Ta, and Ti), with negative εNd(t) values (−2.41 to −1.08), indicating a mantle wedge source affected by subducted components. Geological evidence indicates that the Longmen Shan Fault zone, previously defined by its magmatic rocks, was an active OCT belt during the Late Triassic, separating the South China Continent from the Garze–Litang Ocean. Consequently, we propose a model suggesting the tearing of a subducted oceanic slab from the continental lithosphere within the OCT belt under the southernmost segment of the Yushu–Yidun Arc belt, driven by strong slab pull in the Garze–Litang Ocean subduction zone. This model effectively explains the intriguing geology of the Songpan–Garze–Hoh Xil Terrane, in which Late Triassic EOBs and normal arc-related igneous rocks coexist and are accompanied by abundant slab-derived mineralized porphyries. Our findings have global implications for investigating geochemically diverse magmatism and associated mineralization on passive continental margins.

Late Quaternary evolution of the Belan River Basin, Central India

The Marginal Gangetic Plain is the southernmost segment of the Ganga Plain, formed by Himalayan Orogen thrust fold stress flexing the Indian lithosphere. Rivers originating in the Himalayas have received much attention (i.e., Ganga, Yamuna, Kosi, Ghagra, etc.). In contrast, the tributaries draining from the peripheral bulge of the foreland basin have not been studied in detail, although it bears signatures of seismic activity in the basement of the craton. The Belan River, a tributary of the Tons River and sub-tributary of the Ganga River, drains through the Marginal Gangetic Plain. Various tectonic-induced geomorphic signatures have been identified using Landsat Images, such as fluvial incision, river shifting, gorges channel formation, etc., in the Belan River Basin. Geomorphic indices of active tectonics (asymmetry factor, SL index, basin elongation ratio, and long profile), drainage pattern, and lineament study have been performed for the Belan River Basin. These parameters indicate slope adjustment due to the uplift and subsidence of the parts of the basin area. The most recent and extensive valley fills started accumulating at ∼51 ± 7 ka and persisted until around ∼22 ± 2 ka. The aggradation was accompanied by a weakening summer monsoon, followed by incision after 22 ± 2 ka, indicating a return to warmer and wetter conditions and a stronger summer monsoon after the Last Glacial Maximum (LGM). Tectonic exhumation in the southern and south-eastern parts of the Belan River Basin revealed by geomorphic indices. These findings indicate that the drainage networks of the Belan River Basin were built over a tectonically controlled valley and that denudational processes have been transforming the basin for a considerable period.

Characteristics of landscape and erosion in the Eastern Pamir and their implications for regional fault connections

The Tashkurgan normal fault in the Eastern Pamir is located at the strain transition zone between the dextral strike-slip faulting of the Karakoram fault in the south and the east-west extension of the Kongur Shan extensional system (KES) in the north. Thus, its tectonic evolution and relationship with the KES and the Karakoram fault carry important implications for the strain partition and deformation history of the Pamirs. However, the Tashkurgan fault lacks systematic documentation of its evolution and its relationship to other regional tectonic structures. In order to constrain the tectonic activity of the Tashkurgan fault, we analyze the landscape and 10Be-derived erosion rates in the footwall of the Tashkurgan fault. Our results reveal that both the values of the geomorphological indices and erosion rates increase from the northern segment of the fault towards the central segment and then decrease again southwards but with a further increase in the southernmost segment. We interpret that this pattern is closely related to the fault-related uplift of the Tashkurgan fault except that the southernmost segment is mainly affected by fractured rocks. These observations together with previous studies suggest that the Tashkurgan fault is the southern segment of the KES and extension along the KES has been initiated from the north and transferred to the Tashkurgan fault through the Tagharma fault. Therefore, we infer that the normal faulting of the Tashkurgan fault, and probably the whole KES, may lack a kinematic connection to the dextral slip system in the south.

Asymmetric Bilateral Rupture of the 2022 Ms 6.8 Luding Earthquake on a Continental Transform Fault, Tibetan Border, China

Abstract The Xianshuihe (XSH) fault on the eastern boundary of the Tibetan plateau is one of the most active faults in the world. However, the Moxi fault, the southernmost segment of the XSH fault, remained seismically quiescent for over 230 yr, with a large slip rate and high locking degree. Thus, this region is considered to be a seismic gap capable of hosting earthquakes of Mw 7.0 or greater. In September 2022, the Ms 6.8 Luding earthquake occurred on the Moxi fault, resulting in destructive landslide damage, with 93 people dead and 25 missing. Using regional and global seismic recordings, we integrate the multiple-point-source analysis, finite-fault inversions, and backprojection imaging to investigate in detail the rupture process associated with the 2022 Luding, China, earthquake. Our results show that this event is characterized by an asymmetric bilateral rupture with three episodes: (1) an initial bilateral rupture propagation mainly toward north-northwest; (2) major rupture propagating toward south-southeast and up-dip direction; (3) south-southeast-rupture propagation speed accelerates to ∼2 km/s. Over 70% of the seismic moment is released at shallow depths &amp;lt;12 km thus explaining significant damaging effects. Furthermore, the seismic potential of the remaining locked portions of the XSH and Anninghe faults is still high and positive increases of the Coulomb stress on these faults due to the Luding earthquake might bring them closer to future failure.

A Comparative Analysis of Lineaments Extracted from Landsat 8 OLI &amp; ASTER-GDEM Data, in Talmakent Area, Western High Atlas, Morocco

Structural mapping by remote sensing is an essential component to completing the geological mapping of the Talmaknet region. It allows us to understand the regional tectonics of the southernmost segment of the Hercynian of Western Morocco, exploiting Geographic Information Systems (GIS) and satellite imagery processing techniques in order to obtain a good lineament discrimination. To this end, the approach adopted was based on preliminary processing of Landsat 8 OLI satellite imagery combined with a morpho-structural shading analysis technique, using the Digital Elevation Model (DEM), and applying several techniques of image enhancement and directional filters to the different treatments to improve the visibility of the linear structures in the images.Given the interesting number of fractions collected, a statistical and directional study was carried out on the synthetic maps to show the distribution and orientation of the lineaments. This analysis indicates a dominant NE-SW oriented geological fracturing, which agrees with the existing geological data in the area, confirming the effectiveness of GIS and remote sensing techniques in structural geology studies, especially to have a detailed mapping of the linear structures in the study area.

Plant species diversity in the riparian forests of the Moyar River in southern India

Riparian forests are among the most rapidly disappearing vegetation types throughout the world. River Moyar cascades through gorges and links the Western Ghats with the southernmost segments of the Eastern Ghats. Considering the relatively well-preserved state of the Moyar riparian vegetation and being amongst the least explored forests in southern India, an assessment of angiosperm diversity was undertaken. The study used an array of belt transects along and perpendicular to the river course so that the entire elevation gradient of the gallery could be covered. A total of 172 species representing 126 genera belonging to 47 families, including 100 monotypic genera and 17 monotypic families, were recorded from both the transects in the study area. 131 woody angiosperm species representing 100 genera in 41 families were recorded along the river. The perpendicular gallery transects recorded 111 woody angiosperm species representing 86 genera in 36 families. Thirteen monotypic families were found in both transects, and 70 species were found in both transects. The dominant families with the maximum species were Fabaceae, Rubiaceae and Phyllanthaceae. Shannon diversity index ranged between 2.0 to 3.27 along the river transects and 1.51 to 2.67 along the galleries. The study concludes that Moyar riparian zone merits high conservation value as it supports significant species diversity, including red-listed species and habitat-specific plants, and functions as a vital wildlife corridor in the landscape.

Extension Dynamics of the Northern Fonualei Rift and Spreading Center and the Southern Mangatolu Triple Junction in the Lau Basin at 16°S

AbstractDue to the complexity of 2D magnetic anomaly maps north of 18°S and the sparsity of seismic data, the tectonic evolution of the northern Lau Basin has not yet been unraveled. We use a multi‐method approach to reconstruct the formation of the basin at ∼16°S by compiling seismic, magnetic, gravimetric and geochemical data along a 185 km‐long crustal transect. We identified a crustal zonation which preserves the level of subduction input at the time of the crust's formation. Paired with the seafloor magnetization, the crustal zonation enabled us to qualitatively approximate the dynamic spreading history of the region. Further assessment of the recent tectonic activity and the degree of tectonic overprinting visible in the crust both suggest a complex tectonic history including a dynamically moving spreading center and the reorganizing of the local magma supply. Comparing the compiled data sets has revealed substantial differences in the opening mechanisms of the two arms of the Overlapping Spreading Center (OSC) that is made up by the northernmost tip of the Fonualei Rift and Spreading Center in the east and the southernmost segment of the Mangatolu Triple Junction in the west. The observed transition from a predominantly tectonic opening mechanism at the eastern OSC arm to a magmatic opening mechanism at the western OSC arm coincides with an equally sharp transition from and strongly subduction influenced crust to a crust with virtually no subduction input. The degree of subduction input alters the geochemical composition, as well as the lithospheric stress response.

Hydrothermal Genesis of Fe–Mn Crust in the Southernmost Segment of the Mohns Ridge, Norwegian Sea: REE Geochemistry and Sr and Nd Isotopic Composition

Hydrothermal Genesis of Fe–Mn Crust in the Southernmost Segment of the Mohns Ridge, Norwegian Sea: REE Geochemistry and Sr and Nd Isotopic Composition

Unradiogenic lead isotopic signatures of the source mantle beneath the southernmost segment of the Central Indian Ridge

Numerous studies have shown that the mid-ocean-ridge basalts (MORBs) distributed in the southern Central Indian Ridge (CIR) exhibit the DUPAL anomaly; yet, the corresponding temporal variations of this signal remain underexplored in this region. Here, we report new Sr–Nd–Pb isotope data for the MORB which forms the southernmost segment of the CIR (CIR-1) and interpret these together with previously reported elemental data. Our results indicate that the MORB from the current spreading axis of the CIR-1 segment is produced by a mixture of the partial melt of the depleted MORB mantle and melt derived from the lower continental crust (LCC). This interpretation is consistent with the previously proposed model for Indian MORBs that exhibit the DUPAL anomaly. In contrast, the MORB from the off-axis area shows a depleted trace element composition, particularly for highly incompatible elements. This can be explained by the mixture of the partial melt of a highly trace element-depleted mantle that has not been previously defined in this area and melt from the LCC and C component. Furthermore, high 87 Sr/ 86 Sr, Δ8/4, and Δ7/4 values indicate that the source mantle of the MORB in the off-axis area of the CIR-1 segment has undergone at least two melting events, including the formation of the current MORB. The first melting event occurred approximately 500 Ma and resulted in lower parent/daughter ratios (Th/Pb and U/Pb) in the residual mantle, yielding an unradiogenic lead isotope composition. Overall, our results suggest that a highly depleted mantle domain that underwent an ancient melting event lies under the northern CIR-1 segment, contributing to the formation of MORBs with an unradiogenic lead isotopic signature. • We sampled on- and off-axis mid-ocean-ridge basalt (MORB) from CIR-1. • Depleted mantle partial melt and LCC melt mix to form the on-axis MORB. • In contrast, the off-axis MORB is derived from trace element-depleted mantle. • Trace element-depleted melt could be mixed with melts of the LCC and C component. • A highly depleted mantle domain that underwent ancient melting is suggested.

Lithospheric dripping in a soft collision zone: Insights from late Paleozoic magmatism suites of the eastern Central Asian Orogenic Belt

The closure of Paleo-Asian Ocean is considered to have occurred along the Solonker Suture in the southernmost segment of the Central Asian Orogenic Belt (CAOB), the largest Phanerozoic accretionary orogen on the globe. The suture branches to the east to form the northern Hegenshan–Heihe Suture and the southern Solonker–Changchun Suture. The Hegenshan–Heihe Suture is an ideal natural laboratory for studying the post-collisional geodynamic processes operating in a soft collision zone driven by divergent double-sided subduction. Here we report results from an integrated study of the petrology, geochronology, geochemistry, and Sr–Nd–Hf isotopic compositions of the Early Carboniferous–Early Permian magmatic suite in the Hailar Basin of the Xing’an–Erguna Block. The Early Carboniferous igneous rocks are represented by 356–349 Ma andesitic tuffs, exhibiting typical subduction-related features, such as enrichment in large-ion lithophile elements and depletion in high-field-strength elements. These features, together with the relatively depleted Sr–Nd–Hf isotopic compositions, constant Nb/Y values, but highly variable Rb/Y and Ba values indicate that these rocks were generated by partial melting of a depleted mantle wedge metasomatized by slab-derived fluids. The Late Carboniferous–Early Permian magmatic suite (317–295 Ma) is characterized by high Sr contents (313–1080 ppm) and low Y contents (5–13 ppm), and these can be subdivided into calc-alkaline adakitic rocks and high-K calc-alkaline adakitic rocks. The calc-alkaline adakitic rocks have higher values of Sr/Y, (Sm/Yb)source normalized, and Mg#, and lower values of Y, Ybsource normalized, and K2O/Na2O than the high-K calc-alkaline adakitic rocks, which suggests that the former was generated by partial melting of foundered lower continental crust and the latter by partial melting of normal lower continental crust. Based on our new data, in conjunction with those in previous studies, we conclude that the tectonic evolution of the Hegenshan–Heihe Suture involved Early Carboniferous double-sided subduction of the Nenjiang Ocean, latest Early Carboniferous soft collision between the Xing’an–Erguna and Songliao blocks, and Late Carboniferous–Early Permian post-collisional extension. We also propose a new geodynamic scenario in which removal of the lithospheric root might have occurred in a soft collision zone during the post-collision period via repeated and localized lithospheric dripping, which results from combinedeffects of hydration weakening of the lithosphere caused by pre-collision subduction and asthenospheric stirring triggered by slab break-off.

The role of the pre-Alpine polycrystalline basement in the paleogeographic configuration of multiple Neotethyan oceanic basins

AbstractThe study provides a deeper understanding of the early Mesozoic paleogeogeographic spatial-temporal relationship by studying the two Adria-Europe intervening basement blocks. The Drina-Ivanjica and Pelagonian crustal fragments play important role in the internal early Alpine oceanic constitution further controlling the late Jurassic emplacement of Tethyan Dinaric-Hellenic ophiolites. The proposed paleogeographic reassessment is driven by the new paleocontinental inheritance data associated with the Variscan – pre-Variscan basement terranes. The recently published data suggest an Avalonian-type inheritance of the Pelagonian basement block which indicates a different pre-Variscan plate-tectonic journey, including separate spatial arrangement during Variscan amalgamation. In turn, Cadomian-type basement inheritance has been documented within the sliced Adria microplate. Thus, the Avalonian inheritance place the Pelagonian block away from the Apulia/Adria (Dinarides). In the investigated context of Paleozoic-Mesozoic paleogeographic transition, the Pelagonian block may represent a segment of the Cimmerian ribbon continent or southernmost segment of the Variscan Europe. With regards the nearby Adria microplate, a Triassic-Jurassic oceanic opening led to the decoupling (spreading away from the main Adria microplate) of the Drina-Ivanjica block. The rifting is in line with the simultaneous yet opposite or westward-directed drift of the Pelagonides. The breakup of south European Variscan configuration eventually result in the spatial alignment of the two basement fragments referred to as the “Drina–Pelagonide continental splinter”. By linking the paleogeographic pre-Jurassic–Jurassic relationship between these continental units, the two landlocked Neotethyan Vardar s.l. basins are extrapolated, “Dinaric Tethys” / Inner Dinaric-(Mirdita-Pindos) and the main Vardar Ocean (Western Vardar Zone).

Three-dimensional magnetotelluric model along seismic profile 2T: An improved view on crustal structure in central Slovakia (Western Carpathians)

We present the crustal-scale geophysical model based on the magnetotelluric method focused on 3-D model¬ling of the seismic 2T profile crossing the major Western Carpathian tectonic units in central Slovakia. The results of the 3-D modelling show substantial improvement in previous 2-D models of deep crustal structure in central Slovakia, mainly of the physically distinct tectonic segments and major geo-electrical regional structures like the zone of the Carpathian Conductivity Anomaly, which indicates the occurrence of the large-scale shear zone in the contact zone of the European platform and Inner Western Carpathians. High detail geo-electrical data in 3-D magnetotelluric (MT) cross section also allowed a better interpretation of other conductive anomalies. In the final integ-rated interpretation (combination of 3-D geo-electrical model, gravity data and seismic reflectors), it is shown that frontal part of the Inner Western Carpathians plate exhibits the transpressional tectonic style of the back-thrust Outer Western Carpathians (Flysch Belt) and Pieniny Klippen Belt units over the progressing Inner Western Carpathian thrust wedge. These back-thrusts form the southern branch of the accretionary structural fan – a large-scale transpressional flower structure typical mainly but not only for oblique con-vergent regimes. The southernmost segment of the profile with high whole-crust conductivity due to a higher heat flow caused by young volcanic activity indicates partial melting in the middle and lower crust.

Termination of a Trench-Linked Strike-Slip Fault Zone in the Sumatra–Java Forearc Basin and Accretionary Wedge Complex

This paper presents a review of several published seismic reflection and seismicity data and analyzes of high-resolution bathymetry data to revise the exact location and reveal detail characteristics of a strike-slip fault zone that formed the southernmost segment of the Sumatran Fault (SF). Previous works interpreted this fault segment as a horst structure to the south of a pull-apart basin. We observe a clear linear trace of dissected seafloor parallels to SF in the high-resolution bathymetric map. This structure extends from the south of a pull-apart basin in the northwest to the Sunda accretionary wedge farther southeast. This lineament exhibits a narrow valley and a linear ridge that in the subsurface are interpreted as negative and positive flower structures, respectively. The structure exhibits a vertical fault plane and appears to have deformed the accretionary wedge sediments and basement at depth. A cluster of shallow seismicity is observed along this NW-trending fault zone, indicating the activity of this zone. Here, we proposed this strike-slip fault as the Ujung Kulon Fault that marks the southeasternmost segment of the SF zone. This segment deformed the area of the Sumatra-Java forearc basin and terminated in accretionary wedge near the trench. The accumulated strain within UKF may trigger large earthquake in the future, close to the highly populated areas in the coast of Sumatra and Java.Keywords: Strike-slip fault, Sumatra Fault, Ujung Kulon Fault, segmentation, earthquake.

Provenance of the Permo–Carboniferous sediments in the northern Alxa and its tectonic implications for the southernmost Central Asian Orogenic Belt

As the southernmost segment of the Central Asian Orogenic Belt (CAOB), the northern Alxa orogenic belt (NAOB) connects the southeastern and southwestern segments of the CAOB. The NAOB amalgamated with the closure of the Paleo-Asian Ocean; however, the closure time of the Paleo-Asian Ocean is still on great debate. In this study, we reported new detrital zircon U–Pb geochronology and Hf–O isotopes for the Permo–Carboniferous sediments in the northern Alxa to constrain the provenance and its tectonic implications. The Permo–Carbonifereous Amushan Formation is composed of volcanic-carbonite-clastic rocks and was deposited in a shallow marine environment. Based on the zircon U–Pb geochronology, the Amushan Formation was deposited in the late Carboniferous to early Permian, but some outcrops of volcanic and clastic rocks in the Quaganqulu area were likely formed in the middle to late Permian. The integrated zircon age spectrum for the clastic rocks shows a wide range from late Archean to Paleoproterozoic, Mesoproterozoic (with a peak age at 1458 ​Ma), early Neoproterozoic (with peak ages of 988 ​Ma and 929 ​Ma), early Paleozoic (with a peak age at 447 ​Ma) and late Paleozoic (with a peak age at 294 ​Ma). Combined with the zircon Hf–O isotopes, the provenance was considered to be the Alxa Block, the Shalazhashan terrane and the Zhusileng–Hangwula block (and the southern Beishan orogenic belt). The multiple source regions to the south and north of the Paleo-Asian Ocean indicate the closure of this ocean before the late Carboniferous. The absence or small proportion of depositional age-approximated zircons in most samples makes their age spectra similar to extensional basins. Combined with the intra-plate volcanism, the deposits were considered to be formed in extensional settings. Accordingly, after the closure of the Paleo-Asian Ocean, the NAOB stepped into an extensional stage.

Nutrient over-enrichment and light limitation of seagrass communities in the Indian River Lagoon, an urbanized subtropical estuary

Historically, extensive seagrass meadows were common throughout the Indian River Lagoon (IRL) in east-central Florida, USA. Between 2011 and 2017, widespread catastrophic seagrass losses (~95%) occurred in the IRL following unprecedented harmful algal blooms (HABs), including persistent brown tides (Aureoumbra lagunensis). Little is known about how dissolved nutrients and chlorophyll a are related to light limitation or how biochemical factors, such as the elemental composition (C:N:P) and stable isotope signatures (δ13C, δ15N), of seagrasses within the IRL relate to coverage. Accordingly, we conducted a survey from 2013 to 2015 at 20 sites to better understand these relationships. Results showed a negative correlation between DIN and salinity, indicating freshwater inputs as a DIN source. Seawater N:P ratios and chlorophyll a concentrations were higher in the urbanized, poorly-flushed northern IRL segments. Kd values were higher in the wet season and often exceeded seagrass light requirements (0.8 m−1) for restoration, demonstrating light limitation. Species distribution varied by location. Halodule wrightii was ubiquitous, whereas Syringodium filiforme was not found in the northernmost segments. Thalassia testudinum was only present in the two southernmost segments that had the lowest TDN and highest light availability (Kd). Blade %N and %P also frequently exceeded critical values of 1.8% and 0.2%, respectively, especially in the northern segments. Further, δ15N was positively correlated with ammonium, suggesting wastewater as a major N source. The δ13C values indicated a trend of increasing light limitation from south to north, which helps explain the recent catastrophic loss of seagrasses in the northern IRL. Overall, elemental composition reflected high N-availability and seagrass species distributions were relatable to spatial trends in N and light limitation. For effective restoration, resource managers must reduce N-loading to the IRL to diminish HABs and increase light availability. Regular biochemical monitoring of seagrass tissue should also be implemented during restoration efforts.

Insights into the geometry and faulting style of the causative faults of the M6.7 1805 and M6.7 1930 earthquakes in the Southern Apennines (Italy) from coseismic hydrological changes

I study the coseismic hydrological changes associated with two M6.7 earthquakes that occurred in 1805 and in 1930 in the Southern Apennines of Italy to provide additional constraints on the geometry and mechanism of their causative faults. The two earthquakes originated remarkable datasets of 84 and 52 observations of hydrological changes. I discounted those observations potentially affected by the geomorphological, climatic and methodological contamination of data. Then, I compared the distribution of the streamflow changes with the coseismic strain field induced by a number of seismogenic sources proposed for the two earthquakes. A first important outcome of this study is that it provides evidence against some of the seismogenic structures previously associated with the 1805 and 1930 earthquakes. In addition, for the 1805 earthquake, I find that ~300°-striking, NE-dipping, ∼30–35 km-long seismogenic structures consisting of four to five surface-rupturing normal fault segments best explain (with almost 75% consistency) the distribution of the observed hydrological data. However, I also find that almost 100% of the observations are consistent with the deformation imposed by the three southernmost segments and suggest that most of the seismic release during the 1805 event was provided by those segments only. For the 1930 earthquake, I find that the pattern of the coseismic hydrological changes is best fitted by the only fault characterized by ~E-W trending, oblique slipping, NE-dipping, and a shallow top of fault. The depth of the top of the fault plays an important role in the generation of more pronounced fields of deformation, even for those faults that do not rupture up to the free surface, as in the 1930 case. This study confirms that the hydrological signatures of earthquake strain provide supplemental constraints in the estimating geometry and faulting style of major historical earthquakes that produced large and documented amounts of hydrological data.

Late Quaternary slip rates for the southern Elsinore fault in the Coyote Mountains, southern California from analysis of alluvial fan landforms and clast provenance, soils, and U-series ages of pedogenic carbonate

Offset alluvial fans along the Elsinore fault in the south-central Coyote Mountains were studied to resolve an average late Quaternary slip rate for this major western strand of the San Andreas fault system in southern California. Alluvial fans and their offsets were mapped using high-resolution DEMs combined with field observations of fan-surface morphology and the character of the soils developed in each fan remnant. Clast assemblage data was used to determine the source of each alluvial fan upstream of the fault, and U-series dating of pedogenic carbonate was used to estimate minimum ages of the alluvial fan surfaces. Forty U-Th dates on pedogenic carbonate confirm the utility of the technique for dating late Pleistocene alluvium in arid regions and suggest that age variation among late Pleistocene fans grouped on the basis of soils and geomorphic criteria may be significant. Based on these data, the southernmost segment of the Elsinore fault has sustained a slip rate of 2.4 ± 0.4 mm/y for the past 60–70 ka and probably for the past 150 ka. Because displacement in the most recent surface rupture increases northwest of our slip rate sites, this rate is likely a minimum for the southern Elsinore fault, with the actual rate more likely close to 3 mm/y in the central part of the range. These new data confirm that slip gradients along individual fault segments must be considered when estimating pre-Holocene slip rates for seismic hazard estimates. These new results show that the southern Elsinore fault accounts for about 6% of the total relative motion between North America and the Pacific lithospheric plates in southernmost California. Assessment of previous estimates of slip in the most recent event suggests earthquakes of about Mw 6.8 and, when combined with the slip rate data, a recurrence of such events about every thousand years.

The Effect of Arc Proximity on Hydrothermal Activity Along Spreading Centers: New Evidence From the Mariana Back Arc (12.7°N–18.3°N)

AbstractBack‐arc spreading centers (BASCs) form a distinct class of ocean spreading ridges distinguished by steep along‐axis gradients in spreading rate and by additional magma supplied through subduction. These characteristics can affect the population and distribution of hydrothermal activity on BASCs compared to mid‐ocean ridges (MORs). To investigate this hypothesis, we comprehensively explored 600 km of the southern half of the Mariana BASC. We used water column mapping and seafloor imaging to identify 19 active vent sites, an increase of 13 over the current listing in the InterRidge Database (IRDB), on the bathymetric highs of 7 of the 11 segments. We identified both high and low (i.e., characterized by a weak or negligible particle plume) temperature discharge occurring on segment types spanning dominantly magmatic to dominantly tectonic. Active sites are concentrated on the two southernmost segments, where distance to the adjacent arc is shortest (&lt;40 km), spreading rate is highest (&gt;48 mm/yr), and tectonic extension is pervasive. Re‐examination of hydrothermal data from other BASCs supports the generalization that hydrothermal site density increases on segments &lt;90 km from an adjacent arc. Although exploration quality varies greatly among BASCs, present data suggest that, for a given spreading rate, the mean spatial density of hydrothermal activity varies little between MORs and BASCs. The present global database, however, may be misleading. On both BASCs and MORs, the spatial density of hydrothermal sites mapped by high‐quality water‐column surveys is 2–7 times greater than predicted by the existing IRDB trend of site density versus spreading rate.