Convex-upward Profile Research Articles

New Growth Mechanism of Dust Grains in Protoplanetary Disks with Magnetically Driven Disk Winds

Abstract We discovered a new growth mode of dust grains to kilometer-size bodies in protoplanetary disks that evolve via viscous accretion and magnetically driven disk winds (MDWs). We solved an approximate coagulation equation of dust grains with time-evolving disks that consist of both gas and solid components using a one-dimensional model. With grain growth, all solid particles initially drift inward toward the central star due to the gas drag force. However, the radial profile of gas pressure, P, is modified by the MDW that disperses the gas in an inside-out manner. Consequently, a local concentration of solid particles is created by the converging radial flux of drifting dust grains at the location with a convex-upward profile of P. When the dimensionless stopping time, St, exceeds unity there, the solid particles spontaneously reach the growth-dominated state because of the positive feedback between the suppressed radial drift and the enhanced accumulation of dust particles that drift from the outer part. Once the solid particles are in the drift-limited state, the above-mentioned condition of St for dust growth is equivalent to Σd/Σg ≳ η, where Σd/Σg is the dust-to-gas surface-density ratio and η is the dimensionless radial pressure-gradient force. As a consequence of the successful growth of dust grains, a ring-like structure containing planetesimal-size bodies is formed at the inner part of the protoplanetary disks. Such a ring-shaped concentration of planetesimals is expected to play a vital role in the subsequent planet formation.

Quantitative morphometric analysis of the Jama River profile in a tectonically active margin (Northwestern Ecuador)

The longitudinal profile of a river channel can be described in function of mathematical expressions. The logarithmic fit is the most used method to describe the relative equilibrium of a channel elevation profile. Rivers showing zones of high channel gradient and convex-upward profiles can be evaluated in function of the offset distance with respect to the logarithmic curve. The Jama River profile has been constructed using differential GPS data for the downstream reach and 4-m-grid DEM data base for the remaining headward profile. The resulting longitudinal profile shows a prominent knickpoint of about 80 m in elevation characterized by sharp local convexity. The offset distance in vertical direction from the logarithmic plot is interpreted as the successive accumulation of multiple uplift episodes associated with the Jama Fault System activity. The horizontal offset is suggested to represent the remaining retreated distance for the upstream propagation of the Jama knickpoint. The highly resistant Cretaceous rocks outcropping along the Jama knickpoint reach acts as an inflection point between two subprofiles that show well-fitted logarithmic curves, thereby representing a temporary partitioned equilibrium along the Jama River profile.

The Impact of Well-Field Configuration on Contaminant Mass Removal and Plume Persistence for Homogeneous versus Layered Systems.

A three-dimensional numerical model was used to simulate the impact of different well-field configurations on pump-and-treat mass removal efficiency for large groundwater contaminant plumes residing in homogeneous and layered domains. Four well-field configurations were tested, Longitudinal, Distributed, Downgradient, and natural gradient (with no extraction wells). The reductions in contaminant mass discharge (CMDR) as a function of mass removal (MR) were characterized to assess remediation efficiency. Systems whose CDMR-MR profiles are below the 1:1 relationship curve are associated with more efficient well-field configurations. For simulations conducted with the homogeneous domain, the CMDR-MR curves shift leftward, from convex-downward profiles for natural gradient and Longitudinal to first-order behaviour for Distributed, and further leftward to a sigmoidal profile for the Downgradient well-field configuration. These results reveal the maximum potential impacts of well-field configuration on mass-removal behaviour, which is attributed to mass-transfer constraints associated with regions of low flow. In contrast, for the simulations conducted with the layered domain, the CMDR-MR relationships for the different well-field configurations exhibit convex-upward profiles. The nonideal mass-removal behaviour in this case is influenced by both well-field configuration and back diffusion associated with low-permeability units.

Extensive Middle Amazonian mantling of debris aprons and plains in Deuteronilus Mensae, Mars: Implications for the record of mid-latitude glaciation

The mid-latitudes of Mars are host to a record of recent episodes of accumulations of ice-rich materials. The record includes debris aprons, interpreted to be debris-covered glaciers, that may represent the preserved remnants of a much more extensive ice sheet. We assessed the possibility of former glacial extents by examining debris aprons and the surrounding plains in Deuteronilus Mensae. Geomorphic units and stratigraphic relationships were mapped and documented from Mars Reconnaissance Orbiter (MRO) Context (CTX) and High Resolution Imaging Science Experiment (HiRISE) camera images, and crater retention ages were estimated from crater size–frequency distributions. Three major units are observed within the study area: debris aprons, lower plains, and upper plains. Debris aprons exhibit characteristics typical for these features documented elsewhere and in previous studies, including integrated flow lineations and patterns, convex-upward profiles, and knobby and brain terrain surface textures. A lower bound on the age for debris aprons is estimated to be 0.9Ga. Debris aprons are superposed on a lower plains unit having a lower bound age of 3.3–3.5Ga. A 50–100m thick upper plains unit superposes both debris apron landforms and lower plains units and has a best-fit minimum age of 0.6Ga. The upper plains unit exhibits characteristics of atmospherically-emplaced mantle material, including fine-grained nature, sublimation textures, cyclic layering, draping character, and widespread spatial distribution. Fracturing and subsequent sublimation/erosion of upper plains on debris aprons has contributed to many of the surface textures on debris aprons. The upper plains unit has also been eroded from the lower plains and plateaus, evidenced by isolated blocks of upper plains in the interiors of craters and on the walls and tops of plateaus. While no conclusive evidence diagnostic of former cold-based ice sheets are observed in the plains within the study region, such landforms and units may have been poorly developed or absent, as is often the case on Earth, and would have been covered and reworked by later mantling episodes. These observations suggest that emplacement of thick ice-rich mantle deposits extended at least to near the Early/Middle Amazonian boundary and overlapped with the waning stages of glaciation in Deuteronilus Mensae.

Avulsion of the Fangliao submarine canyon off southwestern Taiwan as revealed by morphological analysis and numerical simulation

This paper discusses submarine morphology as a means of understanding the avulsion of the Fangliao canyon. Multichannel seismic profiles and bathymetric data were used to perform a morphological analysis. In response to tectonic uplifting, the Fangliao canyon was divided into two separate segments and two distinct trends. Initially, in the shelf edge–upper slope region the canyon extends downslope in a N–S direction. Then the canyon course bends to the SW at a depth of about 630m because of a mud diapir uplift and then returns to and maintains a N–S course with the canyon mouth at a depth of 900m, which is its present-day course. The Fangliao ridge, a mud diapiric ridge, lies east of the lower Fangliao canyon and extends upslope into the upper Fangliao canyon with a generally straight course and a nearly N–S trend. Because of the uplift of the Fangliao ridge a major step of the convex-upward profile, ~150m above the canyon floor, resulted in the east Fangliao ridge canyon. The gravity flows are unable to rise up against the high gradient and, thus, have had to abandon the segment of paleocanyon once flowing along the trend of the Fangliao ridge. The internal seismic characteristics of the terraces located along the eastern wall of the Fangliao canyon suggest deposition of terrace mainly occurs in the inner bend of the Fangliao canyon. The sedimentary sequences of the terrace are divided into two units: (1) the lower sedimentary unit is characterized by oblique, high-amplitude reflections and is considered as coarse-grained canyon fills resulting from aggradation related to the canyon avulsion events; and (2) the upper sedimentary unit consists mainly of low-amplitude reflections that are interpreted as inner levee resulting from spillover deposition. Numerical simulations were performed to discuss avulsion of the Fangliao canyon. Initially, the course of the Fangliao canyon had a N–S trend and followed the regional slope (i.e., the direction gravity currents would flow). Subsequently, the mud diapir uplifted the segment of the paleocanyon of the Fangliao canyon once flowing along the present-day Fangliao ridge and diverted its course to the present-day Fangliao canyon. The model suggests that the uplift rate exceeds the erosion rate in the east Fangliao ridge canyon, and hence the sediment flows in the canyon may be unable to pass through the 150-m major step. This new model suggests that the avulsion of the Fangliao canyon is marked by a significant bend at the boundary between the upper and lower canyons.

The geometry of volcano flank terraces on Mars

Flank terraces are subtle, expansive structures on the slopes of many large Martian shield volcanoes. Several terrace formation hypotheses — including self-loading, lithospheric flexure, magma chamber tumescence, volcano spreading, and shallow gravitational slumping — have been suggested. Terraces are not readily visible on photogeological data; consequently, terrace geometry has not yet been comprehensively described. Terrace provenance, therefore, is poorly understood. We used three-dimensional Mars Orbiter Laser Altimeter (MOLA) data to characterise the geometry of these elusive structures, with a view to understanding better the role that flank terraces play in the tectonic evolution of volcanoes on Mars. Terraces have a broad, convex-upward profile in section, and a systematic “fish scale” imbricate stacking pattern in plan. They are visible at all elevations, on at least nine disparate Martian volcanoes. Terrace-like features also occur on three shield volcanoes on Earth, an observation not recorded before. Analysis of a suite of morphometric parameters for flank terraces showed that they are scale-invariant, with similar proportions to thrust faults on Earth. We compared predicted formation geometries to our terrace observations, and found that only lithospheric flexure can fully account for the morphology, distribution, and timing of terraces. As a volcano flexes into the lithosphere beneath it, its upper surface will experience a net reduction in area, resulting in the formation of outward verging thrusts. We conclude, therefore, that flank terraces are fundamental volcanotectonic structures, that they are the surface expressions of thrust faults, probably formed by lithospheric flexure, and that they are not restricted to Mars.

Conceptual Model for Cross-shore Sediment Transport due to Tides and Waves over Intertidal Flat

Conceptual model for geomorphology and sediment transport due to tides and waves over intertidal mudflat has been proposed based on the field measurements of water depth, flow velocity and suspended sediment concentration from July 2004 to January 2008. Suspended-load flux and bed-load ability are expressed relative to the ratio of significant wave height to water depth. The maintenance mechanism for typical convex-upward profile over intertidal mudflat can be explained by the bed-load superiority for suspended-load using the conceptual model.

Sedimentology of Modern, Inclined Heterolithic Stratification (IHS) in the Macrotidal Han River Delta, Korea

ABSTRACT An occurrence of inclined heterolithic stratification (IHS) is described from a tidal point bar in a 40-m-deep distributary of the macrotidal (tidal range 3.6-7.8 m), Han River delta, Korea. The channel bank demonstrates a convex-upward profile with intermittent presence of wave-formed scarps and terraces near the low-water level. The vertical succession of IHS is approximately 25 m thick and dips into the channel with angles reaching 14°. The IHS overlies 15 m of trough cross-bedded sand deposited in the channel thalweg and lower point bar. Even though the channel as a whole is ebb dominated, the preserved cross bedding is predominantly flood directed because the mutually evasive nature of the ebb and flood currents causes the point-bar surface to be flood dominated. This pattern may be a common feature of tidal point bars. The IHS itself consists of interstratified fine sand, sandy silt, and silt with an upward-fining textural trend. Tidal rhythmites are well developed in the middle and upper intertidal zone, and may also be present in the subtidal zone, but are poorly developed near the low-water level because of wave action. Seasonal discharge variations of the Han River are not obvious in the deposits, because the large size, distal location, and energetic tidal environment of the studied channel reduces the impact of river-stage fluctuations. Despite the moderate salinity levels, bioturbation is rare, except in the upper intertidal zone, because of the rapid sedimentation and energetic conditions.

Sand Particle Movement on Migrating Combined-Flow Ripples

ABSTRACT We studied sand particle movement on migrating combined-flow ripples in an experimental flume. The water-surface waves propagated against the flow in the flume. The resultant combined-flow ripples have characteristic rounded profiles. Motion pictures reveal that vortices, generated only close to the lee sides of ripples, are responsible for the rounded profile. These vortices excavate rounded troughs and lift sand particles. The lee sides are nourished by the sand, contributing a convex-upward profile. The flow that separates at the crests is dragged into the vortices at the troughs, so that there is no clear reattachment point with focused erosion, as is the case for current ripples. Sand that fulls to the bed from a floating cloud over the stoss side nourishes the convex upward roun ed stoss side. The relative strength of the oscillatory flows controls the positions at which the vortices lift sand particles to deposit, and determine whether the ripple profiles are rounded or sharp. A lower relative oscillatory flow velocity yields smaller vortices, which cause rounded ripple profiles. Conversely, a higher relative oscillatory flow velocity results in larger vortices and sharper crests. The movement of sand particles is unlike that on ripples beneath either purely oscillatory flows or unidirectional flows.

Quaternary Styles of California Submarine Fans: ABSTRACT

The morphology and sedimentation patterns of large, deep-ocean submarine fans along the California coast, like Monterey fan, differ from that of smaller borderland or slope-basin submarine fans like the Navy and La Jolla fans. Small borderland fans feature areas of channels, isolated depressions, and a convex upward profile which are characteristic of the area defined as a suprafan and regarded as the location of active sand deposition. Large submarine fans are not simply scaled-up versions of small fans but seem to have certain features that suggest a composite of many small fans. On large fans, the basin shape and basin topographic features are a significant factor in the location of active turbidite depositional areas and the duration of these areas as principal sites of deposition. Bathymetric highs act as dams, restricting fan progradation, or deflecting the transport of fan sediments. Continued deposition commonly results in breeching topographic barriers, producing a rapid shift of primary depocenters to more distal regions. What was formerly a lower-fan environment may become the site of a middle-fan depositional lobe, and lower-fan deposition moves further seaward. Owing to these abrupt large-scale changes o fan deposition, it is difficult to recognize classic middle-fan environments on large submarine fans. Extensive channel End_Page 443------------------------------ systems result in sediment bypassing the old middle-fan region. At the lower end of the main valleys on Monterey fan, channels and isolated depressions are observed but the characteristic morphology of suprafans is not clearly developed. Large sediment waves are present on the upper fan valley levee and at various locations on Monterey fan, but are absent on the smaller Navy and La Jolla fans. End_of_Article - Last_Page 444------------