Normal Oceanic Lithosphere Research Articles

Oceanic Plateau and Subduction Zone Jump: Two‐Dimensional Thermo‐Mechanical Modeling

AbstractOceanic plateaus are characterized by overthickened crust and rheologically strong and chemically depleted mantle root with respect to normal oceanic lithosphere. The buoyant nature makes them have a great potential to resist subduction and initiate a new subduction zone behind the plateaus, yet the geodynamic process remains enigmatic. Here, we use 2D thermo‐mechanical modeling method to investigate the role of an oceanic plateau in subduction zone reorganization. We systematically examine the rheological strength and compositional density of oceanic plateau mantle and the size of oceanic plateau. Three distinct subduction styles are observed: (a) steep subduction without subduction zone reorganization, favored by a small‐sized (≤150 km) or lowly depleted oceanic plateau; (b) breakoff of subducting slab followed by flat‐slab subduction, which is conditional on a moderate‐sized (≥300 km) oceanic plateau with high degree depletion but relatively low strength; and (c) initiation of a new subduction zone behind the plateau (subduction zone jump), facilitated by a moderate‐sized oceanic plateau with a strongly depleted mantle root. The geodynamic results demonstrate that the overriding lithosphere undergoes a tectonic force switch from compression to tension in ∼5 Myr as the subduction zone is reorganized. This can be reflected in the crustal deformation of the overriding plate and used as an indicator to identify past subduction zone jumps. The spatial relationship predicted by the subduction zone jump models explains the Jurassic closure event of the accretion on southern Qiangtang at ∼175 Ma and the intraoceanic infant arc magmatism in the Meso‐Tethys at ∼165 Ma.

Lithospheric strength of the Caroline Islands and its tectonic implications

The Caroline Islands are located in a broad zone near plate boundaries in southwestern Pacific. Accumulating evidence suggests that the hotspot origin alone cannot completely explain the formation of the Caroline Islands. To investigate the tectonic setting of their formation, we calculated the effective elastic thickness (Te) of the lithosphere beneath the Caroline Islands from an analysis of bathymetry and free-air gravity anomaly data by the admittance method. A synthetic model based on the actual bathymetry data of the Caroline Islands was developed for the finite window size biasing correction. The results show that the Te values of the Caroline Islands (4.5–11.5 km) are significantly lower than the Te expected for a normal oceanic lithosphere (23–50 km), and that the Te values can be approximated by the depth to the 150 ± 100°C isotherm. The low Te values indicate that the strength of the lithosphere beneath the Caroline Islands has been weakened by geological process. The thermal anomalies related to the Ontong Java Plateau and the South Pacific Isotopic and Thermal Anomaly, and the lithospheric fractures induced by interaction of plates are probable causes of the lithospheric strength reduction of the Caroline Islands.

Strain rate dependency of oceanic intraplate earthquake b‐values at extremely low strain rates

AbstractWe discovered a clear positive dependence of oceanic intraplate earthquake (OCEQ) b‐values on the age of the oceanic lithosphere. OCEQ b‐values in the youngest (<10 Ma) oceanic lithosphere are around 1.0, while those in middle to old (>20 Ma) oceanic lithosphere exceed 1.5, which is significantly higher than the average worldwide earthquake b‐value (around 1.0). On the other hand, the b‐value of intraplate earthquakes in the Ninety East‐Sumatra orogen, where oceanic lithosphere has an anomalously higher strain rate compared with normal oceanic lithosphere, is 0.93, which is significantly lower than the OCEQ b‐value (about 1.9) with the same age (50–110 Ma). Thus, the variation in b‐values relates to the strain rate of the oceanic lithosphere and is not caused by a difference in thermal structure. We revealed a negative strain rate dependency of the b‐value at extremely low strain rates (<2 × 10−10/year), which can clearly explain the above b‐values. We propose that the OCEQ b‐value depends strongly on strain rate (either directly or indirectly) at extremely low strain rates. The high OCEQ b‐values (>1.5) in oceanic lithosphere >20 Ma old imply that future improvement in seismic observation will capture many smaller magnitude OCEQs, which will provide valuable information on the evolution of the oceanic lithosphere and the driving mechanism of plate tectonics.

Global equivalent magnetization of the oceanic lithosphere

As a by-product of the construction of a new World Digital Magnetic Anomaly Map over oceanic areas, we use an original approach based on the global forward modeling of seafloor spreading magnetic anomalies and their comparison to the available marine magnetic data to derive the first map of the equivalent magnetization over the World's ocean. This map reveals consistent patterns related to the age of the oceanic lithosphere, the spreading rate at which it was formed, and the presence of mantle thermal anomalies which affects seafloor spreading and the resulting lithosphere. As for the age, the equivalent magnetization decreases significantly during the first 10–15 Myr after its formation, probably due to the alteration of crustal magnetic minerals under pervasive hydrothermal alteration, then increases regularly between 20 and 70 Ma, reflecting variations in the field strength or source effects such as the acquisition of a secondary magnetization. As for the spreading rate, the equivalent magnetization is twice as strong in areas formed at fast rate than in those formed at slow rate, with a threshold at ∼40 km/Myr, in agreement with an independent global analysis of the amplitude of Anomaly 25. This result, combined with those from the study of the anomalous skewness of marine magnetic anomalies, allows building a unified model for the magnetic structure of normal oceanic lithosphere as a function of spreading rate. Finally, specific areas affected by thermal mantle anomalies at the time of their formation exhibit peculiar equivalent magnetization signatures, such as the cold Australian–Antarctic Discordance, marked by a lower magnetization, and several hotspots, marked by a high magnetization.

Ophiolitic rocks of the Indo-Myanmar Ranges, NE India: relicts of an inverted and tectonically imbricated hyper-extended continental margin basin?

Abstract The Indo-Myanmar Ranges (IMR) of NE India are host to various ophiolitic rocks, including metamorphosed Alpine-type harzburgite and lherzolite. Compared to abyssal peridotites of normal oceanic lithosphere, these ultramafic rocks are enriched in trace and rare earth elements. Spilitic pillow lavas along with mafic dykes and sills locally intruded into the serpentinized ultramafic rocks and associated pelagic sediments exhibit alkaline compositional affinities. Ophiolite formation and emplacement were by a process analogous to that described for mantle exhumation in hyper-extended continental margin settings and ophiolites in parts of the European Alps, involving very slow passive continental margin rifting accompanied by slow upwelling or extensional unroofing of the subcontinental upper mantle up to the seafloor. Preliminary palaeomagnetic measurements conducted on ultramafic rocks within the IMR ophiolite belt give a virtual geomagnetic pole (VGP) at 47° N, 045° E for thermal demagnetization (TDM) measurements and 33° N, 013° E for the alternating field demagnetization (AfD) measurements, requiring an anticlockwise rotation of the ultramafic bodies by 14° during the subduction process. The original trend of the spreading axis of the ophiolites was probably NE–SW, with spreading directed NW–SE. Computation of palaeolatitude of the ultramafic rocks gives an average value of 24.67°. Comparison between the palaeolatitude and the present latitude of the sample sites provides a mere latitudinal shift of less than 1°. Field studies, combined with an analysis of structural and tectonic features in the IMR, suggest a generalized WNW–ESE (east–west) compression and NNE–SSW (north–south) extension contradictory to the NNE–SSW contraction indicated by seismic data. Area balancing techniques employed along sections orientated perpendicular to regional tectonic strike in the IMR reveal systematic variations in the amount of crustal shortening, with a maximum of approximately 60% recorded in the Nagaland–Manipur segment along 25.644° N, 93.826° E–25.076° N, 95.897° E. The amount of shortening gradually decreases away from the axis of maximum shortening and on both sides. Calculations of relative plate motion based on rotation vectors given by different workers for various plate pairs represented in the region reveal that the interaction between the Indian and Myanmar plates can ideally produce the structural and tectonic features of this range. Dextral shear coupled to oblique subduction of the Indian Plate below the Myanmar Plate can best explain all of the structural and tectonic features present in the IMR.

Geochemistry of volcanic glasses from the Louisville Seamount Trail (IODP Expedition 330): Implications for eruption environments and mantle melting

AbstractVolcanic glasses recovered from four guyots during drilling along the Louisville Seamount Trail, southwest Pacific, have been analyzed for major, trace, and volatile elements (H2O, CO2, S, and Cl), and oxygen isotopes. Compared to other oceanic island settings, they are geochemically homogeneous, providing no evidence of the tholeiitic stage that characterizes Hawai'i. The degrees and depth of partial melting remained constant over 1–3 Ma represented by the drill holes, and along‐chain over several million years. The only exception is Hadar Guyot with compositions that suggest small degree preferential melting of an enriched source, possibly because it erupted on the oldest and thickest lithosphere. Incompatible element enriched glass from late‐stage volcaniclastics implies lower degrees of melting as the volcanoes moved off the melting anomaly. Volcaniclastic glasses from throughout the igneous basement are degassed suggesting generation during shallow submarine eruptions (<20 mbsl) or as subaerial flows entered the sea. Drill depths may no longer reflect relative age due to postquench downslope movement. Higher volatile contents in late‐stage volcaniclastics indicate submarine eruptions at 118–258 mbsl and subsidence of the edifices below sea level by the time they erupted, or generation in flank eruptions. Glass from intrusion margins suggests emplacement ∼100 m below the surface. The required uplift to achieve these paleo‐quench depths and the subsequent subsidence to reach their current depths exceeds that expected for normal oceanic lithosphere, consistent with the Louisville melting anomaly being <100°C hotter than normal asthenosphere at 50–70 Ma when the guyots were erupted.

Spatial variations of effective elastic thickness over the Ninetyeast Ridge and implications for its structure and tectonic evolution

We present new data on the strength of oceanic lithosphere along the Ninetyeast Ridge (NER) from two independent methods: spectral analysis (Bouguer coherence) using the fan wavelet transform technique, and spatial analysis (flexure inversion) with the convolution method. The two methods provide effective elastic thickness (T-e) patterns that broadly complement each other, and correlate well with known surface structures and regional-scale features. Furthermore, our study presents a new high resolution database on the Moho configuration, which obeys flexural isostasy, and exhibit regional correlations with the T-e variations. A continuous ridge structure with a much lower T-e value than that of normal oceanic lithosphere provides strong support for the hotspot theory. The derived T-e values vary over the northern (higher T-e similar to 10-20 km), central (anomalously low T-e similar to 0-5 km), and southern (low T-e similar to 5 km) segments of the NER. The lack of correlation of the T-e value with the progressive aging of the lithosphere implies differences in thermo-mechanical setting of the crust and underlying mantle in different parts of the NER, again indicating diversity in their evolution. The anomalously low T-e and deeper Moho (similar to 22 km) estimates of the central NER (between 0.5 degrees N and 17 degrees S) are attributed to the interaction of a hotspot with the Wharton spreading ridge that caused significant thermal rejuvenation and hence weakening of the lithosphere. The higher mechanical strength values in the northern NER (north of 0.5 degrees N) may support the idea of off-ridge emplacement and a relatively large plate motion at the time of volcanism. The low T-e and deeper Moho (similar to 22 km) estimates in the southern part (south of 17 degrees S) suggest that the lithosphere was weak and therefore younger at the time of volcanism, and this supports the idea that the southern NER was emplaced on the edge of the Indian plate. (C) 2013 Elsevier B.V. All rights reserved.

Late Riphean Borisikha ophiolite (Yenisei Ridge): U-Pb zircon age and tectonic setting

The Borisikha ophiolite, composed of serpentinites and amphibolites, is an extension of the Isakovka ophiolite belt in the near-Yenisei part of the Yenisei Ridge. It is a slightly disturbed ∼3.5 km thick fragment of the ophiolite association; its crustal part is composed of metamorphosed wehrlite, troctolite, and gabbro cumulates. The cumulate composition and the geochemical features, which do not show any suprasubductional overprinting, indicate that the ophiolite is a normal oceanic lithosphere. Zircons extracted from pegmatoid amphibolite show sectorial magmatic zoning, low U contents, and magmatic Th/U values. The zircon age determined on a SHRIMP-II mass spectrometer is 682±13 Ma, which corresponds to the time of formation of oceanic crust. In the second half of the Late Riphean, the oceanic lithosphere that included the Borisikha ophiolite was subducted beneath the margin of the Siberian continent. At the same time, rifting, formation of half-grabens, and alkaline magmatism took place at the rear of the Yenisei Ridge. Similar events took place in southeastern East Sayan. Correlation between the coeval Late Baikalian structures along the southern and western margins of the Siberian Platform calls for special study.

Subsidence of normal oceanic lithosphere, apparent thermal expansivity, and seafloor flattening

Seafloor topography has been a key observational constraint on the thermal evolution of oceanic lithosphere, which is the top boundary layer of convection in Earth's mantle. At least for the first ~70 Myr, the age progression of seafloor depth is known to follow the prediction of half-space cooling, and the subsidence rate is commonly believed to be ~350 m Ma−1/2. Here we show that, based on a new statistical analysis of global bathymetry, the average subsidence rate of normal oceanic lithosphere is likely to be ~320 m Ma−1/2, i.e., ~10% lower than the conventional value. We define the ‘normal’ seafloor as regions uncorrelated with anomalous crust such as hotspots and oceanic plateaus, but the lower subsidence rate appears to be a stable estimate, not depending on how exactly we define the normal seafloor. This low subsidence rate can still be explained by half-space cooling with realistic mantle properties, if the effective thermal expansivity of a viscoelastic mantle is taken into account. In light of a revised model of half-space cooling, the normal seafloor unperturbed by the emplacement of anomalous crust exists for all ages, and the so-called seafloor flattening seems to be mostly caused by hotspots and oceanic plateaus.

Role of oceanic plateaus in the initiation of subduction and origin of continental crust

Archean tonalite-trondhjemite-granodiorite (TTG) magmatic suites represent the oldest coherent pieces of felsic continental crust. We present results of long-duration dehydration-melting experiments on amphibolites to show that melting depths of >48 km are required to have sufficient garnet in the residuum for generating the degree of heavy rare earth element (REE) depletion documented in Early Archean TTG. This depth constraint is inconsistent with early crust evolution models that posit melting at the base of oceanic plateaus or oceanic crust to explain the origin of Early Archean continental crust. Alternative explanations for this early crust, e.g., subduction models, are problematic in that they do not readily provide a mechanism for initiation of subduction in a hotter Archean Earth. We hypothesize that intraoceanic subduction systems in the Archean originated due to gravitational instabilities produced by compositional and density contrasts between converging oceanic plateau and normal oceanic lithosphere. Our model differs from earlier models of subduction initiation in that subduction of oceanic lithosphere occurs through the ‘hot’ mantle residuum (protomantle lithosphere) at the base of newly formed oceanic plateau crust. Under a high Archean geothermal gradient, subducted oceanic crust would melt to produce TTG. This model explains the origin of subduction systems, TTG, TTG–mafic and/or ultramafic magma association, stabilization of continental crust, and the broadly coeval formation of cratons and their lithospheric roots.

海洋リソスフェアの熱水変質に伴う元素移動

The hydrothermal circulation of seawater in the oceanic lithosphere is an important factor controlling seawater chemistry, compositions of subducted materials returned to the mantle and microbial activity. We summarize the results of hydrothermally altered rocks taken directly from the ocean floor in terms of major and trace elements combined with petrographic descriptions. Hydrothermal circulation starts at the spreading axis where magmatic heat from a basaltic crustal formation is available (high temperature of > 350°C). Low-temperature alteration (< 150°C) may persist for > a million of years through the ridge flanks. Due to ridge flanks occupying large regions of the seafloor, changes in chemistry, mineralogy and physical properties of the oceanic lithosphere are accompanied by geochemical fluxes that may be even larger than those at the ridge axis. Two deep drill holes, DSDP/ODP 504B and 1256D, allow an examination of downhole variations of hydrothermal alteration in basaltic rocks, and dolerite in the extrusive and sheeted dike sequence. Recent direct sampling from the ocean floor reveals that gabbros and peridotites crop out in significant amounts on the ocean floor, particularly in the slow-spreading ridges. The chemical behavior of these originally deep-seated rocks during hydrothermal circulation thus has a large impact on global mass budgets for many elements. Previous studies on the ocean floor have been mainly conducted in the Atlantic Ocean and the Pacific Ocean. We present our results on hydrothermally altered basaltic rocks, gabbros and peridotites recovered from the Indian Ocean. Basaltic samples dredged from the first segment of the Southwest Indian Ridge near the Rodriguez Triple Junction are classified into three types—a fresh lavas, low-temperature altered rocks and high-temperature altered rocks. Petrological and geochemical features of these rocks are basically comparable to those of the basaltic rocks in DSDP/ODP Hole 504B, which suggests generalities in alteration processes and chemical exchange fluxes during hydrothermal activity across all world oceans. Gabbros and peridotites were sampled from an oceanic core complex, which was composed of tectonically exposed footwalls of detachment faults, from the Central Indian Ridge. Less deformed serpentinized and gabbros were recovered from the ridge-facing slope, whereas highly deformed schist-mylonites of a mixture of these rocks were recovered from the top of the surface (i.e., detachment fault). Efficient localization of strain was probably due to the formation of secondary minerals (e.g., talc, chlorite, serpentine) onto large, discrete shear zones where fluid was introduced locally. In-situ microanalysis of trace elements of the primary minerals and their secondary minerals revealed that selective elements, such as Rb, Sr, Ba, Pb and U, are enriched in the secondary minerals. Although oceanic core complexes are places that allow cross-sectional samplings of deep-seated rocks (i.e., gabbros and peridotites) in the oceanic lithosphere, we should keep in mind the implications of the results for the normal oceanic lithosphere. To understand the nature of the oceanic lithosphere, a close linkage between the ophiolite study and a number of deep holes in the oceanic lithosphere, including a deep hole through the crust-mantle boundary, is required.

Why the Sacramento Delta area differs from other parts of the great valley: Numerical modeling of thermal structure and thermal subsidence of forearc basins

Data on present-day heat flow, subsidence history, and paleotemperature for the Sacramento Delta region, California, have been employed to constrain a numerical model of tectonic subsidence and thermal evo- lution of forearc basins. The model assumes an oceanic basement with an initial thermal profile dependent on its age subjected to refrigeration caused by a subducting slab. Subsidence in the Sacramento Delta region appears to be close to that expected for a forearc basin underlain by normal oceanic lithosphere of age 150 Ma, demonstrating that effects from both the initial thermal profile and the subduction process are necessary and sufficient. Subsidence at the eastern and northern borders of the Sacramento Valley is considerably less, approx- imating subsidence expected from the dynamics of the subduction zone alone. These results, together with other geophysical data, show that Sacramento Delta lithosphere, being thinner and having undergone deeper subsid- ence, must differ from lithosphere of the transitional type under other parts of the Sacramento Valley. Thermal modeling allows evaluation of the rheological properties of the lithosphere. Strength diagrams based on our thermal model show that, even under relatively slow deformation (10 -17 s -1 ), the upper part of the delta crys- talline crust (down to 20-22 km) can fail in brittle fashion, which is in agreement with deeper earthquake occur- rence. Hypocentral depths of earthquakes under the Sacramento Delta region extend to nearly 20 km, whereas, in the Coast Ranges to the west, depths are typically less than 12-15 km. The greater width of the seismogenic zone in this area raises the possibility that, for fault segments of comparable length, earthquakes of somewhat greater magnitude might occur than in the Coast Ranges to the west.

Controls on back‐arc basin formation

The relationship between subduction and back‐arc spreading has been well known since the early days of plate tectonics. However, the reasons why back‐arc basins are associated with some subduction systems but not all has remained elusive. We examine the kinematic controls on subduction and back‐arc basins for both the present‐day and Cenozoic to differentiate between the major competing hypotheses for back‐arc basin formation and to explain their temporal and spatial distribution. Our new data set of subduction and back‐arc basin parameters uses a new set of paleo‐oceanic age grids (Müller et al., 2005) associated with a moving Atlantic‐Indian Ocean hot spot reference frame (O'Neill et al., 2005). The plate model includes detailed reconstructed spreading histories of back‐arc basins based on marine geophysical and satellite gravity data. Our combined rotation and oceanic paleo‐age model provides the age distribution of subducting lithosphere through space and time, convergence rates, and the absolute motion of the downgoing and overriding plates. We find that back‐arc basins develop when the age of subducting normal oceanic lithosphere is greater than 55 million years. Additionally, we establish an age‐dip relationship showing that the intermediate dip angle of the subducting slab is always greater than 30° with back‐arc spreading. Our results suggest that back‐arc basin formation is always preceded by an absolute motion of the overriding plate away from the subduction hinge, thereby creating accommodation space between the overriding and subducting plates. Once back‐arc extension is established, it continues regardless of overriding plate motion, indicating back‐arc spreading is not a simple consequence of overriding plate behaviour. The landward migration of the overriding plate as a precursor to back‐arc extension may indicate that extension on the overriding plate is influenced by the oceanward lateral flow of the mantle. However, once back‐arc extension is established, rollback of the subduction hinge appears to be the primary force responsible for the continued creation of accommodation space. Our analysis indicates the driving mechanism for back‐arc extension is a combination of surface kinematics, properties of the downgoing slab, the effect of lateral mantle flow on the slab, and mantle wedge dynamics.

Petrology, geochemistry and paleogeographic reconstruction of the East Sulawesi Ophiolite, Indonesia

The East Sulawesi Ophiolite (ESO) is tectonically dismembered and widely distributed in Central and East Sulawesi. It comprises, from base to top, residual mantle peridotite and mafic–ultramafic cumulate through layered to isotropic gabbro, to sheeted dolerites and basaltic volcanic rocks. Residual peridotite is dominantly spinel lherzolite intercalated with harzburgite and dunite. Ultramafic rocks from different locations display significant differences in rock composition and mineral. However, the clinopyroxene of peridotite displays REE pattern similarities with those of mid-ocean ridge (MOR) origin, rather than those of suprasubduction zone (SSZ) origin. The gabbroic unit consists of massive gabbro, layered gabbro, mafic and ultramafic cumulate and anorthosite. The observed crystallization sequence of gabbroic unit, which is olivine→(spinel)→plagioclase→clinopyroxene→(orthopyroxene)→(hornblende), and the mineral chemistry data indicate that the ESO gabbro has similarities with MOR setting. Major and trace element geochemistry of basalt and dolerite suggests MOR, oceanic plateau and minor SSZ origins. A possible oceanic plateau origin is supported by the following: (i) the 15-km thickness is comparable with the thickness of oceanic plateau rather than normal oceanic lithosphere; (ii) there are no or only minor olivine phenocrysts in the basalt; and (iii) predominance of aphyric texture in the basalts. The REE pattern of ESO basalt exhibits N-MORB-like signatures. However, a negative Nb anomaly in the trace element spider diagram may be attributed to mantle heterogeneity of an OPB source. The geochemical variations and disparities for both peridotite and basalt and the noncogenetic relationship between crust and mantle sections in several locations suggest that the ESO may have been formed at one tectonic setting and was later overprinted by magmatism in different environments through its birth to emplacement. A possible Cretaceous origin of an oceanic plateau component of the ESO is indicated on the basis of calculated paleopositions using plate trajectory analyses together with previously published paleolatitude data. The ESO can be traced back to the proximity of the presently active region of the SW Pacific Superplume.

Initiation of Subduction Zones as a Consequence of Lateral Compositional Buoyancy Contrast within the Lithosphere: a Petrological Perspective

Tonga and Mariana fore-arc peridotites, inferred to represent their respective sub-arc mantle lithospheres, are compositionally highly depleted (low Fe/Mg) and thus physically buoyant relative to abyssal peridotites representing normal oceanic lithosphere (high Fe/Mg) formed at ocean ridges. The observation that the depletion of these fore-arc lithospheres is unrelated to, and pre-dates, the inception of present-day western Pacific subduction zones demonstrates the pre-existence of compositional buoyancy contrast at the sites of these subduction zones. These observations allow us to suggest that lateral compositional buoyancy contrast within the oceanic lithosphere creates the favoured and necessary condition for subduction initiation. Edges of buoyant oceanic plateaux, for example, mark a compositional buoyancy contrast within the oceanic lithosphere. These edges under deviatoric compression (e.g. ridge push) could develop reverse faults with combined forces in excess of the oceanic lithosphere strength, allowing the dense normal oceanic lithosphere to sink into the asthenosphere beneath the buoyant overriding oceanic plateaux, i.e. the initiation of subduction zones. We term this concept the ‘oceanic plateau model’. This model explains many other observations and offers testable hypotheses on important geodynamic problems on a global scale. These include (1) the origin of the 43Ma bend along the Hawaii‐ Emperor Seamount Chain in the Pacific, (2) mechanisms of ophiolite emplacement, (3) continental accretion, etc. Subduction initiation is not unique to oceanic plateaux, but the plateau model well illustrates the importance of the compositional buoyancy contrast within the lithosphere for subduction initiation. Most portions of passive continental margins, such as in the Atlantic where large compositional buoyancy contrast exists, are the loci of future subduction zones.

Marine controlled‐source electromagnetic sounding: 2. The PEGASUS experiment

The marine controlled‐source electromagnetic sounding method developed over the past 15 years at Scripps Institution of Oceanography employs a towed seafloor electric dipole transmitter of moment 4 × 104 Am and multiple free‐vehicle seafloor electric field recorders. A survey of 40 Ma normal oceanic lithosphere in the northeast Pacific using frequencies of 0.25 to 24 Hz and synchronous stacking of 0.25‐ to 12‐hour‐duration detected signals at transmitter‐receiver ranges between 5 and 95 km. One‐dimensional electrical conductivity structure is recovered from the data using the Occam process of nonlinear regularized inversion. Repeated inversion of a model terminated with an essentially infinite conductor or resistor demonstrates that the maximum depth of inference for this experiment is about 30 km, well into the upper mantle, with bounds placed on conductivity to depths of 60 km. Structure shallower than about 1 km is comparable to that obtained by a similar experiment on the East Pacific Rise and by borehole logging, with a sharp increase in resistivity at depths of 600–800 m, although strictly our experiment is sensitive only to integrated square root of conductivity, or total attenuation, in the surface layers. The lower crust and upper mantle has a resistivity between 2 and 7 × 104 Ω m and a transverse resistance of at least 109 Ω m2, suggesting at most 0.3% volume fraction of free water in the lower crust and some form of conductivity enhancement over mineral conductivity in the uppermost mantle. Although resolution is weak, below 30 km our data are compatible with a dry olivine model of mantle conductivity‐temperature.

Lithospheric thinning by midplate mantle plumes and the thermal history of hot plume material ponded at sublithospheric depths

Mantle plumes supply hot material to the asthenospheric channel which spreads beneath initially normal oceanic lithosphere at midplate hotspots. The fate of this material and the thermal structure of the lithosphere depend on the vigor of secondary convection through the steep thermal gradient above the ponded plume material. It has been proposed either that (1) secondary convection is so vigorous that material hotter than the normal mantle adiabat is cooled a short distance downstream of the hotspot such that the geotherm resembles that of younger lithosphere or (2) that the plume material retains its excess temperature for a significant time and the lithosphere thins slowly. A parameterized method of modeling transient secondary convection is developed to examine this question. Calculations indicate that secondary convection is likely to be sluggish unless partial melting greatly lowers viscosity. In addition, vigorous secondary convection would greatly retard upstream asthenospheric flow of plume material beneath the nose of the Hawaiian swell and asthenospheric flow of hot material away from hotspots on slowly moving plates like Cape Verde. This conclusion is compatible with limited seismic evidence that indicates that the lithosphere is not detectably thinned beneath the Hawaiian swell.

Lithospheric buoyancy and collisional orogenesis: Subduction of oceanic plateaus, continental margins, island arcs, spreading ridges, and seamounts

The sizes of continental blocks, basaltic oceanic plateaus, and island arcs that would cause collisional orogenesis when they enter a subduction zone are calculated in an analysis based upon the assumption of local isostasy and the assumption that plate subduction is primarily driven by the negative buoyancy of the lithosphere. Buoyancy analysis indicates that the bulk density contrast between 80-m.y.-old oceanic lithosphere capped by a 7-km-thick basaltic crust and the less dense underlying asthenosphere is on the order of 0.04 gm/cm 3 . Oceanic lithosphere that is ∼10 m.y. old is the youngest that is more dense than the asthenosphere and hence inherently susceptible to subduction. Subduction zone metamorphism causes the crustal layer of basalt/gabbro to transform into more dense amphibolite and eclogite. Where eclogite formation is extensive, the descending oceanic lithosphere increases in bulk density by as much as 0.04 gm/cm 3 . Lithosphere that is 100 km thick with a 30-km-thick granitic continental crust resists Subduction because it is ∼0.09 gm/cm 3 less dense than the asthenosphere. Contrasts in lithospheric bulk density (crust + mantle) of 3 are the difference between whether subduction is nearly inevitable (as for normal ocean crust) or greatly resisted (as for thick, ancient continents). Collisional orogenesis is defined as a plate interaction of the sort that causes a rearrangement of plate motions, generally with the initiation of a new subduction zone and the creation of mountains. Buoyancy analysis indicates that only bodies of continental and oceanic island are crust that are > ∼15 km thick make the lithosphere buoyant enough to jam a subduction zone. Oceanic island arc complexes built upon ocean crust typically must be active for more than ∼20 m.y. to attain crustal thicknesses so that their attempted subduction causes collisional orogenesis. Oceanic plateaus where basaltic crust as much as ∼17 km thick caps 100-km-thick lithosphere are inherenty subductable and actually less buoyant than normal oceanic lithosphere following subduction metamorphism. Basaltic plateaus must have crustal thicknesses >∼30 km to typically cause collisional orogenesis during subduction. Short subducting seamounts (

On the strength of oceanic fracture zones and their influence on the intraplate stress field

We use the locations and source mechanisms of oceanic intraplate earthquakes to test the hypothesis that the strength of oceanic fracture zones is less than that of normal oceanic lithosphere. If fracture zones are weaker than the surrounding lithosphere, they should be sites of concentrated intraplate seismicity, and the lithospheric stress field should be perturbed in their vicinity. The 77 earthquakes selected for the study have well‐determined focal mechanisms and epicenters in regions where fracture zones are well mapped. We have searched for dependence of faulting style, fault orientation, or principal stress direction on the distance from the nearest fracture zone. If fracture zones were generally weaker than the surrounding lithosphere, one of the principal horizontal stresses would be oriented nearly perpendicular to the fracture zone; we find no evidence that principal stresses near fracture zones are oriented preferentially in this manner. There is a slight tendency for earthquakes to occur near fracture zones, and patterns of fault orientation and sense of slip support the view that differential cooling and horizontal contraction on fracture zones may contribute seismogenic stress. These patterns are not demonstrable at high significance, however. If oceanic transform fault zones are weak relative to the surrounding lithosphere, these results suggest that the strength of the fracture zones increases within a few million years after cessation of transform motion to levels comparable with that of normal oceanic lithosphere.

Earthquakes and ophiolite emplacement in the Molucca Sea Collision Zone, Indonesia

The Molucca Sea in eastern Indonesia is the site of an active arc‐arc collision, and among its interesting features is the exposure of ophiolitic rocks on islands of the Talaud‐Mayu Ridge (TMR), a largely submarine ridge formed of highly deformed rocks that bisects the collision zone. To explore the relationship between earthquakes and uplift of the ophiolite, the centroid depths and fault plane solutions of 18 large earthquakes occurring in the past 20 years beneath the TMR are constrained by inversion of their teleseismic, long‐period P and SH waveforms. Centroid depths range from 16 to 36 km, but uncertainties allow a range of 10 to 45 km. All events show thrust faulting; nodal planes strike parallel to the NNE trending TMR and dip 40±9° to the ESE and 53±9° to the WNW. Published seismic refraction and gravity data support the inference that the WNW dipping, steeper nodal planes are the fault planes. Dips do not change with depth, indicating either that the fault flattens below 40 km depth or that the steeply dipping fault penetrates the entire thickness of the lithosphere. I conclude that the Molucca Sea ophiolite is being lifted by high‐angle thrust faults (45° to 60°) that extend at least 15 km into the upper mantle. Summing seismic moment tensors and assuming uniformly distributed deformation suggest that the closure across the Molucca Sea collision zone may account for 14% to 59% of the Pacific‐Eurasia convergence vector or 15% to 63% of the Philippine‐Eurasia convergence. Moment tensor sums provide estimates of crustal thickening rates beneath the TMR of 7 to 20 mm/yr and these imply uplift rates of 2 to 6 mm/yr if the increase in crustal thickness is isostatically compensated. Hence the ophiolite and an enormous volume of mélange are being lifted higher than the flanks of the island arcs and will be emplaced on them by gravity. I suggest that ophiolites occur in this tectonic setting because the concave down shape of the subducted oceanic lithosphere beneath the Molucca Sea increases its effective buoyancy, preventing it from sinking out of the way of the encroaching island arcs. This case suggests a family of convergent margin settings in which normal oceanic lithosphere is buoyed up so that ophiolites can be stripped from it and emplaced on a continental margin or island arc. These settings include those where (1) slab geometry restricts asthenospheric flow away from the convergent margin, (2) subduction of younger, less dense lithosphere decreases its negative buoyancy, (3) a passive continental margin resists subduction, or (4) the presence of a thick thermal boundary layer beneath a passive continental margin inhibits asthenospheric flow necessary to accommodate the sinking oceanic lithosphere.