Alexander N. Shor

Mud waves in the Argentine Basin and their relationship to regional bottom circulation patterns

Abstract Multi-narrow beam echosoundings from the central Argentine Basin of the South Atlantic Ocean define orientations of abyssal mud waves over an area of 100,000 km 2 . Mud wave heights vary from 80 m, and wave spacings vary from 1 to more than 10 km, with typical values of 4–6 km. Sub-bottom reflectors demonstrate that many of these bed forms have migrated with time. At least four distinct mud wave fields can be identified. Mud waves on the southern flank of the Ewing sediment drift strike northwest and migrate toward the northeast (upslope). Waves on the northern Zapiola Drift strike northeast and migrate toward the southeast. A field of symmetrical and/or relict waves with variable orientations lies between the Ewing and Zapiola Drifts. Mud waves on the southwestern flank of the Zapiola Drift strike west-northwest and migrate north-northeast (upslope). Wave orientations vary with respect to the regional contours, bottom slope and flow direction; however, bottom current flow directions are poorly defined. Analysis of historical bottom photographs from the Argentine Basin suggests that bottom currents locally flow around the sediment drifts. Sediment drifts and mud waves are best developed offshore where surface eddy activity is less intense and bottom flows are probably steadier, both in speed and direction. Mud waves require 10s or 100s of thousands of years to develop. Thus the waves reflect a long-term response to environmental conditions rather than a short-term response to discrete events.

Structure and topography of the Siqueiros transform fault system: Evidence for the development of intra-transform spreading centers

The Siqueiros transform fault system, which offsets the East Pacific Rise between 8°20′N–8°30′N, has been mapped with the Sea MARC II sonar system and is found to consist of four intra-transform spreading centers and five strike-slip faults. The bathymetric and side-looking sonar data define the total width of the transform domain to be ≈20km. The transform domain includes prominent topographic features that are related to either seafloor spreading processes at the short spreading centers or shearing along the bounding faults. The spreading axes and the seafloor on the flanks of each small spreading center comprise morphological and structural features which suggest that the two western spreading centers are older than the eastern spreading centers. Structural data for the Clipperton, Orozco and Siqueiros transforms, indicate that the relative plate motion geometry of the Pacific-Cocos plate boundary has been stable for the past ≈1.5 Ma. Because the seafloor spreading fabric on the flanks of the western spreading centers is ≈500 000 years old and parallels the present EPR abyssal hill trend (350°) we conclude that a small change in plate motion was not the cause for intra-transform spreading center development in Siqueiros. We suggest that the impetus for the development of intra-transform spreading centers along the Siqueiros transform system was provided by the interaction of small melt anomalies in the mantle (SMAM) with deepseated, throughgoing lithospheric fractures within the shear zone. Initially, eruption sites may have been preferentially located along strike-slip faults and/or along cross-faults that eventually developed into pull-apart basins. Spreading centers C and D in the eastern portion of Siqueiros are in this initial pull-apart stage. Continued intrusion and volcanism along a short ridge within a pull-apart basin may lead to the formation of a stable, small intra-transform spreading center that creates a narrow swath of ridge-parallel structures within the transform domain. The morphology and structure of the axes and flanks of spreading centers A and B in the western and central portion of Siqueiros reflect this type of evolution and suggest that magmatism associated with these intra-transform spreading centers has been active for the past ≈0.5–1.0 Ma.

A 220 km2 recently erupted lava field on the East Pacific Rise near lat 8°S

SeaMARC II records show a 220 km2 area of unusually high reflectivity on the East Pacific Rise (EPR) near lat 8°S which we interpret to be a recently erupted lava field. This is the only segment of the EPR exhibiting high reflectivity over such a large area in our continuous SeaMARC II coverage from lat 8°N to 18°N and 3°S to 23°S. The lavas appear to issue from the axial neovolcanic zone between lat 8°15′ and 8°17′S. The lava field extends up to 18km off-axis and inundates scarps up to 100 m high. On the basis of measurements of scarp heights just outside areas where they are buried, we estimate the average thickness of the field to be 70 ±20 m. Thus, the volume of the lava field is 15 ±4 km3. In comparison, the average annual volcanic budget for the Earth is approximately 4-5 km3, and the largest historic basaltic eruption (from the Icelandic Laki fissure in 1783) was approximately 12.3 km3. When did the eruption occur? Comparative studies of acoustic reflectivity and photographed bottom characteristics elsewhere along the EPR suggest that the lava field has little or no sediment cover and is very young. This is also one of the only areas on the EPR where earthquakes occur that do not appear to be caused by transform faulting. If the earthquakes are related to the lava field, then major eruptions may have occurred in 1964, 1965, and 1969. A large helium plume near lat 13°S may be caused in part by eruptions of the lat 8°S lava field. This is consistent with estimates of currents at 2-3 km depth, which are such that a plume generated at lat 8°S would eventually appear off-axis at lat 13°S. Our observations suggest that this spreading segment, which is bounded by small ridge-axis discontinuities at lat 8°05′S and 8°47′S, may have been the most volcanically active segment along the EPR during the past 25 yr.

Late Quaternary slumps and debris flows on the Scotian Slope

The Scotian Slope just west of Verrill Canyon was surveyed using the Sea MARC I deep-towed sidescan-sonar system, high-resolution seismic-reflection profiles, and 25 piston cores. The generally smooth continental slope has a gradient of ∼2.5°. It is crossed by 2 small valleys that are 1 km wide and 100 m deep. Much of the sea bed shows evidence of surficial sliding that removed 10–20 m of sediment, and the slide scars give the sea bed a steplike morphology. From the 600-m isobath to ∼1,800 m, there extend 2 zones of disturbed sediment. These disturbed zones have the rough surface and transparent acoustic character previously regarded as characteristic of debris flows, but sidescan-sonar images and cores suggest that they are principally rotational slide deposits, with true debris flows at their distal limits. Streamlined erosional depressions near the downslope edge of the debris flows cut both the flow and sediment farther down-slope. These may have been produced by a turbidity current associated with the debris flows. Sorted coarse sand in piston cores provides further evidence of current activity. This widespread sediment failure on relatively low slopes was probably the result of a large earthquake that can be dated from cores as occurring between 5000 and 12000 yr B.P.

Sediment slides and turbidity currents on the Laurentian Fan: Sidescan sonar investigations near the epicenter of the 1929 Grand Banks earthquake

The epicenter area of the 1929 Grand Banks earthquake on the continental slope south of Newfoundland has been investigated using Sea MARC I, a deeply towed, midrange sidescan sonar with a 4.5-kHz subbottom profiler. Shallow slides pass downslope into debris flows on the muddy continental slope east of the epicenter. At the head of the Eastern Valley of the Laurentian Fan, west of the epicenter, arcuate slide scars cut undisturbed upper-slope sediment and lead downslope to a lineated erosional seabed. At a water depth of about 1600 m, this erosional seabed passes into extensive fields of 100-m-wavelength gravel waves situated on the broad, irregular valley floor. The gravel waves become better developed downslope and extend at least to water depths of 3000 m. All these morphological features appear fresh on the sidescan sonograms, suggesting that they date from the 1929 earthquake event, and the distribution of slides corresponds to the area of instantaneous cable breaks in 1929. The upper limit of erosion on valley walls suggests that the 1929 turbidity current was less than 300 m thick. Timing of cable breaks downfan suggests that flow velocities were sufficient to rework gravel deposits into large bedforms during waning flow stages over elevated areas of the valley floor. Similar cross-bedded coarse sands and gravels are common in ancient channel deposits.

Morphology of abyssal mudwaves at project MUDWAVES sites in the Argentine Basin

Abstract Extensive fields of mudwaves are present in the Argentine Basin. Mudwaves and water parameters at three sites were studied to provide detailed information on bathymetry, mudwave structure, sediment parameters and bottom water flow characteristics. Two sites, 5 and 6, are discussed in this paper. Crests of mudwaves at Site 5 on the north flank of the Zapiola Drift are oriented 45° anti-clockwise from the measured westward flow direction and migrate to the left of the flow direction. Mudwaves at Site 6 on the south flank of the Zapiola Drift are oriented 20° anti-clockwise from the inferred southeast bottom flow direction, and also migrate to the left of direction. Evidence from current meters and surface sediments suggests that the mudwaves at Site 5 are active today, while those at Site 6 have been active in the Holocene, but perhaps not within the last 100 years. Mudwave orientation anti-clockwise to mean flow, preferential sediment accumulation on the left wave flank, and apparent sediment accumulation on the wave crest agree with a model for mudwave growth presented by Blumsack and Weatherly (1989, Deep-Sea Research, 36, 1327–1339). Cross-wave asymmetry in sediment accumulation pattern is sufficient to cause a 10% change in surface sediment bulk density over the wave profile. The record of mudwave migration obtained from Sites 5 and 6 in the southern Argentine Basin and from Site 7 in the northern Argentine Basin suggests that bottom water flow has changed over a number of time scales during the last 200,000 years. Thus the analysis of abyssal bed forms can provide independent evidence about past changes in bottom flow.

Dense biological communities at 3850 m on the Laurentian Fan and their relationship to the deposits of the 1929 Grand Banks earthquake

During Alvin dives on the Laurentian Fan aimed at exploring the nature of the deposit of the 1929 Grand Banks earthquake and turbidity current, large, dense communities of living vesicomyid and thyasirid clams, gastropods, and other epifaunal taxa similar to those found in hydrothermal and cold seep environments were unexpectedly discovered. The communities are at 3800–3900 m in a passive margin setting, with no apparent mechanism for enhanced fluid flow. The communities occur near the crests of ‘gravel waves’, depositional bedforms created during the passage of the turbidity current, and on the slope and crest of a steep (20–30°) scarp of outcropping valley floor material. We speculate that these communities have established themselves since 1929 and that they are sustained by chemosynthetic processes. The reduced compounds fueling the chemosynthesis presumably are derived from older, organic-rich fan valley floor sediments that were exposed by the 1929 event.

Submarine geology of the Hilina slump and morpho-structural evolution of Kilauea volcano, Hawaii

Abstract Marine geophysical data, including SEA BEAM bathymetry, HAWAII MR1 sidescan, and seismic reflection profiles, along with recent robot submersible observations and samples, were acquired over the offshore continuation of the mobile Kilauea volcano south flank. This slope comprises the three active hot spot volcanoes Mauna Loa, Kilauea, and Loihi seamount and is the locus of the Hawaiian hot spot. The south flank is the site of frequent low-intensity seismicity as well as episodic large-magnitude earthquakes. Its sub-aerial portion creeps seaward at a rate of approximately 10 cm/year. The Hilina slump is the only large submarine landslide in the Hawaiian Archipelago thought to be active, and this study is one of the first to more highly resolve submarine slide features there. The slump is classified into four distinct zones from nearshore to the islands base. Estimates of size based on these data indicate a slumped area of 2100 km 2 and a volume of 10,000–12,000 km 3 , equivalent to about 10% of the entire island edifice. The overall picture gained from these data sets is one of mass wasting of the neovolcanic terrain as it builds upward and seaward, though reinforcement by young and pre-Hawaii seamounts adjacent to the pedestal is apparent. Extensive lava delta deposits are formed by hyaloclastites and detritus from recent lava flows into the sea. These deposits dominate the upper submarine slope offshore of Kilauea, with pillow breccia revealed at mid-depths. Along the lower flanks, massive outcrops of volcanically derived sedimentary rocks were found underlying Kilauea, thus necessitating a rethinking of previous models of volcanic island development. The morphologic and structural evolutionary model for Kilauea volcano and the Hilina slump proposed here attempts to incorporate this revelation. A hazard assessment for the Hilina slump is presented where it is suggested that displacement of the south flank to date has been restrained by a still developing northeast lateral submarine boundary. When it does fully mature, the south flank may be more subject to land slips triggered by large, long duration earthquakes and thus Kilauea may undergo more frequent episodes of failure with increased displacements.

Australian-Antarctic discordance

The Australian-Antarctic discordance is a region of anomalous geophysical and geochemical properties along the mid-ocean ridge system. It includes the isotopic boundary between Pacific Ocean and Indian Ocean basalts. Its lavas have compositions consistent with low mantle temperatures and a relatively low overall extent of melting. These characteristics have been attributed to downward flow in the underlying mantle. New bathymetric and side-scan sonar data show that (1) the spreading axis within the discordance is predominantly characterized by a broad rift valley and segmentation characteristics typical of slow-spreading centers, (2) the isotopic boundary appears to be associated with unusual, chaotic sea floor, and (3) the spreading axis east of the discordance is characterized by an axial ridge typical of fast-spreading centers. These extreme variations, at an essentially constant (intermediate) spreading rate are consistent with differences in melt supply and mantle properties along the spreading axis within and east of the discordance, as suggested in previous studies.

Contourite or turbidite?: magnetic fabric of fine-grained Quaternary sediments, Nova Scotia continental rise

Summary Samples of three piston cores and one gravity core from the Nova Scotia continental rise (depths 4210–4925 m) have been examined to differentiate parallel-to-slope and downslope depositional processes in Quaternary deposits from a region presently influenced by a strong contour current. Measurement of anisotropy of magnetic susceptibility of samples of a red-brown, silt-laminated lutite ‘contourite’ facies shows grain alignments which are consistent with both parallel-to-slope (contour current) flow and downslope (turbidity current) flow. We believe that these results provide support for the hypothesis that ‘geologically significant’ contour currents have influenced continental rise deposition during the Pleistocene. However, our observation that both alongslope and downslope alignments are present in lithologically similar units clearly demonstrates the need for studies on the relationship between lithofacies and process in this geological setting.