Landward Wall Research Articles

Londinium's Landward Wall: Material Acquisition, Supply and Construction

ABSTRACTThe construction of a free-standing stone wall was a significant occasion in Londinium's history, remarkable for the quantity of masonry used and for the continuing additions to the defences over at least three identifiable phases. Since the local geology in the London Basin does not offer suitable building stone, Londinium's walls offer an exceptional example by which to examine the logistics of construction and the transportation of materials in the context of Romano-British building projects. We examine the sources of the materials used, their transport and the scale of labour and investment involved in the construction of the Landward Wall using an energetics-based methodology. Finally, we provide new insights into Londinium's Landward Wall and the socio-economic and practical implications of its construction. Supplementary material is available online (https://doi.org/10.1017/S0068113X21000088) and comprises technical data related to the architectural energetics.

Transitions in microbial communities along two sediment cores collected from the landward walls of the New Britain trench

The landward wall of the New Britain Trench (NBT) is an active continental margin. Unlike passive margins, it lacks a continental rise and the abyssal plain, and has a steep continental slope ending in an oceanic trench. Although the trench systems are receiving more and more attentions, our understanding of the microbiology of the NBT and its trench walls is limited. In this study, we collected a 70 cm gravity core (4524 m depth) and a 30 cm box core (4130 m depth) from the western and eastern landward walls of the NBT, respectively. Diversity and composition of microbial communities were investigated at high resolution in 2 cm depth intervals. In two cores, obvious vertical transitions from the aerobic to anaerobic but distinct dominant anaerobic microbes were observed. In the eastern trench wall, anaerobic fermenters, such as Anaerolineales and Alphaproteobacteria, were dominant, but the western contained abundant Dehalococcoidia, and Gammaproteobacteria, such as Alteromonadales and Oceanospirillales. Combined with the results of higher bacteria abundance and probable shallower aerobic active zone in the eastern core, such patterns of species composition might be related with richer input of labile allochthonous terrestrial organic matter by mass wasting processes in the steeper eastern wall. Our analysis also revealed highly diversified Dehalococcoidia including some new phylotypes, extending the current understandings on Dehalococcoidia diversity. This study fills the gap of benthic microbial study in the NBT, and highlights the need of multi-point sampling in trench studies for the great geographical variations along the trench wall.

Impact of Seawalls on Loggerhead Sea Turtle (Caretta caretta) Nesting and Hatching Success

Abstract As coastal development becomes increasingly threatened by erosion, installation of armoring such as seawalls has been applied to protect property by permanently relocating the position of a dune. The physical impact of seawalls to beach ecosystems is relatively well-understood, but the impact to sea turtle nesting remains unclear. We investigated the impact using observations of loggerhead sea turtle nesting in Florida at a seaward wall over 7 years, and a more landward wall over 3 years. Nesting patterns indicated that passive erosion at seawalls likely caused fewer turtles to attempt to nest on armored beach when compared with unarmored beach. Nests placed in front of seawalls were more likely to be washed away in storms. Placement of walls further from the shoreline may only delay the impact to nesting turtles by a few years. Armoring is expected to multiply as sea levels rise and storms become more frequent; thus, the availability of appropriate nesting habitat for loggerhead sea turtles rema...

Effectiveness of Small Onshore Seawall in Reducing Forces Induced by Tsunami Bore: Large Scale Experimental Study

Tsunami force and pressure distributions on a rigid wall fronted by a small seawall were determined experimentally in a large-scale wave flume. Six different seawall heights were examined, two of which were exposed to a range of solitary wave heights. The same experiment was done without a seawall for comparison. The measured wave profile contained incident offshore, incident broken, reflected broken, and transmitted wave heights measured using wire resistance and ultrasonic wave gauges. Small individual seawalls increased reflection of the incoming broken bore front and reduced force on the rigid landward wall. These findings agree well with published field reconnaissance on small seawalls in Thailand that showed a correlation between seawalls and reduced damage on landward structures.

Comment on “Discussions of ODP Leg 205 and drilling of Middle America Seismogenic Zone”

Von Huene et al. [2003] outline a plan to study convergent margin processes in the Middle America Trench area. They describe the area as mainly an “erosional convergent margin,” which they represent as a fundamental feature of the circum‐Pacific. The concept of tectonic erosion at this type of hypothetical margin has been reviewed by von Huene and Scholl [1991]. They portray tectonic erosion (or “subduction erosion”) to be occurring in a thrust fault zone located at the base of the landward wall of a trench. Tectonic erosion and subduction assisted by pore‐pressure phenomena, alluded to in the subject article, are used to explain the perceived absence of material in the forearc—for example, absence of byproducts of thrust faulting such as accreted oceanic rocks and accreted trench fill. The concept of tectonic erosion is based on and requires the major thrust fault of the plate tectonic model. The fault accommodates the assumed long‐term (millions of years), continuous, rapid convergence of the plates. This model convergence balances reported seafloor spreading.

日本列島周辺の海底地質 その３． 南海舟状海盆（トラフ）及びその付近

The Nankai Trough is regarded as a trench from the structural point of view, in spite of its shallowness and flatness. The topography of the sea floor of the Nankai Trough and its vicinity shows typical features consisting of the continental shelf, the upper slope (shelf slope), the deep-sea terrace, the lower slope (landward wall of the trough), the trough floor, and the ocean basin. The upper slope is comparatively monotonic. The deep-sea terrace is a large structural basin lying at the foot of the upper slope, behind the outer ridge rising on the continental slope. The lower slope from the outer margin of the deep-sea terrace to the trough floor consists of accretions which form ridges and troughs. This is divided into smaller scale units by the structure of faults and folds. Sediments on the sea bed of the Nankai Trough have been deformed in conformity with the formation of the hollow of the trough. The Nankai Trough and its related topographies, namely the upper slope, the outer ridge shifting landward on the whole, and the ridge-and-trough zone, were formed by the subduction of the Philippine Sea plate. Topographic trends of the Nankai Trough area are deviated largely at the boundary, which can be estimated by distribution of hypocenters, between sub-slabs in the subducting Philippine Sea plate. The age of formation of the major topographies of the Nankai Trough and its vicinity falls within the post-Pliocene.

Deep biological communities in the subduction zone of Japan from bottom photographs taken during “nautile” dives in the Kaiko project

Twenty-seven dives of the submersible “Nautile” in the subduction zone around Japan conducted in the French-Japanese Project Kaiko proved that fairly luxuriant benthic communities dominated by deep-sea giant clams of the genus Calyptogena (family Vesicomyidae) were consistently present on the accretionary prism at abyssal depths. Benthic communities characterized by three hitherto undescribed bivalves of the genus Calyptogena were found between depths of about 3800 and 4020 m at the mouth of Tenryu Canyon and at the top of basement swell of the Zenisu Ridge, both situated in the eastern Nankai subduction zone. Sporadic but discrete patches of organisms characterized by one more undescribed bivalve belonging to the genus Calyptogena were observed and collected between depths of 5130 and 5960 m on the landward wall of the Japan and Kouriles Trenches. Photographic inventories were prepared semiquantitatively using each series of bottom photographs taken in these areas with bow cameras of the submersible “Nautile”. Observations on the sporadic but dense distribution of the clams and other characteristic associated organisms match well with the scheme that communities sustained by chemosynthetic energy sources can be present at connate water seepages in subduction zones. These are to date the deepest record of benthic communities supposedly associated with chemosynthetic processes.

Spatial distribution of microearthquakes beneath the Japan Trench from ocean bottom seismographic observations

The area of the Japan Trench, about 40°N off the Tôhoku District of north-eastern Honshu, Japan, was for the first time extensively covered by an ocean bottom seismograph (OBS) array. Seven OBSs were available for analyses of natural earthquakes and the observation period lasted two weeks during 1980 July and August. During this period, each OBS registered between 300 and 600 earthquakes. About 100 events were simultaneously recorded by at least three OBSs. Hypocentres were calculated by using a generalized inversion technique. Sixty-five events, mostly with magnitudes (M) from 1.5 to 3.0, which were not located by a land network, were determined to be within or near the area covered by the OBS array. The OBS array had nearly uniform detectability for earthquakes with M over 2.0 within the latitude range of 39.5° — 41.5°N and the longitude range of 143° — 146.5°E. Within this area there is a highly active region of microseismicity beneath the seaward wall of the Japan Trench. The seaward activity is concentrated within 100 km of the trench axis and is low beneath the Northwest Pacific Basin. A seismicity gap was found just westward of the trench axis, beneath the landward wall of the trench at a water depth of between 6000 and 4000 m. Seismic activity continues landward from the region west of the gap. The OBS array enabled us to determine the focal depth of microearthquake activity with better resolution than was previously possible. The results show that the microearthquake activity is concentrated within the upper part of the oceanic lithosphere. On the seaward side, most events occur at a depth of less than 30 km beneath the trench wall, while the activity on the landward side starts to deepen westward exactly at the trench axis. This paper reports the first direct observations to clarify the spatial distribution of microearthquakes associated with the bending and/or subducting of the oceanic plate just beneath the Japan Trench axis.

Heat‐flow determination in three DSDP boreholes near the Japan Trench

The first deep borehole determinations of temperature gradients and heat flow on the landward wall of the Japan Trench and forearc were made on IPOD DSDP leg 57. These heat flow values are based on temperature logs corrected to equilibrium, using a detailed model of the drilling disturbance. Heat flow values on a deeply submerged marine terrace, landward of the trench slope break are 28 and 32 mW m−2. A measurement in the midslope terrace basin on the landward wall of the trench yielded a value of 22 mW m−2. The results are in good agreement with earlier seafloor measurements and indicate that most of the forearc area is characterized by heat flow about one half of that over oceanic lithosphere seaward of the trench. Our observations indicate only a small increase of heat flow from the trench to the volcanic arc, in agreement with thermal models, which suggests that the subduction of the relatively cold oceanic plate continues to dominate the temperature structure for distances of up to 250 km landward of the trench. The temperature profile in the borehole on the midslope terrace indicates possible vertical flow of pore waters. Hundreds of conductivity determinations were made using a new technique.

Mariana Arc structure inferred from gravity and seismic data

A two‐dimensional gravity model of the lithosphere was constructed along a seismic refraction line near 18°N latitude. Included in the model are crustal layers constrained by seismic refraction results, an estimate of the gravity anomaly caused by the subducting slab, and a model of the low‐density mantle beneath the Mariana Trough. With a reasonable anomaly assumed for the slab it is shown that the gravity anomaly caused by the low‐density mantle is greatest over the axial bathymetric high and tapers off to the sides. With the bottom of the low‐density mantle set at 200 km the density contrast is −0.033 g/cm3. Other depths and densities are tried as well. Several notable anomalies are found on the crustal layers. East of the trench, the crust has been thinned slightly to account for an outer gravity high. Behind the landward wall of the trench, a small, low‐density body is modeled to explain a slight offset of the minimum of the free air anomaly from the trench axis. A 50‐mGal jump on the observed gravity over the volcanic line is explained by an unusual configuration of the frontal arc Moho.

Gravity anomalies and the origin of the Puerto Rico Trench

The Puerto Rico Trench is assumed to be caused by a downwards bending of the Atlantic lithosphere. The gravitational body force that acts on a flap of lithosphere, hanging in the aesthenosphere and more dense than it, pulls the lithosphere down at the trench. Gravity anomalies are modelled assuming that the crustal thicknesses of the Atlantic and Caribbean Oceans are constant (but different) and that the Atlantic Ocean floor seaward of the outer rise, Puerto Rico, and the Caribbean Sea floor are each in isostatic equlibrium. The residual gravity anomalies are then consistent with the existence of a subcrustal dense mass, that could be the hanging slab of lithosphere. The value of this excess mass depends upon more arbitrary assumptions for the crustal mass in the Puerto Rico Trench and its landward wall, but if the other assumptions above are realistic, the dense mass is required and is adequate to bend the surface down at the trench.

Visual Observations of the Sea Floor Subduction Line in the Middle-America Trench

Four dives were made to the floor of the Middle-America Trench with the U.S. Navy's deep research submersible DSV Turtle . The area investigated is located between Costa Rica and the Cocos Ridge where the depth of the trench floor does not exceed the 2000-meter capability of the submersible. At the axis of the trench floor a series of steep northeast-facing scarps 10 to 20 meters high lie parallel to the trench axis. Here oceanic crust appears to have been carried down by near-vertical normal vaults of small displacement. Between these small scarps and the landward wall of the trench a narrow line of recent deformation interrupts a smooth apron. Unconsolidated sediments are thrust in sharply serrated piles and cut by sharp-edged chasms. This line of deformation is interpreted as the present sea floor trace of crustal subduction.

Visual Observations of the Sea Floor Subduction Line in the Middle-America Trench

Four dives were made to the floor of the Middle-America Trench with the U.S. Navy's deep research submersible DSV Turtle. The area investigated is located between Costa Rica and the Cocos Ridge where the depth of the trench floor does not exceed the 2000-meter capability of the submersible. At the axis of the trench floor a series of steep northeast-facing scarps 10 to 20 meters high lie parallel to the trench axis. Here oceanic crust appears to have been carried down by near-vertical normal vaults of small displacement. Between these small scarps and the landward wall of the trench a narrow line of recent deformation interrupts a smooth apron. Unconsolidated sediments are thrust in sharply serrated piles and cut by sharp-edged chasms. This line of deformation is interpreted as the present sea floor trace of crustal subduction.

Sedimentology, structural geology, and tectonics of the Shikoku subduction zone, southwestern Japan

The Shikoku subduction zone is developed along the Nankai Trough where the Philippine plate is underthrust beneath the Asian plate. The landward wall of the Nankai Trough consists of horizontal parallel ridges and basins that trend northeastward. A Deep Sea Drilling Project site (Leg 31, site 298) on the landward flank of the deepest ridge penetrated 525 m of beds in normal stratigraphic position and 86 m of overturned beds (all of Quaternary age), indicating an overturned anticline. The tight, overturned anticline, which trends parallel to the Nankai Trough, has an inter-limb angle of 9°, an axial surface inclined 9° to 14° landward, and a convergently fanning axial plane fracture cleavage. A coarsening-upward turbidite sequence defines a trench facies and demonstrates direct accretion of deposits from this environment. The convergence rate in the Shikoku subduction zone is estimated to be from 1 to 2 cm/yr, with a strain rate of about 10 −13 /sec. Tectonic consolidation has reduced the volume of the subducted and accreted rocks at least one-third. Olistrostromes form as a direct consequence of fold evolution in the submarine environment and can be immediately underthrust, thereby developing a structural fabric.

Transfer of Nazca Ridge Pelagic Sediments to the Peru Continental Margin

A complex set of lithologies, including calcareous oozes, hemipelagites, and turbidites, was recovered from the landward wall of the Peru Trench at its intersection with the Nazca Ridge. The sequence occurs at a water depth of 4,900 m and overlies an acoustic basement in the lowermost continental slope. Early Pliocene calcareous ooze overlies Pliocene to Quaternary ooze; both of these deposits are sandwiched between late Pleistocene (⩽ 400,000 yr), organic-rich turbidites and hemipelagic deposits typical of the Peru Trench and margin. Planktonic and benthic foraminiferal assemblages indicate that the early Pliocene ooze originally was deposited on the Nazca Ridge above the calcium carbonate compensation depth (4,000 m) and to the west of the cool Peru-Chile Current. The Pliocene-Pleistocene ooze contains a temperate fauna associated with the Peru-Chile Current. Block faulting at the terminus of the Nazca Ridge displaced the calcareous ooze 1,900 m from the top of the ridge to the trench below. Apparently these lithologies were then folded against or thrust beneath the lower continental slope within the past 400,000 yr. The stratigraphic sequence and the physiographic setting of the Nazca Ridge–Peru Trench intersection indicate convergence of the Nazca Ridge with the South American block. A minimum convergence rate of 0.8 cm/yr is calculated for the Pleistocene based upon the past and present geographic positions of the calcareous ooze. The best estimate of the rate is 2.8 cm/yr.