Lin Jian

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  • Article
    The role of magmatism in the thinning and breakup of the South China Sea continental margin: Special Topic: the South China Sea Ocean Drilling
    (Oxford University Press, 2019-08-13) Sun, Zhen ; Lin, Jian ; Qiu, Ning ; Jian, Zhimin ; Wang, PinXian ; Pang, Xiong ; Zheng, Jinyun ; Zhu, Benduo
    Magmatism plays a key role in the process of continental margin breakup and ocean formation. Even in the extremely magma-poor Iberia and Newfoundland margin, studies of field outcrops have shown that syn-rift magmatism had participated in rifting from a very early stage and contributed directly to the rifting process. The final transition from exhumed continental mantle to the ocean formation is also triggered by the accumulation and eruption of magma [1]. Therefore, Atlantic-type passive continental margins are classified into two end-members: magma-poor (non-volcanic) and magma-rich (volcanic). The differences between them lie in whether a large amount of intrusive and extrusive magmatism from the mantle plume/hotspot is involved in the syn-rift and breakup stages. A magma-rich margin [2] should include the following characteristics: (i) a high-velocity lower crust (HVLC) caused by syn-rift mafic magma underplating; (ii) continental crust intruded by abundant sills and dikes; (iii) a large volume of seaward-dipping reflectors (SDRs) caused by flood basalt eruption or tuffs. All other margins are classified as magma-poor margins.
  • Article
    A geological model for the structure of ridge segments in slow spreading ocean crust
    (American Geophysical Union, 1994-06-10) Tucholke, Brian E. ; Lin, Jian
    First-order (transform) and second-order ridge-axis discontinuities create a fundamental segmentation of the lithosphere along mid-ocean ridges, and in slow spreading crust they commonly are associated with exposure of subvolcanic crust and upper mantle. We analyzed available morphological, gravity, and rock sample data from the Atlantic Ocean to determine whether consistent structural patterns occur at these discontinuities and to constrain the processes that control the patterns. The results show that along their older, inside-corner sides, both first-and second-order discontinuities are characterized by thinned crust and/or mantle exposures as well as by irregular fault patterns and a paucity of volcanic features. Crust on young, outside-corner sides of discontinuities has more normal thickness, regular fault patterns, and common volcanic forms. These patterns are consistent with tectonic thinning of crust at inside corners by low-angle detachment faults as previously suggested for transform discontinuities by Dick et al. [1981] and Karson [1990]. Volcanic upper crust accretes in the hanging wall of the detachment, is stripped from the inside-corner footwall, and is carried to the outside comer. Gravity and morphological data suggest that detachment faulting is a relatively continuous, long-lived process in crust spreading at <25–30 mm/yr, that it rnay be intermittent at intermediate rates of 25–40 mm/yr, and that it is unlikely to occur at faster rates. Detachment surfaces are dissected by later, high-angle faults formed during crustal uplift into the rift mountains; these faults can cut through the entire crust and may be the kinds of faults imaged by seismic reflection profiling over Cretaceous North Atlantic crust. Off-axis variations in gravity anomalies indicate that slow spreading crust experiences cyclic magmatic/amagmatic extension and that a typical cycle is about 2 m.y. long. During magmatic phases the footwall of the detachment fault probably exposes lower crustal gabbros, although these rocks locally may have an unconformable volcanic carapace. During amagmatic extension the detachment may dip steeply through the crust, providing a mechanism whereby upper mantle ultramafic rocks can be exhumed very rapidly, perhaps in as little as 0.5 m.y. Together, detachment faulting and cyclic magmatic/amagmatic extension create strongly heterogeneous lithosphere both along and across isochrons in slow spreading ocean crust.
  • Article
    Deep high-temperature hydrothermal circulation in a detachment faulting system on the ultra-slow spreading ridge
    (Nature Research, 2020-03-10) Tao, Chunhui ; Seyfried, William E. ; Lowell, Robert P. ; Liu, Yunlong ; Liang, Jin ; Guo, Zhikui ; Ding, Kang ; Zhang, Huatian ; Liu, Jia ; Qiu, Lei ; Egorov, Igor ; Liao, Shili ; Zhao, Minghui ; Zhou, JianPing ; Deng, Xianming ; Li, Huaiming ; Wang, Hanchuang ; Cai, Wei ; Zhang, Guoyin ; Zhou, Hongwei ; Lin, Jian ; Li, Wei
    Coupled magmatic and tectonic activity plays an important role in high-temperature hydrothermal circulation at mid-ocean ridges. The circulation patterns for such systems have been elucidated by microearthquakes and geochemical data over a broad spectrum of spreading rates, but such data have not been generally available for ultra-slow spreading ridges. Here we report new geophysical and fluid geochemical data for high-temperature active hydrothermal venting at Dragon Horn area (49.7°E) on the Southwest Indian Ridge. Twin detachment faults penetrating to the depth of 13 ± 2 km below the seafloor were identified based on the microearthquakes. The geochemical composition of the hydrothermal fluids suggests a long reaction path involving both mafic and ultramafic lithologies. Combined with numerical simulations, our results demonstrate that these hydrothermal fluids could circulate ~ 6 km deeper than the Moho boundary and to much greater depths than those at Trans-Atlantic Geotraverse and Logachev-1 hydrothermal fields on the Mid-Atlantic Ridge.
  • Article
    Crustal Evolution of the Mid-Atlantic Ridge near the Fifteen-Twenty Fracture Zone in the last 5 Ma
    (American Geophysical Union, 2003-03-08) Fujiwara, Toshiya ; Lin, Jian ; Matsumoto, Takeshi ; Kelemen, Peter B. ; Tucholke, Brian E. ; Casey, John F.
    The Mid-Atlantic Ridge around the Fifteen-Twenty Fracture Zone is unique in that outcrops of lower crust and mantle rocks are extensive on both flanks of the axial valley walls over an unusually long distance along-axis, indicating a high ratio of tectonic to magmatic extension. On the basis of newly collected multibeam bathymetry, magnetic, and gravity data, we investigate crustal evolution of this unique section of the Mid-Atlantic Ridge over the last 5 Ma. The northern and southern edges of the study area, away from the fracture zone, contain long abyssal hills with small spacing and fault throw, well lineated and high-amplitude magnetic signals, and residual mantle Bouguer anomaly (RMBA) lows, all of which suggest relatively robust magmatic extension. In contrast, crust in two ridge segments immediately north of the fracture zone and two immediately to the south is characterized by rugged and blocky topography, by low-amplitude and discontinuous magnetization stripes, and by RMBA highs that imply thin crust throughout the last 5 Ma. Over these segments, morphology is typically asymmetric across the spreading axis, indicating significant tectonic thinning of crust caused by faults that have persistently dipped in only one direction. North of the fracture zone, however, megamullions are that thought to have formed by slip on long-lived normal faults are found on both ridge flanks at different ages and within the same spreading segment. This unusual partitioning of megamullions can be explained either by a ridge jump or by polarity reversal of the detachment fault following formation of the first megamullion.
  • Article
    The latest spreading periods of the south china sea: new constraints from macrostructure analysis of IODP expedition 349 cores and geophysical data
    (American Geophysical Union, 2019-08-29) Sun, Zhen ; Ding, Weiwei ; Zhao, Xixi ; Qiu, Ning ; Lin, Jian ; Li, Chun‐Feng
    Macrostructures preserved in deformed rocks are essential for the understanding of their evolution, especially when the deformation is weak and hard to discriminate in regional scale or purely through geophysical data. In order to resolve the inconsistency between NS trending fracture zones and NE oriented spreading fabrics of the South China Sea during the latest spreading stage, we analyzed macrostructures identifiable from the basalt and consolidated sediment samples of the Integrated Ocean Drilling Program (IODP) Sites U1431 and U1433. These two sites are close to the East and Southwest relict spreading ridges and provide critical information on the latest spreading stages. The structures in the basalt of both sites suggest two dominant orientations of NS and NE. At U1431, sediments show mainly WNW trending slickensides, different from that of basalt. At U1433, no structures were found in postspreading sediment. Thus, NE and NS trending structures in basalt are most possibly formed by seafloor spreading. Crosscutting relationship suggests that NE trending structures formed first, followed by NS and finally WNW trending structures. These observations are consistent with geophysical features. Magnetic anomalies and ocean bottom seismometer velocity suggest that the latest relict ridge of the East Subbasin coincides with the EW trending seamount chain. Located between the relict ridges of East and Southwest Subbasins, NS trending Zhongnan‐Liyue Fracture Zone had acted as the latest transform fault. Based on the above evidences, we proposed that the South China Sea may have experienced a short period of NS oriented spreading after earlier SE spreading. These results resolve the previous inconsistencies.
  • Article
    Megamullions and mullion structure defining oceanic metamorphic core complexes on the Mid-Atlantic Ridge
    (American Geophysical Union, 1998-05-10) Tucholke, Brian E. ; Lin, Jian ; Kleinrock, Martin C.
    In a study of geological and geophysical data from the Mid-Atlantic Ridge, we have identified 17 large, domed edifices (megamullions) that have surfaces corrugated by distinctive mullion structure and that are developed within inside-corner tectonic settings at ends of spreading segments. The edifices have elevated residual gravity anomalies, and limited sampling has recovered gabbros and serpentinites, suggesting that they expose extensive cross sections of the oceanic crust and upper mantle. Oceanic megamullions are comparable to continental metamorphic core complexes in scale and structure, and they may originate by similar processes. The megamullions are interpreted to be rotated footwall blocks of low-angle detachment faults, and they provide the best evidence to date for the common development and longevity (∼1–2 m.y.) of such faults in ocean crust. Prolonged slip on a detachment fault probably occurs when a spreading segment experiences a lengthy phase of relatively amagmatic extension. During these periods it is easier to maintain slip on an existing fault at the segment end than it is to break a new fault in the strong rift-valley lithosphere; slip on the detachment fault probably is facilitated by fault weakening related to deep lithospheric changes in deformation mechanism and mantle serpentinization. At the segment center, minor, episodic magmatism may continue to weaken the axial lithosphere and thus sustain inward jumping of faults. A detachment fault will be terminated when magmatism becomes robust enough to reach the segment end, weaken the axial lithosphere, and promote inward fault jumps there. This mechanism may be generally important in controlling the longevity of normal faults at segment ends and thus in accounting for variable and intermittent development of inside-corner highs.
  • Preprint
    Managing tsunamis through early warning systems : a multidisciplinary approach
    ( 2010-11) Jin, Di ; Lin, Jian
    This study attempts to identify the key factors that will make a tsunami warning system most effective, to develop a framework in which results of natural science and engineering research can be effectively integrated into coastal natural hazard planning, and to develop a numerical example that illustrates how benefit-cost analysis may be used to assess early warning systems. Results of the study suggest that while the science of tsunami wave propagation and inundation is relatively advanced, our knowledge on the relationships between tsunami generation and undersea earthquakes, volcanic eruptions, and landslides remains poor, resulting in significant uncertainties in tsunami forecasting. Probabilities of damaging tsunamis for many coastal regions are still unknown, making tsunami risk assessment and management difficult. Thus it is essential to develop new techniques to identify paleo-tsunami events and to compile and develop size and frequency information on historical tsunamis for different locations. An effective tsunami early warning system must include not only the ocean technologies for accurately detecting an emerging tsunami, but also a civil communication system through which the population can be timely warned by the local government and other sources. Since minimizing the evacuation time is a key factor to make a warning system effective, adequate pre-event education and preparation of the population must be a critical component of the system. Results of a numerical example of the South Pacific region suggest that investments in a tsunami warning system in the region may lead to significant economic benefits.
  • Article
    Effects of hotspot‐induced long‐wavelength mantle melting variations on magmatic segmentation at the Reykjanes Ridge: insights from 3D geodynamic modeling
    (American Geophysical Union, 2022-03-10) Zha, Caicai ; Lin, Jian ; Zhou, Zhiyuan ; Xu, Min ; Zhang, Xubo
    Spatial variations in mantle melting induced by the Iceland hotspot have strong effects on meso-scale mantle upwelling and crustal production along the slow-spreading Reykjanes Ridge. The ridge-hotspot interaction has been recorded by diachronous V-shaped ridges and troughs extending away from Iceland, as well as by changes in ridge segmentation since 37 Ma. The origins of V-shaped structures are widely debated, while the causes of the gradual erasion of ridge segments bounded by transform faults are rarely investigated. Through 3D time-dependent geodynamic modeling, this study investigates how the hotspot-induced regional mantle melting variations affect ridge segmentation. Periodic temperature perturbations were initially imposed beneath the melting zone to trigger buoyant upwelling cells, which corresponded to the offset ridge segments at the Reykjanes Ridge. Iceland hotspot-induced long-wavelength mantle melting variations were generated by applying a regional linear temperature gradient at the bottom of the model domain. Modeling reveals a two-stage evolution of the buoyant upwelling cells that characterizes the segmentation transition at the Reykjanes Ridge. In Stage 1, the regional mantle melting variations trigger along-axis pressure-driven mantle flow, which alters the segment-scale mantle upwelling and promotes the propagation of segment boundaries away from the region with relatively higher mantle temperature. In Stage 2, buoyant upwelling cells are destroyed progressively as along-axis mantle flow dominants, leaving V-shaped diachronous boundaries between the segmented and unsegmented crust. These results advance our understanding of the effects of long-wavelength mantle melting variations induced by regional mantle heterogeneities on ridge segment evolution at slow-spreading ridges.
  • Article
    Stress interaction between subduction earthquakes and forearc strike-slip faults : modeling and application to the northern Caribbean plate boundary
    (American Geophysical Union, 2004-12-24) ten Brink, Uri S. ; Lin, Jian
    Strike-slip faults in the forearc region of a subduction zone often present significant seismic hazard because of their proximity to population centers. We explore the interaction between thrust events on the subduction interface and strike-slip faults within the forearc region using three-dimensional models of static Coulomb stress change. Model results reveal that subduction earthquakes with slip vectors subparallel to the trench axis enhance the Coulomb stress on strike-slip faults adjacent to the trench but reduce the stress on faults farther back in the forearc region. In contrast, subduction events with slip vectors perpendicular to the trench axis enhance the Coulomb stress on strike-slip faults farther back in the forearc, while reducing the stress adjacent to the trench. A significant contribution to Coulomb stress increase on strike-slip faults in the back region of the forearc comes from “unclamping” of the fault, i.e., reduction in normal stress due to thrust motion on the subduction interface. We argue that although Coulomb stress changes from individual subduction earthquakes are ephemeral, their cumulative effects on the pattern of lithosphere deformation in the forearc region are significant. We use the Coulomb stress models to explain the contrasting deformation pattern between two adjacent segments of the Caribbean subduction zone. Subduction earthquakes with slip vectors nearly perpendicular to the Caribbean trench axis is dominant in the Hispaniola segment, where the strike-slip faults are more than 60 km inland from the trench. In contrast, subduction slip motion is nearly parallel to the Caribbean trench axis along the Puerto Rico segment, where the strike-slip fault is less than 15 km from the trench. This observed jump from a strike-slip fault close to the trench axis in the Puerto Rico segment to the inland faults in Hispaniola is explained by different distributions of Coulomb stress in the forearc region of the two segments, as a result of the change from the nearly trench parallel slip on the Puerto Rico subduction interface to the more perpendicular subduction slip beneath Hispaniola. The observations and modeling suggest that subduction-induced strike-slip seismic hazard to Puerto Rico may be smaller than previously assumed but the hazard to Hispaniola remains high.
  • Article
    Post-seismic viscoelastic deformation and stress transfer after the 1960 M9.5 Valdivia, Chile earthquake : effects on the 2010 M8.8 Maule, Chile earthquake
    (Oxford University Press, 2014-03-04) Ding, Min ; Lin, Jian
    After the 1960 M9.5 Valdivia, Chile earthquake, three types of geodetic observations were made during four time periods at nearby locations. These post-seismic observations were previously explained by post-seismic afterslip on the downdip extension of the 1960 rupture plane. In this study, we demonstrate that the post-seismic observations can be explained alternatively by volumetric viscoelastic relaxation of the asthenosphere mantle. In searching for the best-fitting viscosity model, we invert for two variables, the thickness of the elastic lithosphere, He, and the effective Maxwell decay time of the asthenosphere mantle, TM, assuming a 100-km-thick asthenosphere mantle. The best solutions to fit the observations in four sequential time periods, 1960–1964, 1960–1968, 1965–1973 and 1980–2010, each yield a similar He value of about 65 km but significantly increasing TM values of 0.7, 6, 10 and 80 yr, respectively. We calculate the corresponding viscoelastic Coulomb stress increase since 1960 on the future rupture plane of the 2010 M8.8 Maule, Chile earthquake. The calculated viscoelastic stress increase on the 2010 rupture plane varies gradually from 13.1 bars at the southern end to 0.1 bars at the northern end. In contrast, the stress increase caused by an afterslip model has a similar spatial distribution but slightly smaller values of 0.1–3.2 bars on the 2010 rupture plane.
  • Article
    A reduced crustal magnetization zone near the first observed active hydrothermal vent field on the Southwest Indian Ridge
    (American Geophysical Union, 2010-09-21) Zhu, Jian ; Lin, Jian ; Chen, Yongshun J. ; Tao, Chunhui ; German, Christopher R. ; Yoerger, Dana R. ; Tivey, Maurice A.
    Inversion of near-bottom magnetic data reveals a well-defined low crustal magnetization zone (LMZ) near a local topographic high (37°47′S, 49°39′E) on the ultraslow-spreading Southwest Indian Ridge (SWIR). The magnetic data were collected by the autonomous underwater vehicle ABE on board R/V DaYangYiHao in February-March 2007. The first active hydrothermal vent field observed on the SWIR is located in Area A within and adjacent to the LMZ at the local topographic high, implying that this LMZ may be the result of hydrothermal alteration of magnetic minerals. The maximum reduction in crustal magnetization is 3 A/M. The spatial extent of the LMZ is estimated to be at least 6.7 × 104 m2, which is larger than that of the LMZs at the TAG vent field on the Mid-Atlantic Ridge (MAR), as well as the Relict Field, Bastille, Dante-Grotto, and New Field vent-sites on the Juan de Fuca Ridge (JdF). The calculated magnetic moment, i.e., the product of the spatial extent and amplitude of crustal magnetization reduction is at least −3 × 107 Am2 for the LMZ on the SWIR, while that for the TAG field on the MAR is −8 × 107 Am2 and that for the four individual vent fields on the JdF range from −5 × 107 to −3 × 107 Am2. Together these results indicate that crustal demagnetization is a common feature of basalt-hosted hydrothermal vent fields at mid-ocean ridges of all spreading rates. Furthermore, the crustal demagnetization of the Area A on the ultraslow-spreading SWIR is comparable in strength to that of the TAG area on the slow-spreading MAR.
  • Article
    Mantle heterogeneity and melting processes in the South China Sea: thermal and melting models constrained by oceanic crustal thickness and basalt geochemistry
    (American Geophysical Union, 2021-01-13) Zhang, Xubo ; Lin, Jian ; Behn, Mark D.
    We simulate mantle flow, thermal structure, and melting processes beneath the ridge axis of the South China Sea (SCS), combining the nominally anhydrous melting and fractional crystallization model, to study mantle heterogeneity and basin evolution. The model results are constrained by seismically determined crustal thickness and major element composition of fossil ridge axis basalts. The effects of half-spreading rate, mantle potential temperature, mantle source composition, and the pattern of melt migration on the crustal thickness and magma chemical composition are systematically investigated. For the SCS, the east and southwest (SW) subbasins have comparable crustal thickness, but the east subbasin has higher FeO and Na2O contents compared to the SW subbasin. The estimated best fitting mantle potential temperatures in the east and SW subbasins are 1,360 ± 15 °C and 1,350 ± 25 °C, respectively. The mantle in the east subbasin (site U1431) prior to the cessation of seafloor spreading is composed primarily of the depleted mid-ocean ridge basalt mantle (DMM), and is slightly contaminated by eclogite/pyroxenite-rich component. However, the mantle source composition of the SW subbasin (sites U1433 and U1434) contains a small percentage (2–5%) of lower continental crust. Basalt samples at the northern margin of the east subbasin (site U1500) shows similar chemical characteristics with that of the SW subbasin. We suggest that the basin-scale variability in the mantle heterogeneity of the SCS can be explained by a single model in which the contamination by the lower continental crust is gradually diluted by melting of DMM as the ridge moves away from the rifted margin.
  • Article
    Segmentation and crustal structure of the western Mid-Atlantic Ridge flank, 25°25′–27°10′N and 0–29 m.y.
    (American Geophysical Union, 1997-05-10) Tucholke, Brian E. ; Lin, Jian ; Kleinrock, Martin C. ; Tivey, Maurice A. ; Reed, Thomas B. ; Goff, John A. ; Jaroslow, Gary E.
    We conducted a detailed geological-geophysical survey of the west flank of the Mid-Atlantic Ridge between 25°25′N and 27°10′N and from the ridge axis out to 29 Ma crust, acquiring Hydrosweep multibeam bathymetry, HAWAII MR1 sidescan-sonar imagery, gravity, magnetics, and single-channel seismic reflection profiles. The survey covered all or part of nine spreading segments bounded by mostly nontransform, right-stepping discontinuities which are subparallel to flow lines but which migrated independently of one another. Some discontinuities alternated between small right- and left-stepping offsets or exhibited zero offset for up to 3–4 m.y. Despite these changes, the spreading segments have been long-lived and extend 20 m.y. or more across isochrons. A large shift (∼9°) in relative plate motion about 24–22 Ma caused significant changes in segmentation pattern. The nature of this plate-boundary response, together with the persistence of segments through periods of zero offset at their bounding discontinuities, suggest that the position and longevity of segments are controlled primarily by the subaxial position of buoyant mantle diapirs or focused zones of rising melt. Within segments, there are distinct differences in seafloor depth, morphology, residual mantle Bouguer gravity anomaly, and apparent crustal thickness between inside-corner and outside-corner crust. This demands fundamentally asymmetric crustal accretion and extension across the ridge axis, which we attribute to low-angle, detachment faulting near segment ends. Cyclic variations in residual gravity over the crossisochron run of segments also suggest crustal-thickness changes of at least 1–2 km every 2–3 m.y. These are interpreted to be caused by episodes of magmatic versus relatively amagmatic extension, controlled by retention and quasiperiodic release of melt from the upwelling mantle. Detachment faulting appears to be especially effective in exhuming lower crust to upper mantle at inside corners during relatively amagmatic episodes, creating crustal domes analogous to “turtleback” metamorphic core complexes that are formed by low-angle, detachment faulting in subaerial extensional environments.
  • Article
    Ages and magnetic structures of the South China Sea constrained by deep tow magnetic surveys and IODP Expedition 349
    (John Wiley & Sons, 2014-12-27) Li, Chun-Feng ; Xu, Xing ; Lin, Jian ; Sun, Zhen ; Zhu, Jian ; Yao, Yongjian ; Zhao, Xixi ; Liu, Qingsong ; Kulhanek, Denise K. ; Wang, Jian ; Song, Taoran ; Zhao, Junfeng ; Qiu, Ning ; Guan, Yongxian ; Zhou, Zhiyuan ; Williams, Trevor ; Bao, Rui ; Briais, Anne ; Brown, Elizabeth A. ; Chen, Yifeng ; Clift, Peter D. ; Colwell, Frederick S. ; Dadd, Kelsie A. ; Ding, Weiwei ; Almeida, Ivan Hernandez ; Huang, Xiao-Long ; Hyun, Sangmin ; Jiang, Tao ; Koppers, Anthony A. P. ; Li, Qianyu ; Liu, Chuanlian ; Liu, Zhifei ; Nagai, Renata H. ; Peleo-Alampay, Alyssa ; Su, Xin ; Tejada, Maria Luisa G. ; Trinh, Hai Son ; Yeh, Yi-Ching ; Zhang, Chuanlun ; Zhang, Fan ; Zhang, Guo-Liang
    Combined analyses of deep tow magnetic anomalies and International Ocean Discovery Program Expedition 349 cores show that initial seafloor spreading started around 33 Ma in the northeastern South China Sea (SCS), but varied slightly by 1–2 Myr along the northern continent-ocean boundary (COB). A southward ridge jump of ∼20 km occurred around 23.6 Ma in the East Subbasin; this timing also slightly varied along the ridge and was coeval to the onset of seafloor spreading in the Southwest Subbasin, which propagated for about 400 km southwestward from ∼23.6 to ∼21.5 Ma. The terminal age of seafloor spreading is ∼15 Ma in the East Subbasin and ∼16 Ma in the Southwest Subbasin. The full spreading rate in the East Subbasin varied largely from ∼20 to ∼80 km/Myr, but mostly decreased with time except for the period between ∼26.0 Ma and the ridge jump (∼23.6 Ma), within which the rate was the fastest at ∼70 km/Myr on average. The spreading rates are not correlated, in most cases, to magnetic anomaly amplitudes that reflect basement magnetization contrasts. Shipboard magnetic measurements reveal at least one magnetic reversal in the top 100 m of basaltic layers, in addition to large vertical intensity variations. These complexities are caused by late-stage lava flows that are magnetized in a different polarity from the primary basaltic layer emplaced during the main phase of crustal accretion. Deep tow magnetic modeling also reveals this smearing in basement magnetizations by incorporating a contamination coefficient of 0.5, which partly alleviates the problem of assuming a magnetic blocking model of constant thickness and uniform magnetization. The primary contribution to magnetic anomalies of the SCS is not in the top 100 m of the igneous basement.
  • Article
    Deep outer-rise faults in the Southern Mariana Subduction Zone indicated by a machine-learning-based high-resolution earthquake catalog
    (American Geophysical Union, 2022-06-06) Chen, Han ; Yang, Hongfeng ; Zhu, Gaohua ; Xu, Min ; Lin, Jian ; You, Qingyu
    Outer-rise faults are predominantly concentrated near ocean trenches due to subducted plate bending. These faults play crucial roles in the hydration of subducted plates and the consequent subducting processes. However, it has not yet been possible to develop high-resolution structures of outer-rise faults due to the lack of near-field observations. In this study we deployed an ocean bottom seismometer (OBS) network near the Challenger Deep in the Southernmost Mariana Trench, between December 2016 and June 2017, covering both the overriding and subducting plates. We applied a machine-learning phase detector (EQTransformer) to the OBS data and found more than 1,975 earthquakes. An identified outer-rise event cluster revealed an outer-rise fault penetrating to depths of 50 km, which was inferred as a normal fault based on the extensional depth from tomographic images in the region, shedding new lights on water input at the southmost Mariana subduction zone.
  • Dataset
    Long-Term evolution of non-transform discontinuities at the Mid-Atlantic Ridge, 24°N - 27°30′N
    ( 2018-12) Zheng, Tingting ; Tucholke, Brian E. ; Lin, Jian
    We studied long-term evolution of non-transform discontinuities (NTDs) on the Mid-Atlantic Ridge from 0 to ~20-25 Ma crust using plate reconstructions of multibeam bathymetry, long-range HMR1 sidescan sonar, residual mantle Bouguer gravity anomaly, and gravity-derived crustal thickness. NTDs have propagated north and south with respect to flowlines of relative plate motion and both rapidly and slowly compared to the half spreading rate, and at times they have been quasi-stable. Fast, short-term (<2 m.y.) propagation is driven by reduced magma supply (increased extension) in the propagating ridge tip when NTD ridge-axis offsets are small (<~ 5 km). Slow propagation can be much longer term. Some NTDs show classic structures of rift propagation including inner and outer pseudofaults and crustal blocks transferred between ridge flanks by discontinuous jumps of the propagating ridge tip. In all cases crustal transfer occurs within the NTD valley. Aside from ridge-axis offset, the evolution of NTDs appears to be controlled by three factors: (1) Gross volume and distribution of magma supplied to ridge segments as controlled by 3D heterogeneities in mantle fertility and/or dynamic upwelling; this controls fundamental ridge segmentation. (2) The lithospheric plumbing system through which magma is delivered to the crust. (3) The consequent focusing of tectonic extension in magma-poor parts of spreading segments, typically at segment ends, which can drive propagation. We also observe long-wavelength (5-10 m.y.) RMBA asymmetry between the conjugate ridge flanks, and we attribute this to asymmetric distribution of density anomalies in the upper mantle.
  • Article
    Mechanism of progressive broad deformation from oceanic transform valley to off-transform faulting and rifting
    (Elsevier, 2022-01-25) Zhang, Fan ; Lin, Jian ; Zhou, Zhiyuan ; Yang, Hongfeng ; Morgan, Jason P.
    Oceanic transform faults (TFs) are commonly viewed as single, narrow strike-slip seismic faults that offset two mid-ocean ridge segments. However, broad zones of complex deformation are ubiquitous at TFs. Here, we propose a new conceptual model for the progressive deformation within broad zones at oceanic TFs through detailed morphological, seismic, and stress analyses. We argue that, under across-transform extension due to a change in plate motion, plate deformation occurs first along high-angle transtensional faults (TTFs) within the transform valleys. Off-transform normal faults (ONFs) form when across-transform deviatoric extensional stresses exceed the yield strength of the adjacent oceanic lithosphere. With further extension, these normal faults can develop into off-transform rift zones (ORZs), some of which can further develop into transform plate boundaries. We illustrate that such progressive complex deformation is an inherent feature of oceanic TFs. The new conceptual model provides a unifying theory to explain the observed broad deformation at global transform systems.
  • Article
    Continental interior and edge breakup at convergent margins induced by subduction direction reversal: a numerical modeling study applied to the South China Sea margin
    (American Geophysical Union, 2020-10-06) Li, Fucheng ; Sun, Zhen ; Yang, Hongfeng ; Lin, Jian ; Stock, Joann M. ; Zhao, Zhongxiang ; Xu, Hehua ; Sun, Longtao
    The dynamics of continental breakup at convergent margins has been described as the results of backarc opening caused by slab rollback or drag force induced by subduction direction reversal. Although the rollback hypothesis has been intensively studied, our understanding of the consequence of subduction direction reversal remains limited. Using thermo‐mechanical modeling based on constraints from the South China Sea (SCS) region, we investigate how subduction direction reversal controls the breakup of convergent margins. The numerical results show that two distinct breakup modes, namely, continental interior and edge breakup (“edge” refers to continent above the plate boundary interface), may develop depending on the “maturity” of the convergent margin and the age of the oceanic lithosphere. For a slab age of ~15 to ~45 Ma, increasing the duration of subduction promotes the continental interior breakup mode, where a large block of the continental material is separated from the overriding plate. In contrast, the continental edge breakup mode develops when the subduction is a short‐duration event, and in this mode, a wide zone of less continuous continental fragments and tearing of the subducted slab occur. These two modes are consistent with the interior (relic late Mesozoic arc) and edge (relic forearc) rifting characteristics in the western and eastern SCS margin, suggesting that variation in the northwest‐directed subduction duration of the Proto‐SCS might be a reason for the differential breakup locus along the strike of the SCS margin. Besides, a two‐segment trench associated with the northwest‐directed subduction is implied in the present‐day SCS region.
  • Article
    Mineralogical and geochemical features of sulfide chimneys from the 49°39′E hydrothermal field on the Southwest Indian Ridge and their geological inferences
    (Springer, 2011-08-24) Tao, Chunhui ; Li, Huaiming ; Huang, Wei ; Han, XiQiu ; Wu, GuangHai ; Su, Xin ; Zhou, Ning ; Lin, Jian ; He, YongHua ; Zhou, JianPing
    During January–May in 2007, the Chinese research cruise DY115-19 discovered an active hydrothermal field at 49°39′E/37°47′S on the ultraslow spreading Southwest Indian Ridge (SWIR). This was also the first active hydrothermal field found along an ultraslow-spreading ridge. We analyzed mineralogical, textural and geochemical compositions of the sulfide chimneys obtained from the 49°39′E field. Chimney samples show a concentric mineral zone around the fluid channel. The mineral assemblages of the interiors consist mainly of chalcopyrite, with pyrite and sphalerite as minor constitunets. In the intermediate portion, pyrite becomes the dominant mineral, with chalcopyrite and sphalerite as minor constitunets. For the outer wall, the majority of minerals are pyrite and sphalerite, with few chalcopyrite. Towards the outer margin of the chimney wall, the mineral grains become small and irregular in shape gradually, while minerals within interstices are abundant. These features are similar to those chimney edifices found on the East Pacific Rise and Mid-Atlantic Ridge. The average contents of Cu, Fe and Zn in our chimney samples were 2.83 wt%, 45.6 wt% and 3.28 wt%, respectively. The average Au and Ag contents were up to 2.0 ppm and 70.2 ppm respectively, higher than the massive sulfides from most hydrothermal fields along mid-ocean ridge. The rare earth elements geochemistry of the sulfide chimneys show a pattern distinctive from the sulfides recovered from typical hydrothermal fields along sediment-starved mid-ocean ridge, with the enrichment of light rare earth elements but the weak, mostly negative, Eu anomaly. This is attributed to the distinct mineralization environment or fluid compositions in this area.
  • Article
    Geometrical effects of a subducted seamount on stopping megathrust ruptures
    (John Wiley & Sons, 2013-05-30) Yang, Hongfeng ; Liu, Yajing ; Lin, Jian
    We have numerically simulated dynamic ruptures along a “slip-weakening” megathrust fault with a subducted seamount of realistic geometry, demonstrating that seamounts can act as a barrier to earthquake ruptures. Such barrier effect is calculated to be stronger for increased seamount normal stress relative to the ambient level, for larger seamount height-to-width ratio, and for shorter seamount-to-nucleation distance. As the seamount height increases from 0 to 40% of its basal width, the required increase in the effective normal stress on the seamount to stop ruptures drops by as much as ~20%. We further demonstrate that when a seamount is subducted adjacent to the earthquake nucleation zone, coseismic ruptures can be stopped even if the seamount has a lower effective normal stress than the ambient level. These results indicate that subducted seamounts may stop earthquake ruptures for a wide range of seamount normal stress conditions, including the case of the thrust fault being lubricated by seamount-top fluid-rich sediments, as suggested from observations in the Japan and Sunda Trenches.