Median Valley Research Articles

Structure and dynamics of the Ecuador Fracture Zone, Panama Basin

SUMMARYIn this study, multiple geophysical data types are used to investigate the structure and dynamics of the Ecuador Fracture Zone—a complex multistranded strike-slip fault system located in the Panama Basin. Gravity modelling reveals a 25–30-km-wide region of ∼3-km-thick, low-density crust beneath this system and an anomalously low-density region in the uppermost mantle. Along both edges, the transition to the ‘normal’ structure and thickness oceanic crust formed at both the Ecuador and Costa Rica Rifts is abrupt. Within the Ecuador Fracture Zone itself, normal faults bound the median ridges. These faults traverse the entire thickness of accumulated sediment and offset the seabed, while sediment layer geometries document multiple phases of relative uplift, with the most recent phase still ongoing. Active extensional faulting, with an approximately spreading ridge-parallel strike, is also observed in 6–7 Ma Costa Rica Rift crust. The median ridges and the transverse ridge at the eastern edge of the Ecuador Fracture Zone also have contrasting crustal density structures. Both median ridges have a lower density crust than between the intervening valleys, while the transverse ridge crust has an equivalent thickness and density structure to that formed at the Costa Rica Rift. The active median valley basement-cutting normal faults allow seawater ingress and alternation of the crustal footwall, and also flow to mantle depth where, based on gravity modelling, 30–50 per cent serpentinization of mantle peridotite occurs. The resulting serpentinite-driven buoyancy acts as the primary control on the observed median ridge relative vertical tectonism. In contrast, the relative uplift of the transverse ridge results from lithospheric flexure in response to a change in spreading direction between the Ecuador and Costa Rica Rifts. Contrary to the widely accepted assumption that fracture zones are tectonically inactive systems, the Ecuador Fracture Zone provides evidence of extension, serpentinization due to ongoing hydrothermal circulation and relative uplift.

New fossil remains of Rhinocerotidae (Perissodactyla) from the early Late Miocene Tebingan area, central Myanmar

ABSTRACT New fossil remains of Rhinocerotidae, dated from the early Late Miocene, were discovered from the lowermost part of the Irrawaddy Formation in the Tebingan area, Magway Region, central Myanmar. The Tebingan rhinoceros fossils, consisting of isolated teeth, maxillae, and mandibles, are assigned to three taxa: Brachypotherium perimense defined by a constricted protocone, a well-developed parastyle and broad upper teeth, to ‘Brachypotherium’ fatehjangense with a flat ectoloph, crochet, and the absence of tubercle at the entrance of the median valley. Other specimens were assigned to Rhinoceros sp. due to having a developed parastyle, crochet and a strong anterior cingulum and the absence of a constricted protocone and ante-crochet. The evolutionary history of Rhinocerotidae is poorly known in Southeast Asia, and few rhinoceros’ species have been identified from the Irrawaddy Formation. The newly identified Rhinoceros specimens from the early Late Miocene Tebingan area are the oldest fossil records for Rhinoceros in Southeast Asia. The Tebingan mammal fauna is similar to the Nagri and Dhok Pathan faunas of the lower/middle Siwaliks of the Indian subcontinent, indicating the faunal exchange between these two regions in the late Neogene.

Using geomorphometric approach to investigate spatial pattern and intensity of erosional dissection in a block-faulted topography (Orlickie-Bystrzyckie Mountains, Central Europe)

The paper presents an attempt to recognize areas of enhanced erosional dissection, based on a combined analysis of several geomorphometric variables and using four different approaches (rank-based, cold and hot spot analysis, k-means clustering of ‘raw’ data, and k-means clustering of local measures of spatial autocorrelation). The study area includes the Orlickie-Bystrzyckie Mountains Block (OBMB), which is a block-faulted range within the Sudetes Mountains, Central Europe, typified by fault-generated range fronts, remnant planation surfaces and topographic ramps due to large-scale tilting. Geology is diverse, with basement rocks and sedimentary cover, and the relevant timescale is late Cenozoic. Morphometric variables derived from a LiDAR-based DEM include contour length, standard deviation of elevation, median valley depth and standard deviation of curvature, and are considered to account for various morphometric signatures of erosional dissection. While deriving compound measures of dissection, three grid cell sizes were considered. Within the OBMB several areas were identified as erosionally dissected to a considerable degree. Their spatial pattern is complex and reflects several factors simultaneously. Differential uplift appears as the main trigger, but the erosional response varies, depending mainly on the size of runoff contributing area – itself related to the pattern of uplift, and the distance from the margins of elevated areas. Lithology (bedrock resistance to erosion) seems to play a subordinate role. We also discuss various issues related to the procedures themselves, pointing out their advantages and limitations. Procedural setups proposed in this paper are not site-specific and may be used in various topographic and geological settings.

Oceanic crustal structures and temporal variations of magmatic budget during seafloor spreading in the East Sub-basin of the South China Sea

Following magma-poor continental rifting, the South China Sea (SCS) experienced seafloor spreading from ~33 Ma to ~15 Ma with an intervening southward ridge jump at ~25 Ma, generating the oceanic East, Northwest and Southwest Sub-basins. However, the magmatic processes during spreading remain poorly known owing to the lack of oceanic crustal images. Here we present a composite profile comprising three seismic lines that run from the northern continent-ocean transition (COT) to the extinct spreading center of the East Sub-basin to reveal the crustal structures. In the northernmost oceanic portion, the first oceanic crust featuring rough basement with numerous faults, and diffuse and weak Moho reflector, is only 3.9–4.5 km thick, implying relatively low magmatic budget and protracted tectonic extension continued from the magma-poor continental rifting to initial spreading. From the northern COT ocean-ward to the abandoned ridge where the jump event occurred, the basement becomes smooth with subdued faults. The Moho reflector tends to be continuous and strong, and the igneous crust gradually becomes thick (locally up to 8.4 km), suggesting considerably increasing magmatic budget during the early spreading stage. However, in the central portion spanning from the abandoned ridge to the extinct spreading axis, the basement becomes rough with crustal faults again. And the crust gradually becomes thin, to only 3.7 km at the extinct spreading center where a deep median valley is still visible, though severely altered by post-spreading volcanism, implying decreasing magmatic budget and increasing tectonic extension during the late spreading stage. Therefore, the crustal structure variations in the East Sub-basin reflects a prominent increasing to decreasing change of magmatic budget during spreading with the peak around the intervening ridge jump. Thin, tectonized crust and deep median valley around the extinct spreading center further support slow seafloor spreading during the last spreading stage of the SCS which ended at ~15–16 Ma, and also suggest that magma contribution from the Hainan mantle plume might be very limited if any.

Seismic Evidence for Tectonically Dominated Seafloor Spreading in the Southwest Sub‐basin of the South China Sea

AbstractSeafloor spreading can be explained by different dynamic mechanisms, magmatically or tectonically dominated. The Southwest Sub‐basin, located at the southwest tip of the propagating seafloor spreading of the South China Sea, remains unclear for its spreading regime owing to poor and debated knowledge of the crustal structures. Here two multichannel seismic lines and one oceanic bottom seismometer line across this subbasin are reprocessed and analyzed with focus on crustal imaging. The sediments are usually 0.5–1.0 km thick over the abyssal basin and slightly thicker in the few grabens. The thickest (3.3 km) sediments are found in the fossil spreading center. The basement is fairly rough and highly faulted by ubiquitous crustal faults. The fossil spreading center is characterized by a deep median valley, similar to that of magma‐poor spreading cases. Both multichannel seismic lines show a few intermittent and diffusive reflectors at 1.5‐ to 3.6‐km depth below the fragmented basement. These reflectors, correlating well with the velocities of 6.8–7.2 km/s obtained from velocity inversion of oceanic bottom seismometer data, are interpreted as Moho reflections. Thus, the inferred crustal thickness is only 1.5–3.6 km excluding the sediments, which is much thinner than usual. The underlying mantle with velocities lower than 8.0 km/s might be serpentinized with water infiltration along these crustal faults. Therefore, it is proposed that the spreading of the Southwest Sub‐basin was tectonically dominated.

Lithospheric Structure and Tectonic Processes Constrained by Microearthquake Activity at the Central Ultraslow‐Spreading Southwest Indian Ridge (49.2° to 50.8°E)

AbstractBeneath ultraslow‐spreading ridges, the oceanic lithosphere remains poorly understood. Using recordings from a temporary array of ocean bottom seismometers, we here report an ~17‐days‐long microearthquake study on two segments (27 and 28) of the ultraslow‐spreading Southwest Indian Ridge (49.2° to 50.8°E). A total of 214 locatable microearthquakes are recorded; seismic activity appears to be concentrated within the west median valley at Segment 28 and adjacent nontransform discontinuities. Earthquakes reach a maximum depth of ~20 km beneath the seafloor, and they mainly occur in the mantle, implying a cold and thick brittle lithosphere. The relatively uniform brittle/ductile boundary beneath Segment 28 suggests that there is no focused melting in this region. The majority of earthquakes is located below the Moho interface, and a 5‐km‐thick aseismic zone is present beneath Segment 28 and adjacent nontransform discontinuities. At the Dragon Flag hydrothermal vent field along Segment 28, the presence of a detachment fault has been inferred from geomorphic features and seismic tomography. Our seismicity data show that this detachment fault deeply penetrates into the mantle with a steeply dipping (~65°) interface, and it appears to rotate to a lower angle in the upper crust, with ~55° of rollover. There is a virtual seismic gap beneath magmatic Segment 27, which may be connected to the presence of an axial magma chamber beneath the spreading center and focused melting; in this scenario, the increased magma supply produces a broad, elevated temperature environment, which suppresses earthquake generation.

Nature of crust in the central Red Sea

A transition between continental crust in the northern Red Sea and oceanic crust in the southern Red Sea coincides broadly with a southward increase in plate tectonic separation rate and with a decrease in upper mantle seismic velocity. We re-evaluate here the nature of crust in the intervening central Red Sea with the results of legacy seismic refraction experiments and recently released marine gravity anomalies derived from satellite altimeter measurements. In the refraction data, collected east of Thetis Deep, velocities of 6.6–6.9kms−1 of a deep refracting layer, which are similar to measured velocities of unaltered gabbro samples, extend outside the deep to 65km from the axis. The new version of the marine gravity field reveals trends crossing the central Red Sea. Whereas some of them connect with major lineaments in the surrounding African-Arabian shield, those around Thetis Deep die out towards the coastlines. They can be paired across the ridge and lie slightly oblique to plate motions, as is typical of oceanic fracture zones or non-transform discontinuities migrating away from hotspots. Taken together these observations support the view that an oceanic rather than extended continental crust underlies this part of the central Red Sea.The crestal mountains around the median valleys of slow-spreading ridges are typically 500–1000m lower at spreading discontinuities. Around Thetis Deep, the similar pattern in the gravity field to those of slow-spreading ridges suggests that the crestal mountains may variably block or impede flowage of evaporites towards the spreading centre, whereas the discontinuities may mark areas where flowage is unobstructed. Limited multibeam data collected in transits outside Thetis Deep show oblique fabrics as expected from these predicted movements.

Constraints on the shallow velocity structure of the Lucky Strike Volcano, Mid-Atlantic Ridge, from downward continued multichannel streamer data

The shallow velocity structure of the Lucky Strike segment of the Mid-Atlantic Ridge is investigated using seismic refraction and reflection techniques applied to downward continued multichannel streamer data. We present a three-dimensional velocity model beneath the Lucky Strike Volcano with unprecedented spatial resolutions of a few hundred meters. These new constraints reveal large lateral variations in P wave velocity structure beneath this feature. Throughout the study area, uppermost crustal velocities are significantly lower than those inferred from lower resolution ocean bottom seismometer studies, with the lowest values (1.8–2.2 km/s) found beneath the three central volcanic cones. Within the central volcano, distinct shallow units are mapped that likely represent a systematic process such as burial of older altered surfaces. We infer that the entire upper part of the central volcano is young relative to the underlying median valley floor and that there has been little increase in the layer 2A velocities since emplacement. Layer 2A thins significantly across the axial valley bounding faults likely as the result of footwall uplift. The upper crustal velocities increase with age, on average, at a rate of ~0.875 km/s/Myr, similar to previous measurements at fast-spreading ridges, suggesting hydrothermal sealing of small-scale porosity is progressing at normal to enhanced rates.

Cretaceous to present kinematics of the Indian, African and Seychelles plates

An iterative inverse model of seafloor spreading data from the Mascarene and Madagascar basins and the flanks of the Carlsberg Ridge describes a continuous history of Indian–African Plate divergence since 84 Ma. Visual-fit modelling of conjugate magnetic anomaly data from near the Seychelles platform and Laxmi Ridge documents rapid rotation of a Seychelles Plate about a nearby Euler pole in Palaeocene times. As the Euler pole migrated during this rotation, the Amirante Trench on the western side of the plate accommodated first convergence and later divergence with the African Plate. The unusual present-day morphology of the Amirante Trench and neighbouring Amirante Banks can be related to crustal thickening by thrusting and folding during the convergent phase and the subsequent development of a spreading centre with a median valley during the divergent phase. The model fits FZ trends in the north Arabian and east Somali basins, suggesting that they formed in India–Africa Plate divergence. Seafloor fabric in and between the basins shows that they initially hosted a segmented spreading ridge that accommodated slow plate divergence until 71–69 Ma, and that upon arrival of the Deccan–Réunion plume and an increase to faster plate divergence rates in the period 69–65 Ma, segments of the ridge lengthened and propagated. Ridge propagation into the Indian continental margin led first to the formation of the Laxmi Basin, which accompanied extensive volcanism onshore at the Deccan Traps and offshore at the Saurashtra High and Somnath Ridge. A second propagation episode initiated the ancestral Carlsberg Ridge at which Seychelles–India and India–Africa Plate motions were accommodated. With the completion of this propagation, the plate boundaries in the Mascarene Basin were abandoned. Seafloor spreading between this time and the present has been accommodated solely at the Carlsberg Ridge.

Moytirra: Discovery of the first known deep‐sea hydrothermal vent field on the slow‐spreading Mid‐Atlantic Ridge north of the Azores

Geological, biological, morphological, and hydrochemical data are presented for the newly discovered Moytirra vent field at 45oN. This is the only high temperature hydrothermal vent known between the Azores and Iceland, in the North Atlantic and is located on a slow to ultraslow‐spreading mid‐ocean ridge uniquely situated on the 300 m high fault scarp of the eastern axial wall, 3.5 km from the axial volcanic ridge crest. Furthermore, the Moytirra vent field is, unusually for tectonically controlled hydrothermal vents systems, basalt hosted and perched midway up on the median valley wall and presumably heated by an off‐axis magma chamber. The Moytirra vent field consists of an alignment of four sites of venting, three actively emitting “black smoke,” producing a complex of chimneys and beehive diffusers. The largest chimney is 18 m tall and vigorously venting. The vent fauna described here are the only ones documented for the North Atlantic (Azores to Reykjanes Ridge) and significantly expands our knowledge of North Atlantic biodiversity. The surfaces of the vent chimneys are occupied by aggregations of gastropods (Peltospira sp.) and populations of alvinocaridid shrimp (Mirocaris sp. with Rimicaris sp. also present). Other fauna present include bythograeid crabs (Segonzacia sp.) and zoarcid fish (Pachycara sp.), but bathymodiolin mussels and actinostolid anemones were not observed in the vent field. The discovery of the Moytirra vent field therefore expands the known latitudinal distributions of several vent‐endemic genera in the north Atlantic, and reveals faunal affinities with vents south of the Azores rather than north of Iceland.

Microseismicity of the Mid‐Atlantic Ridge at 7°S–8°15′S and at the Logatchev Massif oceanic core complex at 14°40′N–14°50′N

Lithospheric formation at slow spreading rates is heterogeneous with multiple modalities, favoring symmetric spreading where magmatism dominates or core complex and inside corner high formation where tectonics dominate. We report microseismicity from three deployments of seismic networks at the Mid‐Atlantic Ridge (MAR). Two networks surveyed the MAR near 7°S in the vicinity of the Ascension transform fault. Three inside corner high settings were investigated. However, they remained seismically largely inactive and major seismic activity occurred along the center of the median valley. In contrast, at the Logatchev Massif core complex at 14°45′N seismicity was sparse within the center of the median valley but concentrated along the eastern rift mountains just west of the serpentine hosted Logatchev hydrothermal vent field. To the north and south of the massif, however, seismic activity occurred along the ridge axis, emphasizing the asymmetry of seismicity at the Logatchev segment. Focal mechanisms indicated a large number of reverse faulting events occurring in the vicinity of the vent field at 3–5 km depth, which we interpret to reflect volume expansion accompanying serpentinization. At shallower depth of 2–4 km, some earthquakes in the vicinity of the vent field showed normal faulting behavior, suggesting that normal faults facilitates hydrothermal circulation feeding the vent field. Further, a second set of cross‐cutting faults occurred, indicating that the surface location of the field is controlled by local fault systems.

A geophysical study of oceanic core complexes and surrounding terrain, Mid‐Atlantic Ridge 13°N–14°N

We describe a geophysical study of oceanic core complexes (OCC) and surrounding seafloor on the Mid‐Atlantic Ridge at 13°N–14°N and off‐axis to ∼1.9 Myr. Data include a detailed, deep‐towed side scan sonar, magnetic field and bathymetry survey, supplemented by concurrent sea‐surface bathymetry, magnetic field and gravity measurements. Using side scan and bathymetry, we infer areas and relative ages of seafloor volcanism, revealing a complex pattern of melt accretion across the median valley including close to its walls. We estimate tectonic and magmatic extension throughout the area, and find that average tectonic extension since chron 2 on plates containing OCCs is up to three times that on their conjugates. Deep‐towed magnetic data reveal asymmetric spreading (faster on OCC‐containing plates) and crustal magnetization that is highly heterogeneous on a scale of ∼5 km, suggesting that exhumed domes of OCCs have highly variable lithologies, perhaps comprising both serpentinized peridotite and gabbro. Improved fits to magnetic data are provided by models incorporating ∼45°of OCC footwall rotation. An axial zone of normal magnetization, of presumed Brunhes epoch, has highly variable width and amplitude, with parts of the ridge axis displaying very low or apparently reversed magnetization. Gravity requires that OCCs have dense cores capped by lower density zones several kilometers thick. Gravity data indicate longer term patterns of crustal thickness and melt distribution that are broadly consistent with numerical models of OCC formation and show that waxing magmatism may terminate OCCs.

The 3-D geometry of detachment faulting at mid-ocean ridges

Seismic images of Cretaceous slow spreading crust from the eastern Central Atlantic provide new constraints on the process of seafloor spreading and the importance of detachment faulting. The seismic depth sections image detachment faults that appear to have exhumed footwall massifs of similar geometry to massifs of plutonic and mantle rocks mapped at the present Mid-Atlantic Ridge. The detachments are consistent with the structure and microearthquakes of the Mid-Atlantic Ridge at 26°N, with the footwall rotation inferred from paleomagnetic data and with numerical modeling of oceanic detachments. Other seismically imaged detachments have similar dimensions and geometry, but are covered by a layer of small fault blocks. The detachment types differ in whether or not the fault locks up in the subsurface, probably controlled by the fault strength, the elastic thickness, and whether the exhumed footwall is partly covered by basalts. Toward the segment middle, decreasing mantle serpentinization, decreasing elastic thickness, and thicker median valley basalts all increase the likelihood that the fault locks up, and a new fault propagates upwards from the still active root zone, transferring a slice of the hanging wall to the footwall, to be rafted with the footwall out of the median valley. As a result an oceanic detachment fault, exhuming the footwall at a segment end to form an oceanic core complex, may disappear laterally beneath rafted blocks; detachment faulting may be more widespread at slow spreading ridges than interpreted from seafloor mapping.

Structure and development of an axial volcanic ridge: Mid-Atlantic Ridge, 45°N

We describe the most comprehensive and detailed high resolution survey of an axial volcanic ridge (AVR) ever conducted, at 45°N on the Mid-Atlantic Ridge. We use 3 m resolution sidescan sonar, deep-towed magnetic field measurements, video observations from eleven ROV dives, and two very-high-resolution bathymetry and magnetic surveys. The most recently active AVR has high topographic relief, high acoustic backscatter, high crustal magnetization and little faulting. It is sharp-crested, 25 × 4 km in extent and 500 m high, and is covered by approximately 8000 volcanic “hummocks” whose detailed nature is revealed for the first time. Each is an individual volcano ≤ 450 m in diameter and ≤ 200 m high, ranging from steep-sided (45°) cones to low domes. Many have suffered significant flank collapse. Hummocks tend to align in rows parallel to the AVR axis, parallel to its NE-trending spurs or, on its lower flanks, sub-normal to the AVR trend. These latter are spaced 1–2 km apart and comprise 1–2 km-long rows of single volcanoes. We infer that their emplacement is controlled by down-flank magma transport, possibly via lava tubes. The AVR contains only one large flat-topped seamount. The flanking median valley floor consists of either older hummocky volcanic terrain or flat-lying, mostly sediment-covered lavas. These typically have low-relief lobate surfaces, inflation and collapse structures, and occasional lava tubes and tumuli. The AVR displays open fissures, mostly along its crest. There is direct evidence for only a few small faults on the AVR, though steep, outward-facing slopes draped by elongate pillows may be small normal faults covered by lava. The surrounding median valley floor is heavily fissured. Normal faults cut it and an older AVR, the latter displaying significant outward facing faults. High crustal magnetization, an approximate proxy for crustal age within the Brunhes, is confined to the active AVR. Magnetic palaeointensity measurements are consistent with ages up to ~ 12 ka for several samples from the active AVR and ≥ 12 ka for one sample from the median valley floor. This is much less than the predicted spreading age, implying distribution of melt off-axis or episodic AVR growth.

Seismic layer 2A variations in the Lucky Strike segment at the Mid‐Atlantic Ridge from reflection measurements

The SISMOMAR experiment carried out seismic measurements of the slow spreading Lucky Strike segment of the Mid‐Atlantic Ridge, located approximately 300 km south of the Azores platform. We present results from a segment scale reflection study of seismic layer 2A that is centered on the Lucky Strike volcano and covers the central 54 km of the 70 km long segment and extends 30 km on both sides of the ridge axis. Our study allows new conclusions about the role of tectonic, magmatic and hydrothermal processes in shaping the upper crustal structure of the Lucky Strike segment. The seismic reflection measurements show an almost constant layer 2A two‐way time within the median valley and an abrupt layer 2A two‐way time decrease off‐axis. This two‐way time decrease is caused by a layer 2A velocity increase on the order of 1 km/s. The uniform two‐way time in the median valley results from a constant thickness layer 2A, which could be caused by the existence of a porosity threshold linked to the lithologic lava/dike boundary. The regularity of the layer 2A/2B interface within the median valley indicates that layer 2A is built entirely inside the median valley. The off‐axis velocity increase is consistent with hydrothermal alteration and the aging of the crust.

Seismic structure of an oceanic core complex at the Mid‐Atlantic Ridge, 22°19′N

We present results from a seismic refraction and wide‐angle experiment surveying an oceanic core complex on the Mid‐Atlantic Ridge at 22°19′N. Oceanic core complexes are settings where petrological sampling found exposed lower crustal and upper mantle rocks, exhumed by asymmetric crustal accretion involving detachment faulting at magmatically starved ridge sections. Tomographic inversion of our seismic data yielded lateral variations of P wave velocity within the upper 3 to 4 km of the lithosphere across the median valley. A joint modeling procedure of seismic P wave travel times and marine gravity field data was used to constrain crustal thickness variations and the structure of the uppermost mantle. A gradual increase of seismic velocities from the median valley to the east is connected to aging of the oceanic crust, while a rapid change of seismic velocities at the western ridge flank indicates profound differences in lithology between conjugated ridge flanks, caused by un‐roofing lower crust rocks. Under the core complex crust is approximately 40% thinner than in the median valley and under the conjugated eastern flank. Clear PmP reflections turning under the western ridge flank suggest the creation of a Moho boundary and hence continuous magmatic accretion during core complex formation.

Anchitherium(Mammalia, Perissodactyla, Equidae) from the early Miocene Hiramaki Formation, Gifu Prefecture, Japan, and its implication for the early diversification of AsianAnchitherium

AnAnchitheriumspecimen with a nearly complete series of the upper cheek teeth (P2–M3), from the upper part (ca.17–18 Ma) of the Hiramaki Formation, Kani Basin, Gifu Prefecture, Japan, was previously referred toAnchitherium“hypohippoides” nomen dubium. Despite that it provides one of the best examples of significant dental characters of AsianAnchitherium, it has remained undescribed and unprepared until recently. Although a paucity of materials from Asia makes the taxonomy of AsianAnchitheriumdifficult to assign, comparison showed that the specimen should be reassigned toAnchitheriumaff.A. gobiense; it differs fromA. aurelianenseand is rather similar toA. gobiensefrom China by virtue of large size and expanded hypostyles. The JapaneseAnchitheriumalso shows distinct features including straight (flattened) ectolophs with narrow mesostyles, rudimentary crochets, and enamel protuberances at the lingual mouth of the median valleys. This combination of accessory features has not been known in AsianAnchitheriumand seems to be rarely observed among the diversified European species. The existence of JapaneseAnchitheriumimplies early species diversification in East Asia that predates a greater diversification in Europe associated with the Middle Miocene Climatic Optimum and supports paleogeographical and paleozoological connection to the Asian mainland under a warm and humid climate prior to the formation of the Japanese archipelago (ca. 16.5 Ma).

Anchitherium (Mammalia, Perissodactyla, Equidae) from the early Miocene Hiramaki Formation, Gifu Prefecture, Japan, and its implication for the early diversification of Asian Anchitherium

An Anchitherium specimen with a nearly complete series of the upper cheek teeth (P2–M3), from the upper part (ca.17–18 Ma) of the Hiramaki Formation, Kani Basin, Gifu Prefecture, Japan, was previously referred to Anchitherium “hypohippoides” nomen dubium. Despite that it provides one of the best examples of significant dental characters of Asian Anchitherium, it has remained undescribed and unprepared until recently. Although a paucity of materials from Asia makes the taxonomy of Asian Anchitherium difficult to assign, comparison showed that the specimen should be reassigned to Anchitherium aff. A. gobiense; it differs from A. aurelianense and is rather similar to A. gobiense from China by virtue of large size and expanded hypostyles. The Japanese Anchitherium also shows distinct features including straight (flattened) ectolophs with narrow mesostyles, rudimentary crochets, and enamel protuberances at the lingual mouth of the median valleys. This combination of accessory features has not been known in Asian Anchitherium and seems to be rarely observed among the diversified European species. The existence of Japanese Anchitherium implies early species diversification in East Asia that predates a greater diversification in Europe associated with the Middle Miocene Climatic Optimum and supports paleogeographical and paleozoological connection to the Asian mainland under a warm and humid climate prior to the formation of the Japanese archipelago (ca. 16.5 Ma).

Upper crustal velocity structure beneath the central Lucky Strike Segment from seismic refraction measurements

We present a three‐dimensional velocity model of the upper crust around the central volcano of the Lucky Strike Segment, Mid‐Atlantic Ridge. The model, constructed from a 3‐D array of air gun shots (37.5 m spacing along line and 100 m between lines) to ocean bottom seismometers fired during a 3‐D seismic reflection survey, shows an off‐axis velocity increase (∼1 km/s), a low‐velocity region within the median valley, and a low‐velocity anomaly underneath the Lucky Strike volcano. Our observations indicate a porosity decrease of 1%–9% (corresponding to a velocity increase of ∼0.5–1 km/s) over a distance of 8 km from the ridge axis (∼0.7 Ma) and a porosity decrease of 4%–11% (corresponding to a velocity increase of ∼2 km/s) between a depth of 0.5 and 1.75 km below seafloor. A sinusoidal variation in the traveltime residuals indicates the presence of azimuthal anisotropy with cracks aligned approximately along the ridge axis. We favor an interpretation in which upper crustal porosities are created by a combination of magmatic accretion (lava–sheeted dike boundary) and active extension (faults, fractures, and fissures). The porosity variation with depth probably depends on pore space collapse, hydrothermal alteration, and a change of stress accommodation. The off‐axis porosities are possibly influenced by both hydrothermal precipitation and the aging of the crust.

Crustal velocity structure of the Lucky Strike segment of the Mid‐Atlantic Ridge at 37°N from seismic refraction measurements

We estimate the seismic structure of the slow spreading Lucky Strike segment of the Mid‐Atlantic Ridge, located approximately 300 km south of the Azores platform, using seismic reflection and seismic refraction data acquired in June 2005 as a part of the Seismic Study for Monitoring of the Mid‐Atlantic Ridge (SISMOMAR) survey. The three‐dimensional velocity model shows an upper crustal low‐velocity anomaly running parallel to the ridge axis, which is limited by the median valley bounding faults. The velocity models also show a low‐velocity anomaly underlying the axial melt lens reflector located at the segment center below the Lucky Strike volcano. This lower crustal low‐velocity region can be explained by elevated temperatures and possibly small amounts of melt. The lower crustal low‐velocity anomaly and the axial melt lens reflector constrain the geometry of the magma chamber responsible for the construction of the Lucky Strike volcano. The presence of this magma chamber and thick crust at the segment center are consistent with a focused melt supply to the segment center.