Seismotectonics of the Fiji Plateau and Lau Basin (original) (raw)
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Shallow seismicity and tectonics of the central and northern Lau Basin
The Lau basin is an active backarc comprising several spreading centers and microplates rapidly evolving in time. As such, crustal earthquakes within the Lau Basin derive from intrabasin tectonic, volcanic, and hydrothermal sources. Because of high mantle attenuation, small earthquakes from the basin are seldom recorded on land seismographs and it has been difficult to study the pattern of crustal seismicity. However, the 1994 LABATTS ocean bottom seismograph experiment in the Lau basin recorded more than 100,000 local T-and body phases allowing for a detailed examination of basin seismotectonics. Nearly 1000 events are located within the basin proper, with another 2000 associated with the forearc and the aseismic front. We identify two previously undocumented tectonic features, a triple junction in the northwestern basin and a nanoplate in the central Lau basin. The single most seismically active feature within the Lau basin is the Lau extensional transform zone (LETZ) just north of the Central Lau Spreading Center. Significant seismicity also occurs near the northern extension of the Eastern Lau Spreading Center and along a southeast extension of the LETZ delineating the boundary of a previously unknown nanoplate. Swarm activity is common in the backarc basin and dominates regions associated with actively reorganizing tectonics. We see no evidence of deformation along the southern boundary of the Niuafo'ou microplate, where 4.5 cm/yr of strain is predicted. It is possible that the plate boundary extends eastward from the Fonualei rift (FR) tip towards the trench creating a Niuatoputapu-Tonga plate division rather than westward implied by the currently accepted Niuafo'ou-Tonga plate system, but more data are necessary from the FR to test that hypothesis. Instead, we suggest that the Niuafo'ou-Tonga pole is~1.5°further north than previously proposed, reducing the predicted strain in this region to b 2 cm/yr.
The Lau basin is an active backarc comprising several spreading centers and microplates rapidly evolving in time. As such, crustal earthquakes within the Lau Basin derive from intrabasin tectonic, volcanic, and hydrothermal sources. Because of high mantle attenuation, small earthquakes from the basin are seldom recorded on land seismographs and it has been difficult to study the pattern of crustal seismicity. However, the 1994 LABATTS ocean bottom seismograph experiment in the Lau basin recorded more than 100,000 local T-and body phases allowing for a detailed examination of basin seismotectonics. Nearly 1000 events are located within the basin proper, with another 2000 associated with the forearc and the aseismic front. We identify two previously undocumented tectonic features, a triple junction in the northwestern basin and a nanoplate in the central Lau basin. The single most seismically active feature within the Lau basin is the Lau extensional transform zone (LETZ) just north of the Central Lau Spreading Center. Significant seismicity also occurs near the northern extension of the Eastern Lau Spreading Center and along a southeast extension of the LETZ delineating the boundary of a previously unknown nanoplate. Swarm activity is common in the backarc basin and dominates regions associated with actively reorganizing tectonics. We see no evidence of deformation along the southern boundary of the Niuafo'ou microplate, where 4.5 cm/yr of strain is predicted. It is possible that the plate boundary extends eastward from the Fonualei rift (FR) tip towards the trench creating a Niuatoputapu-Tonga plate division rather than westward implied by the currently accepted Niuafo'ou-Tonga plate system, but more data are necessary from the FR to test that hypothesis. Instead, we suggest that the Niuafo'ou-Tonga pole is~1.5°further north than previously proposed, reducing the predicted strain in this region to b 2 cm/yr.
1] The Tonga arc and associated Lau basin exhibit many geologically important processes that link subduction and mantle flow with plate separation and crustal production. We create seismic tomograms of the Tonga-Lau region by jointly inverting for Vp and Vp/Vs structure using data from the LABATTS ocean bottom seismograph experiment and several island deployments to better constrain dynamic processes in the mantle wedge. Jointly using P and S data can help distinguish between the various mechanisms responsible for seismic velocity anomalies such as temperature and the presence of melt and/ or volatiles. Because high attenuation in the wedge limits the S wave data set, we focus on 2-D inversions beneath the linear OBS array where resolution is best and also parameterize the solution in terms of the Vp/ Vs ratio. As expected, the subducting slab has fast Vp and Vs and a low Vp/Vs ratio, consistent with the cold downgoing plate. The Central Lau Spreading Center (CLSC) exhibits stronger anomalies in Vp/Vs than in Vp, with the anomalies larger than would be predicted purely by temperature variations. The CLSC anomaly extends >100 km to the west of the axis, suggesting a broad region of melt production driven by passive upwelling from plate separation rather than active upwelling mechanisms. The anomaly is asymmetric about the axis, suggesting that slab-induced corner flow possibly influences mantle dynamics several hundred kilometers away from the arc. There is a strong anomaly beneath the volcanic arc that gradually deepens as it trends toward the back arc, likely outlining a hydrated region of melt production that feeds the volcanic front. Hydration possibly continues throughout the wedge to at least 400 km depth. The Lau ridge exhibits a thicker lithosphere relative to the rest of the Basin, while the Fiji platform likely has a thinner lithosphere than the Lau Ridge from more recent extension. There is also a reasonable likelihood of a small degree of partial melt in the uppermost mantle beneath the platform.
Front. Earth Sci., 2021
The transition from subduction to transform motion along horizontal terminations of trenches is associated with tearing of the subducting slab and strike-slip tectonics in the overriding plate.One prominent example is the northern Tonga subduction zone, where abundant strike-slip faulting in the NE Lau back-arc basin is associated with transform motion along the northern plate boundary and asymmetric slab rollback. Here, we address the fundamental question: how does this subduction-transform motion influence the structural and magmatic evolution of the back-arc region? To answer this, we undertake the first comprehensive study of the geology and geodynamics of this region through analyses of morphotectonics (remotepredictive geologic mapping) and fault kinematics interpreted from ship-based multibeam bathymetry and Centroid-Moment Tensor data.Our results highlight two notable features of the NE Lau Basin: 1) the occurrence of widely distributed off-axis volcanism, in contrast to typical ridge-centered back-arc volcanism, and 2) fault kinematics dominated by shallow-crustal strike slip-faulting (rather than normal faulting) extending over ∼120 km from the transform boundary. The orientations of these strike-slip faults are consistent with reactivation of earlier-formed normal faults in a sinistral megashear zone. Notably, two distinct sets of Riedelmegashears are identified, indicating a recent counter-clockwise rotation of part of the stress field in the back-arc region closest to the arc. Importantly, the Riedel structures identified in this study directly control the development of complex volcanic-compositional provinces, which are characterized by variably-oriented spreading centers, off-axis volcanic ridges, extensive lava flows, and point source rear-arc volcanoes. This study adds to our understanding of the geologic and structural evolution of modern backarc systems, including the association between subduction-transform motions and the siting and style of seafloor volcanism.
Propagating rift and overlapping spreading center in the North Fiji Basin
Marine Geology, 1994
Ruellan, E., Huchon, P., Auzende, J.-M. and Gr~.cia, E., 1994. Propagating rift and overlapping spreading center in the North Fiji Basin. In: J.-M. Auzende and T. Urabe (Editors), North Fiji Basin: STAmMeR French-Japanese Program. Mar. Geol., 116: 37-56.
Seismotectonics of the Arthur's Pass region, South Island, New Zealand
Geological Society of America Bulletin, 1978
The spatial distribution of the seismicity of the Arthur's Pass region in South Island, New Zealand, shows that earthquakes occur within the crust in definite subparallel zones, each with an east-northeast trend. Composite focal mechanism solutions indicate that the compressive axis is oriented approximately west-northwest, with an average slip direction of N72°E. Comparisons between the 1973 study described here and one conducted in 1972 reveal no differences in the spatial and temporal distributions of the seismicity. Tectonically, the region can be considered as one in which transference of motion between the Alpine fault and the Hikurangi Trench is occurring. This tectonic situation, coupled with the shoaling of the Benioff zone associated with a southward migration of the Hikurangi Trench, suggests that the deformation in the Arthur's Pass region has occurred within the past 2 m.y. This region has marked similarities with the Transverse Ranges area of the San Andreas fault zone in southern California. Both regions are located where two different tectonic and seismic "styles" merge-namely, where a master fault, with its associated seismic activity consisting of infrequent great earthquakes separated by long periods of quiescence, branches into a system of splay faults having a moderate to high level of continuous activity, with the occurrence of large earthquakes. The regions do differ, however, in that the long quiet zone on the Alpine fault is oblique to the slip vector between plates, whereas similar zones of the San Andreas fault are both parallel and nonparallel to the slip vector. This suggests that long quiet zones of strike-slip faults are not a conse
Journal of Geophysical Research, 1973
Shallow earthquakes that occurred during 10 years between the New Hebrides and Fiji islands are relocated by using a digital computer. The spatial distribution of the earthquakes may outline plate boundaries in the Fiji plateau; these boundaries, however, are diffuse and could be broad zones of deformation. In the center of the plateau west of Fiji times of P and S waves traveling in the uppermost mantle indicate velocities of 7.70 and 4.30 km/ sec, respectively. Along the seismically active margins of the plateau P velocities are 7.30-7.40 km/sec. These velocities are considerably lower than P and S velocities of about 8.45 and 4.75 km/sec, respectively, of the normal oceanic basins of the Pacific plate to the north and east of the plateau. The zone of low velocity beneath the Fiji plateau and its boundaries seems to coincide, with a high seismic wave attenuation zone that exists in the uppermost mantle between the Fiji and New Hebrides islands. These observations and other geophysical and geological aspects of the Fiji plateau clearly imply that the different lithospheric plates between the two opposite-facing lithospheric consumption zones of Tonga and New Hebrides arcs were recently generated and are not part of the oceanic Pacific plate.
1] The Tonga arc and associated Lau basin exhibit many geologically important processes that link subduction and mantle flow with plate separation and crustal production. We create seismic tomograms of the Tonga-Lau region by jointly inverting for Vp and Vp/Vs structure using data from the LABATTS ocean bottom seismograph experiment and several island deployments to better constrain dynamic processes in the mantle wedge. Jointly using P and S data can help distinguish between the various mechanisms responsible for seismic velocity anomalies such as temperature and the presence of melt and/ or volatiles. Because high attenuation in the wedge limits the S wave data set, we focus on 2-D inversions beneath the linear OBS array where resolution is best and also parameterize the solution in terms of the Vp/ Vs ratio. As expected, the subducting slab has fast Vp and Vs and a low Vp/Vs ratio, consistent with the cold downgoing plate. The Central Lau Spreading Center (CLSC) exhibits stronger anomalies in Vp/Vs than in Vp, with the anomalies larger than would be predicted purely by temperature variations. The CLSC anomaly extends >100 km to the west of the axis, suggesting a broad region of melt production driven by passive upwelling from plate separation rather than active upwelling mechanisms. The anomaly is asymmetric about the axis, suggesting that slab-induced corner flow possibly influences mantle dynamics several hundred kilometers away from the arc. There is a strong anomaly beneath the volcanic arc that gradually deepens as it trends toward the back arc, likely outlining a hydrated region of melt production that feeds the volcanic front. Hydration possibly continues throughout the wedge to at least 400 km depth. The Lau ridge exhibits a thicker lithosphere relative to the rest of the Basin, while the Fiji platform likely has a thinner lithosphere than the Lau Ridge from more recent extension. There is also a reasonable likelihood of a small degree of partial melt in the uppermost mantle beneath the platform.
Seismological evidences for a slab detachment in the Tonga subduction zone
The Tonga Wadati-Benioff zone is characterized by a large seismicity gap beneath the Lau Basin that raises the question of the slab continuity between the shallow to intermediate part (60-300 km) and the deep part (400-700 km). To address this problem, we investigated the Wadati-Benioff Zone geometry and stress regime through a detailed analysis of the spatial distribution of moment tensors and variation of the stress tensor, using the global seismicity [Engdahl, E., Van der Hilst, R., Buland, R., 1998. Global teleseismic earthquake relocation with improved travel times and procedures for depth determination. Bull. Seism. Soc. Am. 88, 722-743.] and the Centroid Moment Tensor solutions (CMT) catalogs [Dziewonski, A.M., Chou, T., Woodhouse, J.H., 1981. Determination of earthquake source parameters from waveform data for studies of global and regional seismicity. J. Geophys. Res. 86, 2825Res. 86, -2852. The stress tensors were computed using the Gephart's program . An improved method for determining the regional stress tensor using earthquake focal mechanism data: application to the San Fernando earthquake sequence. J. Geophys. Res. 89,[9305][9306][9307][9308][9309][9310][9311][9312][9313][9314][9315][9316][9317][9318][9319][9320]. The stress inversion results indicate that between 21°S and 27°S, and depths down to 700 km the slab is under homogeneous down dip compressional stress regime, while north of 21°S we found strong variations of the stress orientations between the intermediate and deep portions of the slab. We also show that between 14°and 19°S, the stresses at intermediate depth (60-300 km) can be resolved into two slab parallel domains, a thin upper part of the slab that is under downdip compression and the lower part that is under downdip extension. This pattern of two zones with opposite mechanical behavior is characteristic of a subducted plate with a free lower limit that does not interact with the 670-km depth boundary. These results together with the large seismicity gap within the slab argue for a slab detachment.