Asymmetric deep crustal structure across the Juan de Fuca Ridge

Rohr, Kristin M. M.; Milkereit, B.; Yorath, C J

doi:10.1130/0091-7613(1988)016<0533:adcsat>2.3.co;2

Cited by 101 publications

(95 citation statements)

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“…The shallow crustal event documented both on-and off-axis is therefore most likely a porosity horizon, which may exist within the volcanic section as either a mechanical compaction boundary [Spudich and Orcutt, 1980], a hydrothermal precipitation or cracking front [Rohr et al, 1988;Wilkens et al, 1991] or a transition from rubbly to more massive lava flows [Harding et al, 1989]. However, it is difficult to convincingly associate the layer 2A/2B transition with an intra-volcanic porosity boundary (see Christeson et al [1996] & Chapter 5).…”

Section: Discussionmentioning

confidence: 99%

“…The tectonic deepening of a fracture front off-axis is also difficult to reconcile with seafloor observations, which show the onset of large-scale fracturing to occur outside the zone where layer 2A thickening begins [Bicknell et al, 1987;Edwards et al, 1991;Carbotte et al, 1997]. The layer 2A horizon may alternatively be hydrothermal or metamorphic in origin [Rohr et al, 1988;Vera et al, 1990;Wilkens et al, 1991]. The primary evidence in support of this argument comes from DSDP jODP Hole 504B, where a transition in seismic velocity [Becker et al, 1988], acoustic attenuation [Wilcock et al, 1992]' permeability [Anderson et al, 1985] and porosity [Becker et al, 1982] is recorded within the extrusive section at basement depth of 100-200 m. However, as Harding et al [1993] and Christeson et al [1994a] point out, this site is situated in crust that has matured considerably during its 6…”

Section: Geological Interpretationmentioning

confidence: 99%

“…A more limited number of seismic studies have examined the upper crust built at the intermediate spreading Juan de Fuca Ridge (JDFR) [Poujol and Jacobson, 1988;Rohr et al, 1988;White and Clowes, 1990;Cudrack and Clowes, 1993;McDonald et al, 1994]. They reveal a thicker, more variable layer 2A than at the EPR.…”

Section: Rise (Epr)mentioning

confidence: 99%

“…Refraction studies of the Endeavor segment at 48°N [White and Clowes, 1990;Cudrack and Clowes, 1993] find layer 2A to be 200-650 m thick, with velocities of 2.5-2.8 km/s. An abrupt increase in velocity beneath this layer is equated with an intermittent reflector imaged 0.3-0.5 s beneath the seafloor along a coincident multichannel line [Rohr et al, 1988]. Refraction data from the Cleft segment at 45°N also document a highly variable layer 2A (200-550 m thick, "-'2.7 km/s) that is ,,-,350 m thick beneath the rise axis, but exhibits no systematic cross-or along-axis trends McDonald et al [1994].…”

Section: Rise (Epr)mentioning

confidence: 99%

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Seismic and magnetic constraints on the structure of upper oceanic crust at fast and slow spreading ridges

Hussenoeder¹

1998

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The upper ocean crust contains a comprehensive record of the shallow geological processes active along the world's mid-ocean ridge system. This thesis examines the magnetic and seismic structure of the upper crust at two contrasting ridges-the fast spreading East Pacific Rise (EPR) and the slow spreading Mid-Atlantic Ridge (MAR)-to build a more complete understanding about the roles of volcanic emplacement, tectonic disruption and hydrothermal alteration in the near-ridge environment.A technique that inverts potential field measurements. directly from an uneven observation track is developed and applied to near-bottom magnetic data from the spreading segments north of the Kane transform on the MAR. It is concluded that the central anomaly magnetization high marks the locus of focused volcanic emplacement. A cyclic faulting model is proposed to explain the oscillatory magnetization pattern associated with discrete blocks of crust being transported out of the rift valley between intensely altered fault zones. Seismic waveform and amplitude analyses of the magma sill along the EPR reveal it to be a thin «100 m) body of partial melt. These characteristics have important implications for melt availability and transport within the cycle of eruption and replenishment. A genetic algorithm-based seismic waveform inversion technique is developed and applied to on-and near-axis multichannel data from 17°20 's on the EPR and the spreading segment south of the Oceanographer transform (MAR) to map and compare for the first time the detailed velocity structure of the upper crust at two different spreading rates. Combined with conventionally processed seismic profiles, our results show that, while final extrusive thickness is comparable at all spreading ridges (300-500 m), the style of thickening may vary. While a thin (:S100 m) extrusive carapace quadruples in thickness within 1-4 km of the EPR crest, the extrusive section at the MAR achieves its final thickness within the inner valley. Both show evidence for a narrow zone of volcanic emplacement. Vigorous hydrothermalism at the EPR may produce a more rapid increase in basement velocities relative to the MAR. Rapid modification of the extrusive/dike transition at both ridges indicates that hydrothermalism is enhanced in this interval. Along-axis transport of lavas may thicken the extrusive pile at slow spreading segment ends, strengthening the magnetic highs generated by lava chemistry. Biographical SketchIn order to wax only partially rhapsodic, let me start at my formative age as a marine geophysicist ... the age of six. The first hint that I wasn't going to grow up to be a successful businessman was dropped in Germany, where my best friend and I rejected the idea of opening a lemonade stand in favor of selling fossils and geodes on the street corner. We soon went bankrupt. Our village in southern Germany was not far from Solnhofen, home of the famed Archaeopteryx. The countryside was full of geological treasures exposed in streambeds or along roadsides. Our final m...

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Section: Discussionmentioning

confidence: 99%

Section: Geological Interpretationmentioning

confidence: 99%

Section: Rise (Epr)mentioning

confidence: 99%

Section: Rise (Epr)mentioning

confidence: 99%

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Seismic and magnetic constraints on the structure of upper oceanic crust at fast and slow spreading ridges

Hussenoeder¹

1998

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“…Few seismic constraints exist on the changes in crustal structure or thickness between the Juan de Fuca and Gorda Ridges. Most recent seismic work on the Juan de Fuca Ridge has focused on the structure of the middle and upper crust [McClain and Lewis, 1982;Morton et al, 1987;Cudrak, 1988;Rohr et al, 1988;White and Clowes, 1990;Christeson et al, 1993;Cudrak and Clowes, 1993;McDona ld et al, 1994]. T he thermal structure of the crust can be estimated from the presence or absence of a magma body in the crust.…”

Section: Hooft and Detrlck: Juan De Fuca Axial Morphology Variationsmentioning

confidence: 99%

The influence of magma supply and eruptive processes on axial morphology, crustal construction and magma chambers

Hooft¹

1996

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The influence of porosity and crack morphology on seismic velocity and permeability in the upper oceanic crust

Carlson

2014

Geochem Geophys Geosyst

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[1] The purpose of this study is to explore the relationships between porosity, crack morphology, seismic velocity, and permeability in the upper oceanic crust. In a theoretical model, velocity is a quadratic function of porosity U, and the coefficients are functions of the fractional area of asperity contact A f across the cracks, a measure of crack morphology; thus, v 5 v(A f , U). Fits to data sets from the lava and dike sections in Holes 504B and 1256D yield A f 5 0.096(8) and 0.045(3), with rms errors of 1.7 and 0.3%, respectively. Lower values of A f account for the high porosities and low velocities observed in Layer 2A. After 0.2 Ma, the porosities of both Layer 2A and the deeper part of the lava pile remain in the range 8 6 4%. The observed increase of Layer 2A velocities after 0.2 Ma is caused largely by changing crack morphology as opposed to decreasing porosity; at 8% porosity, an increase of A f from 0.003 to 0.1 accounts for an increase of velocity from 3 to >5 km/s. An empirical log-linear relationship between permeability j and v suggests a relationship between j and A f as well. This relationship between j and v can be explained if the permeability coefficient j o is a closely constrained quadratic function of the area of asperity contact A f . A f is thus the primary variable affecting both seismic velocities and the permeability of the uppermost oceanic crust.

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Asymmetric deep crustal structure across the Juan de Fuca Ridge

Cited by 101 publications

References 0 publications

Seismic and magnetic constraints on the structure of upper oceanic crust at fast and slow spreading ridges

Seismic and magnetic constraints on the structure of upper oceanic crust at fast and slow spreading ridges

The influence of magma supply and eruptive processes on axial morphology, crustal construction and magma chambers

The influence of porosity and crack morphology on seismic velocity and permeability in the upper oceanic crust

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