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Journal Article
Morris, A, Gee JS, Pressling N, John BE, MacLeod CJ, Grimes CB, Searle RC.  2009.  Footwall rotation in an oceanic core complex quantified using reoriented Integrated Ocean Drilling Program core samples. Earth and Planetary Science Letters. 287:217-228.   10.1016/j.epsl.2009.08.007   AbstractWebsite

Oceanic core complexes expose lower crustal and upper mantle rocks on the seafloor by tectonic unroofing in the footwalls of large-slip detachment faults. The common occurrence of these structures in slow and ultra-slow spread oceanic crust suggests that they accommodate a significant component of plate divergence. However, the subsurface geometry of detachment faults in oceanic core complexes remains unclear. Competing models involve either: (a) displacement on planar, low-angle faults with little tectonic rotation; or (b) progressive shallowing by rotation of initially steeply dipping faults as a result of flexural unloading (the "rolling-hinge" model). We address this debate using palaeomagnetic remanences as markers for tectonic rotation within a unique 1.4 km long footwall section of gabbroic rocks recovered by Integrated Ocean Drilling Program (IODP) sampling at Atlantis Massif oceanic core complex on the Mid-Atlantic Ridge (MAR). These rocks contain a complex record of multipolarity magnetizations that are unrelated to alteration and igneous stratigraphy in the sampled section and are inferred to result from progressive cooling of the footwall section over geomagnetic polarity chrons C1r.2r, C1r.1n (Jaramillo) and C1r.1r. For the first time we have independently reoriented drill-core samples of lower crustal gabbros, that were initially azimuthally unconstrained, to a true geographic reference frame by correlating structures in individual core pieces with those identified from oriented imagery of the borehole wall. This allows reorientation of the palaeomagnetic data, placing far more rigorous constraints on the tectonic history than those possible using only palaeomagnetic inclination data. Analysis of the reoriented high temperature reversed component of magnetization indicates a 46 degrees +/- 6 degrees anticlockwise rotation of the footwall around a MAR-parallel horizontal axis trending 011 degrees +/- 6 degrees. Reoriented lower temperature components of normal and reversed polarity suggest that much of this rotation occurred after the end of the Jaramillo chron (0.99 Ma). The data provide unequivocal confirmation of the key prediction of flexural, rolling-hinge models for oceanic core complexes, whereby oceanic detachment faults initiate at higher dips and rotate to their present day low-angle geometries as displacement increases. (C) 2009 Elsevier B.V. All rights reserved.

Gee, J, Schneider DA, Kent DV.  1996.  Marine magnetic anomalies as recorders of geomagnetic intensity variations. Earth and Planetary Science Letters. 144:327-335.   10.1016/s0012-821x(96)00184-7   AbstractWebsite

In addition to providing a robust record of past geomagnetic polarity reversals, marine magnetic anomalies often show shorter wavelength variations, which may provide information on geomagnetic intensity variations within intervals of constant polarity. To evaluate this possible geomagnetic signal, we compare sea surface profiles of the Central Anomaly with synthetic profiles based on Brunhes age (0-0.78 Ma) paleointensity records derived from deep sea sediments. The similarity of the synthetic profiles and observed profiles from the ultra-fast spreading southern East Pacific Rise suggests that geomagnetic intensity variations play an important role in the magnetization of the oceanic crust. This interpretation is further supported by systematic variations in the pattern of the Central Anomaly at slower spreading ridges, which are entirely consistent with a progressively smoother record of the sediment-derived paleointensity. If the sedimentary records, as calibrated to available absolute paleointensity data, accurately record variations in dipole intensity over the Brunhes, it follows that much of the Brunhes was characterized by geomagnetic intensities lower than either the mean dipole moment for the past 10 ka or the average for the period from 0.05 to 5.0 Ma. Furthermore, the sediment paleointensity records reflect the significant increase in geomagnetic intensity, from a low of similar to 2 x 10(22) Am-2 near 40 ka to a peak value (11 x 10(22) Am-2) at similar to 3 ka, that has been well documented from absolute paleointensity determinations, We suggest that geomagnetic intensity variations may be the most important cause of the rapid changes in the source layer magnetization near the ridge crest and the resultant Central Anomaly Magnetic High.

Staudigel, H, Gee J, Tauxe L, Varga RJ.  1992.  Shallow Intrusive Directions of Sheeted Dikes in the Troodos Ophiolite - Anisotropy of Magnetic-Susceptibility and Structural Data. Geology. 20:841-844.   10.1130/0091-7613(1992)020<0841:sidosd>2.3.co;2   AbstractWebsite

Sheeted dikes play a central role in the formation of oceanic crust. It is commonly assumed that sheeted dikes intrude vertically upward, from elongated mid-ocean ridge (MOR) magma chambers, but there are no direct observational data bearing on this hypothesis. This assumption contrasts with the intrusive behavior of subaerial volcanoes where magmas rise into shallow central magma chambers that laterally feed vertically oriented dikes. We have studied intrusive directions of sheeted dikes in a structural analogue to oceanic crust, the Troodos ophiolite. Structural and magnetic fabric data of 65 dikes provide consistent results and suggest a broad distribution of shallow (<20-degrees) to nearly vertical, upward magma-transport directions. These data suggest that horizontal emplacement has to be considered for sheeted dikes at MORs, implying more centralized MOR plumbing systems than previously thought. Such plumbing systems provide ample opportunity for complex mixing, fractionation, and contamination of MOR lavas in magma chambers and tabular magma-storage volumes. Whether the MOR magma supply is linear or centralized also has a fundamental effect on crustal accretion processes and the geometry of hydrothermal convection systems.