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Koppers, AAP, Yamazaki T, Geldmacher J, Gee JS, Pressling N, Hoshi H, Anderson L, Beier C, Buchs DM, Chen LH, Cohen BE, Deschamps F, Dorais MJ, Ebuna D, Ehmann S, Fitton JG, Fulton PM, Ganbat E, Hamelin C, Hanyu T, Kalnins L, Kell J, Machida S, Mahoney JJ, Moriya K, Nichols ARL, Rausch S, Sano SI, Sylvan JB, Williams R.  2012.  Limited latitudinal mantle plume motion for the Louisville hotspot. Nature Geoscience. 5:911-917.   10.1038/ngeo1638   AbstractWebsite

Hotspots that form above upwelling plumes of hot material from the deep mantle typically leave narrow trails of volcanic seamounts as a tectonic plate moves over their location. These seamount trails are excellent recorders of Earth's deep processes and allow us to untangle ancient mantle plume motions. During ascent it is likely that mantle plumes are pushed away from their vertical upwelling trajectories by mantle convection forces. It has been proposed that a large-scale lateral displacement, termed the mantle wind, existed in the Pacific between about 80 and 50 million years ago, and shifted the Hawaiian mantle plume southwards by about 15 degrees of latitude. Here we use Ar-40/Ar-39 age dating and palaeomagnetic inclination data from four seamounts associated with the Louisville hotspot in the South Pacific Ocean to show that this hotspot has been relatively stable in terms of its location. Specifically, the Louisville hotspot-the southern hemisphere counterpart of Hawai'i-has remained within 3-5 degrees of its present-day latitude of about 51 degrees S between 70 and 50 million years ago. Although we cannot exclude a more significant southward motion before that time, we suggest that the Louisville and Hawaiian hotspots are moving independently, and not as part of a large-scale mantle wind in the Pacific.

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.

Bowles, J, Gee JS, Kent DV, Perfit MR, Soule SA, Fornari DJ.  2006.  Paleointensity applications to timing and extent of eruptive activity, 9 degrees-10 degrees N East Pacific Rise. Geochemistry Geophysics Geosystems. 7   10.1029/2005gc001141   AbstractWebsite

[ 1] Placing accurate age constraints on near-axis lava flows has become increasingly important given the structural and volcanic complexity of the neovolcanic zone at fast spreading ridges. Geomagnetic paleointensity of submarine basaltic glass (SBG) holds promise for placing quantitative age constraints on near-axis flows. In one of the first extensive tests of paleointensity as a dating tool or temporal marker we present the results of over 550 successful SBG paleointensity estimates from 189 near-axis (< 4 km) sites at the East Pacific Rise, 9 degrees - 10 degrees N. Paleointensities range from 6 to 53 mu T and spatially correspond to the pattern expected from known temporal variations in the geomagnetic field. Samples within and adjacent to the axial summit trough (AST) have values approximately equal to or slightly higher than the present-day. Samples out to 1 - 3 km from the AST have values higher than the present-day, and samples farther off axis have values lower than the present-day. The on-axis samples (< 500 m from the AST) provide a test case for using models of paleofield variation for the past few hundred years as an absolute dating technique. Results from samples collected near a well-documented eruption in 1991 - 1992 suggest there may be a small negative bias in the paleointensity estimates, limiting resolution of the dating technique. Possible explanations for such a bias include local field anomalies produced by preexisting magnetic terrain; anomalously high magnetic unblocking temperatures, leading to a small cooling rate bias; and/or the possibility of a chemical remanence produced by in situ alteration of samples likely to have complicated thermal histories. Paleointensity remains useful in approximating age differences in young flows, and a clear along-axis paleointensity contrast near 9 degrees 50'N is suggestive of a similar to 150 - 200 year age difference. Paleointensity values of off-axis samples are generally consistent with rough age interpretations based on side scan data. Furthermore, spatial patterns in the paleointensity suggest extensive off-axis flow emplacement may occur infrequently, with recurrence intervals of 10 - 20 kyr. Results of a stochastic model of lava emplacement show that this can be achieved with a single distribution of flows, with flow size linked to time between eruptions.

Parker, RL, Gee JS.  2002.  Calibration of the pass-through magnetometer - II. Application. Geophysical Journal International. 150:140-152.   10.1046/j.1365-246X.2002.01692.x   AbstractWebsite

We describe the experimental procedure we use to calibrate a cryogenic pass-through magnetometer. The procedure is designed to characterize the magnetometer sensitivity as a function of position within the sensing region. Then we extend a theory developed in an earlier paper to cover inexact observations and apply it to the data set. The theory allows the calculation of a smooth, harmonic, internally consistent interpolating function for each of the nine components of the response tensor of the magnetometer. With these functions we can calculate the response to a dipole source in any orientation and position, and predict the magnetometer signal from any kind of specimen. The magnetometer in the paleomagnetic laboratory onboard the research vessel Joides Resolution is the subject of one such experiment and we present the results. The variation with position of sensitivity is displayed in a series of plane slices through the magnetometer. We discover from the calibration model that the X and Z coils are misaligned so that the magnetic centre of the coils is displaced from the geometric centre by approximately 0.7 cm. We synthesize the signal expected from the magnetometer when a variety of simple cores are measured. We find that, unless appropriate corrections are made, changes in magnetization direction can appear as variations in magnetic intensity, and conversely, fluctuations in the magnetization strength can produce apparent swings in declination and inclination. The magnitude of these effects is not small and is certainly worth taking into account in the interpretation of records from this kind of instrument. In a pilot study on data from a core measured with the shipboard magnetometer, we observe some large distortions, particularly in declination, that are attributable to uncorrected instrumental effects.

Bowers, NE, Cande SC, Gee JS, Hildebrand JA, Parker RL.  2001.  Fluctuations of the paleomagnetic field during chron C5 as recorded in near-bottom marine magnetic anomaly data. Journal of Geophysical Research-Solid Earth. 106:26379-26396.   10.1029/2001jb000278   AbstractWebsite

Near-bottom magnetic data contain information on paleomagnetic field fluctuations during chron C5 as observed in both the North and South Pacific. The North Pacific data include 12 survey lines collected with a spatial separation of up to 120 kin, and the South Pacific data consist of a single long line collected on the west flank of the East Pacific Rise (EPR) at 19 degreesS. The North Pacific magnetic profiles reveal a pattern of linear, short-wavelength (2 to 5 km) anomalies (tiny wiggles) that are highly correlated over the shortest (3.8 km) to longest (120 km) separations in the survey. Magnetic inversions incorporating basement topography show that these anomalies are not caused by the small topographic relief. The character of the near-bottom magnetic profile from anomaly 5 on the west flank of the EPR, formed at a spreading rate more than twice that of the North Pacific, displays a remark-able similarity to the individual and stacked lines from the North Pacific survey area, Over distances corresponding to 1 m.y., 19 lows in the magnetic anomaly profile can be correlated between the North and South Pacific lines. Modeling the lows as due to short polarity events suggests that they may be caused by rapid swings of the magnetic field between normal and reversed polarities with little or no time in the reversed state. Owing to the implausibly high number of reversals required to account for these anomalies and the lack of any time in the reversed state, we conclude that the near-bottom signal is primarily a record of pateointensity fluctuations during chron C5. Spectral analysis of the North Pacific near bottom lines shows that the signal is equivalent to a paleointensity curve with a temporal resolution of 40 to 60 kyr, while measurements of the smallest separations of correlatable dips in the field suggest a temporal resolution of 36 kyr.

Gee, JS, Cande SC, Hildebrand JA, Donnelly K, Parker RL.  2000.  Geomagnetic intensity variations over the past 780 kyr obtained from near-seafloor magnetic anomalies. Nature. 408:827-832.   10.1038/35048513   AbstractWebsite

Knowledge of past variations in the intensity of the Earth's magnetic field provides an important constraint on models of the geodynamo. A record of absolute palaeointensity for the past 50 kyr has been compiled from archaeomagnetic and volcanic materials, and relative palaeointensities over the past 800 kyr have been obtained from sedimentary sequences. But a long-term record of geomagnetic intensity should also be carried by the thermoremanence of the oceanic crust. Here we show that near-seafloor magnetic anomalies recorded over the southern East Pacific Rise are well correlated with independent estimates of geomagnetic intensity during the past 780 kyr. Moreover, the pattern of absolute palaeointensity of seafloor glass samples from the same area agrees with the well-documented dipole intensity pattern for the past 50 kyr. A comparison of palaeointensities derived from seafloor glass samples with global intensity variations thus allows us to estimate the ages of surficial lava flows in this region. The record of geomagnetic intensity preserved in the oceanic crust should provide a higher-time-resolution record of crustal accretion processes at mid-ocean ridges than has previously been obtainable.

Juarez, MT, Tauxe L, Gee JS, Pick T.  1998.  The intensity of the Earth's magnetic field over the past 160 million years. Nature. 394:878-881.   10.1038/29746   AbstractWebsite

In contrast to our detailed knowledge of the directional behaviour of the Earth's magnetic field during geological and historical times(1,2), data constraining the past intensity of the field remain relatively scarce. This is mainly due to the difficulty in obtaining reliable palaeointensity measurements, a problem that is intrinsic to the geological materials which record the Earth's magnetic field. Although the palaeointensity database has grown modestly over recent years(3-5), these data are restricted to a few geographical locations and more than one-third of the data record the field over only the past 5 Myr-the most recent database(5) covering the time interval from 5 to 160 Myr contains only about 100 palaeointensity measurements. Here we present 21 new data points from the interval 5-160 Myr obtained from submarine basalt glasses collected from locations throughout the world's oceans. Whereas previous estimates for the average dipole moment were comparable to that of the Earth's present field(6), the new data suggest an average dipole moment of (4.2 +/- 2.3) x 10(22) A m(2), or approximately half the present magnetic-field intensity. This lower average value should provide an important constraint for future efforts to model the convective processes in the Earth's core which have been responsible for generating the magnetic field.

Tauxe, L, Gee J, Gallet Y, Pick T, Bown T.  1994.  Magnetostratigraphy of the Willwood Formation, Bighorn Basin, Wyoming - New Constraints on the Location of Paleocene Eocene Boundary. Earth and Planetary Science Letters. 125:159-172.   10.1016/0012-821x(94)90213-5   AbstractWebsite

The lower Eocene Willwood Formation in the Bighorn Basin of Wyoming preserves a rich and diverse mammalian and floral record. The paleomagnetic behavior of the sequence of floodplain paleosols of varying degrees of maturation ranges from excellent to poor. We present a magnetostratigraphic section for a composite section near Worland, Wyoming, by using a set of strict criteria for interpreting the step-wise alternating field and thermal demagnetization data of 266 samples from 90 sites throughout the composite section. Correlation to the geomagnetic reversal time scale was achieved by combining magnetostratigraphic and biostratigraphic data from this section, from a section in the Clark's Fork Basin in northern Wyoming, and from DSDP Site 550, with the isotopic date determined on a tuff near the top of our section. Our correlation suggests that the Bighorn Basin composite section in the Worland area spans from within Chron C24r to near the top of Chron C24n, or from approximately 55 to 52 Ma. This correlation places the Paleocene/Eocene boundary within the vicinity of the base of the section. Cryptochron C24r.6 of Cande and Kent is tentatively identified some 100 m above the base of the section. The temporal framework provided here enables correlation of the mammalian biostratigraphy of the Bighorn Basin to other continental sequences as well as to marine records. It also provides independent chronological information for the calculation of sediment accumulation rates to constrain soil maturation rates. We exclude an age as young as 53 Ma for the Paleocene/Eocene boundary and support older ages, as recommended in recent time scales. The location of a tuff dated at 52.8 +/- 0.3 Ma at the older boundary C24n.1 is consistent with the, age of 52.5 Ma estimated by Cande and Kent and inconsistent with that of 53.7 Ma, from Harland et al.

Gee, J, Staudigel H, Tauxe L, Pick T, Gallet Y.  1993.  Magnetization of the La Palma Seamount Series - Implications For Seamount Paleoples. Journal of Geophysical Research-Solid Earth. 98:11743-11767.   10.1029/93jb00932   AbstractWebsite

Paleopoles determined from seamount magnetic anomalies constitute the major data source for the Pacific apparent polar wander path, but relatively little is known about the processes of remanence acquisition in seamounts. Since magnetic anomalies reflect both natural remanence (NRM) and the induced field, it is important first to assess whether the NRM is likely to represent an original field direction and second to constrain the magnitude of the induced component. To this end, we present paleomagnetic data from an uplifted, subaerially exposed section through a seamount on La Palma, Canary Islands. The Pliocene Seamount Series of La Palma comprises a >6 km sequence of alkalic extrusives and intrusives which includes all lithologies likely to be volumetrically important in seamounts. The structural tilt of the Seamount Series allows separation of early thermal or chemical remanence from magnetization components acquired after tilting (e.g., viscous remanence). The NRM provides a poor indication of the original magnetization direction, although the characteristic magnetization of many La Palma samples is compatible with the original pretilt direction. Hydrothermal alteration has resulted in the production of Ti-poor magnetite and an increasing contribution of hematite with increasing degree of alteration. More importantly, well-defined magnetization directions which deviate from any reasonable geomagnetic direction at La Palma can be attributed to hydrothermal alteration in a different polarity than prevalent during the original magnetization. Based on a comparison of the magnitude of low-stability components of magnetization and laboratory acquisition of viscous remanence and previous estimates of the induced magnetization, we conclude that viscous and induced magnetization probably account for 15-25% of the total magnetization of seamounts. The resulting paleopole bias is a function of the polarity and paleolatitude of the seamount and ranges from 4-degrees to 16-degrees for Cretaceous seamounts in the Pacific.

Klootwijk, CT, Gee JS, Peirce JW, Smith GM, McFadden PL.  1992.  An Early India-Asia Contact - Paleomagnetic Constraints from Ninetyeast Ridge, ODP Leg 121. Geology. 20:395-398.   10.1130/0091-7613(1992)020<0395:aeiacp>;2   AbstractWebsite

New paleomagnetic results from sedimentary rock and basement of the Ninetyeast Ridge (Ocean Drilling Program Leg 121, Sites 756-758) detail the northward movement of the Indian plate for the past 80 m.y. Analysis of the combined paleolatitude-age profile indicates a distinct reduction in India's northward movement rate at 55+ Ma, interpreted as completion of suturing of Greater India and Asia. India's northward motion slowed from 18-19.5 cm/yr to 4.5 cm/yr for the location of Site 758. Comparison of this profile with paleomagnetic data from the wider Himalayan region indicates that initial contact between northwestern Greater India and southern Asia was already established by Cretaceous-Tertiary time. This supports a possible causal link between the India-Asia convergence and the Deccan Traps extrusion.

Klootwijk, CT, Gee JS, Peirce JW, Smith GM.  1992.  Neogene Evolution of the Himalayan Tibetan Region - Constraints from ODP Site-758, Northern Ninetyeast Ridge - Bearing on Climatic-Change. Palaeogeography Palaeoclimatology Palaeoecology. 95:95-110.   10.1016/0031-0182(92)90167-4   AbstractWebsite

Magnetic susceptibility, remanence and lithostratigraphic profiles for the Neogene-Quaternary sequence at Site 758 (ODP Leg 121) on the northern Ninetyeast Ridge show distinct changes, dated from biostratigraphy and detailed magnetostratigraphy, at 17.5, 10.4-10.0, 8.8, 6.5, 5.4-5.1, 2.7-2.5, 1.9, and 1.2-1.1 Ma. These magnetic and lithologic changes appear to reflect changes in the supply and character of terrigenous material from the Himalayan-Tibetan region resulting from changes in gradient of the Ganges, Brahmaputra and probably the ancient Indus drainage systems. The sedimentary changes can be correlated with changes in uplift-sensitive markers such as the oceanic Sr-87/Sr-86 ratio and monsoonal induced upwelling, but not clearly so with sealevel variations. We interpret these sedimentary changes, therefore, to primarily reflect changes in the tectonic evolution of the Himalayan-Tibetan region. The changes in the distal marine sedimentary record of the northern Ninetyeast Ridge are compared with isotopic control on the timing of Himalayan-Tibetan tectonic phases and magnetostratigraphic control on their reflection in the proximal Siwalik molasse record. This comparison indicates that the distal Ninetyeast Ridge record can be used to detail and to place minimal age constraints on tectonic phases in the wider Himalayan region and on evolution of the proximal molasse sequence, with a time lag determined for the four earliest changes at less than 1 m.y. The changes at 17.5 Ma and 5.4-5.1 Ma can be interpreted in terms of the causative chain: enhanced plate motion double-line arrow pointing right uplift and sedimentation change double-line arrow pointing right climatic change, supporting arguments that the Late Cainozoic global climatic deterioration is driven by uplift of large plateaus such as the Himalayan-Tibetan region and the Western Cordillera.

Klootwijk, CT, Gee JS, Peirce JW, Smith GM.  1992.  The origin of Ninetyeast Ridge: palaeomagnetic constraints from ODP Leg 121. Journal of Southeast Asian Earth Sciences. 7:247-252.   10.1016/0743-9547(92)90004-u   AbstractWebsite

Palaeomagnetic results from a lower Turonian to Middle Eocene ash-rich sedimentary sequence on Broken Ridge (ODP Sites 752–755) and ash and basalt sequences on Ninetyeast Ridge (90ER) (Sites 756–758) (1) support a Kerguelen hotspot origin for the 90ER volcanic pile; (2) are consistent with models for a large-scale southward jump of the Southeast Indian Ridge (SEIR) west of the 90ER at about 58 Ma; and (3) suggest a possible Amsterdam-St Paul hotspot origin for magnetic overprints on the 90ER.

Klootwijk, CT, Gee J, Smith GM, Pierce JW.  1991.  Constraints on the India-Asia convergence; paleomagnetic results from Ninetyeast Ridge. Proceedings of the Ocean Drilling Program, Scientific Results. 121:777-884.   10.2973/   Abstract

This study details the Late Cretaceous and Tertiary northward movement of the Indian plate. Breaks in India's northward movement rate are identified, dated, and correlated with the evolution of the India-Asia convergence. Paleolatitudinal constraints on the origin of Ninetyeast Ridge are discussed, and limited magnetostratigraphic detail is provided.Nearly 1500 sediment and basement samples from Sites 756, 757, and 758 on Ninetyeast Ridge were studied through detailed alternating field and thermal demagnetization. Primary and various secondary magnetization components were identified. Breakpoint intervalsintheprimarypaleolatitudepatternforcommon-Site758wereidentified at2.7,6.7,18.5,about53,63.5-67,and68-74.5 Ma. Only the breakpoint interval at about 53 Ma reliably reflects a reduction in India's northward movement rate. The onset of this probably gradual slowdown was dated at 55 Ma (minimal age) based on the intersection of weighted linear regression lines. At the locationofcommon-Site758,northwardmovementslowedfrom 18-19.5cm/yr(fromatleast65to55Ma)to4.5cm/yr(from55 to at least 20 Ma). Reanalysis of earlier DSDP/ODP paleolatitude data from the Indian plate gives a comparable date (53 Ma) for this reduction in northward velocity.Comparison of our Ninetyeast Ridge data and Himalayan paleomagnetic data indicates that the initial contact of Greater India and Asia mayhave already been established by Cretaceous/Tertiary boundary time. The geological record of the convergence zone and the Indian plate supports the notion that the Deccan Traps extrusion may have resulted from the ensuing deformation of the Indian plate. W e interpret the breakpoint at 55+ Ma to reflect completion of the eastward progressive India-Asia suturing process.Neogene phases in the evolution of the convergence zone were correlated with significant changes in the susceptibility, NRM intensity, and lithostratigraphic profile of Site 758.These changes are interpreted to reflect and postdate tectonic phases in the evolutionofthewiderHimalayanandsouthernTibetanregion.Thechangesweredatedandinterpretedasfollows: 17.5Ma,initial uplift of the Higher Himalaya following initiation of intercontinental underthrusting; 10-10.4 Ma, increased uplift and onset of Middle Siwaliks sedimentation; 8.8 Ma, probable reduction in influx corresponding with the Nagri Formation to Dhok Pathan Formation changeover; 6.5 Ma, major tectonic phase evident throughout the wider Himalayan region and northern Indian Ocean; 5.1-5.4 Ma, onset of oroclinal bending of the Himalayan Arc,of extensional tectonism in southern Tibet, and of Upper Siwalik sedimentation; 2.5-2.7 and 1.9 Ma, major phases of uplift of the Himalayan and Tibetan region culminating in the present-day high relief.The basal ash sequence and upper flow sequence of Site 758 and the basal ash sequence of Site 757 indicate paleolatitudes at about 50°S. These support a Kerguelen hot spot origin for Ninetyeast Ridge. Consistently aberrant inclinations in the basalt sequence ofSite757mayberelatedtoasouthwardridgejumpataboutthetime(58Ma)thatthesebasaltswereerupted.Thebasalt sequence of Site 756 indicates a lower paleolatitude (about 43°S), as do parts of the basalt sequence of Site 758 which also have reversed polarity overprints. The low paleolatitudes for Site 756 may be explained by late-stage volcanism north of the Kerguelen hot spot or the influence of the Amsterdam-St. Paul hot spot.

Pospichal, JJ, Dehn J, Driscoll N, van Eijden AJM, Farrell J, Fourtanier E, Gamson PD, Gee J, Janecek T, Jenkins GD, Klootwijk CT, Nomura R, Owen RM, Rea DK, Resiwati P, Smit J, Smith GM.  1991.  Cretaceous-Paleogene biomagnetostratigraphy of sites 752-755, Broken Ridge; a synthesis. Proceedings of the Ocean Drilling Program, Scientific Results. 121:721-742.   10.2973/   Abstract

Broken Ridge, in the eastern Indian Ocean, is a shallow-water volcanic platform which formed during the Early to middle Cretaceous at which time it comprised the northern portion of the Kerguelen-Heard Plateau. Rifting during the middle Eocene and subsequent seafloor spreading has moved Broken Ridge about 20°N to its present location. The sedimentary section of Broken Ridge includes Turonian-lower Eocene limestone and chalk with volcanic ash, an interval of detrital sands and gravels associated with middle Eocene rifting and uplift, and a middle-late Oligocene unconformity overlain by a thin section of Neogene-Holocene pelagic calcareous ooze. This paper summarizes the available post-cruise biostratigraphic and magnetostratigraphic data for the Cretaceous- Paleogene section on Broken Ridge. The synthesis of this information permits a more precise interpretation of the timing of events in the history of Broken Ridge, in particular the timing and duration of the middle Eocene rifting event. Paleontologic data support rapid flexural uplift of Broken Ridge in response to mechanical rather than thermal forces. Other highlights of the section include a complete Cretaceous/Tertiary boundary and an opportunity for first-order correlation of Paleogene diatom stratigraphy with that of the calcareous groups.

Pringle, Staudigel H, Gee J.  1991.  Jasper Seamount: Seven million years of volcanism. Geology. 19:364-368.   10.1130/0091-7613(1991)019<0364:jssmyo>;2   AbstractWebsite

Jasper Seamount is a young, mid-sized (690 km super(3)) oceanic intraplate volcano located abut 500 km west-southwest of San Diego, California. Reliable super(40)Ar/ super(39)Ar age data were obtained for several milligram-sized samples of 4 to 10 Ma plagioclase by using a defocused laser beam to clean the samples before fusion. Gee and Staudigel suggested that Jasper Seamount consists of a transitional to tholeiitic shield volcano formed by flank transitional series lavas, overlain by flank alkalic series lavas and summit alkalic series lavas. Twenty-nine individual super(40)Ar/ super(39)Ar laser fusion analyses on nine samples confirm the stratigraphy: 10.3-10.0 Ma for the flank transitional series, 8.7-7.5 Ma for the flank alkalic series, and 4.8-4.1 Ma for the summit alkalic series. The alkalinity of the lavas clearly increases with time, and there appear to be 1 to 3 m.y. hiatuses between each series. The age data are consistent with the complex magnetic anomaly of Jasper; however, the dominant reversed polarity inferred from the anomaly suggests that most of the seamount formed at ca. 11 Ma, prior to the onset of Chron C5N.