Ridge Intersection Research Articles

Local Extraction of 3D Time‐Dependent Vector Field Topology

AbstractWe present an approach to local extraction of 3D time‐dependent vector field topology. In this concept, Lagrangian coherent structures, which represent the separating manifolds in time‐dependent transport, correspond to generalized streak manifolds seeded along hyperbolic path surfaces (HPSs). Instead of expensive and numerically challenging direct computation of the HPSs by intersection of ridges in the forward and backward finite‐time Lyapunov exponent (FTLE) fields, our approach employs local extraction of respective candidates in the four‐dimensional space‐time domain. These candidates are subsequently refined toward the hyperbolic path surfaces, which provides unsteady equivalents of saddle‐type critical points, periodic orbits, and bifurcation lines from steady, traditional vector field topology. In contrast to FTLE‐based methods, we obtain an explicit geometric representation of the topological skeleton of the flow, which for steady flows coincides with the hyperbolic invariant manifolds of vector field topology. We evaluate our approach on analytical flows, as well as data from computational fluid dynamics, using the FTLE as a ground truth superset, i.e., we also show that FTLE ridges exhibit several types of false positives.

Effect of Teaching Session on Resident Ability to Identify Anatomic Landmarks and Anterior Cruciate Ligament Footprint: A Study Using 3-Dimensional Modeling.

Background:Femoral tunnel positioning in anterior cruciate ligament reconstruction (ACLR) is an intricate procedure that requires highly specific surgical skills.Purpose:To report the ability of residents to identify femoral landmarks and the native ACL footprint before and after a structured formal teaching session as a reflection of overall surgical skill training for orthopaedic surgery residents in Canada.Study Design:Controlled laboratory study.Methods:A total of 13 senior orthopaedic residents were asked to identify a femoral landmark and an ACL footprint on ten 3-dimensional (3D)–printed knee models before and after a teaching session during the fall of 2018. The 3D models were made based on actual patients with different anatomic morphologic features. ImageJ software was used to quantify the measurements, which were then analyzed through use of descriptive statistics.Results:Before and after the teaching session, residents attempted to identify a specific anatomic location (bifurcate and intercondylar ridge intersection) with a mean error per participant ranging from 5.00 to 10.95 mm and 4.79 to 12.13 mm in magnitude, respectively. Furthermore, before and after the teaching session, residents identified the specific position to perform the surgical procedure (ACL femoral footprint), with a mean error per participant ranging from 4.58 to 8.80 mm and 3.87 to 11.07 mm in magnitude, respectively. The teaching session resulted in no significant improvement in identification of either the intersection of the bifurcate and intercondylar ridges (P = .9343 in the proximal-distal axis and P = .8133 in the anteroposterior axis) or the center of the femoral footprint (P = .7761 in the proximal-distal axis and P = .9742 in the anteroposterior axis).Conclusion:Although a formal teaching session was combined with a hands-on session that entailed real surgical instrumentation and fresh cadaveric specimens, the intervention seemed to have no direct impact on senior residents’ performance or their ability to demonstrate the material taught. This puts into question the format and efficacy of present teaching methods. Also, it is possible that the 3D spatial perception required to perform these skills is not something that can be taught effectively through a teaching session or at all. Further investigation is required regarding the effectiveness and application of surgical skill laboratories and simulations on the competencies of orthopaedic residents.

Large-scale structure of the Doldrums multi-fault transform system (7-8ºN Equatorial Atlantic): preliminary results from the 45th expedition of the R/V A.N. Strakhov

The Equatorial portion of the Mid Atlantic Ridge is displaced by a series of large offset oceanic transforms, also called “megatransforms”. These transformdomains are characterized by a wide zone of deformation that may include different conjugated fault systems and intra-transform spreading centers (ITRs).Among these megatransforms, the Doldrums system (7-8oN) is arguably the less studied, although it may be considered the most magmatically active. Newgeophysical data and rock samples were recently collected during the 45th expedition of the R/V Akademik Nikolaj Strakhov. Preliminary cruise results allowto reconstruct the large-scale structure and the tectonic evolution of this poorly-known feature of the Equatorial Atlantic. Swath bathymetry data, coupled withextensive dredging, were collected along the entire megatransform domain, covering an area of approximately 29,000 km2. The new data clearly indicate thatthe Doldrums is an extremely complex transform system that includes 4 active ITRs bounded by 5 fracture zones. Although the axial depth decreases toward thecentral part of the system, recent volcanism is significantly more abundant in the central ITRs when compared to that of the peripheral ITRs. Our preliminaryinterpretation is that a region of intense mantle melting is located in the central part of the Doldrums system as consequence of either a general transtensive regimeor the occurrence of a more fertile mantle domain. Large regions of basement exposure characterize the transform valleys and the ridge-transform intersections.We speculate that different mechanisms may be responsible for the exposure of basement rocks. These include the uplift of slivers of oceanic lithosphereby tectonic tilting (median and transverse ridges formation), the denudation of deformed gabbro and peridotite by detachment faulting at inner corner highs, andthe exposure of deep-seated rocks at the footwall of high-angle normal faults at the intersection of mid-ocean ridges with transform valleys

Ocean Floor Observation and Bathymetry System (OFOBS): A New Towed Camera/Sonar System for Deep-Sea Habitat Surveys

Towed camera systems are commonly used to collect photo and video images of the deep seafloor for a wide variety of purposes, from pure exploratory research to the development of management plans. Ongoing technological developments are increasing the quantity and quality of data collected from the deep seafloor. Despite these improvements, the area of seafloor, which towed systems can survey, optically remains limited by the rapid attenuation of visible wavelengths within water. We present an overview of a new towed camera platform integrating additional acoustical devices: the ocean floor observation and bathymetry system (OFOBS). The towed system maintains continuous direct communication via fiber optic cable with a support vessel, operational at depths up to 6000 m. In addition to collecting seafloor photo and video data, OFOBS gathers sidescan data over a 100-m swath width. OFOBS functionality is further augmented by a forward looking sonar, used to aid in hazard avoidance and real-time course correction. Data collected during the first field deployments of OFOBS, at a range of seamounts on the Langseth Ridge/Gakkel Ridge intersection (86° N, 61° E) in the high Arctic in September 2016, are presented to demonstrate the functionality of the system. Collected from a location with near continuous ice cover, this explanatory data set highlights the advantages of the system for deep-sea survey work in environments currently difficult to access for the majority of subsurface research platforms.

Endometriosis origin from primordial germ cells

ABSTRACTEndometriosis is defined by the presence of endometrial ectopia. Multiple hypotheses have been postulated to explain the etiology of endometriosis to understand various clinical evidences. The etiology of endometriosis is still unclear.The primary question to understanding the etiology of endometrial ectopia (endometriosis) is determining the origin of eutopic (normally cited) endometrium.According to the new theory, primordial germ cells migrate from hypoblast (yolk sac close to the allantois) to the gonadal ridges. The gonadal ridges which composed of primordial germ cells derive to the: eutopic endometrium, ovary, ovarian ligament and ligamentum teres uteri.There are 2 principal processes in uterine organogenesis: the intersection of gonadal ridges with mesonephral ducts to form the uterine folds with an endometrial cavity and the fusion of the both uterine folds together to form the unicavital (normal) uterus. In the uterine folds there are closer cell-to-cell communications, polypotential germ cells differentiate and grow into myometrium and endometrial layers.Some of the polypotential germ cells fail to reach the ridges and stay in the peritoneal cavity, where they may be transforming into external endometrial heterotopies.The main insight in the etiology of endometriosis is polypotential germ cells origin, which may explain its potency, pathogenesis and expansion.

Making and breaking an island arc: A new perspective from the Oligocene Kyushu‐Palau arc, Philippine Sea

The Kyushu‐Palau Ridge (KPR) is a 2600 km long remnant island arc that is separated from the active Izu‐Bonin‐Mariana (IBM) arc by a series of spreading and rift basins. We present 40Ar/39Ar ages and geochemical data for the entire length of the Kyushu‐Palau arc as well as for the conjugate arc which is stranded within the IBM fore arc. New 40Ar/39Ar ages indicate that the KPR was active between 25 and 48 Ma, but the majority of the exposed volcanism occurred in the final phase, between 25 and 28 Ma. Rifting of the Kyushu‐Palau arc to form the Shikoku and Parece Vela basins occurred simultaneously along the length of the arc (circa 25 Ma), and at a similar distance from the trench. Unlike the IBM, the KPR has only limited systematic along‐arc geochemical trends. Two geochemical components within the KPR indicate an origin in the suprasubduction mantle. First, EM‐1‐like lavas are identified in a restricted section of the arc, suggesting a localized heterogeneity. Second, EM‐2‐like arc volcanoes formed on juvenile West Philippine Basin crust, potentially reflecting ingress of mantle from the then active EM‐2 province which lies in the west. Another geochemical heterogeneity is found at the KPR–Daito Ridge intersection where the arc developed on preexisting Cretaceous Daito Ridge crust. The geochemical characteristics at this intersection likely result from the involvement of sub–Daito Ridge lithospheric mantle. Subduction flux beneath the KPR generally matches post–45 Ma Eocene/Oligocene lavas in the IBM fore arc, involving fluids and melts derived from altered igneous crust.

Early Silurian mafic–ultramafic and granitic plutonism in contemporaneous flysch, Magerøy, northern Norway: U–Pb ages and regional significance

The Caledonian Magerøy sequence comprises Earliest Silurian clastic metasedimentary rocks, which underwent folding and regional metamorphism and were intruded by a mafic–ultramafic complex and various granitic plutons. U–Pb ages of 438.2 ± 0.7 Ma for gabbro in the Honningsvåg Igneous Complex, 438.4 ± 0.9 Ma for an associated granite and 437.7 ± 1.6 Ma for the Finnvik granite coincide within error with the age of deposition inferred from fossils demonstrating a very rapid geological evolution. Plutons of peraluminous affinity are somewhat younger at 436.0 ± 1.0 Ma (Skarsvåg granite) and 434.5 ± 1.5 Ma (a granitic dyke in the Skarsvåg Nappe). The association of flysch deposition, folding and mafic–felsic magmatism suggests formation at a trench–ridge intersection. These Early Silurian events are the expression of a period of major magmatism in a variety of settings all along the Caledonides, probably reflecting rapid convergence and subduction during closure of the Iapetus Ocean. Re-evaluation of the palaeomagnetic signature of the Honningsvåg Igneous Complex indicates that at the time of formation the suite was located in an equatorial position, probably close to the Laurentian margin, and was subsequently translated southward by some 1350 km prior to and during emplacement onto the Baltic margin.

Early Carboniferous trigonoceratid nautilids from the Pitkin Formation of Arkansas, Midcontinent North America

ABSTRACT Two new species of trigonoceratid nautilids from the middle Chesterian (Early Carboniferous) Pitkin Formation of Arkansas, Midcontinent North America are described. Epistroboceras pitkinense sp. nov. is most similar to E. caneyense Niko and Mapes, 2004, but is distinguished mainly by its dorsally shifted umbilical angle. Stroboceras gordoni sp. nov. is characterized by the narrow lg 2 (second lateral groove) in the lateral grooves, the two bilaterally symmetrical lateral ridges, and the weak nodes at intersections of ridges and the growth lines. Species previously described or assigned to Stroboceras are re-examined. Similarities in the shell morphology between the trigonoceratid nautilid Stroboceras and the isochronous Early Carboniferous ammonoid Eumorphoceras probably provide an example of synchronized convergent evolution.

Geochemical and Isotopic Heterogeneities along an Island Arc-Spreading Ridge Intersection: Evidence from the Lewis Hills, Bay of Islands Ophiolite, Newfoundland

This study focuses on the origin of magma heterogeneity and the genesis of refractory, boninite-type magmas along an arc---ridge intersection, exposed in the Lewis Hills (Bay of Islands Ophiolite). The Lewis Hills contain the fossil fracture zone contact between a split island arc and its related marginal oceanic basin. Three types of intrusions, which are closely related to this narrow tectonic boundary, have been investigated. Parental melts in equilibrium with the ultramafic cumulates of the Pyroxenite Suite are inferred to have high MgO contents and low Al2O3, Na2O and TiO2 contents. The trace element signatures of these Pyroxenite Suite parental melts indicate a re-enriched, highly depleted source with 0 1 mid-ocean ridge basalt (MORB) abundances of the heavy rare earth elements (HREE). Initial eNd values of the Pyroxenite Suite range from y1 5 to ‡0 6, which overlap those observed for the island arc. Furthermore, the Pyroxenite Suite parental melts bear strong similarities to boninite-type equilibrium melts from island arc-related pyroxenitic dykes and harzburgites. Basaltic dykes split into two groups. Group I dykes have 0 6 MORB abundances of the HREE, and initial eNd values ranging from ‡5 4 to ‡7 5. Thus, they have a strong geochemical affinity with basalts derived from the marginal basin spreading ridge. Group II dykes have comparatively lower trace element abundances ( 0 3 MORB abundances of HREE), and slightly lower initial eNd values (‡5 4 to ‡5 9). The geochemical characteristics of the Group II dykes are transitional between those of Group I dykes and the Pyroxenite Suite parental melts. Cumulates from the Late Intrusion Suite are similarly transitional, with eNd values ranging from ‡2 9 to ‡4 6. We suggest that the magma heterogeneity observed in the Lewis Hills is due to the involvement of two compositionally distinct mantle sources, which are the sub-island lithospheric mantle and the asthenospheric marginal basin mantle. It is likely that the refractory, boninite-type parental melts of the Pyroxenite Suite result from remelting of the sub-arc lithospheric mantle at an arc---ridge intersection. Furthermore, it is suggested that the thermal-dynamic conditions of the transtensional transform fault have provided the prerequisite for generating magma heterogeneity, as a result of mixing relationships between arc-related and marginal basin-related magmas.

Mantle plumes and other structures of Siberian and East-European Platforms related to localization of giant mineral deposits

The structure of subcontinental mantle convection was discovered using direct and indirect indicators of mantle convection cell (MCC) boundaries (Kravchenko & Khain, 1990, 1996, 1997). The boundaries serve as hotlines, i.e., zones of ascent of hot mantle matter and plumes trained by triple junctions of MCC. This discovery let us predict the localization of Noril'sk type (Cu-Ni-Pt) ore deposits at the South-West of the Baikal rift, in the middle current of the Lena river and at North (Polar) and South of the Ural, and the localization of Tomtor type (REE-Nb-P) ore deposits in the intersections of Moho ridges (up to 15 km high) in the Northern Siberian Platform (Kravchenko et al., 1997; Khain & Kravchenko, 1999).The prediction related to the Polar Ural seems to be confirmed (there occurs the Pt-bearing dunite-clinopyroxenite massif Seum-Keu with diameter 120 km). The structure of the East-European Platform is of another type, which determines other, more complicated conditions of ore deposit localizations. The East-European Platform has a round, peripheral uplifted zone. Along this zone all known ore deposits, including giant ore deposits (Pechenga, Khibina, Lovozero, and others) are localized. The Ural geosyncline, related to one of four global lineament zones, is situated firmly to the east boundaries of the Platform. But laws of ore deposits localizations are the same for both the Siberian and East-European Platforms. They are the localization of ore deposits in the meridional rift zones, developed since early Riphean and repeatedly reactivated, and their disposition in intersections of transform faults or other rifts.

Oceanic inside corner detachments of the Limassol Forest area, Troodos ophiolite, Cyprus

Abstract: Flat-lying extensional detachment faults have been imaged in the inside corner regions of ridge–transform intersections on the Mid-Atlantic Ridge. Exposed detachment surfaces are 10 km or more across, and are corrugated in the direction of spreading, as are continental detachments. Beneath the detachments lie core complexes of peridotite and gabbro; these are overlain by blocks of crustal material. We argue here that similar detachments are an essential component of the Limassol Forest area of the Troodos ophiolite in Cyprus, which lies south of the Arakapas Fault zone, previously recognized as a palaeo-transform fault, and here interpreted as a transform fault that evolved into a fracture zone. In the Limassol Forest, core complexes of mantle peridotite can be shown to have been exposed at the sea floor, or to have been covered by overlapping crustal blocks, separated from the peridotite core and from each other by low-angle extensional faults. The extension can be shown to have occurred shortly after crustal construction, and the already extended terrain was then intruded by swarms of dykes and plutons. We interpret these relations as arising when crust is constructed in an inside corner area, extended by detachment faulting, deformed further during slip along the transform, and then intruded by new magma as it passes the second spreading centre. The structurally deeper parts of the crustal blocks that overlie the detachment lie broadly towards the west, indicating that the spreading axis lay in that direction. The ophiolite north of the transform is much less extended, and we interpret this as a section of outside corner crust. In this interpretation, the Troodos ophiolite formed to the east (in its current orientation) of a ridge–transform–ridge intersection, in which the transform had a dextral offset and sinistral slip. The part of the ophiolite that forms the Limassol Forest was produced at the western inside corner, and spread eastwards until it passed the second spreading axis, at which point the ophiolite north of the Arakapas Fault was created and welded to the Limassol Forest when the transform became a fracture zone.

Possible giant metamorphic core complex at the center of Artemis Corona, Venus

Hundreds of circular features on Venus known as coronae are characterized by annular fractures and commonly associated radial fractures and lava flows. Coronae are thought to have been produced by buoyant mantle diapirs that flatten and spread at the base of the lithosphere and cause fracturing, uplift, and magmatism. The interior of Artemis Corona, by far the largest corona at 2100 km diameter, is divided in half by a northeast-trending deformation belt that contains numerous rounded ridges resembling antiforms. The largest of these ridges, located at the center of Artemis Corona, is ∼5 km high on its steep northwest flank where it is adjacent to a flat-bottomed, 10-km-wide trough interpreted as a rift valley. The 280-km-long antiformal ridge is marked by perpendicular grooves that cross the ∼50-km-wide ridge and extend southeastward as far as 120 km across adjacent plains. The grooves abruptly terminate northwestward at the rift trough. The large antiformal ridge terminates southwestward at a transform shear zone that parallels the grooves. These features—rift valley, antiformal uplift, grooves, and transform shear zone—are morphologically and geometrically similar to grooved, elevated, submarine metamorphic core complexes on the inside corners of ridge-transform intersections of slow-spreading ridges on Earth. As with submarine core complexes, the grooved surface on Venus is interpreted as the footwall of a large-displacement normal fault, and the grooves are inferred to be the product of plastic molding of the footwall to irregularities on the underside of the hanging wall followed by tectonic exhumation of the molded grooves and conveyer-belt–like transport up and over the large antiform and across the southeastern plains. According to this interpretation, the trend of the grooves records the direction of extension, which is perpendicular to the thrusts at the leading edge of the annular thrust belt 1000 km to the southeast. Both may have formed at the same time as a result of uniform southeastward displacement of the southeastern half of the interior of Artemis Corona. The location of this grooved core complex at the center of Artemis Corona may reflect genesis above the buoyant, ascending tail of the corona-producing mantle diapir.

Tectonic segmentation of the North Andean margin: impact of the Carnegie Ridge collision

The North Andean convergent margin is a region of intense crustal deformation, with six great subduction earthquakes M w≥7.8 this century. The regional pattern of seismicity and volcanism shows a high degree of segmentation along strike of the Andes. Segments of steep slab subduction alternate with aseismic regions and segments of flat slab subduction. This segmentation is related to heterogeneity on the subducting Nazca Plate. In particular, the influence of the Carnegie Ridge collision is investigated. Four distinct seismotectonic regions can be distinguished: Region 1 – from 6°N to 2.5°N with steep ESE-dipping subduction and a narrow volcanic arc; Region 2 – from 2.5°N to 1°S showing an intermediate-depth seismic gap and a broad volcanic arc; Region 3 – from 1°S to 2°S with steep NE-dipping subduction, and a narrow volcanic arc; Region 4 – south of 2°S with flat subduction and no modern volcanic arc. The Carnegie Ridge has been colliding with the margin since at least 2 Ma based on examination of the basement uplift signal along trench-parallel transects. The subducted prolongation of Carnegie Ridge may extend up to 500 km from the trench as suggested by the seismic gap and the perturbed, broad volcanic arc. These findings conflict with previous tectonic models suggesting that the Carnegie Ridge entered the trench at 1 Ma. Furthermore, the anomalous geochemical (adakitic) signature of the volcanoes in the broad Ecuador volcanic arc and the seismicity pattern are proposed to be caused by lithospheric tears separating the buoyant, shallowly subducting Carnegie Ridge from segments of steep subduction in Regions 1 and 3. It is further suggested that Carnegie Ridge supports a local `flat slab' segment similar to that observed in Peru. The impact of the Carnegie Ridge collision on the upper plate causes transpressional deformation, extending inboard to beyond the volcanic arc with a modern level of seismicity comparable to the San Andreas fault system. The pattern of instrumental and historical seismicity indicates (1) great earthquakes on the northern and southern flanks of the colliding ridge, (2) a slight reduction in observed seismicity at the trench–ridge intersection, (3) increased stress far into the continent, and (4) a NNE displacement of the N. Andes block, to be further effects of the collision.

Structure and geology of the continental shelf of the Laptev Sea, Eastern Russian Arctic

The Laptev Sea is of great significance for studying the processes of the initial breakup of continents. It is the southern termination of the Gakkel spreading ridge and thus the location of structural features resulting from a continental margin/spreading ridge intersection. The present-day understanding of the Laptev Shelf geology is based on the Russian multichannel seismic reflection data and extrapolation of the terrestrial geology. Geologic and plate-kinematic data are used to constrain the interpretation of the seismic reflection data. The Laptev Rift System consists of several deep subsided rifts and high standing blocks of the basement. From west to east these are: the West Laptev and South Laptev rift basins, Ust' Lena Rift, East Laptev and Stolbovoi horsts, Bel'kov-Svyatoi Nos and Anisin rifts. The central and eastern parts of the shelf have the greatest contrasts in the gravity field ranging from −60 mGal over the rifts to 50 mGal over the horsts. The rifts contain up to five seismic stratigraphic units bounded by clear regional reflectors and underlain by folded heterogeneous basement. They are suggested to be Late Cretaceous to Holocene in age and reflect different stages of spreading ridge/continental margin interaction. The estimated total thickness of the rift-related sediments varies between 4 and 8–10 km while the sedimentary cover on the uplifts is significantly reduced and generally does not exceed 1–2 km. An eastward decrease of the total thickness of the sedimentary sections from about 10 km in the South Laptev Basin to 4–5 km in the Bel'kov-Svyatoi Nos Rift and the simplicity of the entire rift structure may indicate a rejuvenation of the rifts in the same direction. The entire rift system is covered by the uppermost seismic unit, which probably reflects a deceleration of the rifting during the last reorganization of the North American/Eurasian plate interaction since about 2 Ma.

Rift propagation, detachment faulting, and associated magmatism in Bransfield Strait, Antarctic Peninsula

Multichannel seismic data from Bransfield Strait, a young (<4 Ma?) marginal basin located at the NW tip of the Antarctic Peninsula, reveal a complex array of rifting styles short of clearly defined seafloor spreading. The strait is undergoing basinwide extension. While enriched mid‐ocean ridge basalts have been dredged from topographic highs in the NE part of the basin, seismic evidence does not support the formation of oceanic crust there. Acoustic signatures of volcanic crust at known axial deep highs extend laterally subseafloor; their distribution and style support propagation of volcanism and associated extension from NE to SW. Active extensional structures along both margins control a strong physiographic asymmetry. The South Shetland Islands margin is composed of a few, closely spaced, large‐offset normal faults; the gentler Antarctic Peninsula margin exhibits broader, distributed extension. To the NE, normal fault polarity reversal structures (NFPRs) dominate; listric faults sole into NW dipping, low‐angle reflectors interpreted as detachments. To the SW, larger NFPRs and convex‐up geometries up to ∼20 km across suggest broad uplift rather than detachment‐based extension. Such uplift may precedes seafloor volcanism and rapid associated deflation along detachments. NW‐SE trending accommodation zones also segment the basin along‐strike. Bransfield Strait extension may result both from plate boundary tectonics at the South Shetland Trench and at the evolving Antarctic‐Scotia plate boundary. Extension was probably initiated by onset of subduction rollback at the trench, as a slab window opened from the SW. Augmented extension in the NE part of the basin may be a response to left‐lateral transtension associated with reorganization of the triple junction at the Shackleton Fracture Zone‐South Scotia Ridge intersection.

Precise ages and isotopic constraints for the Lewis Hills (Bay of Islands Ophiolite): Preservation of an arc–spreading ridge intersection

Research Article| December 01, 1998 Precise ages and isotopic constraints for the Lewis Hills (Bay of Islands Ophiolite): Preservation of an arc–spreading ridge intersection Michaela Kurth; Michaela Kurth 1Institut für Mineralogie und Geochemie, Universität Köln, Zülpicher Strasse 49b, 50674 Köln, Germany2Max-Planck-Institut für Chemie, Abteilung Geochemie, Postfach 3060, 55020 Mainz, Germany Search for other works by this author on: GSW Google Scholar Andreas Sassen; Andreas Sassen 1Institut für Mineralogie und Geochemie, Universität Köln, Zülpicher Strasse 49b, 50674 Köln, Germany2Max-Planck-Institut für Chemie, Abteilung Geochemie, Postfach 3060, 55020 Mainz, Germany Search for other works by this author on: GSW Google Scholar Günter Suhr; Günter Suhr 1Institut für Mineralogie und Geochemie, Universität Köln, Zülpicher Strasse 49b, 50674 Köln, Germany2Max-Planck-Institut für Chemie, Abteilung Geochemie, Postfach 3060, 55020 Mainz, Germany Search for other works by this author on: GSW Google Scholar Klaus Mezger Klaus Mezger 2Max-Planck-Institut für Chemie, Abteilung Geochemie, Postfach 3060, 55020 Mainz, Germany3Institut für Mineralogie, Universität Münster, Corrensstrasse 24, 48149 Münster, Germany Search for other works by this author on: GSW Google Scholar Author and Article Information Michaela Kurth 1Institut für Mineralogie und Geochemie, Universität Köln, Zülpicher Strasse 49b, 50674 Köln, Germany2Max-Planck-Institut für Chemie, Abteilung Geochemie, Postfach 3060, 55020 Mainz, Germany Andreas Sassen 1Institut für Mineralogie und Geochemie, Universität Köln, Zülpicher Strasse 49b, 50674 Köln, Germany2Max-Planck-Institut für Chemie, Abteilung Geochemie, Postfach 3060, 55020 Mainz, Germany Günter Suhr 1Institut für Mineralogie und Geochemie, Universität Köln, Zülpicher Strasse 49b, 50674 Köln, Germany2Max-Planck-Institut für Chemie, Abteilung Geochemie, Postfach 3060, 55020 Mainz, Germany Klaus Mezger 2Max-Planck-Institut für Chemie, Abteilung Geochemie, Postfach 3060, 55020 Mainz, Germany3Institut für Mineralogie, Universität Münster, Corrensstrasse 24, 48149 Münster, Germany Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1998) 26 (12): 1127–1130. https://doi.org/10.1130/0091-7613(1998)026<1127:PAAICF>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Michaela Kurth, Andreas Sassen, Günter Suhr, Klaus Mezger; Precise ages and isotopic constraints for the Lewis Hills (Bay of Islands Ophiolite): Preservation of an arc–spreading ridge intersection. Geology 1998;; 26 (12): 1127–1130. doi: https://doi.org/10.1130/0091-7613(1998)026<1127:PAAICF>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The Lewis Hills, the southernmost massif of the Bay of Islands Ophiolite, exposes a prominent shear zone. The origin of this shear zone is pivotal with respect to the interpretation of the tectonic setting of the Lewis Hills and the genesis of the Bay of Islands Ophiolite. Different models suggest that the shear zone represents a fracture zone that separates either oceanic lithospheres of similar origin or the lithospheres of an island arc and a marginal basin. A contrasting model disputes the fracture zone setting. New U-Pb zircon ages of the Lewis Hills combined with Sm-Nd systematics are used to resolve this controversy. In the western part of the Lewis Hills (Western Lewis Hills), magmatic zircons from two trondhjemite bodies yield U-Pb ages of 500.6 ± 2.0 Ma and 503.7 ± 3.2 Ma. Initial ϵNd(502 Ma) values for trondhjemites and gabbros range from −1.5 to +2.0. Both the mineral ages and Nd signature are similar to those of the island-arc –related Little Port Complex, which is located to the north of the Western Lewis Hills. In contrast, apatite from a gabbro of the eastern part of the Lewis Hills (Eastern Lewis Hills) yields a concordant U-Pb age of 485.0 ± 1.0 Ma. This gabbro has an ϵNd(485 Ma) of +7.4. The age and isotope signature link the Eastern Lewis Hills with the northern marginal-basin–related massifs of the Bay of Islands Ophiolite. The significant differences of ages and isotope characteristics of the Western and Eastern Lewis Hills support the interpretation of the shear zone as the major tectonic boundary between an island arc and a marginal basin. This complex tectonic setting may have resulted in a wide spectrum of magma compositions that involved both island arc and marginal basin sources. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Localization of Martian ridges by impact craters: Mechanical and chronological implications

The eastern flank of the Tharsis dome (Mars) is affected by compressional ridges. Two conceptual models exist for these structures: (1) the ridges are the expression of a superficial shortening and are rooted on a rheological boundary located in the megaregolith or (2) the ridges result from a global shortening on a lithospheric scale and are rooted on the deep thermal brittle‐ductile transition inside the lithosphere. It is shown that the position of the ridges is strongly influenced by the existence of impact craters. The distribution of crater‐ridge distances has been measured in the Coprates area. This distribution shows that the intersection of ridges and impact craters with a diameter larger than 4 km is four times as frequent as a homogeneous and random distribution of craters relative to what ridges would produce. Very detailed geometric studies of relationships between intersecting ridges and craters show that these craters are older than the ridges. This localization implies that (1) ridges are superficial structures rooted at shallow depths in the megaregolith, and (2) the ridging phase occurred some time after the deposit of the ridged plain unit material, probably during the upper Hesperian. Detailed geometrical studies of ridge edges and graben/ridge width relationships suggest that ridges are bounded by two reverse faults with different offsets, and that the rheological boundary on which ridges and graben are rooted is around 1.5 km deep. Three ways are possible for an impact crater to localize strain in a regional uniaxial compressional stress field: (1) the strength of impacted material is lower than the strength of the surrounding nonaffected rocks, (2) the lithostatic pressure underneath the crater is lower than the lithostatic pressure at the same depth in the surroundings due to the existence of the crater cavity, and (3) the stress is concentrated on the crater borders if craters can be assimilated to holes in a shortened elastic plate. In all three cases, the initiation of the ridges occurs in the area where a low‐strength and/or stress concentration envelope around the crater is nearest to the low‐strength decollement level.

Emergence and petrology of the Mendocino Ridge

The Mendocino Fracture Zone, a 3,000-km-long transform fault, extends from the San Andreas Fault at Cape Mendocino, California due west into the central Pacific basin. The shallow crest of this fracture zone, known as the Mendocino Ridge, rises to within 1,100 m of the sea surface at 270 km west of the California Coast. Rounded basalt pebbles and cobbles, indicative of a beach environment, are the dominant lithology at two locations on the crest of Mendocino Ridge and a40Ar/39 Ar incremental heating age of 11.0 ± 1.0 million years was determined for one of the these cobbles. This basalt must have been erupted on the Gorda Ridge because the crust immediately to the south of the fracture zone is older than 27 Ma. This age also implies that the crest of Mendocino Ridge was at sea level and would have blocked Pacific Ocean eastern boundary currents and affected the climate of the North American continent at some time since the late Miocene. Basalts from the Mendocino Fracture Zone (MFZ) are FeTi basalts similar to those commonly found at intersections of mid-ocean ridges and fracture zones. These basalts are chemically distinct from the nearby Gorda Ridge but they could have been derived from the same mantle source as the Gorda Ridge basalts. The location of the 11 Ma basalt suggests that Mendocino Ridge was transferred from the Gorda Plate to the Pacific Plate and the southern end of Gorda Ridge was truncated by a northward jump in the transform fault of MFZ.

Rifting in the northern Norwegian‐Greenland Sea: Thermal tests of asymmetric spreading

The analysis of heat flow, seismic, and topographic data collected in the northern Norwegian‐Greenland Sea reveals an asymmetric evolution of the Eurasian and North American plates. These data are compared to predictions from three kinematic models of extension which produce asymmetry about the Knipovich Ridge: (1) uniform asymmetric pure shear, (2) lithospheric simple shear, and (3) rift/ridge jumping. The data are consistent with a range of deformation scenarios, from one ridge jump occurring at about 25 m.y. after the initiation of spreading to continuous asymmetric extension. The simple shear model can match the data only when a detachment fault dips more steeply than 45° under Svalbard. Tectonic and heat flow evidence suggests that asymmetry may have evolved from a combination of all three models. When the Mohns Ridge (propagating to the east) encountered the preexisting northward trending Spitsbergen Shear Zone, the direction of ridge propagation shifted to the north, being influenced by the abrupt change in the regional stress field at the shear zone‐ridge axis intersection. As a result, the nascent Knipovich Ridge entered into and propagated along the shear zone. Consequently, formerly active shear faults became the new detachment surfaces along which new crust is minted and asymmetrically extended. The high level of deviatoric stress about the Mohns and Knipovich Ridge intersection may cause a gradual eastward migration of the Knipovich Ridge, resulting in multiple zones of magma intrusion. “Off‐axial” bands of high heat flow and volcanism located along the Barents Sea and Svalbard margin, along the Yermak Plateau, and on continental Svalbard just southeast of the Yermak Plateau may be evidence for this migration. Propagation from the Nansen Ridge may have entered the same shear zone from the north, explaining the creation of the small Molloy Ridge and off‐axis volcanism on the Yermak Plateau.

LARGE EARTHQUAKES IN THE TONGA REGION ASSOCIATED WITH SUBDUCTION OF THE LOUISVILLE RIDGE

Subduction of the Louisville Ridge influences both interplate and intraplate seismicity in the southern Tonga region (22°S–26°S). Five earthquakes with diverse mechanisms and seismic moments greater than 1 × 1027 dyn cm have occurred in this region since 1975. These include two outer rise, one underthrusting, and two intermediate depth events, all of which occurred in close proximity to the subducting ridge. In order to understand better the nature of these events and to determine the role played by the Louisville Ridge in the subduction process, detailed rupture models have been determined for each earthquake by body and surface wave analysis. Three events occurred at shallow depths near the oblique intersection of the Louisville Ridge and the Tonga are. These are the compressional (thrust) outer rise event of October 11, 1975 (Mw = 7.4); the tensional (normal) outer rise event of October 10, 1977 (Mw = 7.4); and the subsequent underthrusting event on December 19, 1982 (Mw = 7.5), the latter being the largest documented interplate event in Tonga. The intermediate depth events of June 22, 1977 (d = 70 km, Mw = 8.1) and April 13, 1980 (d = 160 km, Mw = 7.6) both have normal fault mechanisms; however; the 1977 event has a downdip tension axis, whereas the 1980 event has a downdip compression axis. The 1977 event is unusual due to its large size and the fact that most intermediate depth activity in Tonga involves downdip compression. The body wave radiation for the 1977 event indicates concentrated rupture in the depth range 70–90 km, near the downdip projection of the Louisville Ridge. It appears that the Louisville Ridge perturbs the regional stress regime at both intermediate and shallow depths in the slab. The ridge appears to be a buoyant feature that produces locally strong interplate coupling to the north of its junction with the Tonga trench, which accounts for the 1982 thrust event and the preceding 1975 compressional outer rise event located seaward of the thrust rupture zone. South of the ridge intersection the plate interface appears to be less strongly coupled: tensional stresses due to slab pull are communicated to shallow depths, resulting in the October 10, 1977 (Mw = 7.4), tensional outer rise event. The locally enhanced interplate coupling, along with a downdip stress barrier to the intraplate compressional stresses that arise from deep resistance to the slab subduction, are two effects of the ridge that appear to explain the accumulation of in‐plate tension in the region of the June 22, 1977, event, the April 13, 1980, downdip compressional event reflects a return to in‐plate compression following the great 1977 rupture which relaxed the tensional stress. The unusual stress regime of the subducting ridge and its oblique trajectory down the plate are thus responsible for the diversity of large earthquake occurrence in the southern Tonga arc.