GP23A-0022
Magnetic Anomaly Modeling in the Bay of Bengal
The thick sediment deposits carried by the several rivers in India, into the Bay of Bengal mask the underlying crust and pose severe restrictions in mapping the structural configuration and constructing the geodynamical history of the Bay of Bengal. In addition to this, the subduction zone around the Andaman and Sumatran region causes the oceanic lithosphere to be under stress, resulting in intense deformation. Geopotential data offer the unique opportunity of resolving some of these issues. Satellite derived free air gravity data, marine magnetic data, available seismic profiles and isopach maps over the Bay of Bengal are utilized in the present paper. Seismic profiles in the Indian offshore together with the isopach map of the region are utilized to constrain the model parameters for reproducing the satellite derived free air gravity anomalies over a profile along 11 degrees N latitude from the east coast of India up to the Andaman region. This helps determine the configuration of the 85 degrees East Ridge. Using thickness parameters derived from this gravity model as a constrain, an attempt is made to model the NGDC marine magnetic data along this long profile of around 1100 km length, by incorporating remanent magnetization in addition to the induced field. The magnetic profile along 11 degrees N is very well reproduced when the strike angle of the spreading anomalies matched the value obtained by the Scientists of the National Institute of Oceanography, India. However, we find that only three reversals are required to be incorporated into the model, with the 85E Ridge being reversely magnetised. A similar process of modeling is applied to the second profile along 10 degrees North latitude. Along the two-modeled profiles, the magnetic characteristics of the sea floor to the east of 85 E ridge are similar but they show different characteristic to the west. The results of these models will be interpreted in terms of the Sea floor spreading of the region.
GP23A-0023
Oldest age of South China Sea seafloor spreading identified by marine magnetic anomaly
The oldest age of the South China Sea (SCS) seafloor spreading was previously believed to be ca. 32 (magnetic anomaly C11). New magnetic data acquired in the northernmost SCS however obviously shows the existence of oceanic-crust-related E-W trending magnetic polarity reversal patterns. Through magnetic modeling, the oldest SCS oceanic crust was dated at Late Eocene, as old as 37 Ma (magnetic anomaly C17), with a half-spreading rate of 40 mm/yr. The corresponding continent-ocean boundary (COB) in the northern SCS is therefore put a little bit northward. The SCS oceanic crust is limited in the west by the presence of a relatively low magnetization zone, corresponding to outermost thinned portion of the continental crust. The northward extension of the oldest SCS oceanic crust is found to be terminated by a NW-SE trending transform fault, called the Luzon-Ryukyu Transform Plate Boundary (LRTPB). The LRTPB is supposed to be a former left-lateral transform fault connecting the former southeast-dipping Manila Trench in the south and the northwest-dipping Ryukyu Trench in the north. The oldest age of the SCS oceanic crust identified in the northernmost SCS by marine magnetic anomalies indicates that the initiation of the SCS seafloor spreading occurred in the northeastern and southeastern corners. However, an older SCS oceanic crust than 37 Ma cannot be excluded. The spreading age younger towards southwest indicates that there was a SCS mid-ocean ridge propagation southwestward or a different stages of the SCS seafloor spreading; the SCS seafloor spreading occurred first in the east and then in the west.
GP23A-0024
Marine Magnetic Anomaly and Magnetization of Oceanic Plate around the Japan Trench in the Northwestern Pacific
We have newly collected dense magnetic data around the Japan Trench in the northwestern Pacific. We present characteristics of the complied magnetic anomaly and crustal magnetization variation. The Pacific Plate in the study area has a series of parallel magnetic anomalies (Japanese Lineation Set), identified as chron M14-M7 (140-127 Ma). These anomalies are well lineated, in the direction of WSW-ENE, and have high-amplitudes of ~500-1000 nT, peak-to-trough, in the seaward slope of the trench. The northwestern margin of the Pacific Plate is being subducted beneath the northern Japanese Islands at the Japan Trench. The amplitudes of anomalies gradually decrease towards the land from the trench axis. To correct for effects of seafloor topography, crustal magnetization was calculated. Densely distributed seismic survey profiles enable us to constrain the depth of subducting oceanic crust, and to correct for the depth effects. The resultant crustal magnetization indicates along-lineation variation with a period of ~50-200 km. Wakes of the higher magnetization, sub-perpendicular to the magnetic lineation, could suggest non-transform discontinuities that originate at a mid-ocean ridge. The low magnetization appears on the seaward trench slope where horst-graben structure is developed. This result may suggest that the lower magnetization relates to formation of the horst-graben structure associated with plate bending and normal faulting. The across-lineation variation may indicate temporal variation of the Cretaceous field intensity. The ~135 Ma crust has higher magnetization and decreases towards ~130 Ma. The magnetization gradually decreases as the plate subduction proceeded. The apparent decay could reflect destruction and mechanical disorganization and/or chemical demagnetization of the topmost part of the oceanic crust along the plate boundary.
GP23A-0025
Fossil Overlapping Spreading Centre in the Central Pacific at the Trace of the Pacific-Cocos-Nazca Triple Point
The Central Pacific region between 125° W/5° N and 110° W/10° S which contains the "plate tectonic mirror image" to the region of the Cocos plate currently being subducted beneath Central America has been the target of an investigation using multibeam bathymetry, magnetics and gravity. One of the main points of interest was the formation of the triple junction of Pacific, Cocos and Nacza plates in the Late Oligocene/Early Miocene (around 24 Myrs). This event indicates the breakup of the Farallon plate on the eastern side of the East-Pacific-Rise into Cocos and Nazca plates. At this time the first Cocos-Nazca spreading centre (CNS-1) a precursor of the currently active spreading centre (CNS-3) between Cocos and Nazca plates was initiated. The breakup process is a result of changing spreading directions caused by global changes in plate movement directions. This is documented in the area of investigation in the Central Pacific in changes in the strike direction of magnetic lineations from north to south and morphological structures of the seafloor. Irregularities in the magnetic lineation pattern in a small area in the centre of the investigation area and curved morphological structures on the oceanic seafloor are interpreted in terms of remnants of a fossil overlapping spreading centre. The overlapping spreading centre was formed as a consequence of shiftings in the spreading axis which subsequently are straightened out by processes like the one described.
GP23A-0026
A Plea for Marine Vector Magnetometers: Cruise Experiences From the Central Pacific
The research cruise SONNE-180 was carried out in the Central Pacific at about $120°W just south of the Equator. The research area on the Pacific plate is conjugated to the break-Up of the Farallon plate into Cocos and Nazca plates 23 m.y.\ ago. This presentation is focusing on instrumental and methodological developments while the resulting plate tectonic reconstruction of the East Pacific is presented in a separate paper by Barckhausen. The Magnetometer array consisted of i) two Overhauser sensors (SeaSpy) recording the total magnetic field in gradiometer mode, ii) a newly developed marine vector magnetometer (BGR) which can be deployed on the same cable together with the two gradiometer sensors, and iii) a second, independent vector magnetometer system (SIO). Processed vector magnetometer yielded identical time series in the total magnetic field with the Overhauser sensors. Also the vertical component of both vector magnetometers is in a good agreement. Furthermore, applying the gradiometer mode to a Fluxgate-Overhauser pair of sensors worked as well as the standard Overhauser-Overhauser gradiometer. A new processing scheme has been applied to the data of all sensors containing a band pass filter in time domain in order to limit purely on wavelengths related to seafloor spreading anomalies. Those pre-processed total magnetic field data entered the anomaly reconstruction free of external temporal variations by summing up the gradiometer differences. Euler rotations by inclinometer angles provided the vertical component of the vector magnetometers. Furthermore, vector magnetometer data enabled processing in the spectral domain (multitaper method applied here). Fitting synthetic anomaly patterns to the observed data is much more straightforward for the vertical component than for the small total field anomalies of about half amplitude. Furthermore, synthetic models are better constrained because they have to predict simultaneously all components. In summary, marine vector magnetometers are an important step ahead in marine magnetics due to additional component information and increased possibilities in analysis and modelling, especially in regions where total field anomalies are small.
GP23A-0027
Seafloor Dating Within The Brunhes Period With Deep-Sea Vector Magnetic Anomalies
Marine magnetic anomalies provide a robust record of the past geomagnetic reversal. Classical sea-surface magnetic anomaly studies are generally restricted to the analysis of polarity reversals, although a systematic secondary signal, made of shorter wavelength and lower amplitude anomalies, superimposes to the major anomalies. The consistent pattern of these tiny wiggles worldwide strongly suggests a geomagnetic origin for these anomalies, either short polarity intervals or large-scale fluctuations in the past geomagnetic field intensity. If they are precisely dated, these short wavelength geomagnetic variations provide a new tool to determine the age of the seafloor with a high resolution, especially for young mid-ocean ridge basalt. In order to collect high resolution magnetic data within the Brunhes period, a deep-sea vector magnetic survey was carried out across the Central Indian Ridge (CIR, 19° S) with the French submersible Nautile. We correct the magnetic effects of the submersible and obtain four magnetic anomalies profiles from ridge axis to the Brunhes-Matuyama boundary. We estimate the seafloor magnetization by comparing the deep-sea magnetic anomalies to synthetic anomalies computed with a realistic geometry (topography, submersible path). This estimated magnetization is generally similar to the Natural Remanent Magnetization (NRM) measured on the rock samples that were collected during Nautile dives, suggesting that the seafloor magnetization deduced from the deep-sea magnetic anomalies is reliable. Beyond the NRM measurements, rock magnetic measurements show that the samples are petrologically and magnetically homogenous, with pseudo-single domain tetanomagnetite. Variations within pillow lavas are minimized by coring the samples at a constant distance from the glassy rim. Although varying alteration degrees result in scattered data, the NRM and paleointensity obtained by Thellier method are well correlated and suggest that variations in magnetization are mostly due to the fluctuations of paleointensity. The seafloor magnetization variations obtained across the CIR compares well with the geomagnetic paleointensity curve obtained from sediment cores for the Brunhes period (Guyodo and Valet, Nature 1999). Similar patterns are clearly recognized on both, suggesting that the seafloor can be dated at very high resolution through the geomagnetic intensity variations.
GP23A-0028
Magnetic Properties and Absolute Paleointensity of Upper Oceanic Crust Generated by Superfast Seafloor Spreading, ODP Leg 209.
We investigate the magnetic mineralogy and absolute paleointensity of oceanic basalt samples from Hole 1256D, cored during Ocean Drilling Program (ODP) Leg 206. Hole 1256D is located on the Cocos Plate about 5 km east of the transition zone between marine magnetic anomalies 5Bn.2n and 5Br (~15 Ma). During Leg 206, the hole penetrated 502 m into basalts of the upper oceanic crust that was generated by superfast seafloor spreading (>200 mm/yr) along the East Pacific Rise. Rock magnetic investigations included continuous low field (k-T) thermomagnetic analyses, alternating field (AF) and thermal demagnetization, optical microscopy, saturation isothermal remanent magnetization (SIRM), and magnetic grain size analyses. Following the removal of a drilling overprint, AF and thermal demagnetization paths for most samples decay linearly to the origin on orthogonal vector end point diagrams, suggesting that a stable characteristic remanent magnetization component can be resolved. Optical microscopy and k-T (Curie points) identified titanomagnetites and titanomaghemites as the main magnetic carriers and grain size studies indicate that the carriers are either single domain (SD) and/or pseudosingle domain (PSD) in nature. Using the modified Thellier-Coe double heating method, we determined absolute paleointensity determinations for 51 specimens sampled from different ''stratigraphic'' levels of the core. pTRM checks were performed systematically one temperature step down the last pTRM acquisition in order to document magnetomineralogical changes during heating. The temperature was incremented by steps of 50°C between room temperature and 500°C and every 25-30°C for higher temperatures. The paleointensity determinations were obtained from the slope of the Arai diagrams. Special care was taken to interpret the Arai diagrams within the same range of temperatures lower than 300°C unless a clear and unique slope was present over a higher range of temperatures. Only about 10 percent of the samples yielded acceptable results. The paleofield estimated from these samples ranges between 28 to 16 micro Teslas (i.e., VADM of 6 to 4 x $10^{22} A/m^{2}$), which is concordant with the average paleofield intensity for the period between 0-160 Myr (4 ± 2 x$10^{22} A/m^{2}$) and half of the strength of the present field (~8x$10^{22} A/m^{2}$).
GP23A-0029
The Cretaceous Quiet Zone: How Quiet?
Geomagnetic field intensity variations deduced from magnetostratigraphic data are mainly restricted to the past few million years. As a consequence, many important questions, such as the long-term evolution of the geomagnetic field intensity and the temporal distribution of excursions, remain unsolved yet. The possibility to recover these fluctuations over a long time interval from marine magnetic anomalies is therefore of particular interest. Within the period documented by these anomalies, the Cretaceous Normal Superchron (CNS) presents a major interest in geomagnetism as little is known on the characteristics of the geomagnetic field during this event, except that it apparently did not reverse for about 35 Myr. Cruise Magofond 3 of R/V Suroit (July-August 2005) was dedicated to the CNS, with a target area on the Cretaceous Quiet Zone off Western Africa, on the eastern flank of the Mid-Atlantic Ridge. Both sea-surface and deep tow magnetic anomaly profiles have been collected. Magnetic observatory data from M'Bour (Senegal) and Guimar (Canary Islands) have been regularly transmitted to the ship in order to check for any external magnetic field disturbance which may have affected the data. In addition, seismic reflection data were acquired to insure that the observed anomalies are not caused by the basement topography and, eventually, to estimate and correct such an effect. Altogether, these data brings new constraints on the variability of the geomagnetic field during the superchron. As a preliminary result, they show the occurrence of several consistent short-wavelength magnetic anomalies which may be linked either to short reversed polarity intervals or to excursions. In particular, the ISEA reversed polarity subchron at the beginning of the CNS seems to be present in most profiles. Other observed anomalies may also depict subchrons and would therefore challenge the concept of a non-reversing geodynamo during the exceptionally long CNS.
GP23A-0030
Magnetic Anomalies Between 35 N and 55 N in the North Atlantic: Identification and Implications
In this work we present a new approach to the detailed identification and interpretation of the magnetic isochrones 5, 6, 13, 18, 20, 22, 22, 23, 24, 25, 26, 28, 29, 30, 32, 33 and 33r on the Eurasian as well as on the American plate, between 55° N and 35° N in the North Atlantic. The identifications were based on a continuous reduction to the pole new technique. From the detailed reconstruction of magnetic isochrones and flow lines, we show that the East Azores Fracture Zone cannot be considered as homologous to the Pico Fracture Zone, implying that a significant amount of the Azores plateau was formed as an independent lithospheric band accreted to the Eurasian plate between the time of anomaly 22 (49 Ma) and ~5 (10 Ma). Based on magnetic anomalies and morphological constrains we reconstruct the previous configurations of the Azores plateau. In particular, we extend a previous study of the Mid-Atlantic Ridge (MAR) for the period 0-10 Ma to a wider time interval into a schematic evolution model for the Azores Triple Junction area. The new isochrones identifications and computation of Euler rotation poles have also implications in the movement of the ancient Iberia plate. Contrary to the currently accepted belief that its movement with respect to the Eurasia plate ended about the time of anomaly 6 (20 Ma), we found no evidences of any movement after the time of anomaly 18 (~38 Ma).
GP23A-0031
Revised Late Cretaceous and Early Cenozoic Apparent Polar Wander Path for the Pacific Plate
The current apparent polar wander path (APWP) of the Pacific plate suffers from a general lack of detail and has been calculated using some data with questionable reliability. This is especially true of the data set for the Late Cretaceous and Cenozoic which has come largely from seamount anomaly inversions and seafloor magnetic anomaly skewness. In an effort to increase the detail and reliability of the Pacific plate APWP, we used a larger and more diverse data set to calculate four mean poles for the latest Cretaceous and Paleogene. We combined four types of data in order to test data reliability and consistency, and found good agreement among different data types. Over half of the data comes from piston and DSDP/ODP sediment core paleocolatitudes, with the rest made up of paleocolatitudes from DSDP/ODP basalt cores, declinations from seamount anomaly inversions and effective inclinations from magnetic anomaly skewness analyses. Our four mean paleomagnetic poles represent the Oligocene, Eocene, Paleocene and Maastrichtian at 29, 44, 61 and 69 Ma, respectively. The 29 Ma pole is located at $80.1° N, $24.4° E, the 44 Ma pole at $74.4° N, $356.0° E, the 61 Ma pole at $72.2° N, $5.8° E, and the 69 Ma pole at $72.3° N, $355.7° E. The large numbers of data included in this compilation allow for reasonably compact error bounds and the good agreement between data types implies small systematic error. Although a significant percentage of the data are from azimuthally-Unoriented cores, which do not provide constraint on paleodeclination, a wide longitudinal distribution of sites, as well as the use of declinations from seamount anomaly inversions gave reasonably good control on pole paleolongitude. While the new APWP exhibits the expected northward motion of the Pacific plate, it also shows a stillstand from the Late Cretaceous until approximately 44 Ma. This stillstand suggests no northward motion of the Pacific plate during this time, a concept at odds with accepted models of Pacific plate motion. The APWP is consistent with DSDP/ODP basalt paleomagnetic data that suggest the Hawaiian-Emperor hotspot moved south during formation of the Emperor Chain, but it implies an amount of motion greater than $20°, if the entire latitude shift is attributed to hotspot drift.
GP23A-0032
A Revised Cretaceous-Cenozoic Apparent Polar Wander Path for the Pacific Plate and its Tectonic Implications
We have revised calculated pole positions of the Pacific apparent polar wander path (APWP) for the Cretaceous and Cenozoic. Previous poles on the Pacific APWP have been constrained primarily by seamount magnetic anomaly inversions and skewness of seafloor spreading magnetic anomalies, both with shortcomings related to interpretation of magnetic anomalies. We compiled a diverse data set consisting of paleocolatitudes determined from sediment piston cores, sediment and basalt cores from ocean drilling, paleodeclinations from seamount magnetic anomalyinversions, and effective inclinations from magnetic lineation skewness. By combining several types of data, potential bias by any one type is reduced. The APWP shows ~40$^{o}$ of northward movement since Early Cretaceous (~123 Ma) time. Jurassic data are few and suggest southward APW prior to an Early Cretaceous turnaround. Although the timing of the turnaround is poorly constrained, it appears coincident with the timing of Ontong Java Plateau formation, suggesting a link between the two events. Interestingly, a paleomagnetic pole for sites on the Ontong Java Plateau is significantly different from the pole calculated from contemporaneous data elsewhere on the Pacific plate. The difference implies that the plateau is on lithosphere that has moved north ~15$^{o}$ less than the rest of the plate and suggests parts of the Cretaceous Quiet Zone Pacific plate may have been amalgamated from surrounding plates. Contrary to prior interpretations, the APWP does not show an Early to mid-Cretaceous stillstand, but slow northward polar motion until ~92 Ma instead. APW between ~92-83 Ma is rapid, coincident with the polar shift described by Sager and Koppers [Science, v. 287, p. 455, 2000]. Although the rapid shift ended by Chron 33r (~83 Ma), the beginning is poorly constrained by sparse data and thus the drift rate is uncertain. From Chron 33r to ~44 Ma, there is a polar stillstand, and afterwards the poles drift northward to the spin axis. The Late Cretaceous to early Cenozoic stillstand is curious in that it corresponds almost exactly to the duration of formation of the Emperor Seamounts, suggesting a link. Rotated into the Antarctic plate reference frame, Late Cretaceous-Cenozoic Pacific poles show a ~10$^{o}$ offset from the Antarctic APWP, implying problems with the Pacific-Antarctic plate motion circuit. The stillstand implies no northward movement of the Pacific plate for ~35 Myr, at odds with accepted models of plate motion containing significant northward drift at this time. The APW reinforces conclusions that the Emperor Chain melting anomaly moved south (e.g., Tarduno et al., Science, v. 301, p. 1064, 2003), but our results imply greater overall motion (>20$^{o}$). Data from the Pacific alone do not allow us to say how the southward motion occurred; whether it was entirely through melting anomaly motion or whether other phenomena, such as true polar wander or changing non-dipole components in the geomagnetic field also played a role.
GP23A-0033
Paleomagnetism of the Marble Bar Chert Member, Western Australia: implications for an Apparent Polar Wander Path for Pilbara craton during Archean
aleomagnetic study is conducted on the early Archean Marble Bar Chert Member (3454 - 3471 Ma) in Pilbara craton, northwestern Australia, to reconstruct an apparent polar wander path (APWP) for this craton and understand for geodynamic feature during early Archean time. The Archean Biosphere Drilling Project (ABDP) drilled a continuous 270 m long oriented core from the Towers Formation, includes the Marble Bar Chert Member. Stepwise thermal demagnetization (ThD) for 431 discrete specimens, extracted from 158.5 to 182.0 m in depth of the drilled core, revealed two distinct magnetic components (LT and MT). The MT component is divided into two depth groups, which provide the mean paleomagnetic directions of MB1 and MB2. The MB1, MB2, and published paleomagnetic poles of early Archean rocks from the Pilbara craton displays continuous shift of the paleomagnetic poles along the stratigraphic sequence of them. This indicates the MT component preserving the primary magnetization acquired during sedimentation. Based on the result of this study and the other published paleomagnetic poles, the simplest early to late Archean APWP for the Pilbara craton is reconstructed. The fast shift of the paleomagnetic poles was observed between MB1 and MB2, which corresponds to the 4.1° of paleolatitude change and approximately 460 km of latitudinal drift of the Pilbara craton. The estimated speed of the lateral drift is 13.6 -129 cm/y, which is significantly large compared with current plate motion velocities. Plate tectonic process, relating the anomalous active convection of the mantle during Archean time, likely explains the significant fast lateral drift of the Pilbara craton. On the other hand, true polar wander (TPW) also can explain the significant paleolatitude change. Although, an angular velocity of TPW estimated as 1.0° - 9.4°/Myr is extremely lager that reported velocity of the TPW (0.5°/Myr), inertial interchange TPW might explain the fast lateral drift of the Pilbara craton.
GP23A-0034
High Resolution Apparent Polar Wander Path from Kimmeridgian-Aptian Umbro-Marchean Sections (Northern Apennines, Italy)
A new high resolution apparent polar wander path (APWP) segment have been obtained from four Kimmeridgian to lower Aptian Northern Apennines (Italy) sections. Apennines are a fold and thrust belt developed during Neogene in response to the convergence between Africa and Europe and are formed by Meso-Cenozoic sedimentary succession, deposited along the Tethyan margin of Adria. On the basis of paleomagnetic data, it has been shown that Adria is a promontory of the African plate since at least Permian times. Whereas paleomagnetic data from Africa are of poor quality for this period, we used data from Adria to improve quality of the APWP in the 150-120 Ma range. We collected 861 samples from the four sections, with a sampling space varying from 30 to 50 cm. The cores have been measured in the paleomagnetic laboratories of the Istituto Nazionale di Geofisica e Vulcanologia of Rome and of the Institut de Physique du Globe of Paris by 2G DC-SQUID cryogenic magnetometers. Samples were cleaned both using thermal and alternating field demagnetization, depending on their lithologies and on their paleomagnetic behavior. The sections show a detailed record of polarity chrons from M21n to M14 and from M9 to M0, however with a magnetostratigraphic recording gap between M14n to M11. The magnetic stratigraphy obtained is consistent with the sequence of polarity chrons as inferred from oceanic magnetic anomaly analysis. We investigated the evolution of directions vs. time. The use of the paleomagnetic data for determination of APWP is hampered by the large local tectonic rotations linked to the Apennines tectonics. To solve this problem, we computed relative rotation between sections and realigned them in a common declination reference frame. We synthesized a new high resolution 150 to 120 Ma APWP for Adria and compared it with similar age APWP segment for Africa. The shape of the new segment is very similar to the time-equivalent segment of the synthetic APWP of Besse & Courtillot (2002). We obtained a perfect overlap between the two APWP segments by a $28° counterclockwise (CCW) rotation of our common frame section. This overlap confirms that Adria promontory moved coherently with Africa during this time span, whereas the rotations were introduced during Apenninic orogenesis. Finally, we address the geometry of the earth magnetic field for this important time interval, and discuss, in relation with worldwide plate evolution, the peculiar shape of our APWP, which displays a hairpin turn during Berriasian with absolutely no standstill.
GP23A-0035
Paleomagnetism of Spanish Peaks, Silver Mountain, Associated Dike Swarms and Related Intrusions (South-Central Colorado): Refining the mid-Cenozoic Reference Paleomagnetic Pole
Exposed in the northernmost Raton Basin, the late Eocene to mid-Miocene Spanish Peaks igneous complex includes numerous discrete intrusions, with considerable temporal, spatial, chemical and thickness variations. The volcanic center includes a northern radial dike swarm (Silver Mountain), a set of N80E trending 10+ km long dikes, radial dikes centered on the Spanish Peaks intrusions, and a set of dikes, sills and stocks emplaced parallel to tilted strata along the eastern Sangre de Cristo Mountains. We have collected intrusions at 138 sites and have completed AF and thermal demagnetization for 95 of the sites, with 49 sites to date yielding highest quality, internally consistent and interpretable results. Of the remaining 46, many need remagnetization circle analysis, some have dispersed directions within-site, others have low within-site dispersion but deviate significantly from typical dipole directions, and a few are interpreted to be lightning-struck. The 36 normal polarity sites yield a mean of (Decl. = $010°, Incl. = $62°, α95$ = $3°, k = 55) and 13 reverse polarity sites yield a mean of (Decl. = $183°, Incl. = $-53°, α95$ = $7°, k = 38). The grand mean for 49 sites is (Decl. = $008°, Incl. = $59°, α95$ = $3°, k = 44). Previous work on some of these rocks by Larson and Strangway (JGR, 1969) resulted in a grand mean (Decl. = $351°, Incl. = $6°, α95$ = $13°, k = 27) based on only five sites, all of normal polarity. Our data set passes the McFadden reversals test with a B classification. The VGP grand mean is (81.5N / 307.4E, A95 = $5°, K = 20.5, ASD = 17.9). The current dataset deviates from previously reported estimates of a mid-Cenozoic field direction for North America. Permissible, although not verified, explanations for this discrepancy include incomplete sampling of paleosecular variation or a very modest clockwise rotation of the region. Rock magnetic experiments completed on a representative set of the intrusions indicate for the reliable sites that a geologically stable magnetization is dominantly carried by a mixture of multi-domain and single-domain magnetite.