Hiroki Kamata

Tectonics of an arc-arc junction : an example from Kyushu Island at the junction of the southwest Japan arc and the Ryukyu arc

Abstract Three distinctive geologic events occurred synchronously during the last 6 m.y. on Kyushu Island, lying at the junction of the Southwest Japan Arc and the Ryukyu Arc. These events included: (1) dextral-fault displacement along a 100 km section of the Median Tectonic Line; (2) formation of a rectangular volcano-tectonic depression 70 km long and 40 km wide; and (3) approximately 30° of counterclockwise crustal rotation. We propose an integrated tectonic model that explains these three tectonic features in terms of subduction of the Philippine Sea plate, which resumed at around 6 Ma after a halt of more than 5 m.y. Oblique subduction of the Philippine Sea plate beneath the Southwest Japan Arc detached a fore-arc sliver that was displaced along a dextral fault, the Median Tectonic Line, north of the Nankai Trough. A volcano-tectonic depression, the Hohi volcanic zone, was formed by the pull-apart structure on the western margin of the Median Tectonic Line. By contrast, normal subduction of the Philippine Sea plate under the Ryukyu Arc caused back-arc spreading, observed as crustal extension in the northern Okinawa Trough. The crustal extension caused the counterclockwise rotation for the northernmost Ryukyu Arc including southern Kyushu.

Volcanic and structural history of the Hohi volcanic zone, central Kyushu, Japan

More than 5000 km3 of magmatic material was erupted in Pliocene-Pleistocene times in a volcano-tectonic depression, i. e., the Hohi volcanic zone (HVZ) in central Kyushu, Japan. The eruptive deposits consist mainly of andesite lava flows and large-scale pyroclastic-flow deposits. Their eruptions were accompanied by the formation of an EW-oriented graben (70 km × 45 km) under regional NS extensional stress. Pre-Tertiary basement rocks are absent on the surface of the graben but occur at depth, having subsided up to 3 km. Radiometric ages of volcanic rocks on the surface show zoned isochrons from 5 Ma at the margin to 0.3 Ma in the center of the HVZ. The youngest center of age zonation coincides with a 30 mgal negative Bouguer gravity anomaly. Radiometric ages of rocks from drill cores are older toward the bottom of the graben, reaching a maximum of at least 4 Ma. Volcanic activity concentrated over time toward the center of the graben and buried successively erupted material. Areas of active volcanism in the HVZ became smaller and changed in style during the 5-Ma history of activity. Volcanism of the early stage (5-2 Ma) was characterized by voluminous eruptions of andesitic lava flows that formed lava plateaus and were intruded by EW-oriented feeder dikes, perhaps related to fissure eruptions. In contrast, late-stage volcanism (2-0 Ma) resulted primarily in andesitic to dacitic lava domes with features of monogenetic volcanoes produced at low eruption rates. The HVZ shows unimodal volcanism dominated by andesitic and dacitic lavas with a small amount of rhyolite and only traces of basalt; these characteristics differ from those that typify volcanism in most other extensional areas. Erupted material in the HVZ is of the calc-alkali and high-alkali tholeiite series and shows no significant chemical changes over 5 Ma, except for an increase in K2O after 1.6 Ma. The net horizontal displacement along normal faults indicates that the HVZ widened by about 10%–20% across the graben at an average rate of 0.1 cm/yr. I interpret the HVZ to be neither a pull-apart structure of the pre-Tertiary basement nor the result of propagation of the Okinawa Trough, but rather the earliest stage of rifting when vertical subsidence caused by normal faulting is compensated by filling with volcanic material.

Evolution of the caldera‐forming eruption at Crater Lake, Oregon, indicated by component analysis of lithic fragments

Crater Lake caldera (8 × 10 km), formed 6845 years B. P. (14C age) during the climactic eruption of the volcanic edifice known as Mount Mazama, is intermediate in size between small calderas associated with central vent eruptions and large calderas that have ring fracture vent systems. Our quantitative study of lithic fragments in the ejecta confirms the existing model of changes in vent configuration during the climactic eruption of Mount Mazama. Initial activity was from a single vent that produced a rhyodacite pumice fall from a Plinian column. Altered preexisting volcanic rocks are the predominant lithic type in the Plinian deposit, and their extensive hydrothermal alteration is considered as evidence of their relatively deep origin. The Wineglass Welded Tuff lies atop the Plinian deposit and contains a higher proportion of fresh volcanic rocks, suggesting enlargement of the single vent by slumping of its walls. This same vent enlargement caused the Plinian eruption column to collapse and feed valley-hugging pyroclastic flows that deposited the Wineglass Welded Tuff. When enough material was erupted from the shallow magma chamber that its roof was no longer adequately supported, Mount Mazama collapsed to form the caldera, while highly energetic pyroclastic flows produced the climactic ignimbrite. A lag breccia that represents the proximal facies of the compositionally zoned climactic ignimbrite lies atop the Wineglass Welded Tuff and contains predominantly altered volcanic rocks of deeper origin, accompanied by minor granitoids from the magma chamber walls. Azimuthal differences in lithic component proportions in the lag breccia correlate well with the geology of the caldera walls, indicating that the climactic ignimbrite was ejected by multiple vents along a ring fracture system. Systematic lithic component changes within the lag breccia suggest different quarrying levels that reflect waxing and waning of the discharge rate during the caldera collapse phase of the climactic eruption. Our lithic component analysis demonstrates that calderas that may be too small to experience structural resurgence, such as Crater Lake, nevertheless may form by syneruptive subsidence along ring fractures.

Temporal change in chemistry of magma source under Central Kyushu, southwest Japan : progressive contamination of mantle wedge

Volcanism related to subduction of the Philippine Sea (PHS) plate began in Central Kyushu at 5 Ma, after a pause of igneous activity lasting about 10 m.y. It formed a large volcano-tectonic depression, the Hohi volcanic zone (HVZ), and has continued to the present at a decreasing eruption rate. The products are largely andesite and dacite, which became enriched in K with time. The proportion of tholeiitic to calc alkalic rocks also increases with time. Calc-alkalic high-Mg basaltic andesites (YbBs) were erupted in the early stage of the HVZ activity (5–3 Ma), and high-alumina basalts (KjBs) were erupted in the later stage (2–0 Ma). In contrast to the basalts in the HVZ, Northwest Kyushu basalts (NWKBs) have been erupted on the backarc side of the HVZ since 11 Ma, and hence are not related to the PHS plate subduction. They are mainly high-alkali tholeiitic to alkali basalt that shows no notable chemical change with time. NWKB, YbB, and KjB have MORB-normalized incompatible-element spectra that differ from each other, as is well expressed in both Nb and Sr anomalies. The patterns of KjB and NWKB are typical of those for island-arc basalt (IAB) and ocean-island basalt (OIB), respectively. YbB shows a pattern intermediate between the two. We suggest that the magma source beneath the HVZ changed in composition from an OIB-type mantle to an IAB-type mantle as the subduction of PHS plate advanced. However, the magma source remained fertile under Northwest Kyushu. In order to explain the temporal change of source mantle beneath the HVZ, we propose a model for progressive contamination of the mantle wedge, in which three processes (contamination by a slab-derived component, subtraction of magma from the mantle, and mixing of the mantle residue and slab-derived component) are repeated as subduction continues. As long as the progressive contamination of mantle wedge proceeds, its trace-element composition converges at a steady-state value for a short period. This value does not depend on the initial composition of the mantle wedge but instead on the composition of the slab-derived component. The trace-element composition of the magma produced in such a mantle wedge approaches that of the slab-derived component with time, but the major-element composition is determined by the phase relations of mantle peridotite. The slab-derived component may be basaltic liquid that is partially melted from rutile-bearing eclogite.

History of basin formation and tectonic evolution at the termination of a large transcurrent fault system: deformation mode of central Kyushu, Japana

Abstract The central part of the Kyushu Island is a locus of active arc-volcanism associated with subduction of the Philippine Sea plate. Being located at a bend in the boundary between the Philippine Sea and Eurasian plates, a complicated lateral displacement has generated half-grabens, a zone of strike-slip, and rhomboidal basins filled with volcanic material in central Kyushu. Initial northward subduction in latest Miocene time activated the N-S-trending Kokura-Tagawa Tectonic Line (KTL) as a left-lateral slip fault that bounds the western margin of the area of volcano-tectonic depression, the Hohi volcanic zone, which was initially formed about 6 Ma ago. The depocenter, which is now called Kuju basin, in the Hohi volcanic zone was at the corner of the KTL and the E-W-trending Median Tectonic Line (MTL). Relative convergence direction of the Philippine Sea plate shifted counterclockwise at about 1.5 Ma, and active motion on the KTL declined. At the same time, west-northwestward subduction enhanced right slip on the MTL. Geologic, gravimetric, and seismic data indicate that the MTL has shifted its active trace northward in central Kyushu as far as 10 km. As a result, the depocenter adjacent to the transcurrent fault migrated northeastward in the Hohi volcanic zone, specifically, from Kuju basin (Pliocene) via Shonai basin (early Quaternary) to Beppu Bay basin (late Quaternary). The latest depocenter of the Beppu Bay is surrounded by active faults that clearly delineate a rhomboidal basin on the MTL. Central Kyushu exemplifies the basin forming history and tectonic evolution at the termination of a large transcurrent fault system.

Correlation of widespread tephra deposits based on paleomagnetic directions: Link between a volcanic field and sedimentary sequences in Japan

Abstract Magnetic measurements were made of the Yabakei and the Imaichi pyroclastic-flow deposits of the late Matuyama Chron in central Kyushu and their correlative co-ignimbrite ashes: the Pink and the Azuki tephra deposits in the Kinki district, and the Ku6C and the O7 tephra deposits in Boso Peninsula. Site mean magnetic directions of the Imaichi deposit are all of reversed polarity with declinations deflected slightly to the east. These directions are consistent with those of the Azuki and the Ku6C ashes. Magnetizations of the Yabakei, the Pink, and the O7 ashes are also consistent with one another. All are characterized by shallow inclinations (about 30°) of normal polarity with slightly westerly declinations. The identical paleomagnetic directions of these widespread tephra confirm correlations that have been proposed mainly on the basis of glass and mineral chemistry, and provide a tephrochronological linkage between the source volcanic area and the lacustrine or marine sequences over distances of at least 1000 km. This linkage allows us to correlate the oxygen isotope record of the Kazusa Group in Boso Peninsula with the alternating marine/non-marine lithology of the Osaka Group in the Kinki district. Radiometric dates from the Yabakei and the Imaichi pyroclastic-flow deposits are concordant with the astronomically-calibrated geomagnetic polarity time scale.

Shishimuta caldera, the buried source of the Yabakei pyroclastic flow in the Hohi volcanic zone, Japan

Drill-hole, geochronologic, and gravity data identify the buried Shishimuta caldera beneath post-caldera lava domes and lacustrine deposits in the center of the Hohi volcanic zone. The caldera is the source of the Yabakei pyroclastic flow, which erupted 1.0 Ma ago with a bulk volume of 110 km3. The caldera is a breccia-filled funnel-shaped depression 8 km wide and > 3 km deep with a V-shaped negative Bouguer gravity anomaly up to 36 mgal. Neither ring vents nor resurgence was recognized; instead, post-caldera monogenetic volcanism in an extensional setting dominated the area. The andesitic breccia has a relatively low density and fills the caldera; it possibly formed by fragmentation of disrupted roof rock during the violent Yabakei eruption and related collapse. Fewer normal faults and shallow microearthquakes occur inside the caldera than around it, possibly because rocks beneath the caldera are structurally incoherent. A profile of Shishimuta caldera may be more elongated vertically, and have a more intensely fractured zone, than that of a Valles-type caldera.

The eruptive rate and history of Kuju volcano in Japan during the past 15,000 years

Abstract The eruptive history of Kuju volcano on Kyushu, Japan, during the past 15,000 years has been determined by tephrochronology and 14C dating. Kuju volcano comprises isolated lava domes and cones of hornblende andesite together with aprons of pyroclastic-flow deposits on its flanks. Kuju volcano produced tephras at roughly 1000-yr intervals during the past 5000 years and 70% of the domes and cones have formed during the past 15,000 years. The youngest magmatic activity of Kuju volcano was the 1.6 km3 andesite eruption about 1600 years ago which emplaced a lava dome and block-and-ash flow. Kuju volcano shows a nearly constant long-term eruption rate (0.7–0.4 km3 for 1000 years) during the past 15,000 years. This rate is within the range of estimated average eruption rates of late Quaternary volcanoes in the Japanese Arc, but is about one order of magnitude higher than the eruption rate of Unzen volcano. Kuju volcano has been in phreatic eruption since October 1995. The late Quaternary history of Kuju indicates that it poses a significant volcanic hazard, primarily due to block-and-ash flows from collapsing lava domes.

The proximal facies of the Tosu pyroclastic-flow deposit erupted from Aso caldera, Japan.

The Tosu pyroclastic flow deposit, a low-aspect-ratio ignimbrite (LARI), has widely distributed breccia facies around Aso caldera, Japan. The proximal facies, 9–34 km away from the source, consists of 3 different lithofacies, from bottom to top: a lithic-enriched and fines-depleted (FD) facies, a lithic-enriched (LI) facies with an ash matrix, and a fines- and pumice-enriched (NI) facies. Modes of emplacement of FD, LI, and NI are interpreted as ground layer, 2b-lithic-concentration zone, and normal ignimbrite, respectively. These stratigraphic components in the Tosu originated from the flow head (FD) and the flow body (LI and NI), and were generated by a single column collapse event. Remarkably thick FD and LI, in contrast to thin NI, suggest that due to high mobility most ash and punice fragments in the Tosu were carried and deposited as NI in the distal area. Heavier components were selectively deposited as FD and LI in the proximal area. The rate of falloff of lithic-clast size in the Tosu shows an inflection at 20 km from the source. In a survey of well-documented pyroclastic flows, the inflection distance of a LARI is generally greater than that of a high-aspect-ratio ignimbrite, so that the eruption of the former is probably more intense than the latter.

Deformation of the Wineglass Welded Tuff and the timing of caldera collapse at Crater Lake, Oregon

Abstract Four types of deformation occur in the Wineglass Welded Tuff on the northeast caldera rim of Crater Lake: (a) vertical tension fractures; (b) ooze-outs of fiamme: (c) squeeze-outs of fiamme; and (d) horizontal pull-apart structures. The three types of plastic deformation (b–d) developed in the lower part of the Wineglass Welded Tuff where degree of welding and density are maximum. Deformation originated from concentric normal faulting and landsliding as the caldera collapsed. The degree of deformation of the Wineglass Welded Tuff increases toward the northeast part of the caldera, where plastic deformation occurred more easily because of a higher emplacement temperature probably due to proximity to the vent. The probable glass transition temperature of the Wineglass Welded Tuff suggests that its emplacement temperature was ⩾750°C where the tuff is densely welded. Calculation of the conductive cooling history of the Wineglass Welded Tuff and the preclimactic Cleetwood (lava) flow under assumptions of a initially isothermal sheet and uniform properties suggests that (a) caldera collapse occurred a maximum of 9 days after emplacement of the Wineglass Welded Tuff, and that (b) the period between effusion of the Cleetwood (lava) flow and onset of the climactic eruption was