Role of ridge jumps and ridge propagation in the tectonic evolution of the Lau back-arc basin, southwest Pacific

The igneous geochemistry of lavas and breccias from the basement of Sites 790 and 791, and pumice clasts from the Pliocene-Pleistocene sedimentary section of Sites 788, 790, 791, and 793 were studied. Arc volcanism became silicic about 1.5 m.y. before the inception of rifting in the Sumisu Rift at 2 Ma, but eruption of these silicic magmas reflects changes in stress regime, especially during the last 130,000 yr, rather than crustal anatexis. Arc magmas have had a larger proportion of slab-derived components since the inception of rifting than before, but are otherwise similar. Rift basalts and rhyolites are derived from a different source than are arc andesites to rhyolites. The rift source has less slab-derived material and is an E-MORB-like source, in contrast to an N-MORB-type source overprinted with more slab-derived material beneath the arc. Rift magma types, in the form of rare pumice and lithic clasts, preceded the rift, and the earliest magmas that erupted in the rift already differed from those of the arc. The earliest large rift eruption produced an exotic explosion breccia ("mousse") despite eruption at >1800 mbsl. Although this rock type is attributed primarily to high magmatic water content, the clasts are more MORB-like in trace element and isotopic composition than are modern Mariana Trough basalts. After rifting began, arc volcanism continued to be predominantly silicic, with individual pumice deposits containing clasts that vary in composition by about 5 wt% SiO 2 , or about as much as in historical eruptions of submarine Izu Arc volcanoes. The overall variations in magma composition with time during the inception of arc rifting are broadly similar in the Sumisu Rift and Lau Basin, though newly tapped OIB-type mantle seems to be present earlier during basin formation in the Sumisu than Lau case.

Section: Comparison With the Lau Basinmentioning

confidence: 99%

Petrology and Geochemistry of Pliocene-Pleistocene Volcanic Rocks from the Izu Arc, Leg 126

Gill¹,

Seales²,

Thompson³

et al. 1992

“…Lee Vallier et al, 1991);Sonne in 1984/85 and again in 1987(e.g., von Stackelberg and Shipboard Scientific Party, 1985, 1988Sunkel, 1990;Frenzel et al, 1990); Charles Darwin (e.g., Parson et al, 1990;Collier and Sinha, 1990); Natsushima (Falloon and Crawford, 1991;Falloon et al, 1987); and Akademik Mstislav Keldysh (Falloon et al, 1992). The results accumulated by these cruises now allow the compilation of a good data base of petrological, geochemical, morphological, and geophysical data on which to develop models based on the recognition of patterns of geochemical behavior, interrelated temporally and geographically to the progressive opening of the Lau Basin.…”

Section: Introductionmentioning

confidence: 99%

Regional Geochemistry of the Lau-Tonga Arc and Backarc Systems

Ewart¹,

Bryan²,

Chappell³

et al. 1994

Detailed comparison of mineralogy, and major and trace geochemistry are presented for the modern Lau Basin spreading centers, the Sites 834-839 lavas, the modern Tonga-Kermadec arc volcanics, the northern Tongan boninites, and the Lau Ridge volcanics. The data clearly confirm the variations from near normal mid-ocean-ridge basalt (N-MORB) chemistries (e.g., Site 834, Central Lau Spreading Center) to strongly arc-like (e.g., Site 839, Valu Fa), the latter closely comparable to the modern arc volcanoes. Sites 835 and 836 and the East Lau Spreading Center represent transitional chemistries. Bulk compositions range from andesitic to basaltic, but lavas from Sites 834 and 836 and the Central Lau Spreading Center extend toward more silica-undersaturated compositions. The Valu Fa and modern Tonga-Kermadec arc lavas, in contrast, are dominated by basaltic andesites. The phenocryst and groundmass mineralogies show the strong arc-like affinities of the Site 839 lavas, which are also characterized by the existence of very magnesian olivines (up to Fo 90 _9 2 ) and Cr-rich spinels in Units 3 and 6, and highly anorthitic plagioclases in Units 2 and 9.The regional patterns of mineralogical and geochemical variations are interpreted in terms of two competing processes affecting the inferred magma sources: (1) mantle depletion processes, caused by previous melt extractions linked to backarc magmatism, and (2) enrichment in large-ion-lithophüe elements, caused by a subduction contribution. Ageneral trend of increasing depletion is inferred both eastward across the Lau Basin toward the modern arc, and northward along the Tongan (and Kermadec) Arc. Numerical modeling suggests that multistage magma extraction can explain the low abundances (relative to N-MORB) of elements such as Nb, Ta, and Ti, known to be characteristic of island arc magmas. It is further suggested that a subduction jump following prolonged slab rollback could account for the initiation of the Lau Basin opening, plausibly allowing a later influx of new mantle, as required by the recognition of a two-stage opening of the Lau Basin.

“…Areas of relatively thick sediment cover show the least back scatter and appear darkest on the images. These qualitative interpretations of back-scatter strengths are supported by dredge and core samples collected during part of the Charles Darwin Cruise CD33 (Parson et al, 1989).The GLORIA data are dominated by a number of broad zones of high back scatter, which have previously been identified as lines of recent seafloor generation at spreading ridge axes (Parson et al, 1990). These zones correspond to the segments of the backarc axis termed the Eastern, Intermediate, and Central Lau Spreading Centers.…”

mentioning

confidence: 72%

“…The GLORIA data are dominated by a number of broad zones of high back scatter, which have previously been identified as lines of recent seafloor generation at spreading ridge axes (Parson et al, 1990). These zones correspond to the segments of the backarc axis termed the Eastern, Intermediate, and Central Lau Spreading Centers.…”

Section: Introductionmentioning

confidence: 99%

“…To the north of 19°20'S, the deformation associated with the southerly advance of the Central Lau spreading center propagating rift tip and the concommittant deformation within the Instantaneous Transform Zone (ITZ) has severely disrupted this fabric (Parson et al, 1990). Superimposed on the shear fabrics associated with the ITZ are a series of discrete deep basins, up to 3500-m maximum depth, which appear to form as transient extensional relay rifts between the propagating (CLSC) and annihilated (ELSC) ridge tips.…”

mentioning

confidence: 99%

See 1 more Smart Citation

Morphotectonics of the Lau Basin Seafloor-Implications for the Opening History of Backarc Basins

Parson¹,

Hawkins²,

Hunter³

1992

A new bathymetric chart has been compiled from conventional wide-beam echo-sounder profiles and recently acquired swath bathymetric data for the Lau Basin between 18°30' to 21°S and 175°30' and 178°W. Long-range sidescan sonar data has been interpreted for the same area, and together the two data sets provide powerful imagery revealing the morphological and textural setting for the Ocean Drilling Program Sites 834 through 839. Our interpretation of these data suggests that the Lau Basin may be subdivided into two principal morphotectonic domains on the basis of significantly different topography and structure. The recognition of a fundamental contrast in the crustal character between the earliest formed basin and the most recently formed (1 Ma) seafloor has far-reaching implications for understanding the early geological evolution of all marginal basins, particularly those in backarc settings.Major bathymetric compilations of the Lau Basin have been made by a number of workers, including Hawkins (1974) and Chase (1985); modifications to these base maps have been incorporated into other published reviews of the morphology and geology of the region, the most notable among them being Kroenke (1984). The principal source of data used in the preparation of these charts has been single-track, echo-sounder profiles acquired by survey vessels. The seafloor topography between these lines of primary data has then been interpolated using the subjective skill and judgment of the compiler. Most recently, cruises by French, German, and American survey vessels fitted with multibeam bathymetric swath mapping (SeaBeam) systems have accelerated the capacity to produce more detailed and accurate bathymetric charts, and these latter data are in various stages of publication. The SEAPSO program (Foucher and Shipboard Party, 1986) covered the southern portion of the backarc axis on and near the Valu Fa Ridge, between 20° and 23°S. These data were followed by published results from the Sonne 35 and 48 cruises (von Stackleberg, 1990), who in addition surveyed some of the central Lau Basin. Scripps Institution of Oceanography PAPATUA expedition Leg 04 (Hawkins, 1988) and Leg 05 (Craig and Pareda, 1987) surveyed extensively throughout the area, and ROUNDABOUT expeditions Legs 14 and 15 (Hawkins, 1989) provided additional coverage between 15° and 19°S. Almost all of these data have been acquired since the last major bathymetric revision, and we present here the first major compilation of bathymetry to include all of these new data.As well as including all of the data recorded using swath multibeam bathymetric techniques, we have included the most recent soundings acquired by conventional echo-sounding methods. These include data accessed by means of the National Geophysical Data Center (NGDC), Boulder, CO. The chart presented as Figure 1 (back-pocket foldout) forms a small subset of a regional compilation of the bathymetry of the Lau Basin made at a scale 1:375000, which is in the final stages of preparation (Parson et al., unpu...