Northern Mariana Trough Research Articles

The Fukutoku Volcanic Complex: Implications for the northward extension of Mariana rifting and its tectonic controls on arc volcanism

Fukutoku-Oka-no-Ba (FOB) volcano is one of the most active submarine volcanoes in the Izu-Bonin-Mariana Arc. Historically, explosive eruptions associated with new island formation have occurred, and pumice rafts and discolored water have been frequently observed. Multiple geochemical studies focusing on the pumice erupted from FOB have been conducted to date, however, volcanic and tectonic features around FOB are poorly understood due to the scarcity of bathymetric and geological data. We compiled previously collected multibeam bathymetry data and major element compositions of dredged volcanic rocks to reveal the nature of magmatic and tectonic activity around FOB. Bathymetry data collected by 6 ship-based surveys between 1999 and 2006 define relatively young volcanic structures consisting of undeformed circular craters, steep-sided edifices and flank edifices on Kita-Fukutoku Bank, Kita-Fukutoku Caldera and Minami-Ioto which surround FOB. All the edifices with signs of relatively young volcanism exist on a common basal platform, together forming a large volcanic complex that is 50 km long and 25 km wide. We have named this feature the Fukutoku Volcanic Complex. Previously unreported N-S trending grabens and faults on the complex were identified from inspection of the bathymetry data. Some faults are continuous of the fault scarps bounding the northern Mariana Trough to the south, suggesting they are surface manifestations of the northward extension of Mariana Trough rifting. Vertical offsets of the grabens and faults decrease northward and the northernmost tip of Mariana rifting can now be observed to extend to 24°28′N on Kita-Fukutoku Bank. The basal platform and some young edifices on the Fukutoku Volcanic Complex are elongated or aligned in the N-S direction suggesting that arc volcanism here is tectonically influenced by Mariana rifting. Compiled major element compositions demonstrate two compositional types of magma with trachytic and basaltic rocks erupted around FOB. Although previous geochemical studies implied the association of basaltic magma with the FOB trachytic eruptions, the distribution of basaltic volcanism on the seafloor had not been well-defined. The presence of basaltic rocks around FOB, whose alkali compositions are similar to those of the northern Mariana Trough, and rift-related tectonic morphology suggest that the intrusion of basaltic magmas may be associated with the northward extension of Mariana rifting. Our detailed regional study provides new insights into the interplay of arc volcanism and extensional tectonics in the earliest stage of the Izu-Bonin-Mariana rift system.

An iron isotope perspective on back-arc basin development: Messages from Mariana Trough basalts

The geology and magmatism of the northern Mariana Trough (NMT) represent a type example for the development of a back-arc basin (BAB). It is a mature spreading center in the south and propagating northward via rifting of arc and rear arc lithosphere in response to trench retreat and overriding plate extension in space and time. It is thus essential to fully characterize its tectonomagmatic processes that are potentially of global significance. For this reason, we carried out a comprehensive geochemical study on basaltic samples (glasses) from the NMT axis from 17 to 23°N by analyzing a full suite of major and trace elements, Sr–Nd–Pb–Hf radiogenic isotopes and stable Fe isotopes. The Fe isotopes, in particular, have the potential to offer novel perspectives beyond the aforementioned geochemical variables. From south to north, the δ56Fe of the NMT basalts (NMTB) decrease systematically, varying from a MOR-like value of +0.12‰ to an arc-like value of 0.00‰. Importantly, incompatible element abundances and Sr–Nd–Pb–Hf isotope compositions of the NMTB also show systematic along-latitude variations. Positively correlated with δ56Fe, the abundances of fluid-insoluble elements (e.g., Nb, Ta, Zr, Hf, Ti and heavy rare earth elements) show the south-to-north decrease, suggesting that the NMTB δ56Fe systematics are inherited from the magma source variation. The source variation is most consistent with an asthenosphere source in the south and progressively more depleted harzburgitic lithosphere source of prior melt depletion towards north. On the other hand, there are scattered but significant enrichment trends towards north in terms of Sr–Nd–Pb–Hf radiogenic isotopes and fluid-soluble incompatible elements (e.g., Ba, Rb, Cs, Th, U, K, Pb, Sr and light rare earth elements), which is consistent with south-to-north increase in slab material (e.g., slab fluids, sediments etc.) contribution. These north-to-south magma source systematics defined by the NMTB can be readily understood as a tectonic response to the back-arc basin development with time from initial arc-magmatism dominated by sub-arc lithospheric mantle melting with significant slab material contributions to back-arc rifting and to back-arc spreading center magmatism characterized by increasing asthenospheric mantle melting, decreasing slab material contribution and diminishing sub-arc lithospheric mantle participation. This tectonic-magma source evolution in space and time is manifested by the snapshot of the NMTB magmatism in terms of the geology, petrology and geochemistry.The NMTB δ56Fe values and fluid-insoluble incompatible element abundances are unaffected by slab materials, but controlled by the harzburgitic residues of previous melt extraction depleted in these elements with low δ56Fe at the early arc-magmatism stage in the north and by the asthenosphere of MORB mantle characteristics in the south with well-developed back-arc spreading system. Importantly, these observations affirm the understanding that compositionally depleted and physically buoyant lithosphere is prerequisite as the overriding plate for subduction initiation at its edges.

Spatiotemporal Seismotectonic Implications for the Izu–Bonin–Mariana Subduction Zone from b-Values

AbstractStudies on the physical properties of the entire Izu–Bonin–Mariana (IBM) subduction zone contribute to comprehensive seismotectonic understanding and earthquake potential assessment, especially given previous controversial conclusions. Determining seismic b-value is a method that has been used for other regions and is adopted here to study the spatiotemporal variations along the IBM system. Based on the frequency–magnitude distribution relation log10(N)=a−bM, b-values are mapped within the subduction zone using earthquakes with Mw≥2 after 2005. The b-value anomalies in cross sections indicate detailed seismotectonic characteristics against the regional geological background. The common characteristics from north to south: (1) regional high b-values at shallow depths in the overriding are associated with relatively low temperatures in thermal model, the bottom half of which correspond with highly serpentinized mantle wedge; and (2) low b-values at intermediate depths are associated with high temperatures along the primarily heated hydrated slab. In the Izu–Bonin segment, low b-values around the slab deflection at deep depths respond to stress buildup and shearing instability of metastable olivine in primarily heated hydrated slabs. In the Mariana segment, high b-values beneath the volcanic region at depths from the surface to 50 km and between 50 and 100 km are associated with extension and volcanism and the melting region, respectively. Temporal b-value variations indicate regional changes before and after large events for further seismic risk analysis. Stress drops of large intermediate and deep earthquakes are negligible to local stress state in strong flexure of the incoming slab. The rupture zone around the Pagan region at an approximate depth of 200 km and the region around the rifting–spreading transition in the northern Mariana trough at depths between 180 and 350 km are areas for potential large earthquakes.

Influence of subduction components on magma composition in back‐arc basins: a comparison between the Mariana and Okinawa troughs

AbstractNew Sr, Nd, and Pb isotopic data of seven basalt samples from the middle Mariana Trough were analysed together with existing geochemical data of basalts from the middle Mariana Trough (MMT), the northern Mariana Trough (NMT), the middle Okinawa Trough (MOT), and the southern Okinawa Trough (SOT). Basalts from the Mariana and Okinawa troughs have similarities to island arc lavas and are enriched in large ion lithophile elements but depleted in high field strength elements compared with normal mid‐ocean ridge basalt (N‐MORB). The Sr, Nd, and Pb isotopic compositions of the back‐arc basin basalts maybe modelled by a mixing between end‐member components of N‐MORB‐like mantle and enriched mantle end‐member. Modelling results suggest that as the degree of back‐arc extension increases, the proportion of subduction components in the source magma decreases. The compositions of SOT basalts can be explained by the mixing of subduction components in the proportion 16%, followed by MOT at 14%, NMT at 7%, and MMT at 4%. This result indicates that the contribution of subduction components to the composition of source magmas for back‐arc basalts is closely related to the tectonic evolution of the entire back‐arc system. The proportions of subduction components in basaltic magma sources can be used as an indicator of the extent of evolution of a back‐arc basin or the development of a back‐arc spreading system. Copyright © 2016 John Wiley & Sons, Ltd.

Spreading process of the northern Mariana Trough: Rifting‐spreading transition at 22°N

We have conducted a geophysical survey of the northern Mariana Trough from 19°N to 24°N. The trough evolves southward from incipient rifting to seafloor spreading within this region. This study aims to clarify the location and time of the rifting‐to‐spreading transition, which was controversial previously, and processes of seafloor spreading after the transition. The new data set includes swath bathymetry with side‐scan images and magnetic vector anomaly. The mantle Bouguer gravity anomaly (MBA) was calculated using the free‐air gravity anomaly from satellite altimetry. The rifting‐to‐spreading transition occurs at about 22°N, which is proved by seafloor‐spreading fabric in the bathymetry, clear magnetic lineations, and the bull's‐eye pattern in MBA. Four ridge segments separated by three nontransform discontinuities are recognized between 19°N and 22°N. The northernmost segment has relatively abundant magma supply compared with the other segments, which is estimated from a larger segment length, shallower axial depths with no rift valley, and lower MBA. The next segment to the south is, on the other hand, a magma‐starved segment with a prominent rift valley. Two anomalously deep grabens (called the Central Grabens) formed by amagmatic extension occur near the segment ends. The succession of magma‐rich, magma‐starved, and normal segments with increasing distance from the volcanic arc is the same as the observation in the Lau Basin reported by Martinez and Taylor [2002]. The magnetic anomaly revealed the detailed history of the spreading. The seafloor spreading between 19°N and 20°N began prior to 5 Ma, and that between 20°N and 21°30′N began at about 4 Ma. Spreading half‐rates in the western side of the spreading center were 2 to 3 cm/year before 2.58 Ma south of 21°30′N and during the Matuyama Chron north of 21°30′N, but an average during the Brunhes Chron is 1 cm/year or less. Orientations of the ridge axes, which range from −20° to 0° at present, have rotated about 20° clockwise since the start of the spreading. These changes in rate and direction might be associated with changes in the motion of the Philippine Sea plate. Spreading has been asymmetric in the northern Mariana Trough. The spreading rates of the western side of the spreading center have been significantly larger than the eastern counterpart in general. The asymmetry may have been caused by an interaction of mantle upwelling systems under the volcanic front and the backarc spreading center and would be a characteristic of backarc spreading.

Asymmetric rifting of the northern Mariana Trough

The evolution of rifting in the northern Mariana Trough was studied, based on single‐channel seismic reflection profiles and heat flow. The rift showed structural asymmetry. The northernmost part of the Mariana Trough at 24°N, just south of Minami‐Iwojima Island, is now in an incipient rifting stage and shows a half‐graben structure. The arc crust just behind the volcanic front is cut by a few major east‐dipping normal faults. The major faults extend southward behind the Hiyoshi seamounts around 23°30′N. The rift develops to a full‐graben stage at ∼ 23°N, where the width of the trough increases to 80 km. The trough is comprised of several faulted and tilted blocks of island‐arc crust. Maximum subsidence occurs along a row of small grabens on the eastern margin of the trough. These grabens are separated by arc volcanoes, and their depths increase southward from 2500 m at 23°20′N to 4500 m at 22°N. The strike of each graben is north‐northwest–south‐southeast, which is close to the trend of the remnant West Mariana Ridge, but oblique to the active Mariana arc. Crustal extension becomes concentrated along the eastern margin of the trough as rifting progresses. The transition from rifting to sea floor spreading may occur at ∼ 22°N, where the width of the trough is ∼ 120 km. The possible spreading center lies along the southern extension of the grabens on the eastern margin. The period of back‐arc rifting before spreading begins is estimated to be less than 3 million years. Heat flow is asymmetric in the rift. High heat flow was observed only in or close to the row of grabens along the eastern margin of the trough. The asymmetric pure shear extension model fits the observed heat flow distribution better than the simple shear extension model.

Noble gases in pillow basalt glasses from the northern Mariana Trough back-arc basin

Noble gases in pillow basalt glasses from the northern Mariana Trough back-arc basin

Asymmetric rifting of the northern Mariana Trough

Asymmetric rifting of the northern Mariana Trough

Noble gases in pillow basalt glasses from the northern Mariana Trough back-arc basin

Noble gas concentrations and isotopic compositions have been measured in eight samples of pillow basalt glasses collected from seven different localities along 250 km of the Mariana Trough spreading and rifting axis. The samples have uniform and mid-ocean ridge basalt (MORB)-like 3He/4He values of 9–12 × 10–6 (6.4–8.6 times atmospheric) despite large variations in 4He. Concentrations of the noble gases Ne, Ar, Kr, and Xe show much smaller variations between samples, but larger variations in isotopic compositions of Ne, Ar, and Xe. Excess radiogenic 21Ne is observed in some samples. 40Ar/36Ar varies widely (atmospheric to 1880). Kr is atmospheric in composition for all samples. Some samples show a clear excess 129Xe, which is a well-known MORB signature. Isotopic compositions of the heavier noble gases (Ar, Kr, and Xe) in some samples, however, show more atmospheric components. These data reflect the interaction of a MORB-like magma with an atmospheric component such as seawater or of a depleted mantle source with a water-rich component that was probably derived from the subducting slab.

Noble gas and carbon isotopes in Mariana Trough basalt glasses

Noble gas elemental and isotopic compositions have been measured as well as the abundance of C and its isotopic ratios in 11 glasses from submarine pillow basalts collected from the Mariana Trough. The 3He/ 4He ratios of 8.22 and 8.51 R atm of samples dredged from the central Mariana Trough (∼18°N) agree well with that of the Mid-Ocean Ridge Basalt (MORB) glasses (8.4±0.3 R atm), whereas a mean ratio of 8.06±0.35 R atm in samples from the northern Mariana Trough (∼20°N) is slightly lower than those of MORB. One sample shows apparent excess of 20Ne and 21Ne relative to atmospheric Ne, suggesting incorporation of solar-type Ne in the magma source. There is a positive correlation between 3He/ 4He and 40Ar/ 36Ar ratios, which may be explained by mixing between MORB-type and atmospheric noble gases. Excess 129Xe is observed in the sample which also shows 20Ne and 21Ne excesses. Observed δ 13C values of ∼20°N samples vary from −3.76‰ to −2.80‰, and appear higher than those of MORB, and the corresponding CO 2/ 3He ratios are higher than those of MARA samples at ∼18°N, suggesting C contribution from the subducted slab.

Chemical and Isotopic Composition of Lavas from the Northern Mariana Trough: Implications for Magmagenesis in Back-arc Basins

Chemical and Isotopic Composition of Lavas from the Northern Mariana Trough: Implications for Magmagenesis in Back-arc Basins

Chemical and Isotopic Composition of Lavas from the Northern Mariana Trough: Implications for Magmagenesis in Back-arc Basins

We report the results of a geochemical and isotopic study of mostly basaltic glasses recovered from 25 dredge stations along the northernmost 500 km of the Mariana Trough extension axis. The distribution of samples links regions of seafloor spreading to the south with regions farther north where a progression of rifting styles accompanies the earliest stages of back-arc basin extension. Petrographic, chemical and isotopic compositions of igneous rocks reflect the changing styles of extension, with typical back-arc basin basalts in the south which become increasingly similar to arc lavas to the north. Felsic lavas also appear along the extensional axis in the north. Glassy, sparsely phyric basalts characterize regions of seafloor spreading. Felsic lavas and porphyritic basalts occur in the northern, rifting portion. Geochemical and isotopic compositions distinguish between mature arc portions (Ce/Pb 20; ^(206)Pb/^(204)Pb >18.5, ^(87)Sr/^(86)Sr >0.7032, ^eNd 10, Ba/La +7). Samples from along the extensional axis of the northern Mariana Trough show progressive changes in chemical and isotopic compositions, from back-arc basin basalts that formed by seafloor spreading northward through increasingly arc–like basalts, until lavas that are indistinguishable from arc lavas are encountered in the northernmost portion of the rift. Batch-melting models indicate that northernmost rift lavas reflect higher degrees of melting, with 13 ± 5% melting where seafloor spreading occurs, doubling to 28 ± 8% for the northernmost part of the rift axis. The greater degree of melting in the north reflects the greater amount of water added to the mantle source, reflecting the arc-like nature of the source region and melt generation style characteristic of the initial stages of back-arc basin formation. Our data indicate that F = 0.44W + 0.07, where F is the degree of mantle melting and W is the percent water in the mantle. ‘True’ back-arc basin basalts are generated by adiabatic decompression associated with mantle upwelling in mature extensional settings. Eruption of ‘true’ back-arc basin basalts accompanies seafloor spreading, which begins when the basin is 100–150 km wide. The arc is disrupted during early rift formation, because arc magmatism is captured by the extension axis, but the generation of arc melts by hydrous melting of the mantle wedge continues whether or not back-arc extension is occurring. Back-arc basin seafloor spreading requires development of an upwelling mantle flow regime, allowing melting by adiabatic decompression, similar to that responsible for mid-ocean ridge basalt (MORB). The arc begins to re-form once extension progresses nearly to the point of seafloor spreading.

Volcanologic and tectonic evolution of the Kasuga seamounts, northern Mariana Trough: Alvin submersible investigations

The Kasuga seamounts are active volcanoes situated wholly within the northern part of the backarc basin west of the active Mariana volcanic arc. They lie along a lineament that cuts across the northern portion of the Mariana arc from the trench axis to at least 60 km west of the volcanic front. The distribution of the major edifices probably represents a localized focusing of volcanic activity in the eastern backarc basin as a consequence of both cross-arc rifting and backarc basin formation. Structure of the edifices, distribution of satellite cones, and draping of the fault scarp by lava flows suggest continuing deformation of the volcanoes as a consequence of movement on the fault that bounds a graben to the east of the volcanic chain. Volcaniclastic deposits are common. Most are probably emplaced cold through the water column, but one of these deposits was probably emplaced hot. Hydrothermal processes are active at the summits of at least two of the volcanoes and produce compositionally distinctive fluids. The volcanoes have a broad range of lava compositions from basaltic to dacitic. Detailed studies of the volcanoes using the Alvin submersible show that the two southern edifices consist mainly of primitive, mantle-derived basalts, which range from relatively low-K calcalkaline basalts to high-K absarokites and shoshonites. The highest-K lavas lack many of the trace element characteristics of subduction-related volcanism (e.g. they have low Ba, Sr, Pb and B relative to other incompatible elements). We attribute this lack to derivation from old lithospheric sources during thinning of the arc as rifting proceeds. The intersection of cross-arc fractures related to forearc rifting and arc-parallel faults related to incipient backarc rifting permits primitive low-degree melts to erupt with minimal crustal-level differentiation. These melts preserve information about heterogeneities in the arc lithosphere.

Rifting history of the northern Mariana Trough: SeaMARC II and seismic reflection surveys

SeaMARC II sidescan sonar data and seismic reflection profiles of the northernmost portion of the Mariana Trough suggest a rifting rather than a seafloor‐spreading origin for lithosphere within this active backarc basin of the western Pacific. Variations in morphology along the strike of the backarc basin suggest both a spatial and a temporal variation in the style of extension of the basin. The northern Mariana Trough widens from 70 km near 23°N to 135 km near 21°30′N. Major active fault zones bound both sides of the backarc basin. Active rifting and volcanism are predominant in a roughly 20‐km‐wide active rift zone along the eastern (arc) side of the basin from 24°N to 22°N. The locus of active rifting has remained near the volcanic front of the arc during extension and is probably controlled by lithospheric weakness, which is greatest at the volcanic front. The zone of rifting intersects the active volcanic arc at Nikko Seamount. The distinction between the mechanism for formation of the Mariana Trough proposed here and that of slow mid‐ocean ridge spreading is that no true plate boundary has developed in the northernmost Mariana Trough. Rather, the backarc basin is composed of arc lithosphere intruded by arc and backarc basin magmas. Volcanism both in the rift zone and throughout the basin is fault‐controlled. South of 22°N, the zone of active rifting diverges westward from the arc as the transition to true seafloor spreading is approached. The transition to true mid‐ocean ridge spreading occurs to the south of the survey area described here.

Mode of seafloor spreading in the northern Mariana Trough

A geophysical study was conducted to clarify the mode of seafloor spreading in the northern Mariana Trough north of 20°N. Magnetic anomaly data and single-channel seismic reflection profiles were taken along ship's tracks typically with a 2–4 mile spacing. The transition from back-arc rifting to true seafloor spreading occurs at a structural boundary, which has NE-SW strike, at about 22°N. South of this boundary, clear magnetic lineations occur, and axial rift valleys on the spreading ridges are visible on the seismic profiles. Our identification of the magnetic lineations shows that the accretion of oceanic crust started at about 3.5 Ma with a spreading half-rate of ~ 2 cm/yr. The average spreading rate during the Brunhes chron is of the order of 1 cm/yr. The rate increases northward and is accompanied by a decrease in the relative depths of the axial rift valleys. A ridge-axis discontinuity is found at about 21°N. Two spreading axes overlap each other about 50 km along-strike, and are offset about 30 km. We interpret this discontinuity as an on-going ridge jump from the western axis to the eastern one. The southern end of the western axis is cut by a transform fault but the eastern axis is now propagating northwards. Arc volcanoes comprising the present volcanic front occur on the eastern flank of the spreading ridges, suggesting basinward migration of the volcanic arc. The spreading center of the Mariana Trough is located not at the center but on the eastern, or the volcanic arc, side of the trough in general. Offset is caused by the arcward ridge jump and the basinward migration of the volcanic arc. Asymmetric rifting before the beginning of the accretion of oceanic crust would be another reason for the offset. A row of rift grabens along the eastern margin of the trough north of 22°N, where the extension of island-arc crust is concentrated, continues to the spreading ridge south of 22°N. The arcward offset of spreading ridges is also found in the Lau Basin, suggesting that it is a common feature of the evolution of back-arc basins.

Submarine Topography of Northern Mariana Trough

Submarine Topography of Northern Mariana Trough

Enriched back-arc basin basalts from the northern Mariana Trough: implications for the magmatic evolution of back-arc basins

The composition of basalts erupted at the earliest stages in the evolution of a back-arc basin permit unique insights into the composition and structure of the sub-arc mantle. We report major and trace element chemical data and O-, Sr-, Nd-, and Pb- isotopic analyses for basalts recovered from four dredge hauls and one ALVIN dive in the northern Mariana Trough near 22°N. The petrography and major element chemistry of these basalts (MTB-22) are similar to tholeiites from the widest part of the Trough, near 18°N (MTB-18), except that MTB-22 have slightly more K 2O and slightly less TiO 2. The trace element data exhibit a very strong arc signature in MTB-22, including elevated K, Rb, Sr, Ba, and LREE contents; relatively lowK/Ba and highBa/La andSr/Nd. The Sr- and Nd- isotopic data plot in a field displaced from that of MTB-18 towards Mariana arc lavas, and the Pb-isotopic composition of MTB-22 is indistinguishable from Mariana arc lavas and much more homogeneous than MTB-18. Mixing of 50–90% Mariana arc component with a MORB component is hypothesized. We cannot determine whether this resulted from physical mixing of arc mantle and MORB mantle, or whether the arc component is introduced by metasomatism of MORB-like mantle by fluids released from the subducted lithosphere. The strong arc signature in back-arc melts from the Mariana Trough at 22°N, where the back-arc basin is narrow, supports general models for back-arc basin evolution whereby early back-arc basin basalts have a strong arc component which diminishes in importance relative to MORB as the back-arc basin widens.

Light noble gases in basalt glasses from Mariana Trough

We have measured elemental and isotopic compositions of light noble gases (He, Ne and Ar) of eight glasses in submarine pillow basalts dredged from the northern Mariana Trough. The 3He 4He ratios are quite uniform with the value of (9.4–10,3) × 10 −6 despite large variations in 4He contents. The 4He 20Ne ratios vary significantly from 4.6 to 2.1 × 10 5 and the variation is well explained by mixing between mantle-derived and atmospheric noble gases. Based on the 1/ 20Ne- 4He/ 20Ne diagram, the 4He content in pristine glasses was estimated to be 9.2 × 10 −6 ccSTP/g. A similar correlation is found between the 36Ar content and the 40Ar 36Ar ratio, which leads to the estimation of the 40Ar content. The 4He 40Ar ratio is comparable to that in the vesiculated basalt and considerably higher than the radiogenic production ratio in the primary magma. The observed 3He 4He ratios are somewhat lower than that of average MORB and are higher than those of volcanic gases in subduction zones, suggesting the isotopic signature of He in the back-arc basin is distinct from mid-ocean ridges and island arcs.

Seismotectonics around the Mariana trough

Strike-slip type mechanism solutions, with approximately east-west striking tension axes, were obtained for large shallow earthquakes under the central topographic high of the northern Mariana Trough. These solutions are consistent with the model that spreading is in progress in the direction perpendicular to the structural lineations trending N-S in most of the trough and NNW-SSE in the northern part. Weak coupling at the subduction zone between the Pacific plate and the landward plate (Mariana platelet) is suggested by the occurrence of few thrust type and many normal fault type earthquakes near the Mariana Trench. Spreading striking nearly E-W with an approximate total rate of 6 cm/yr in the widest section of the trough is speculatively inferred from the plate motion model of the Philippine Sea plate with respect to the surrounding plates and from the anchored slab model and the collision effect of the Caroline Ridge with the Philippine Sea plate.