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scott bogue's research |
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Very rapid geomagnetic field change recorded by the partial remagnetization of a lava flow (Click here for a reprint)
Scott W. Bogue and Jonathan M. G. Glen
A new paleomagnetic result from a lava flow with a distinctive, two-part remanence reinforces the controversial hypothesis that geomagnetic change during a polarity reversal can be much faster than normal. The 3.9-m-thick lava (“Flow 20”) is exposed in the Sheep Creek Range (north central Nevada) and was erupted during a reverse-to-normal (R-N) geomagnetic polarity switch at 15.6 Ma. Flow 20 began to acquire a primary thermoremanence while the field was pointing east and down but was soon buried, reheated, and partially-remagnetized in a north-down direction by the 8.2-m-thick flow that succeeded it. A simple conductive cooling calculation shows that the observed remagnetization could not have occurred unless Flow 20 was still warm (about 150°C near its base) when buried and that the 53° change from east-down to north-down field occurred at an average rate of approximately 1°/week, several orders of magnitude faster than typical of secular variation.
Deformation of continental crust along a transform boundary, Coast Mountains, British Columbia
Margaret E. Rusmore, Scott W. Bogue, Karen Dodson, Kenneth A. Farley, and Glenn J. Woodsworth
New structural, paleomagnetic, and apatite (U-Th)/He results from the continental margin inboard of the Queen Charlotte fault (?54°N) delineate patterns of brittle faulting linked to transform development since ?50 Ma. In the core of the orogen, ?250 km from the transform, north striking, dip-slip brittle faults and vertical axis rotation of large crustal domains occurred after ?50 Ma and before intrusion of mafic dikes at 20 Ma. By 20 Ma, dextral faulting was active in the core of the orogen, but extension had migrated toward the transform, continuing there until <9 Ma. Local tilting in the core of the orogen is associated with glacially driven, post-4 Ma exhumation. Integration with previous results shows that post-50 Ma dextral and normal faulting affected a region ?250 km inboard of the transform and ?300 km along strike. Initially widespread, the zone of active extension narrowed and migrated toward the transform ?25 Ma after initiation of the transform, while dextral faulting continued throughout the region. Differential amounts of post-50 Ma extension created oroclines at the southern and northern boundaries of the deformed region. This region approximately corresponds to continental crust that was highly extended just prior to transform initiation. Variation in Neogene crustal tilts weakens interpretations relying on uniform tilting to explain anomalous paleomagnetic inclinations of mid-Cretaceous plutons. Similarities to the Gulf of California suggest that development of a transform in continental crust is aided by previous crustal extension and that initially widespread extension narrows and moves toward the transform as the margin develops. Structural correction of paleomagnetic vectors dispersed about two fold axes and application to the Duke Island (Alaska) ultramafic complex
Scott W. Bogue and C. Sherman Gromme
A new analysis of paleomagnetic data from the mid-Cretaceous (~110 Ma) ultramafic complex at Duke Island (southeast Alaska) supports large poleward transport of the Insular Superterrane relative to North America consistent with the Baja British Columbia hypothesis. Previous paleomagnetic work has shown that the characteristic remanence of the ultramafic complex predates kilometer-scale deformation of the very well developed cumulate layering but that the layering was not everywhere horizontal before the folding. It is possible, however, to estimate paleohorizontal for the Duke Island ultramafic complex because the post-remanence deformation of the intrusion occurred about two well defined and spatially separate fold axes. In such a case, the tectonically rotated paleomagnetic directions should be distributed along small circles centered on each of the two fold axes. The ancient field direction will lie on both small circles and therefore be identifiable as one of their two intersection points. Interpreted this way, the tectonically rotated remanence of the Duke Island ultramafic complex defines a mid-Cretaceous (i.e., ancient) field direction that is within 2° of the paleomagnetic direction found by assuming the cumulate layering was initially horizontal (despite the paleomagnetic evidence to the contrary) and performing the standard structure correction. The inferred mid-Cretaceous paleolatitude of Duke Island is 21.2° (2350 km) anomalous with respect to cratonic North America. This result is concordant with southerly paleolatitudes determined by many other workers from bedded rocks of terranes farther inboard in the Insular and Intermontane superterranes.
Geomagnetic field behavior before and after the Kauai
reverse-normal polarity transition Scott W. Bogue Abstract. New paleomagnetic results from 4 m.y. old lava flows from Kauai, Hawaii, suggest that strong poloidal field is associated with an unusual state of the geodynamo that follows attempts at polarity reversal (successful or not). The new data comprise 50 paleomagnetic sites from superposed lava flows occurring just below and above the Kauai reverse-normal polarity transition. A composite record of 45 distinct field determinations was constructed by combining sites that record similar ancient field and correlating them to previously published results from Kauai. Of the 45 data, 25 include paleointensity estimates derived from double-heating experiments. A comparison of the composite record from Kauai with two similar data sets from volcanic sequences shows that field variability (in direction, intensity, or both) can change substantially across a polarity transition. These changes, however, do not appear to be systematic in sign or magnitude. The only feature common to all three records is high field strength in the posttransitional interval, interpreted here as a transient phenomenon associated with the reversal process.
Bogue, S. W., and J. M. G. Glen (2010), Very rapid geomagnetic field change recorded by the partial remagnetization of a lava flow, Geophys. Res. Lett., 37, L21308, doi:10.1029/2010GL044286. Rusmore, M. E., S. W. Bogue, K. Dodson, K. A. Farley, and G. J. Woodsworth, 2010. Deformation of continental crust along a transform boundary, Coast Mountains, British Columbia, Tectonics, 29, TC4007, doi:10.1029/2009TC002502. Bogue, S. W., and C. S. Gromme, 2004. Structural correction of paleomagnetic vectors dispersed about two fold axes and application to the Duke Island (Alaska) ultramafic complex. Journal of Geophyscial Research 119, B11102, doi:10.1029/2004JB002989. Mukhopadhyay, S., J. C. Lassiter, K. A. Farley, and S. W. Bogue, Geochemistry of Kauai shield-stage lavas: Implications for the chemical evolution of the Hawaiian plume, Geochem. Geophys. Geosyst., 4(1), 1009, doi:10.1029/2002GC000342, 2003.
Farley, K. A, M.E. Rusmore, and S. W. Bogue, 2001. Post-10 Ma uplift and exhumation of the northern Coast Mountains, British Columbia. Geology 29, 99-102.
Bogue, S. W., 2001. Geomagnetic field behavior before and after the Kauai R-N polarity transition. Journal of Geophysical Research 106, 447-462.
Bogue, S. W. and E. Neroda, 1995. Intensity of the geomagnetic field during the Kauai N-R transition. EOS, Transactions, American Geophysical Union 76 (46), 170.
Bogue, S.W., C. S. Gromme, and J. W. Hillhouse, 1995. Paleomagnetism, magnetic anisotropy, and mid-Cretaceous paleolatitude of the Duke Island (Alaska) ultramafic complex. Tectonics 14, 1133-1152.
Bogue, S. W. and H. A. Paul, 1993. Distinctive field behavior following geomagnetic reversals. Geophysical Research Letters 20 (21), 2399-2402.
Bogue, S. W. and R. T. Merrill, 1992. The character of the field during geomagnetic reversals. Annual Review of Earth and Planetary Sciences 20, 181-219.
Bogue, S. W., D. S. Cowan, and J. I. Garver, 1989. Paleomagnetic evidence for poleward transport of Upper Jurassic rocks in the Decatur terrane, San Juan Islands, Washington, Journal of Geophysical Research 94, 10,415-10,427.
Bogue, S. W. and K. A. Hoffman, 1987. Morphology of geomagnetic reversals. Reviews of Geophysics 25, 910-916.
Bogue, S. W., 1987. Magnetostratigraphy of Kauai, Hawaii. Geological Society of America Abstracts with Programs 19, 359-360.
Bogue, S. W. and J. E. Evans, 1986. Discordant paleomagnetic component from the Eocene Chumstick Formation, central Washington State. Geological Society of America Abstracts with Programs 18, 88.
Bogue, S. W. and R. S. Coe, 1984. Transitional paleointensities from Kauai, Hawaii, and geomagnetic reversal models. Journal of Geophysical Research 89, 10,341-10,354.
Bogue, S. W. and R. S. Coe, 1982. Successive paleomagnetic reversal records from Kauai. Nature 295, 399-401.
Bogue, S. W. and R. S. Coe, 1981. Paleomagnetic correlation of Columbia River Basalt flows using secular variation. Journal of Geophysical Research 86, 11,883-11,897.
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