Multichannel seismic-reflection data collected in 1982 in the Tonga Region of the South Pacific Ocean

Examination of seismic reflection profiles from the western half of the Lau backarc basin, together with sedimentological and downhole logging data from Ocean Drilling Program Leg 135 drill sites, has allowed us to assess the sedimentary and structural processes operative during arc rifting, and the nature of the transition from rifting to true backarc spreading. The attenuated arc terrain of the western Lau Basin is characterized by a horst-and-graben morphology. Several of the graben were drilled during Leg 135. Within each of these fault-bounded sub-basins, a twofold sedimentary infill was recognized: a lower sequence of volcaniclastic gravity-flow deposits overlain by an upper sequence dominated by hemipelagic clay-nannofossil oozes. The thickness and age of each sequence varies from basin to basin. The volcaniclastic deposits at each site appear to be of local derivation, from relict high-standing arc fragments and/or volcanic constructs, and little sediment transport has occurred between basins. Tilting of the sedimentary fill within some of the graben, documented on seismic records and from the drill holes, attests to rotational normal faulting on some of the basin-bounding faults. Reference to the lithostratigraphy of the drill sites shows that the principal phases of activity on these faults took place at different times at the different sites. We review the likely regional tectonic and volcanic "events" that might have been responsible for the diachroneity of sedimentation and tectonism at each site, and show that the timing of southward propagation of the Eastern Lau Spreading Center through the attenuated arc crust, and transition from arc rifting to true seafloor spreading, correlates with the peak of extensional tectonism and consequent influx of gravity-flow sediments in each of the sub-basins drilled in the western Lau Basin.

Section: Aims and Methodsmentioning

confidence: 99%

Tectonics and Sedimentation in the Lau Basin (Southwest Pacific)

Parson¹,

Rothwell²,

Macleod³

1994

“…The site is located on the flank of a small elongate domal feature on the inner (Tonga) trench wall. Multichannel-seismic lines acquired by the U.S. Geological Service in 1982 (Mann, 1985;Herzen and Exon, 1985) were combined with singlechannel profiles collected by Parson et al (1989) to assess the regional setting of Site 841. Additional data were collected by JOIDES Resolution during Leg 135 to identify a location where all the objectives of the site could be addressed.…”

Section: Site 841mentioning

confidence: 99%

Data Report: Underway Geophysics: Regional Characteristics of Seismic-Reflection Data for Leg 135

Parson¹

1994

location. Navigation data were compiled in real time and used for track-line displays. Transits Between SitesUnderway geophysical data were collected on Leg 135 during transits to and between drill sites. The data collected include (1) 3.5and 12-kHz precision echo-sounder profiles, (2) total intensity measurements of the Earth's magnetic field, (3) single-channel seismicreflection profiles (collected in the vicinity of, and over, each site), and (4) underway navigation data. Bathymetry Equipment and MethodsBathymetric data were acquired using both the 3.5-and 12-kHz profiling systems and displayed using Raytheon Model 1807M LSR linescan recorders, which were operated generally at a 1-s sweep rate. Transducers for both systems are mounted in a sonar dome installed to improve noise conditions at high ship speeds and in rough weather conditions. The 3.5-kHz system provided some information regarding sub-bottom acoustic stratigraphy, generally with penetration of tens of meters, dependent on sediment type.

“…Multichannel seismic reflection data were collected on the U.S. Geological Survey research vessel S.P. Lee using a Globe Universal Sciences Model 4200 seismic recording system, a Seismic Engineering towed acoustic array or streamer, and a seismic source of five Bolt air guns with a total volume of 19.86 L 3 (Mann, 1985). The acoustic array consisted of 24 active sections, each 100 m long and containing 60 hydrophones.…”

Section: Geophysical Data Collected Over Drill Sitesmentioning

confidence: 99%

“…Multichannel seismic reflection data were processed and migrated for inclusion in Scholl and Valuer (1985) and in preparation for Leg 135. Processing of these data is described by Mann (1985). Multichannel lines 8 and 14 (Site 840) were also reprocessed by Digicon in 1988 for the Committee for Co-ordination of Joint Prospecting for mineral resources in South Pacific offshore areas (CCOP/SOPAC).…”

Section: Geophysical Data Collected Over Drill Sitesmentioning

confidence: 99%

Synthetic Seismograms, Migrated Seismic Reflection Profiles, and Lithologic Correlations for Leg 135 Sites in the Lau Basin and Tonga Arc

Bruns¹,

Geist²,

Lavoie³

1994

At Leg 135 sites where velocity and density logs were acquired, synthetic seismograms are constructed to match seismic reflection data available for the sites. These synthetic seismograms provide a way to correlate the seismic data with the borehole lithology, and to determine what lithologic contrasts are causing the seismic reflections. Single-channel seismic reflection profiles acquired on board the JOIDES Resolution were reprocessed and migrated, and the synthetic seismogram was spliced into the profile at the location of the borehole. For Sites 840 and 841, synthetics were also constructed to match migrated multichannel seismic reflection profiles obtained by the U.S. Geological Survey research vessel S.P. Lee. The match between synthetic and real data is variable; for lines crossing Sites 834, 835, 838, 839, and 840, some excellent correlations can be made; for Site 841, the match of synthetic to real data is poor. Observed reflections are associated with lithologic unit boundaries; with coarse beds such as conglomerate, vitric gravel, and vitric sand; with basalts either within the basement or, as at Site 841, within the sedimentary section; and with sedimentary interbeds in basement basalts. Poor correlations between synthetic and real data are caused by rapid lateral changes in lithology at Site 840, and probably by a combination of rapid lithologic changes and faulting at Site 841.Hawkins, J., Parson, L., Allan, J., et al., 1994. Proc. ODP, Sci. Results, 135: College Station, TX (Ocean Drilling Program).