Patricia Fryer

Mariana blueschist mud volcanism: Implications for conditions within the subduction zone

Several recently discovered active mud volcanoes on the nonaccretionary Mariana convergent plate margin are erupting slab-derived fluids, serpentine mud, and metamorphosed rocks from depths of as great as 25 km. Blueschist materials from the metamorphosed subducted plate are contained in the muds. Pore fluids indicate a depth dependence for decarbonation. The mud volcanoes record in situ conditions along the decollement, including pressure and temperature conditions and physical properties within the subduction zone. Similar mud-flow material occurs worldwide as “sedimentary serpentinite” deposits in accreted fragments of former convergent margins, making this kind of mud volcanism a more important phenomenon in convergent margins than previously recognized.

Origin and emplacement of Mariana forearc seamounts

Large seamounts occur on the outer half of the Mariana forearc. They represent a new class of seamounts consisting of horsts and diapirs of metamorphosed forearc material. The degree of metamorphism in this material depends on the amount of water available and the pressure-temperature regime of the forearc wedge. The major source for the water involved in the metamorphism is most likely the descending slab. Theoretical models for thermal regimes in convergent margins suggest that the lower grade metamorphic facies will be restricted to the outermost part of a forearc. Zeolite and chlorite facies rocks predominate in dredge hauls from horsts on the outer 50 km of the Mariana forearc. Thermal models indicate that higher grade greenschist facies should occur farther from the trench. Seamounts that were probably formed in response to diapiric emplacement of serpentinite predominate from 50 to 120 km from the trench. Uplift of the horsts and emplacement of the serpentinite diapirs were probably facilitated by vertical tectonic movement in response to subduction of plate seamounts and by fracturing of the Mariana forearc.

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

Abstract 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 2 O 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.

Evolution of the Mariana Convergent Plate Margin System

The Mariana convergent plate margin system of the western Pacific provides opportunities for studying the tectonic and geochemical processes of intraoceanic plate subduction without the added complexities of continental geology. The systems relative geologic simplicity and the well-exposed sections of lithosphere in each of its tectonic provinces permit in situ examination of processes critical to understanding subduction tectonics. Its general history provides analogs to ancient convergent margin terranes exposed on land and helps to explain the chemical mass balance in convergent plate margins. The Mariana convergent margins long history of sequential formation of volcanic arcs and extensional back arc basins has created a series of volcanic arcs at the eastern edge of the Philippine Sea plate. The trenchward edge of the overriding plate has a relatively sparse sediment cover. Rocks outcropping on the trenchs inner slope are typical of the early formed suprasubduction zones lithosphere and have been subjected to various processes related to its tectonic history. Pervasive forearc faulting has exposed crust and upper mantle lithosphere. Many large serpentinized peridotite seamounts are within 100 km of the trench axis. From these we can learn the history of regional metamorphism and observe and sample active venting of slab fluids. Ocean drilling recovered suprasubduction zone lava sequences erupted since the Eocene that suggest that the forearc region remains volcanologically dynamic. Seismic studies and seafloor mapping show evidence of deformation throughout forearc evolution. Large portions of uplifted southern forearc are exposed at the larger islands. Active volcanoes at the base of the eastern boundary fault of the Mariana Trough vary in size and composition along strike and record regional differences in source composition. Their locations along strike of the arc are controlled in part by cross-arc structures that also facilitate formation of submarine volcano chains extending from the base of the fault westward into the back arc basin. The western boundary is the West Mariana Ridge, the western portion of the volcanic arc active prior to formation of the Mariana Trough. The trough evolved in a two-stage extension process of rifting and subsequent seafloor spreading. The back arc basin varies along strike from rifted arc lithosphere with scattered volcanoes but no real spreading center in the north to a complex mid-ocean-type spreading center south of 20°N. The change from initial rifting to true seafloor spreading is also evident across the Mariana Trough from rifted topography near the West Mariana Ridge to spreading ridges in the central to eastern basin south of 20°N. This morphologic change indicates an early stage of extension with basin-and-range-type topography predominant and volcanism restricted to fissure eruptions along fault block boundaries. The spreading ridges and abyssal hill morphology evolved later as new lithosphere was generated at elongate volcanic ridges located in the center of rift valleys. The center of extension intersects the active volcanic front differently at either end of the Mariana Trough. In the north, extension is by rifting of arc lithosphere where it intersects the arc. In the south a major strike-slip fault extends from the trench axis across the forearc, through the volcanic arc, and into the back arc basin. Arc magmas apparently leak along this fault zone into the forearc and the back arc spreading center. The complexity of interrelated tectonism and magmatism in this convergent margin is daunting, but studies of arc systems such as this provide the best hope of interpreting many of the exposed terranes accreted to continents. Comparison of subaerial terranes with recent studies of intraoceanic convergent margins will add to our understanding of plate interactions and of the evolution of the volcanic arcs and extensional back arc basins generated within such environments.

Petrology and geochemistry of lavas from the Sumisu and Torishima backarc rifts

Thirteen dredge hauls from the active Sumisu and Torishima rift grabens west of the Izu-Bonin arc at about 30°–31°N, 140°E, recovered a suite of tholeiitic basalts to sodic rhyolites. Volcanism occurs along tensional faults within and bounding the rift grabens and along the transfer zones between adjoining rift segments. The Sumisu and Torishima rift lavas differ significantly from the lavas of the adjacent arc volcanic centers in having lowerAl2O3/Na2O,Ba/Zr,V/Ti, andBa/Ce and higher abundances of the rare earth elements. The rift lavas also have characteristics of backarc basin basalts, in that they are enriched in Al2O3, and depleted in total iron and TiO2 relative to mid-ocean ridge basalt, characteristics which are consistent with a higher water content in the source. Thus, the model of a progressive change in backarc basin basalt composition from arc-like to mid-ocean ridge-like, as a function of evolution of the basin, as has been suggested from many backarc regions, is not generally applicable. The comparison of the Sumisu and Torishima rift lavas with Mariana backarc basin lavas indicates that backarc basin basalts differ in composition from one basin to another. The comparison of these backarc basin suites with mid-ocean ridge suites from similar axial depths indicates that the overall control over the spectrum of backarc basin basalt compositions may be different extents of melting of the mantle. The Sumisu and Torishima rift lavas formed by a slightly higher extent of melting than the Mariana backarc basin basalts, a phenomenon which is related to the depth of the ridge segments. Furthermore, the data suggest a systematically higher extent of melting in the arc lavas than in the backarc lavas for both of these arc/backarc systems, consistent with a greater flux of water beneath the arc than beneath the backarc region.

Evolution of backarc rifting: Mariana Trough, 20°–24°N

The Mariana Trough is an actively opening backarc basin in the western Pacific. The trough formed by extension which longitudinally split an earlier arc massif creating a crescent-shaped basin between the remnant West Mariana Ridge and eastern active volcanoes of the Mariana Arc. Opening increases southward from the Volcano Islands near 24°N where the two arcs join. At 18°N in the central Mariana Trough the basin is widest and may be opening primarily by seafloor spreading. We present a synthesis of closely spaced shipboard gravity, magnetic, and bathymetry measurements from the northern basin (20°–24°N), an area that undergoes a significant progressive southward increase in extension. We have identified three stages in the evolution of rifting in this area: (1) asymmetric rifting from 24°N to 22°15′N where faulting and magmatism have migrated laterally to remain near the active volcanic arc side of the basin, (2) a “localization” of rifting from 22°15′N to ∼21°N where the primary zone of rifting separates from the active volcanic arc, and (3) a further concentration of rifting from 21°N to 20°N leading to the formation of deep tectonic grabens near the center of the basin. This last stage may be a precursor to (or incipient) seafloor spreading. We describe a new type of magnetic lineation resolved in a three-dimensional seafloor magnetization inversion of the trough between 20° and 24°N. The detailed character of these lineations, their association with tectonic structures, and other geophysical observations indicate that they are not seafloor spreading lineations but rather result from magnetic intrusions and volcanism emplaced within preexisting, less magnetic rifted arc crust. The development of these well-defined magnetization bands indicates that the zone of magmatism remains relatively narrow at any one time throughout the opening of the trough, although tectonic deformation appears to be more widely distributed. In addition, the geometry of the magnetization bands with respect to the remnant arc border faults indicates that rifting propagated rapidly northward.

Geophysical characteristics of the southern Mariana Trough, 11°50′N–13°40′N

Despite slow opening rates generally inferred for the Mariana Trough, the southernmost part of the basin has “fast spreading” geophysical and morphologic characteristics that are unlike the features of the basin to the north. A side-scan sonar and geophysical survey maps the eastern part of the basin and the seafloor spreading center between 11°50′N and 13°40′N and identifies the following characteristics: the ridge across-axis profile forms a triangular to rounded high with relief of 100 to 500 m and cross-sectional area variations of 1 to 7 km2; the along-axis mantle Bouguer gravity gradient is 0.2 mGal/km; axial segmentation occurs as overlapping axes and small deviation in trend; no transform fault offsets exist despite significant changes in the trend of the spreading center. Characteristics of the surrounding basin include shallower overall depth than in the north; no well-developed frontal arc high in the southernmost trough; the close proximity of submarine arc-type volcanoes to the spreading center; and tectonic fabric that is at a high angle to the trend of the spreading center on the eastern flank but is concordant on the western flank. These characteristics imply different tectonic and magmatic conditions in the southern trough from the rest of the basin. We propose that these effects are related to (1) the geometry of trench rollback in the southern trough leading to trench-parallel extension generating inward radiating extensional faults; (2) decoupling of the trench-parallel extensional strain by the spreading center so that it primarily affects the eastern flank of the basin; and (3) augmentation of the spreading centers magmatic budget by arc magmatic sources contributing to its fast spreading character. Although these effects may be accentuated in the southern Mariana Trough by the geometry of trench rollback and position of the slab, which here underlies the spreading center, they reflect distinct volcanic and tectonic processes which are varyingly expressed in back arc systems but are not normally found at mid-ocean ridges.

O, Sr, Nd and Pb isotopic composition of the Kasuga Cross-Chain in the Mariana Arc: A new perspective on the K-h relationship

The Kasuga system in the Mariana Arc consists of three submarine volcanoes, Kasuga 1 (K1) along the magmatic front and Kasuga 2 (K2) and Kasuga 3 (K3) extending into the back arc. K1 and nearby Fukujin are dominated by low- and medium-K tholeiitic andesites and basaltic andesites while K2 and K3 lavas are calc-alkaline and consist of primitive medium- and high-K basalts and absarokites (Mg# = 66–80) and subordinate andesites and dacites. Construction of this complex on back-arc basin crust that is no older than 45 Ma permits resolution of the sources and processes responsible for the increase in LIL contents in arc volcanic rocks with distance to the Benioff zone (the K-h relationship). New O, Sr, Nd and Pb isotopic data for fresh lavas from K2 and K3 and Pb isotopic data for K1 are interpreted to better understand the significance of these enrichments. The range inδ18O for four basalts (+5.7 to +6.0‰) and three clinopyroxene separates (5.23–5.57‰) indicates an unmodified mantle source;δ18O for three andesites and dacites (6.0–6.2‰) is consistent with their evolution by shallow fractionation of basalt. K2 and K387Sr/86Sr andeNd data range from 0.70327 to 0.70410 and +2.9 to +6.1. These lavas do not show the elevated87Sr/86Sr for a giveneNd that is characteristic of lavas erupted along a magmatic front; instead they suggest participation of OIB-like mantle. Pb isotopic data also show significant variations, with206Pb/204Pb ranging from 18.77 to 19.13,Δ207Pb= +5to+12, andΔ208Pb= +7to+30. The trend of the Pb isotopic data and mantle-like Ce/Pb (10–23) and Ba/La (10–27) preclude any significant role for subducted crust or sediments in controlling these enrichments. Instead, the inverse variation of magma production rate and isotopic heterogeneity is interpreted to indicate diminished melting with distance from the magmatic front, such that the smaller volcanoes K2 and K3 are dominated by the lowest temperature-melting fraction of a heterogeneous mantle. Diminished water flux results in preferential melting of OIB-type mantle ‘plums’—and attendant enrichments in resultant magmas—with depth to the Benioff zone.

Drilling constraints on lithospheric accretion and evolution at Atlantis Massif, Mid‐Atlantic Ridge 30°N

Expeditions 304 and 305 of the Integrated Ocean Drilling Program cored and logged a 1.4 km section of the domal core of Atlantis Massif. Postdrilling research results summarized here constrain the structure and lithology of the Central Dome of this oceanic core complex. The dominantly gabbroic sequence recovered contrasts with predrilling predictions; application of the ground truth in subsequent geophysical processing has produced self-consistent models for the Central Dome. The presence of many thin interfingered petrologic units indicates that the intrusions forming the domal core were emplaced over a minimum of 100-220 kyr, and not as a single magma pulse. Isotopic and mineralogical alteration is intense in the upper 100 m but decreases in intensity with depth. Below 800 m, alteration is restricted to narrow zones surrounding faults, veins, igneous contacts, and to an interval of locally intense serpentinization in olivine-rich troctolite. Hydration of the lithosphere occurred over the complete range of temperature conditions from granulite to zeolite facies, but was predominantly in the amphibolite and greenschist range. Deformation of the sequence was remarkably localized, despite paleomagnetic indications that the dome has undergone at least 45 degrees rotation, presumably during unroofing via detachment faulting. Both the deformation pattern and the lithology contrast with what is known from seafloor studies on the adjacent Southern Ridge of the massif. There, the detachment capping the domal core deformed a 100 m thick zone and serpentinized peridotite comprises similar to 70% of recovered samples. We develop a working model of the evolution of Atlantis Massif over the past 2 Myr, outlining several stages that could explain the observed similarities and differences between the Central Dome and the Southern Ridge.

Geology of the Mariana Trough

The Mariana Trough is an actively spreading, crescent-shaped, backarc basin located in the western Pacific between the active Mariana volcanic island arc and the West Mariana Ridge, a remnant arc. The geologic evolution of the Mariana Trough varies along strike of the basin from the initial opening phase in the north to the mature seafloor spreading phase in the central latitudes. The opening of the basin began with an initial period of stretching and collapse of the preexisting arc followed by development of ridge/ transform structures along an active volcanic and tectonic zone on the eastern side of the basin. Eventually a true spreading center developed within the basin as the principal volcanic and tectonic zone diverged from the active volcanic front. The current along-strike variations in rifting/ spreading history define distinct geographic regions: the northern rifting apex, the central spreading basin, and the southern platform.