High‐resolution bathymetry and three‐dimensional seismic data along the Cocos Ridge reveal a 245 km2 field of ∼1–4 km in diameter seafloor depressions. The seafloor depressions are part of a ...two‐tiered honeycomb pattern. The lower‐tier depressions have steep faults that truncate strata with chaotic internal reflections consistent with sediment collapse into the depression. These extend into a lens shaped interval just above igneous basement. Overlying these depressions is a second broader set with rough seafloor morphology with gently dipping boundaries defined by pinch‐out stratigraphic patterns. Drilling results indicate that the lens‐shaped zones that host the deeper depressions represent anomalous regions of high porosity, low velocity, and low density within calcareous rich sediment. Analysis of nannofossils from IODP Site U1414 suggests the collapse structures formed during the late Miocene, whereas the younger shallower depressions likely formed between the early Pliocene and the Pliocene‐Pleistocene boundary. Geochemical and petrological analysis at Site U1414 suggests that hydrothermal circulation during the late Miocene led to carbonate dissolution and collapse. Following collapse, focused fluid‐flow and bottom current scouring resulted in formation of the overlying set of depressions and a honeycomb seafloor morphology. Similar sets of depressions along the Carnegie Ridge to the south support the hypothesis that two‐tiered depressions formed in response to processes that occurred broadly across the Panama Basin between the late Miocene and the Pliocene‐Pleistocene transition. Geochemical results at Site U1414, combined with geophysical data, suggest this two‐tiered system of depressions currently guides ongoing fluid outflow.
Plain Language Summary
We characterize a vast field of mega seafloor depressions along the Cocos Ridge using three‐dimensional geophysical data tied to drilling results. The mega depressions display distinct two‐tiered structures that form a honeycomb‐like pattern on the seafloor. By imaging faulting and sediment layers within the depressions, we reveal that the older, steeper‐walled depressions formed through collapse processes, whereas the younger, broader depressions likely formed through a combination of fluid‐flow and current scouring. Drilling results indicate that the older collapses formed during the late Miocene, and geochemical results suggest that hydrothermal circulation, likely along ridge‐parallel faults, dissolved carbonate‐rich sediments and led to collapse. In contrast, drilling results suggest the younger depressions formed sometime between the Pliocene and Pliocene‐Pleistocene boundary. Similar depressions imaged within the Panama Basin and along the Carnegie Ridge to the south suggest widespread Galapagos Hotspot volcanism and hydrothermal circulation led to dissolution and collapse of carbonate‐rich sediments across the eastern equatorial Pacific. Fluid‐flow through the existing collapses and water current activity following the closing of the Central American Seaway likely led to the formation of the younger depressions. Drilling results, seismic reflection indicators of fluid pathways, and indicators of seafloor seepage suggests these depressions are actively guiding fluid outflow.
Key Points
The study provides three‐dimensional seismic tied to drilling results for understanding the development of mega‐depressions along the Cocos Ridge
The depressions formed initially through dissolution and collapse, driven by volcanism and hydrothermal circulation
The younger depressions likely formed through bottom current activity and fluid‐flow, the latter of which appears to be active today
Origin and dynamics of depositionary subduction margins Vannucchi, Paola; Morgan, Jason P.; Silver, Eli A. ...
Geochemistry, geophysics, geosystems,
June 2016, 2016-06-00, 20160601, Letnik:
17, Številka:
6
Journal Article
Recenzirano
Odprti dostop
Here we propose a new framework for forearc evolution that focuses on the potential feedbacks between subduction tectonics, sedimentation, and geomorphology that take place during an extreme event of ...subduction erosion. These feedbacks can lead to the creation of a “depositionary forearc,” a forearc structure that extends the traditional division of forearcs into accretionary or erosive subduction margins by demonstrating a mode of rapid basin accretion during an erosive event at a subduction margin. A depositionary mode of forearc evolution occurs when terrigenous sediments are deposited directly on the forearc while it is being removed from below by subduction erosion. In the most extreme case, an entire forearc can be removed by a single subduction erosion event followed by depositionary replacement without involving transfer of sediments from the incoming plate. We need to further recognize that subduction forearcs are often shaped by interactions between slow, long‐term processes, and sudden extreme events reflecting the sudden influences of large‐scale morphological variations in the incoming plate. Both types of processes contribute to the large‐scale architecture of the forearc, with extreme events associated with a replacive depositionary mode that rapidly creates sections of a typical forearc margin. The persistent upward diversion of the megathrust is likely to affect its geometry, frictional nature, and hydrogeology. Therefore, the stresses along the fault and individual earthquake rupture characteristics are also expected to be more variable in these erosive systems than in systems with long‐lived megathrust surfaces.
Key Points:
The description of a new mode of forearc growth
Forearc grows by deposition of material from “above” when material is removed from “below” by subduction erosion
Sections of forearcs can be totally replaced during extreme subduction erosion events
Chemical compositions of sediment pore waters are presented from 13 piston and gravity cores that were collected on ∼24 Ma crust of the Cocos Plate seaward of the Middle America Trench and near the ...onset of crustal faulting from subduction. Cores were collected mainly within a 1.75 km2 area overlying a buried basement topographic high that supports an elevated heat flux, consistent with seawater transport within the upper volcanic crust. Systematic variations in pore water chemical profiles indicate upward seepage speeds (up to 1.7 cm yr−1 providing a net flux of 0.1 L s−1), constrain the chemical composition of the formation water within the underlying upper basaltic basement, and elucidate diagenetic reactions in the sediment. Relative to seawater, formation water has an elevated temperature (70–80°C) and concentrations or values of Ca, chlorinity, Sr, Ba, Li, Fe, Mn, Si, Cs, D/H, and Mo, and lower concentrations or values of Mg, Na, sulfate, alkalinity, TCO2, K, B, F, phosphate, 87Sr/86Sr, δ13C, δ18O, U, and Rb. Although this site is located only 30 km from the trench axis, there is no chemical evidence for subduction‐related hydrologic influences. Instead, the data are explained by a combination of seawater‐basalt reactions within the upper basement and diffusive exchange with overlying sediment, as part of a shallow, ridge‐flank hydrothermal system. It is unclear why this site has an elevated heat flux relative to neighboring crust, but this may result from variations in crustal properties or modification related to flexural faulting outboard of the trench.
Plain Language Summary
An area of elevated volcanic rock buried below sediment west of the Middle American Trench is unusually warm, with temperatures of 70–80°C compared to more regional‐distributed crust that is vigorously cooled (10–20°C) from circulating seawater (formation water) within the upper volcanic crust. Because of the proximity to subduction‐related crustal faults, sediment cores were collected from this area of warm crust and interstitial sediment pore waters were extracted and analyzed to assess if and how this area with a high heat flux is linked to subduction processes, potentially providing a source of water that serpentinizes the underlying mantle. Chemical data from sediment pore waters confirm upward seepage, the composition of crustal formation water in underlying volcanic rocks, and the extent of reactions as the formation water ascended the sediment column. Crustal formation water appears to be unaffected by nearby plate subduction. Instead, this warm and chemically altered formation water defines a ridge‐flank hydrothermal system, which is distinct from hydrothermal systems to the west and south. How this hydrothermal system became isolated is unknown, but it could be a result of aging and evolving crustal properties or of plate faulting related to subduction processes.
Key Points
Pore water chemical profiles reveal sediment diagenesis, seepage speeds up to 1.7 cm yr−1, and discharge from 1.75 km2 of 0.1 L s−1
Crustal formation water is warm (∼75°C) and chemically altered, stemming from water‐basalt reactions and diffusive exchange with pore water
This ridge‐flank hydrothermal system is hydrologically isolated from the ventilated crust to the west and the trench to the east
In 2011 we acquired a 3D seismic reflection volume across the Costa Rica margin NW of the Osa Peninsula to investigate the complex structure and the development of the seismogenic zone within the ...Costa Rican subduction zone in the vicinity of recent International Ocean Drilling Program (IODP) drilling. In contrast to previous interpretations, these newly acquired seismic images show that the margin wedge is composed of a layered fabric that is consistent with clastic sediments, similar to materials recovered from IODP drilling, that have been thrust and thickened into thrust-bounded folded sequences. These structures are consistent with a balanced sequence that has been frontally accreted in the context of an accretionary model. We interpret these sequences as sediment originally deposited on the subducting crust in a trench basin created by the southward migration of the Cocos–Nazca–Caribbean triple junction, and accreted during recent margin subduction that also accelerated with passage of the triple junction. The margin is composed of relatively rapidly accreted sediment that was added to the margin during a phase of accretion within the last ∼5 Ma that was probably preceded throughout the Neogene by periods of non-accretion or erosion.
•The Costa Rica margin, NW of Osa, appears to be composed of clastic sediments.•Few structures within the Costa Rica margin are consistent with tectonic erosion.•The margin wedge is composed of fault-propagation folds with no deep normal faults.•The margin wedge has regularly been shortening over the past 2.0–4.5 m.y.
We used high‐resolution mapping to document 161 sites of potential fluid seepage on the shelf and slope regions where no geophysical seep indicators had been reported. Identified potential seabed ...seepage sites show both high‐backscatter anomalies and bathymetric expressions, such as pockmarks, mounds, and ridges. Almost all identified seabed features are associated with bright spots and flat spots beneath, as mapped within the 3‐D seismic grid. We obtained EM122 multi‐beam data using closely spaced receiver beams and 4–5 times overlapping multi‐beam swaths, which greatly improved the sounding density and geologic resolvability of the data. At least one location shows an acoustic plume in the water column on a 3.5 kHz profile, and this plume is located along a fault trace and above surface and subsurface seepage indicators. Fluid indicators are largely associated with folds and faults within the sediment section, and many of the faults continue into and offset the reflective basement. A dense pattern of normal faults is seen on the outer shelf in the multi‐beam bathymetry, backscatter, and 3‐D seismic data, and the majority of fluid seepage indicators lie along mapped fault traces. Furthermore, linear mounds, ridges, and pockmark chains are found on the upper, middle, and lower slope regions. The arcuate shape of the shelf edge, projection of the Quepos Ridge, and high density of potential seep sites suggest that this area may be a zone of former seamount/ridge subduction. These results demonstrate a much greater potential seep density and distribution than previously reported across the Costa Rican margin.
Key PointsHigh density of fluid seep indicators discovered off Central AmericaPotential seeps are structurally controlled, many by faults
Mawrth Vallis contains one of the largest exposures of phyllosilicates on Mars. Nontronite, montmorillonite, kaolinite, and hydrated silica have been identified throughout the region using data from ...the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM). In addition, saponite has been identified in one observation within a crater. These individual minerals are identified and distinguished by features at 1.38–1.42, ∼1.91, and 2.17–2.41 μm. There are two main phyllosilicate units in the Mawrth Vallis region. The lowermost unit is nontronite bearing, unconformably overlain by an Al‐phyllosilicate unit containing montmorillonite plus hydrated silica, with a thin layer of kaolinite plus hydrated silica at the top of the unit. These two units are draped by a spectrally unremarkable capping unit. Smectites generally form in neutral to alkaline environments, while kaolinite and hydrated silica typically form in slightly acidic conditions; thus, the observed phyllosilicates may reflect a change in aqueous chemistry. Spectra retrieved near the boundary between the nontronite and Al‐phyllosilicate units exhibit a strong positive slope from 1 to 2 μm, likely from a ferrous component within the rock. This ferrous component indicates either rapid deposition in an oxidizing environment or reducing conditions. Formation of each of the phyllosilicate minerals identified requires liquid water, thus indicating a regional wet period in the Noachian when these units formed. The two main phyllosilicate units may be extensive layers of altered volcanic ash. Other potential formational processes include sediment deposition into a marine or lacustrine basin or pedogenesis.
As many as one-third of mutations in a gene result in the corresponding enzyme having an increased Michaelis constant, or K(m), (decreased binding affinity) for a coenzyme, resulting in a lower rate ...of reaction. About 50 human genetic dis-eases due to defective enzymes can be remedied or ameliorated by the administration of high doses of the vitamin component of the corresponding coenzyme, which at least partially restores enzymatic activity. Several single-nucleotide polymorphisms, in which the variant amino acid reduces coenzyme binding and thus enzymatic activity, are likely to be remediable by raising cellular concentrations of the cofactor through high-dose vitamin therapy. Some examples include the alanine-to-valine substitution at codon 222 (Ala222-->Val) DNA: C-to-T substitution at nucleo-tide 677 (677C-->T) in methylenetetrahydrofolate reductase (NADPH) and the cofactor FAD (in relation to cardiovascular disease, migraines, and rages), the Pro187-->Ser (DNA: 609C-->T) mutation in NAD(P):quinone oxidoreductase 1 NAD(P)H dehy-drogenase (quinone) and FAD (in relation to cancer), the Ala44-->Gly (DNA: 131C-->G) mutation in glucose-6-phosphate 1-dehydrogenase and NADP (in relation to favism and hemolytic anemia), and the Glu487-->Lys mutation (present in one-half of Asians) in aldehyde dehydrogenase (NAD + ) and NAD (in relation to alcohol intolerance, Alzheimer disease, and cancer).
In 2011 we acquired an 11 × 55 km, 3‐D seismic reflection volume across the Costa Rica margin, NW of the Osa Peninsula, to accurately image the subduction thrust in 3‐D, to examine fault zone ...properties, and to infer the hydrogeology that controls fluid accumulation along the thrust. Following processing to remove water column multiples, noise, and acquisition artifacts, we constructed a 3‐D seismic velocity model for Kirchhoff prestack depth migration imaging. Images of the plate boundary thrust show high‐reflection amplitudes underneath the middle to lower slope that we attribute to fluid‐rich, poorly drained portions of the subduction thrust. At ~ 5 km subseafloor, beneath the upper slope, the plate interface abruptly becomes weakly reflective, which we interpret as a transition to a well‐drained subduction thrust. Mineral dehydration during diagenesis may also diminish at 5 km subseafloor to reduce fluid production and contribute to the downdip change from high to low amplitude. There is also a layered fabric and systems of both thrust and normal faults within the overriding plate that form a “plumbing system.” Faults commonly have fault plane reflections and are presumably fluid charged. The faults and layered fabric form three compartmentalized hydrogeologic zones: (1) a shallow NE dipping zone beneath the slope, (2) a steeply SW dipping zone beneath the shelf slope break, and (3) a NE dipping zone beneath the shelf. The more direct pathway in the middle zone drains the subduction thrust more efficiently and contributes to reduced fluid pressure, elevates effective stress, and creates greater potential for unstable coseismic slip.
Key Points
The subduction thrust abruptly transitions downdip from fluid rich to fluid poor
Subduction thrust fluid content is controlled by the overriding plate structure
Downdip fluid loss along the subduction thrust triggers increased seismicity
PDF
