Seismic-Reflection Structure of the Upper Oceanic Crust: Implications from DSDP/ODP Hole 504B, Panama Basin

Collins, J. A.; Brocher, Thomas M.; Purdy, G. M.

doi:10.2973/odp.proc.sr.111.154.1989

Cited by 6 publications

(10 citation statements)

References 36 publications

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“…However, comparison of our results with those of normal oceanic crust (e.g. Carlson & Herrick 1990) and those sampled in Hole 504B (Wilkins & Langseth 1983; Salisbury et al 1985; Moos et al 1986; Christensen et al 1989; Collins et al 1989) (Table 5) would suggest that the velocities in the Troodos sheeted dyke complex are significantly less than those in oceanic crust. However the average thicknesses are equivalent.…”

Section: Discussionmentioning

confidence: 46%

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Geophysical constraints on the crustal architecture of the Troodos ophiolite: results from the IANGASS project

Mackenzie¹,

Maguire²,

Coogan

et al. 2006

Geophysical Journal International

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S U M M A R YWe present results from modelling a combined 160 km seismic wide-angle reflection/refraction and gravity profile sited across the sheeted dykes, lavas and sediments of the northern part of the Troodos ophiolite, Cyprus. The P-wave seismic velocity and density model, including five separate layers, is defined to a depth of ∼10 km. This shows the ophiolite sequence dipping to the east beneath the central, well resolved, 85 km section of the line between Kambos in the west and Kelia in the east. The upper layer (average velocity 2.83 km s −1 ; density 2.21 g cm −3 ) is interpreted as consisting of sediments and the upper extrusive group. Beneath this, accepting that layer boundaries may be gradational beyond the experiment resolution, the layers are identified as the lower extrusive and basal groups (3.57 km s −1 ; 2.31 g cm −3 ), the sheeted dyke complex (SDC) (4.57 km s −1 ; 2.72 g cm −3 ), a gabbroic layer (6.31 km s −1 ; 2.92 g cm −3 ), and an ultramafic cumulate layer and/or serpentinized peridotite (7.00 km s −1 ; 3.00 g cm −3 ). While the thickness (∼6 km) of this layer beneath the proposed gabbroic layer is defined, positive identification of its composition is not possible, owing to the lack of discriminatory S-wave velocity information. The underlying material provides an unreversed velocity of 8.0 km s −1 beneath the western end of the profile. These modelled velocities and densities are compared with seismic models of oceanic crust and their implications for models of the tectonic evolution of the Troodos ophiolite are discussed.Several major faults are interpreted from the models, with throws of up to ∼500 m offsetting both the extrusive-SDC and the SDC-gabbroic layer boundaries. These are all downthrown to the west, there being no evidence from the data to suggest any major faults downthrown to the east. Our results therefore do not support existing tectonic models for a multiple graben structure within the ophiolite recording either the progress of an eastward stepping spreading centre or off-axis graben formation. Instead, assuming that the faults are downthrown towards the ridge axis, the data are consistent with a single spreading centre located to the west of the recording profile.

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Section: Discussionmentioning

confidence: 46%

“… a Salisbury et al . 1985, b Moos et al 1986, c Wilkins & Langseth 1983, d Christensen et al 1989, e Collins et al 1989. …”

Section: Discussionmentioning

confidence: 99%

Geophysical constraints on the crustal architecture of the Troodos ophiolite: results from the IANGASS project

Mackenzie¹,

Maguire²,

Coogan

et al. 2006

Geophysical Journal International

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“…It is the only hole to penetrate through the volcanic section into the underlying sheeted dike complex, and the site has become a reference section for the petrology, geochemistry, hydrothermal alteration, and magnetic and physical properties of the upper oceanic crust (Anderson, Honnorez, et al, 1982;. Previous drilling results from the site (Dick, Erzinger, Stokking, et al, 1992) and seismic evidence (Becker, Sakai, et al, 1988;Collins et al, 1989;Mutter, 1992) suggested that the bottom of the hole lay within the lower portion of the sheeted dike complex, close to the seismic Layer 2/Layer 3 boundary. It has been thought that this transition coincides with the change downward from sheeted dikes to underlying gabbros, or that it is a metamorphic boundary within gabbros or sheeted dikes (e.g., Fox et al, 1973;Christensen and Salisbury, 1975).…”

Section: Introductionmentioning

confidence: 99%

Hydrothermal Alteration of a Section of Upper Oceanic Crust in the Eastern Equatorial Pacific: A Synthesis of Results from Site 504 (DSDP Legs 69, 70, and 83, and ODP Legs 111, 137, 140, and 148)

Alt¹,

Laverne²,

Vanko³

et al. 1996

Proceedings of the Ocean Drilling Program

152

255

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Mineralogical, chemical, and isotopic results from seven drilling legs that visited DSDP/ODP Hole 504B over 14 years are compiled here to present an integrated view of hydrothermal alteration of oceanic crust at Site 504. Hole 504B reaches to 2111 mbsf, through 274.5 m sediment, 571.5 m of volcanic rocks, a 209 m transition zone, and 1050 m into a sheeted dike complex. The volcanic section was altered through a series of processes involving interaction with seawater at low temperatures, with the effects of cold, oxidizing seawater decreasing downward. These processes and their effects on the volcanic section are generally similar to those in other oceanic upper crustal sections.The transition zone and upper dikes were altered in a subsurface mixing zone, where hydrothermal fluids upwelling through the dikes mixed with cooler seawater circulating in the overlying more permeable volcanic rocks. Alteration of the transition zone and upper dikes (down to 1500 mbsf) occurred in a series of stages, reflecting the thermal and chemical evolution of the hydrothermal system from (1) early chlorite, actinolite, albite-oligoclase, and titanite, to (2) quartz, epidote and sulfides, to (3) anhydrite, and finally to (4) zeolites and local calcite. The maximum temperature estimated for the first two stages is 350°-380°C, and the inferred mineral assemblages for these early stages are typical of the greenschist facies.The lower dikes (1500-2111 mbsf) underwent an early, high-temperature (>400°C) alteration stage, resulting in the formation of hornblende and calcic secondary plagioclase, consistent with reactions inferred to occur in deep subsurface reaction zones, where hydrothermal vent fluids acquire their final compositions. Much of the subsequent reactions produced greenschist assemblages at ~300°-400°C. The lower dikes have lost metals and sulfur and are a source of these elements to hydrothermal vent fluids and seafloor sulfide deposits. The lower dikes underwent subsequent alteration stages similar to the upper dikes, with rare epidote + quartz veins recording the presence of upwelling hydrothermal fluids, and limited late off-axis effects (zeolites and prehnite). Anhydrites in the lower dikes indicate more reacted fluid compositions than in the upper dikes.Alteration of the sheeted dikes from Hole 504B is heterogeneous, with recrystallization controlled by fracturing and access of fluids. Defining the position of the seismic Layer 2/3 transition depends upon the scale of observation, but the change at Site 504 occurs within the sheeted dikes and is correlated with progressive changes in porosity and hydrothermal alteration. However, we still do not know the nature of the transition from sheeted dikes to gabbros in in situ ocean crust, or the nature of the inferred fault at the base of Hole 504B and its role in fluid flow and alteration.

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“…12) show that the seismic layer 2A/2B transition corresponds with a rapid decrease in porosity. The similarity of the RAMESESS results with those from the EPR and from the drilled section of layer 2 in ODP Hole 504b (Collins et al 1998) and the Hess Deep (Francheteau et al 1992), all of which equate the layer 2A event with the base of the extrusive igneous section, suggest that the observations from the RAMESSES area have a similar origin.…”

Section: Reflection Events and Their Originsmentioning

confidence: 58%

Morphology and genesis of slow-spreading ridges-seabed scattering and seismic imaging within the oceanic crust

Peirce

Sinha

Topping

et al. 2007

Geophysical Journal International

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SUMMARY A grid of 32 across‐axis and five axis‐parallel multichannel seismic (MCS) reflection profiles were acquired at an axial volcanic ridge (AVR) segment at 57° 45′N, 32° 35′W on the slow‐spreading Reykjanes Ridge, Mid‐Atlantic Ridge, to determine the along‐axis variation and geometry of the axial magmatic system and to investigate the relationship between magma chamber structure, the along‐axis continuity and segmentation of melt supply to the crust, the development of faulting and the thickness of oceanic layer 2A. Seismic reflection profiles acquired at mid‐ocean ridges are prone to being swamped by high amplitude seabed scattered noise which can either mask or be mistaken for intracrustal reflection events. In this paper, we present the results of two approaches to this problem which simulate seabed scatter and which can either be used to remove or simply predict events within processed MCS profiles. The 37 MCS profiles show clear intracrustal seismic events which are related to the structure of oceanic layer 2, to the axial magmatic system and to the faults which dismember each AVR as it ages through its tectono‐magmatic life cycle and which form the median valley walls. The layer 2A event can be mapped around the entirety of the survey area between 0.1 and 0.5 s two‐way traveltime below the seabed, being thickest at AVR centres, and thinning both off‐axis and along‐axis towards AVR tips. Both AVR‐parallel and ridge‐parallel trends are observed, with the pattern of on‐axis layer 2A thickness variation preserved beneath relict AVRs which are rafted off‐axis largely intact. Each active AVR is underlain by a mid‐crustal melt lens reflection extending almost along its entire length. Similar reflection events are observed beneath the offset basins between adjacent AVRs. These are interpreted as new AVRs at the start of their life cycle, developing centrally within the median valley. The east–west spacings of relict AVRs and offset basins is ∼5–7 km, corresponding to a life span of the order of 0.5–0.7 Myr, during which AVRs appear to undergo multiple 20–60 Kyr tectono‐magmatic cycles.

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Seismic-Reflection Structure of the Upper Oceanic Crust: Implications from DSDP/ODP Hole 504B, Panama Basin

Cited by 6 publications

References 36 publications

Geophysical constraints on the crustal architecture of the Troodos ophiolite: results from the IANGASS project

Geophysical constraints on the crustal architecture of the Troodos ophiolite: results from the IANGASS project

Hydrothermal Alteration of a Section of Upper Oceanic Crust in the Eastern Equatorial Pacific: A Synthesis of Results from Site 504 (DSDP Legs 69, 70, and 83, and ODP Legs 111, 137, 140, and 148)

Morphology and genesis of slow-spreading ridges-seabed scattering and seismic imaging within the oceanic crust

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