Magnetic Properties and Oxide Petrography of the Sheeted Dike Complex in Hole 504B

Pariso, Janet E.; Johnson, H. Paul

doi:10.2973/odp.proc.sr.111.132.1989

Cited by 21 publications

(18 citation statements)

References 29 publications

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“…One possible explanation is that the rheological differences between massive, columnar intrusives and more brecciated, horizontally layered volcanics may produce a mechanical boundary between fundamentally different styles of deformation. Magnetic measurements at Hole 504B reveal that the stable inclination of the extrusive section is rotated by over 20° from expected, and that this mismatch is reduced within the transition zone to nearly 0° in the upper dikes [Smith and Banerjee, 1986;Kinoshita et al, 1989;Pariso and Johnson, 1989]. The authors infer from these observations that the volcanic section was tectonically rotated as it moved away from the ridge axis, whereas the sheeted dikes…”

Section: Geological Interpretationmentioning

confidence: 51%

“…This proves to be a difficult task, especially since hydrothermal processes have been linked to a well-established increase in extrusive velocities with age. It is also geologically unreasonable to relate the extremely thin transition zones at sites L1, L2 and L4 to a dike emplacement zone only Smith and Banerjee, 1986;Becker et al, 1989;Pariso and Johnson, 1989].…”

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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: Geological Interpretationmentioning

confidence: 51%

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confidence: 99%

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

Hussenoeder¹

1998

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“…The E/W spreading geometry of 504B is ideal for detecting rotations about ridgeparallel axes with paleomagnetic remanence directions. The dips of chilled margins and paleomagnetic inclinations of sheeted dikes in hole 504B indicate <10°of rotation [Pariso and Johnson, 1989;Allerton et al, 1995]. Although the geometry of Hole 1256D is less suitable for determining rotations, the location in superfast-spread crust provides useful comparison to Pito Deep.…”

Section: Comparison With Other Areasmentioning

confidence: 99%

Paleomagnetic constraints on deformation of superfast-spread oceanic crust exposed at Pito Deep Rift

Horst¹,

Gee²,

Karson³

2011

J. Geophys. Res.

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[1] The uppermost oceanic crust produced at the superfast spreading (∼142 km Ma −1 , full-spreading rate) southern East Pacific Rise (EPR) during the Gauss Chron is exposed in a tectonic window along the northeastern wall of the Pito Deep Rift. Paleomagnetic analysis of fully oriented dike (62) and gabbro (5) samples from two adjacent study areas yield bootstrapped mean remanence directions of 38.9°± 8.1°, −16.7°± 15.6°, n = 23 (Area A) and 30.4°± 8.0°, −25.1°± 12.9°, n = 44 (Area B), both are significantly distinct from the Geocentric Axial Dipole expected direction at 23°S. Regional tectonics and outcrop-scale structural data combined with bootstrapped remanence directions constrain models that involve a sequence of three rotations that result in dikes restored to subvertical orientations related to (1) inward-tilting of crustal blocks during spreading (Area A = 11°, Area B = 22°), (2) clockwise, vertical-axis rotation of the Easter Microplate (A = 46°, B = 44°), and (3) block tilting at Pito Deep Rift (A = 21°, B = 10°). These data support a structural model for accretion at the southern EPR in which outcrop-scale faulting and block rotation accommodates spreading-related subaxial subsidence that is generally less than that observed in crust generated at a fast spreading rate exposed at Hess Deep Rift. These data also support previous estimates for the clockwise rotation of crust adjacent to the Easter Microplate. Dike sample natural remanent magnetization (NRM) has an arithmetic mean of 5.96 A/m ± 3.76, which suggests that they significantly contribute to observed magnetic anomalies from fast-to superfast-spread crust.

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“…A number of lines of evidence suggest that block rotations of this type (resulting in inward-dipping lavas and outward-dipping dikes) may be common in intermediate-to fast-spread crust. These include paleomagnetic studies of oriented dike samples from Hess Deep [Hurst et al, 1994b] and ODP drill cores [Pariso and Johnson, 1989;Schouten and Denham, 2000], interpretations of axial magnetic anomalies [Schouten et al, 1999], and the inward-dipping trajectories of magnetic polarity boundaries along the Blanco Transform Fault [Tivey et al, 1998]. Hooft et al [1996] predicted a similar subsidence pattern based on the presence of a finite Figure 21.…”

Section: Geochemistry Geophysicsmentioning

confidence: 99%

Structure of uppermost fast‐spread oceanic crust exposed at the Hess Deep Rift: Implications for subaxial processes at the East Pacific Rise

Karson

Klein

Hurst

et al. 2002

Geochem Geophys Geosyst

118

178

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The uppermost 2 km of the oceanic crust created at the fast spreading (135 mm yr À1 , full rate) equatorial East Pacific Rise (EPR) is exposed for tens of kilometers along escarpments bounding the Hess Deep Rift. Mosaics of large-scale digital images from the remotely operated vehicle (ROV) Argo II and direct observations from the submersible Alvin document a degree of geological complexity and variability that is not evident from most studies of ophiolites or prevailing models of seafloor spreading. Dramatic variations in the thickness and internal structure are documented in both the basaltic volcanic and sheeted dike rock units. These rock units are characterized by extensive faulting, fine-scale fracturing, and rotations of coherent crustal blocks meters to tens of meters across. The uppermost basaltic lavas are essentially undeformed and have overall gently inclined flow surfaces. Through most of the basaltic lava unit, however, lava flow contacts dip (208-708W) toward the EPR and generally increase in dip downward in the section. Dikes cutting the lavas and in the underlying sheeted dike unit generally dip (908 -408E) away from the EPR. Deeper level gabbroic rocks show little evidence of the intense fracturing typical of the overlying units. We interpret this upper crustal structure as the result of subaxial subsidence within 1-2 km of the EPR that accommodated the thickening of the basaltic lava unit to $500 m. Variations in the thickness of lava and dike units and spatially related structures along the rift escarpments suggest temporal fluctuations in magma supply. These results indicate that substantial brittle deformation accompanied waxing and waning volcanism during the accretion of the crustal section exposed at the Hess Deep Rift. If this type of structure is typical of uppermost oceanic crust generated at the EPR, these processes may be common along fast spreading mid-ocean ridges.

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Magnetic Properties and Oxide Petrography of the Sheeted Dike Complex in Hole 504B

Cited by 21 publications

References 29 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

Paleomagnetic constraints on deformation of superfast-spread oceanic crust exposed at Pito Deep Rift

Structure of uppermost fast‐spread oceanic crust exposed at the Hess Deep Rift: Implications for subaxial processes at the East Pacific Rise

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