Kenneth P. Kodama

Detection of noninteracting single domain particles using first‐order reversal curve diagrams

We present a highly sensitive and accurate method for quantitative detection and characterization of noninteracting or weakly interacting uniaxial single domain particles (UNISD) in rocks and sediments. The method is based on high-resolution measurements of first-order reversal curves (FORCs). UNISD particles have a unique FORC signature that can be used to isolate their contribution among other magnetic components. This signature has a narrow ridge along the Hc axis of the FORC diagram, called the central ridge, which is proportional to the switching field distribution of the particles. Therefore, the central ridge is directly comparable with other magnetic measurements, such as remanent magnetization curves, with the advantage of being fully selective to SD particles, rather than other magnetic components. This selectivity is unmatched by other magnetic unmixing methods, and offers useful applications ranging from characterization of SD particles for paleointensity studies to detecting magnetofossils and ultrafine authigenically precipitated minerals in sediments.

A successful rock magnetic technique for correctng paleomagnetic inclination shallowing: Case study of the Nacimiento Formation, New Mexico

The accuracy of an inclination shallowing correction technique was tested on the remanence of the Paleocene Nacimiento Formation. The correction assumes that changes in paleomagnetic inclination during deformation will be directly related to changes in remanence anisotropy. The remanence anisotropy is also a function of the magnetic anisotropy of the individual magnetic grains. The Nacimiento Formation was selected for this test because its inclination is shallow by 7° to 8° and previous paleomagnetic studies indicate that it has a primary remanence with a well-constrained age and that magnetic overprints are minimal. Tectonics are an unlikely explanation for its shallow inclination. These conditions allow the inclination-corrected remanence to be compared to a well-defined paleomagnetic pole to determine the accuracy of the technique. The characteristic remanence from 20 Nacimiento sites was isolated by alternating field and thermal demagnetization and was corrected using the anisotropy of anhysteretic remanence measured for three to five samples per site. Individual particle anisotropy was determined by drying a magnetic separate/epoxy mixture in a 35–50 mT magnetic field. Laboratory compaction of disaggregated Nacimiento material was also used to yield an effective individual magnetic particle anisotropy. The two techniques resulted in similar values for individual particle anisotropy and similar inclination shallowing corrections. Both corrections were successful, and the inclination-corrected formation mean direction was indistinguishable from the direction predicted by North Americas Paleocene paleopole. The inclination shallowing correction technique reported here should be applied routinely in paleomagnetic studies, provided that the remanence is a primary depositional remanence that has been affected only by syndepositional or early postdepositional processes (e.g., compaction).

Compaction‐corrected inclinations from southern California Cretaceous marine sedimentary rocks indicate no paleolatitudinal offset for the Peninsular Ranges terrane

Paleomagnetic data have been used extensively to delineate the terrane displacement and accretion history of the western margin of North America. However, the anomalously shallow paleomagnetic inclinations used to indicate large-scale northward translation might be alternatively interpreted as due to postmagnetization tilting of batholithic rocks and compaction of marine sediments. To understand the magnitude of burial compaction effects on the post-Cretaceous motion history of the Peninsular Ranges-Baja Borderlands terrane, a rock magnetic, compaction, and paleomagnetic study of the Ladd Formation and the Point Loma Formation from southern California was conducted. The anhysteretic anisotropy of remanence of the characteristic remanence-carrying grains and individual magnetic grain anisotropy were used to correct the inclinations of each formation. Individual magnetic grain anisotropy was determined by both compaction experiments and redeposition of a magnetic separate in a DC magnetic field. Standard paleomagnetic studies of the units indicated that previous Ladd Formation results could be reproduced, and a correction was made at the sample level. We were unable to adequately reproduce earlier results of the Point Loma Formation, so the average remanence anisotropy was used to correct the previously reported mean direction. The mean inclination of the Ladd Formation was conected from 46° (α95 = 8°) to 58° (α95 = 4°), and the mean inclination of the Point Loma Formation was conected from 39.5°±5.4° (normal) and −36.4° ± 16.6° (reversed) to 56.0°±5.1° (normal) and −53.0°±16.7° (reversed). These results suggest that the Peninsular Ranges-Baja Borderland terrane has been part of the western North America since the Late Cretaceous and that clay-containing sedimentary rocks may typically experience from 10° to 15° of inclination shallowing due to burial compaction.

A compaction correction for the paleomagnetism of the Cretaceous Pigeon Point Formation of California

The paleomagnetism of the Cretaceous Pigeon Point Formation turbidites was reexamined to determine whether the 25° of southerly paleolatitudinal offset originally observed (Champion et al., 1984) for these rocks was all, or in part, due to compaction shallowing of their paleomagnetic inclination. The study consisted of two parts: (1) A standard paleomagnetic study, including detailed thermal and alternating field demagnetization, was conducted on oriented cores collected at Pigeon Point, approximately 50 km south of San Francisco, California. The results of this study were combined with the alternating field demagnetized results for samples provided by D. Champion from the initial Pigeon Point paleomagnetic study. The combined data set has a mean direction for Pigeon Point (I=41.6°, D=346.9°) similar to that originally obtained by Champion et al. (1984). (2) Material from the Pigeon Point Formation was disaggregated, given a laboratory analog of a postdepositional remanence, and compacted to pressures as high as 0.13 MPa which caused volume losses up to 53%. The laboratory-compacted samples were alternating field demagnetized, and their magnetic inclination and anisotropy of anhysteretic remanence were both measured. These data were used to derive correction curves, following Jackson et al. (1991), which describe the specific relationship between remanence anisotropy and inclination shallowing for the Pigeon Point Formation. Two correction curves were determined, one assuming that the magnetic particle orientation distribution experienced a prolate deformation after remanence acquisition and one assuming an oblate deformation. These two different corrections were necessary because the anisotropy of anhysteretic remanence indicates a composite fabric due to both a prelithification technically caused lineation and a burial compaction foliation. The anisotropy of anhysteretic remanence measured for each paleomagnetic sample and the correction curves determined from the laboratory compaction experiments indicate that the inclination of the Pigeon Point Formation has been shallowed by burial compaction. The compaction-corrected Pigeon Point mean directions assuming either a prolate (I=53.1°, D=347.2°) or an oblate (I=49.8°, D=346.8°) deformation suggest only 13° to 16° of southerly paleolatitudinal offset for the Pigeon Point Formation in the Cretaceous, not the 25° originally observed (Champion et al., 1984). The resulting paleolatitude for the Pigeon Point Formation could indicate that Salinia served as a link between the cratonic Sierra Nevada arc to the north and the Peninsula Ranges/Baja-Borderlands allochthon to the south. Alternatively, our results suggesting a 10° compaction inclination shallowing for the Pigeon Point turbidites may indicate that many of the paleomagnetic studies placing the Peninsula Ranges/Baja-Borderlands 15° to the south of North America in the Cretaceous and Tertiary may have suffered from a similar effect and that the allochthon has been nearly in place since the Cretaceous.

Flexural flow folding and the paleomagnetic fold test: An example of strain reorientation of remanence in the Mauch Chunk Formation

The relationship between the remanent magnetization and the detailed strain geometry around a first-order fold in the Appalachian Valley and Ridge Province was investigated to examine whether penetrative strains associated with folding can generate a apparent synfolding geometry from a prefolding magnetization. Paleomagnetic results from the Mississippian Mauch Chunk Formation on both limbs of the Frackville Anticline near Lavelle, Pennsylvania, yield two magnetic components, an intermediate unblocking temperature (300°C–600°C) Kiaman remagnetization and a two-polarity high unblocking temperature (650°C–680°C) characteristic magnetization. When the magnetic directions are incrementally corrected for bedding tilt, the intermediate-temperature component is most tightly clustered at 85% unfolding (D=176°, I=3°) and the high-temperature component is most tightly clustered at 75% unfolding (D=184°, I=27°). Mesoscopic and microscopic structural fabric analyses suggest a strain history that includes a significant component of flexural slip/flow folding. In the coarser-grained sandstone units, folding has largely been accommodated by slip on bedding, while in the finer-grained beds, folding has been accommodated by grain-scale deformation mechanisms such as pressure solution and low-temperature plasticity. Finite strain measurements, determined from center-to-center distances between quartz grains, yield strain ellipsoids consistent with this folding model. Inclination of the characteristic component varies as a function of the magnitude of the finite strain. This variation suggests that the characteristic magnetization has been systematically reoriented with respect to bedding during folding. Remanence directions on the south dipping limb have been rotated to shallower inclinations, while those on the north dipping limb have been rotated to steeper directions causing the prefolding magnetization to appear synfolding. These rotations are in agreement with models of rigid particle rotation in deforming viscous media. Unlike the characteristic magnetization, the secondary component appears to be unaffected by the deformation, and its synfolding behavior is interpreted as the acquisition of a secondary magnetization during Alleghenian folding. These results show that it is important to consider penetrative strains when evaluating the significance of apparent synfolding magnetizations.

Compaction-corrected paleomagnetic paleolatitudes for Late Cretaceous rudists along the Cretaceous California margin: Evidence for less than 1500 km of post–Late Cretaceous offset for Baja British Columbia

The paleolatitudinal distribution of bivalve rudists has important significance for the Baja British Columbia (Baja BC) hypothesis that western Canadian superterranes from British Columbia have been displaced 3000 km since Cretaceous time. Rudists are not observed in Baja BC sedimentary rocks, yet they are common in Late Cretaceous strata in California and Baja California, which have the same paleomagnetically determined paleolatitudes (approximately 25°N) as Baja BC rocks of Late Cretaceous age. In order to resolve this contradiction and to delimit more exactly the southern paleolatitudes of Baja BC, paleomagnetic inclinations corrected for the effects of burial compaction were used to determine the paleolatitudinal distribution of rudists along the California margin. Compaction-corrected paleomagnetic data from the Peninsular Ranges and Salinia terranes indicate that rudists were restricted to paleolatitudes between 34° and 40°N. Evidence of coastal upwelling in the latest Cretaceous Marca Shale may explain the northern limit of the rudist distribution. These data suggest that Baja BC was no farther south than 40°N in the Late Cretaceous, thus limiting its post-Cretaceous displacement to less than 1500 km, and that burial compaction has also affected the paleomagnetism of Nanaimo Group sedimentary rocks from Vancouver Island. This result also helps resolve the conflict between paleomagnetic results, which show 1500 km of post–Late Cretaceous offset between the Insular–Coast Plutonic Complex superterrane and the Intermontane superterrane and geologic observations, which can allow only tens of kilometers of offset between these terranes in the Methow- Tyaughton basin.

Using thermochronometry and low‐temperature demagnetization to accurately date Precambrian paleomagnetic poles

Dating magnetizations in Precambrian rocks is increasingly important in the attempt to unravel Precambrian plate configurations and supercontinent assemblages. We used low-temperature demagnetization and modern thermochronometric methods to dissect a multicomponent magnetization of the Glamorgan Gabbro, Ontario, previously studied by Buchan and Dunlop [1976] and Berger and York [1981]. We found that the HbA component is a primary thermoremanent magnetization carried by single-domain magnetite. The new paleomagnetic pole position (32.6°S latitude, 141.9°E longitude) is not significantly different from the published pole; however, the cooling history suggests that the 1015±15 Ma magnetization age is older than previously thought (980 Ma). The new age produces a better fit in Rodinia supercontinent reconstructions. The refined HbB pole (29.9°N latitude, 169.9°E longitude) is carried by multidomain-type magnetite and pyrrhotite. A possible ∼175°C thermal event at ∼600 Ma recorded by K-feldspar could be responsible for remagnetizing the multidomain grains. The new age for Hbs is 220 m.y. younger than the previous estimate, raising questions concerning the ages of similar poles from the Canadian Grenville Province.

Unraveling tectonic and climatic controls on synorogenic growth strata (Northern Apennines, Italy)

We develop a new high-resolution stratigraphic age model to unravel the contributions of tectonic and climatic processes on early to late Pleistocene synorogenic growth strata. We capitalize on excellent, continuous exposures along the flank of the Po foreland in northern Italy to elucidate hydrologic, geomorphic, and sedimentologic processes that are regularly attributed to, but rarely proven to be caused by, glacial-interglacial climatic changes and unsteady rock uplift. We perform our analysis on the Enza section, a succession of marine and terrestrial strata exposed along the Enza River, between Parma and Reggio Emilia, northern Italy. Bedding in the Enza section displays synorogenic growth strata geometry, with bedding dips that range from 2° to 55°, that becomes progressively shallower upsection. We develop an age model that incorporates biostratigraphy, magnetostratigraphy, rock-magnetic cyclostratigraphy, cosmogenic radionuclide burial dating, and optically stimulated luminescence dating and shows that the Enza section spans the interval between 0.04 and 1.65 Ma. Furthermore, the age model pins the time of deposition for several lithostratigraphic units of regional significance and shows that sediment accumulation was unsteady, ranging from 14–31 cm/k.y. in the marine part of the section to 5–362 cm/k.y. in the overlying littoral and terrestrial part of the section. Unsteady deposition is most pronounced in the terrestrial deposits where thick fluvial gravel packages accumulated in short (∼10–15 k.y.) time periods that coincide with Quaternary glacial intervals. There is direct evidence for a dominant tectonic control in the older, marine part of the section. Here, sediment accumulation rates on the limb of the fold growing along this portion of the Northern Apennine mountain front show that between 1.07 and 1.65 Ma, repetitive progradation of neritic sand units directly followed pulses of rapid, punctuated uplift. In contrast, the cyclic terrestrial facies variations in the Enza section reveal that once the section became emergent at ca. 1 Ma and uplift slowed, climate was the dominant control on sediment production and deposition.

The effects of grain‐scale deformation on the Bloomsburg Formation Pole

Previous paleomagnetic results from the Silurian Bloomsburg Formation in the central Appalachian Valley and Ridge Province suggest that the Bloomsburgs characteristic magnetization is a synfolding magnetization acquired during the Acadian Orogeny. However, there is little geological evidence to support extensive Acadian folding in the central Appalachians. A more favorable explanation is that the characteristic magnetization originated as a prefolding Silurian magnetization and has been altered by grain-scale deformation to mimic a synfolding Devonian remagnetization. In order to investigate whether penetrative deformation has altered the Bloomsburg Formations characteristic signal, the relationship between strain and remanence was examined around three central Appalachian Valley and Ridge folds. Structural analysis of mesoscopic and microscopic features along with center-to-center finite strain analysis at these three folds indicates a deformation sequence that includes prefolding layer-parallel shortening overprinted by top-to-the-foreland, bedding-parallel shear and flexural slip/flow folding. Inclination of the characteristic magnetization varies systematically with the magnitude of finite strains. This relationship between strain and remanence suggests that the characteristic magnetization has been progressively rotated toward shallower inclinations by penetrative deformation analogous to rigid particle rotation in a viscously deforming matrix. Using directions from sites with relatively low finite strain values, a new Bloomsburg Formation pole is calculated at 18.7°N, 107.4°E (K = 58.4, A95 = 8.2°). This new pole falls on the Silurian track of the North American apparent polar wander path between the Silurian Wabash Reef pole and the Silurian-Devonian Andreas pole. In addition, these results suggest that previously reported 20°–30° difference in declination of site mean directions north arid south of the Pennsylvania salient may also be the result of grain-scale reorientation of the remanence-carrying grains and not oroclinal bending.

Sensitive moisture response to Holocene millennial-scale climate variations in the Mid-Atlantic region, USA

Millennial-scale climate variability has been increasingly recognized as one of the most prominent features of the Holocene. However, regional responses, especially in terms of moisture conditions, are poorly documented and understood. Here we present lithologic and magnetic evidence from White Lake in northern New Jersey, USA, to show that low lake levels occurred at about 1.3, 3.0, 4.4 and 6.1 ka (1 ka = 1000 cal. yr BP). The low lake levels are indicated by heterogeneous coarse calcareous sediment layers showing strong magnetic intensities. These detrital layers were likely formed during low stands when shallow-water marls were exposed, oxidized, transported and redeposited. This model is supported by laboratory experiments showing that oxidation of marls can enhance magnetic intensities. The dry periods inferred from the low lake levels of White Lake appear to occur concurrently with the cold periods recorded in the North Atlantic sediments. The correlation between millennial-scale dry/wet cycles inferred from lake-level fluctuations of White Lake and cold/warm cycles in North Atlantic sediments suggests sensitive moisture responses to Holocene millennial-scale climate variability. The dry-cold (or wet-warm) association is supported by instrumental records of the last century showing that the Mid-Atlantic region was dominated by wet conditions, while most parts of the conterminous USA experienced droughts, when the North Atlantic Ocean was warm. The consistent moisture responses of the Mid-Atlantic region to temperature changes of the North Atlantic Ocean may have persisted for the past 6000 years.