Sedimentation on the Tonga Forearc Related to Arc Rifting, Subduction Erosion, and Ridge Collision: A Synthesis of Results from Sites 840 and 841

Abstract

The sedimentary succession drilled at Sites 840 and 841 on the Tonga forearc allows the sedimentary evolution of the active margin to be reconstructed since shortly after the initiation of subduction during the mid Eocene. Sedimentation has been dominated by submarine fan deposits, principally volcaniclastic turbidites and mass-flows derived from the volcanic arc. Volcaniclastic sedimentation occurred against a background of pelagic nannofossil sedimentation. A number of upward-fining cycles are recognized and are correlated to regional tectonic events, such as the rifting of the Lau Basin at 5.6 Ma. Episodes of sedimentation dating from 16.0 and 10.0 Ma also correlate well with major falls in eustatic sea level and may be at least partially caused by the resulting enhanced erosion of the arc edifice. The early stages of rifting of the Lau Basin are marked by the formation of a brief hiatus at Site 840 (Horizon A), probably a result of the uplift of the Tonga Platform. Controversy exists as to the degree and timing of the uplift of Site 840 before Lau Basin rifting, with estimates ranging from 2500 to 300 m. Structural information favors a lower value. Breakup of the Tonga Arc during rifting resulted in deposition of dacite-dominated. volcaniclastic mass flows, probably reflecting a maximum in arc volcanism at this time. A pelagic interval at Site 840 suggests that no volcanic arc was present adjacent to the Tonga Platform from 5.0 to 3.0 Ma. This represents the time between separation of the Lau Ridge from the Tonga Platform and the start of activity on the Tofua Arc at 3.0 Ma. The sedimentary successions at both sites provide a record of the arc volcanism despite the reworked nature of the deposits. Probe analyses of volcanic glass grains from Site 840 indicate a consistent low-K tholeiite chemistry from 7.0 Ma to the present, possibly reflecting sediment sourcing from a single volcanic center over long periods of time. Trace and rare-earth-element (REE) analyses of basaltic glass grains indicate that thinning of the arc lithosphere had begun by 7.0 Ma and was the principle cause of a progressive depletion of the high-field-strength (HFSE), REE, and large-ion-lithophile (LILE) elements within the arc magmas before rifting. Magmatic underplating of the Tofua Arc has reversed this trend since that time. Increasing fluid flux from the subducting slab since basin rifting has caused a progressive enrichment in LILEs. Subduction erosion of the underside of the forearc lithosphere has caused continuous subsidence and tilting toward the trench since 37.0 Ma. Enhanced subsidence occurred during rifting of the South Fiji and Lau basins. Collision of the Louisville Ridge with the trench has caused no change in the nature of the sedimentation, but it may have been responsible for up to 300 m of uplift at Site 840.

Structure and deformation of the Kermadec forearc in response to subduction of the Pacific oceanic plate

“…Clift et al 1994), the slight extension and negligible rotation observed along Profile D is evidence of the temporally consistent tectonic erosion caused by the subduction of background Pacific crust at ∼28…”

“…The abundance of volcaniclastic material, dated as late Miocene in age, implies the presence of a single volcanic chain before this time (Ballance et al 1989). This single-arc system remained stable until rifting of the Lau Basin began at ∼7.8 Ma (Clift 1994;Ballance et al 1999), and the Havre Trough opened ∼5 Ma (Malahoff et al 1982;Clift et al 1994). Initiation of rifting in these backarc basins coincides with a peak in the generation of volcaniclastic material.…”

“…Karig 1970;Gillies & Davey 1986). Parson et al (1992) and Clift et al (1994) record a depositional hiatus from 32 to 16 Ma that could have enabled the consolidation and diagenesis of sediments across the forearc and thus, by establishing a density contrast with subsequently deposited sediments, be capable of generating the observed high-amplitude reflection event. While other causes of this contrast are plausible, none are observed in the borehole data to an extent that could have generated the amplitude of this reflection.…”

“…Significant extension and basin rotation are observed at ODP site 841 and in the MCS data collected on the Tonga mid-trench slope, near the present day LRSC collision zone (Parson et al 1992;Clift et al 1994;Stratford et al 2014). Clift & MacLeod (1999) use varying rates of subsidence and tilting to infer that the majority of subduction erosion at this location can be attributed to LRSC subduction.…”

“…The Karig (1970) data failed to image the internal structure of the deposits of the upper forearc, and so they were described as acoustically transparent. Clift et al (1994) recognized full Bouma sequences (Bouma 1962) in cores at ODP site 840, located on the Tonga Platform (Fig. 1), and speculated that turbidite flows dominate sedimentation on the upper-trench slopes.…”