Adam Klaus

New insights into deformation and fluid flow processes in the Nankai Trough accretionary prism: Results of Ocean Drilling Program Leg 190

Moore, G. F., Taira, A., Klaus, A., Becker, L., Boeckel, B., Cragg, B. A., Dean, A., Fergusson, C. L., Henry, P., Hirano, S., Hisamitsu, T. et al. (2001). New insights into deformation and fluid flow processes in the Nankai Trough accretionary prism: Results of Ocean Drilling Program Leg 190. Geochemistry, Geophysics, Geosystems, 2, Article No: 2001GC000166.

Submarine silicic caldera at the front of the Izu-Bonin arc, Japan: Voluminous seafloor eruptions of rhyolite pumice

Myojin Knoll caldera, a submarine rhyolitic center 400 km south of Tokyo, is one of nine silicic calderas along the northern 600 km of the Izu-Bonin(-Ogasawara) arc and the first anywhere to receive detailed, submersible-based study. The caldera, slightly smaller than the Crater Lake structure in Oregon, is 6 × 7 km in diameter; its inner walls are 500–900 m high, and it has a remarkably flat floor at 1400 m below sea level (mbsl). The caldera collapse volume is ∼18 km 3 , suggesting that more than 40 km 3 of pumiceous tephra may have been erupted at the time the caldera formed. Precaldera seafloor eruptions built a broad volcanic edifice consisting of overlapping composite volcanoes made of rhyolitic lavas, shallow intrusions, and a variety of volcaniclastic deposits—including thick accumulations of rhyolitic pumice erupted at 900– 500 mbsl. The caldera-forming eruption produced a 150–200 m deposit of nonwelded, fines-depleted pumice that resembles a colossal layer of popcorn at the top of the caldera wall. Freshly erupted pumice behaved as “sinkers” or “floaters,” depending on the environment in which it cooled. The pumice clasts deposited proximally and exposed in the caldera wall were likely quenched in eruption columns that remained below sea level. This pumice ingested seawater and sank as gases filling its vesicles cooled, particularly as steam in its vesicles condensed to liquid water. Some eruption columns may have broken through the sea surface and entered the air, especially during vigorous phases of the caldera-forming eruption. These pumices had the opportunity to ingest air as they cooled, becoming floaters as they fell back to the sea; these could have been carried distally on the sea surface by the combined effects of ocean currents and wind. The age of the caldera is unknown, but it may be as young as several thousand years. Its magmatic system at depth retains sufficient heat to sustain an actively growing intracaldera Kuroko- type polymetallic sulfide deposit, rich in gold and silver and topped by chimneys emitting fluids as hot as 278 °C. Sufficient time has elapsed, however, for a 250-m-high postcaldera dome to grow on the caldera floor and for the caldera rim to be deeply scalloped by slumping.

Magnetic lineations within Shatsky Rise, northwest Pacific Ocean: Implications for hot spot‐triple junction interaction and oceanic plateau formation

Oceanic plateaus are major ocean features, yet their origins and development are poorly understood. Many are huge piles of basalt, and it is widely accepted that mantle plumes are their source, perhaps from eruptions of the voluminous head of an emerging plume. Shatsky Rise is a basaltic plateau that formed during a period of geomagnetic reversals, unlike many mid-Cretaceous plateaus, so magnetic data can help us understand its tectonic history. In this study, we analyzed magnetic anomaly data from 131 cruises over and around Shatsky Rise and constructed a magnetic lineation chart for tectonic interpretation. A significant finding is that magnetic lineations are traceable through low parts of the rise between volcanic massifs, indicating nearly normal lithosphere, and between large volcanic edifices. Many lineations form bights near the rise axis and show former locations of the Pacific-Izanagi-Farallon triple junction. They indicate that the junction was in a ridge-ridge-ridge (RRR) configuration and closely followed the rise axis from chron M20 (146 Ma) to chron M4 (127 Ma). Lineation and bight geometries indicate that the triple junction jumped northeastward at least nine times, often apparently in response to volcanic activity from the mantle plume. Some jumps, however, appear related to ridge reorientations. Numerous features within the rise are parallel to lineations or lineation offsets, implying that the plateau formed near the triple junction and was modified by ridge faulting. Most of our observations support the hypothesis that Shatsky Rise was formed by a mantle plume that captured a triple junction.

Geochemical evidence for a mid-Cretaceous superplume

Basalt lavas with a high Nb/Y ratio for a given Nb/Zr ratio occur in the Polynesian “superswell” region of the South Pacific, which probably formed by upwelling of a deep-mantle superplume. The distinctive geochemical characteristics of the Polynesian basalts may be attributed to melting of a mantle source that is more enriched in a basaltic (ancient mid-oceanic-ridge basalt) component. Basalts displaying such chemical signatures have been found on Shatsky Rise, the Ontong Java Plateau, and greenstones from subduction-zone complexes of Sakhalin Island. The occurrence of Polynesian-type basalts, together with an estimate of their ages, suggests that the South Pacific superplume was active as long ago as 90–150 Ma. The superplume activity preceded the onset of the superchron, supporting an idea that the superplume acted as a trigger for such a global event.

Explosive deep water basalt in the sumisu backarc rift.

Eruption of 1-million-year-old tholeiitic basalt >1800 meters below sea level (>18 megapascals) in a backarc rift behind the Bonin arc produced a scoriaceous breccia similar in some respects to that formed during subaerial eruptions. Explosion of the magma is thought to have produced frothy agglutinate which welded either on the sea floor or in a submarine eruption column. The resulting 135-meter-thick pyroclastic deposit has paleomagnetic inclinations that are random at a scale of <2.5 meters. High magmatic water content, which is about 1.3 percent by weight after vesiculation, contributed to the explosivity.

Bathymetry of Shatsky Rise, northwest Pacific Ocean: Implications for ocean plateau development at a triple junction

Oceanic plateaus are large igneous edifices thought to have been created by nascent mantle plumes, but owing to sparse data, their origins remain uncertain. Understanding plateau evolution is important because they are significant ocean features and may provide clues about mantle plume dynamics. We constructed a bathymetry map of Shatsky Rise, a large Pacific plateau, combining multibeam and wide-beam echosounder data from 87 cruises and U.S. Navy multibeam contours. The rise consists of three large, isolated volcanic edifices (massifs), surrounded by nearly normal lithosphere, a linear volcanic ridge, and a group of about 80 scattered seamounts. Massif flank slopes are typically gentle (∼1.5°) and often parallel magnetic lineations or fracture zones. The slope angles imply effusive volcanism, similar to flood basalts, whereas the rise shape suggests formation near the Pacific-Izanagi-Farallon triple junction with modification of volcano flanks by spreading-ridge tectonics. Edifice sizes and inferred ages imply a trend of decreasing volume and age from southwest to northeast. Furthermore, gaps between massifs suggest episodic volcanism. Existing data are broadly consistent with the “plume head” hypothesis wherein the largest edifice formed by massive plume head eruptions, the ridge formed from the plume tail, and the two massifs in between represent a transition. Seamounts east of the rise are morphologically distinct, which suggests that they may have been formed by a different source or mechanism than Shatsky Rise proper.

Consolidation patterns during initiation and evolution of a plate-boundary decollement zone: Northern Barbados accretionary prism

Borehole logs from the northern Barbados accretionary prism show that the plate-boundary decollement initiates in a low-density radiolarian claystone. With continued thrusting, the decollement zone consolidates, but in a patchy manner. The logs calibrate a three-dimensional seismic reflection image of the decollement zone and indicate which portions are of low density and enriched in fluid, and which portions have consolidated. The seismic image demonstrates that an underconsolidated patch of the decollement zone connects to a fluid-rich conduit extending down the decollement surface. Fluid migration up this conduit probably supports the open pore structure in the underconsolidated patch.

Structural development of Sumisu Rift, Izu‐Bonin Arc

Geophysical swath mapping, multichannel seismic profiling, and ocean drilling data are used to document the structural evolution of Sumisu Rift and to analyze the pattern of strain resulting from extension of an intraoceanic island arc. The ∼120-km-long, 30–50-km-wide Sumisu Rift is bounded to the north and south by structural and volcanic highs west of the Sumisu and Torishima calderas and longitudinally by curvilinear border fault zones with both convex and concave dip slopes. The zig-zag pattern of normal faults (average strikes 337° and 355°) indicates extension oriented 076°±10°, orthogonal to the volcanic arc. Three oblique transfer zones divide the rift along strike into four segments with different fault trends and uplift/subsidence patterns. Differential strain across the transfer zones is accommodated by interdigitating, rift-parallel faults and sometimes by cross-rift volcanism, rather than by strike- or oblique-slip faults. From estimates of extension (2–5 km), the age of the rift (∼2 Ma), and the accelerating subsidence, we infer that Sumisu Rift is in the early synrift stage of back arc basin formation. Following an early sag phase, half graben formed with synthetically faulted, structural rollovers facing large-offset (2–2.5 km throw) border fault zones. In the three northern rift segments the largest faults are on the arc side and dip 60°–75°W, whereas in the southern segment they are on the west side and dip 25°–50°E. The present “full graben” stage is dominated by hanging wall antithetic faulting, basin widening by footwall collapse, and a concentration of subsidence in an inner rift. The hanging wall collapses, but not necessarily as a result of border fault propagation from adjacent rift segments. Whereas the border faults may penetrate the Theologically weak lithosphere (Te ≈ 3 km), many of the hanging wall and footwall collapse structures are detached only a few kilometers below the seafloor. Back arc volcanism, usually erupted along faults, occurs in the rift and along the protoremnant arc during both stages. Where drilled, the arc margin has been uplifted 1.1±0.5 km concurrently with ∼1.1 km of rift basin subsidence. Extremely high sedimentation rates, up to 6 m/kyr in the inner rift, have kept pace with synrift faulting, created a smooth basin floor, and resulted in sediment thicknesses that mimic the differential basin subsidence. A linear zone of weakness caused by the greater temperatures and crustal thickness along the arc volcanic line controls the initial locus of rifting. Rifts are better developed between the arc edifices; intrusions may be accommodating extensional strain adjacent to the arc volcanoes. No obvious correlations are observed between the rift structures and preexisting cross-arc trends.

Impact-induced mass wasting at the K-T boundary: Blake Nose, western North Atlantic

Seismic reflection data combined with results from ocean drilling document regional-scale slumping associated with the Cretaceous-Tertiary (K-T) impact event. The K-T boundary is biostratigraphically complete at three Ocean Drilling Program sites (1049, 1050, and 1052) located on Blake Nose (30°N, 75°W) off eastern Florida in water depths of 1300–2600 m. Maastrichtian chalk is folded and fractured below the K-T boundary at all three sites, whereas lowermost Paleocene clays and chalks are undeformed. Deformation is pervasive in Maastrichtian at the deepest water site (Site 1049), whereas at the shallower water sites (Sites 1050 and 1052) thick intervals of structurally intact Maastrichtian chalk are separated by thinner intervals of highly deformed sediments. Correlation of core to seismic reflection data indicates that the K-T boundary immediately overlies seismic facies characteristic of mass wasting that extend across most of the ∼55 km distance of the depth transect. Maastrichtian sediments appear to have moved as large slump blocks on the upper part of Blake Nose but deformed more uniformly in the deeper water parts of the transect. We suggest that mass wasting occurred on the Blake Nose >1600 km from the Chicxulub crater in response to the cataclysmic seismicity generated by the impact.

Chronostratigraphic framework for the IODP Expedition 318 cores from the Wilkes Land Margin: constraints for paleoceanographic reconstruction

The Integrated Ocean Drilling Program Expedition 318 to the Wilkes Land margin of Antarctica recovered a sedimentary succession ranging in age from lower Eocene to the Holocene. Excellent stratigraphic control is key to understanding the timing of paleoceanographic events through critical climate intervals. Drill sites recovered the lower and middle Eocene, nearly the entire Oligocene, the Miocene from about 17 Ma, the entire Pliocene and much of the Pleistocene. The paleomagnetic properties are generally suitable for magnetostratigraphic interpretation, with well-behaved demagnetization diagrams, uniform distribution of declinations, and a clear separation into two inclination modes. Although the sequences were discontinuously recovered with many gaps due to coring, and there are hiatuses from sedimentary and tectonic processes, the magnetostratigraphic patterns are in general readily interpretable. Our interpretations are integrated with the diatom, radiolarian, calcareous nannofossils and dinoflagellate cyst (dinocyst) biostratigraphy. The magnetostratigraphy significantly improves the resolution of the chronostratigraphy, particularly in intervals with poor biostratigraphic control. However, Southern Ocean records with reliable magnetostratigraphies are notably scarce, and the data reported here provide an opportunity for improved calibration of the biostratigraphic records. In particular, we provide a rare magnetostratigraphic calibration for dinocyst biostratigraphy in the Paleogene and a substantially improved diatom calibration for the Pliocene. This paper presents the stratigraphic framework for future paleoceanographic proxy records which are being developed for the Wilkes Land margin cores. It further provides tight constraints on the duration of regional hiatuses inferred from seismic surveys of the region.