U33A-0020 1340h
Four Centuries of the Geocentric Axial Dipole Hypothesis
William Gilbert first articulated what has come to be known as the geocentric axial dipole hypothesis. The GAD hypothesis is the principle on which paleogeographic reconstructions rely to constrain paleolatitude. For decades there have been calls for permanent non-dipole contributions to the time averaged field. Recently, these have demanded large contributions of the axial octupole, which, if valid, would call into question the general utility of the GAD hypothesis. In the process of geological recording of the geomagnetic field, ``Earth filters'' distort the directions. Many processes, for example, sedimentary inclination error and random tilting lead to a net shallowing of the observed direction. Therefore inclinations that are shallower than expected from GAD can be explained by recording biases, northward transport, or non-dipole geomagnetic fields. Using paleomagnetic data from the last five million years from well constrained lava flow data allows the construction of a statistical geomagnetic field model. Such a model can predict not only the average expected direction for a given latitude, but also the shape of the distribution of directions produced by secular variation. This allows us to differentiate among the possible explanations for shallow bias. We find no compelling reason to abandon the geocentric dipole hypothesis that has served us well for four centuries.
U33A-0021 1340h
Corrected Paleolatitudes for Pangea in the Early Mesozoic
A series of continental basins that developed during rifting of the Pangea supercontinent in the early Mesozoic are now distributed along the margins of the North Atlantic and their preserved contents (mainly redbeds and CAMP basalts) have often been targets of paleomagnetic studies. A continuous record of paleolatitudinal drift and a geomagnetic polarity time scale for ~35 Myr of the Late Triassic and earliest Jurassic have been derived from several of the basins in eastern North America and provide a precise spatio-temporal framework for detailed paleogeographic analysis. However, reported paleomagnetic directions from Jameson Land in East Greenland are anomalously shallow with respect to coeval sections in North America, a discrepancy that is too large to be explained by uncertainties in the reconstruction of Greenland to North America. Therefore, either the magnetizations of the Jameson Land (and perhaps other early Mesozoic rift basin) sediments are biased by inclination error or the Late Triassic time-averaged field included significant nondipole (axial octupole) contributions. According to a new statistical geomagnetic field model (Tauxe and Kent, 2004) constrained by paleomagnetic data from young lava flows, these two phenomena result in very different distributions of paleomagnetic directions, providing a basis to diagnose and correct for inclination error in sufficiently large paleomagnetic datasets. The resulting congruence of independent data from sedimentary and igneous rocks ranging over thousands of kilometers and 10s of millions of years can be taken as strong support that a geocentric axial dipole field similar to the last 5 Myr was operative more than 200 Myr ago. The corrected paleolatitudes indicate a faster rate of poleward motion of this sector of Pangea and broader continental climate belts in the Late Triassic and earliest Jurassic.
U33A-0022 1340h
The Tarim APWP Paradox
Central Asia boasts one of the world's densest regions sampled for paleomagnetic data thanks to numerous sections of well-exposed rocks possessing stable remanent magnetizations. Such is the case for the Tarim craton, which is represented by a quasi-continuous time sequence of paleomagnetic poles since the Permo-Carboniferous. Most of these poles are derived from studies demonstrating positive fold and-or reversal tests, with N equal to or greater than 6 sites or 50 samples. Samples collected from Permo-Carboniferous rocks usually have reverse polarities, and samples collected from Cretaceous rocks usually have normal polarities, consistent with the geomagnetic polarity time scale. Despite the apparent excellent quality of the paleomagnetic data from Tarim, they impose geologically unrealistic tectonic displacements when compared to the Eurasian and-or Indian APWPs. This leads to the Tarim APWP paradox: is there a problem (inclination shallowing, overprinting, etc.) with the plentiful Tarim data, or is the Eurasian APWP not representative of the land east of the Ural Mountains? If the latter is true, then previous tectonic reconstructions must be reconsidered. If the former is true, then when/how can we rely on the paleomagnetic data? We present arguments showing that both scenarios have their pros and cons.
U33A-0023 1340h
Detecting Non-Geocentric Axial Dipole Structure in the Time-Averaged Field
The approximation of Earth's magnetic field by a geocentric axial dipole (GAD) is central to applications of paleomagnetism in global tectonics. Quantifying non-GAD contributions to the geomagnetic field is not only important because of the consequences for tectonic studies, but is essential to understanding the role of inner core growth and core-mantle boundary influences on field generation. Significant departures from GAD, in particular those that can be represented by a zonal octupole field, have been suggested over long time periods (10$^8$ -- 10$^9$ yr) prior to 250 Myr ago. However, biased estimates of the time-averaged field direction are obtained from unit vectors (i.e., in the absence of paleointensity data); deviations of inclination from that predicted by a GAD field are well approximated by a zonal octupole contribution. Simulations from statistical models for paleosecular variation that match 0--5 Myr paleodirection and paleointensity data indicate that the biases in predicted inclinations are on the order of 2$^o$. Increased secular variation would result in larger biases, so this effect may be important during periods of low paleointensity and/or earlier in Earth's history when the inner core was smaller. Over shorter time intervals (10$^6$ yr), the time-averaged field shows smaller, but observable departures from GAD. Although a zonal quadrupole contribution has been considered robust, proposed longitudinal (non-zonal) structure in the time-averaged field has been challenged on the grounds of inadequate spatial and temporal data coverage, data quality, and contamination by local tectonic effects. It is now possible to compile regional data sets comprising paleodirection and occasionally paleointensity data from tens to hundreds of sites, and spanning the period 0--5 Ma. These include Hawaii, Reunion, Japan, French Polynesia, New Zealand, and North America. In addition, a recent sampling program has focused on obtaining paleomagnetic data from 0--5 Ma lava flows from previously under-sampled high latitude and the southern hemisphere regions. New time-averaged field models constructed from 0--5 Ma normal polarity data have improved data coverage in the southern hemisphere and suggest the presence of southern hemisphere flux lobes, undetectable with previous data sets. We use 0--5 Ma data sets, new time-averaged field models, and statistical models to examine the conditions required regionally and globally to detect non-GAD field structure.
U33A-0024 1340h
Rapid true polar wander: A quixotic search?
Studies in the 1960's emphasized that polar wander could be a significant component of apparent polar wander curves. These concerns were assuaged when subsequent tests revealed that polar wander components were very small and overwhelmed by plate motion. Nevertheless, the concept of large and rapid polar wander has been surprisingly resilient. It gained new support in the 1980's when polar wander was defined in a fixed hotspot frame of reference and called ``true polar wander" (TPW). This new TPW was correlated to a wide variety of phenomena, from the rate of geomagnetic reversals to anomalous volcanism, but the linkages often differed sharply between studies. And renewed investigations, including paleomagnetic studies of the Emperor seamounts (ODP Leg 197), have emphasized that hotspot drift can be relatively rapid (over 40 mm/yr). We trace problems in some past and recent TPW hypotheses to an overreliance on the hotspot reference frame. The failure to scrutinize independently the two key components (APWPs and hotspot tracks) in directional space has led to illusory short-term spurts of TPW and overestimates of long-term rates. We review tests that can be used to detect these artifacts. The exclusion of hotspot drift reduces total ``TPW" displacements so that they are near the level of other competing explanations that might account for variation in global paleomagnetic pole positions (e.g. reconstruction uncertainities and inadequate geomagnetic field averaging). This vanishingly small amount of polar wander suggests that locally developing mantle mass heterogeneities may have been naturally balanced by global mantle flow in the post-Jurassic Earth.
U33A-0025 1340h
Theoretical Constraints on True Polar Wander
It is now standard to attribute most of the apparent polar motion to continental drift caused by plate tectonics. However, an important component of this apparent motion is the true motion of the pole relative to geographic axes, i.e. true polar wander (TPW). For the present geological epoch, long term TPW is small compared to APW, but simple theoretical considerations suggest that it could have been large(r) in other epochs and could have been responsible for 90$\deg$ inertial interchange events. In this work, we use a simple Maxwell model to analytically describe how changes in mass anomalies translate into TPW. Unlike previous work, our goal is to derive simple analytical estimates of TPW based on the characteristic amplitudes and timescales for changes in the moment of inertia. We find estimates for both the amplitude and speed of TPW as a function of various Earth properties. Our analysis shows that there are four variables of primary importance: the (geological) timescale of the forcing $\tau$$_{force}$, the viscosity structure of the Earth which yields a combined viscous relaxation time $\tau$$_{relax}$, the characteristic amplitude of the non-hydrostatic changes in moment of inertia $\Delta$C$_{dyn}$, and the added moment of inertia due to the equatorial bulge $\Delta$C$_{rot}$. Although the amplitude of the forcing is relatively small, substantial TPW arises because the viscous relaxation time is small compared to geological time scales. However, the maximum velocity of TPW is not sensitive to the geologic timescale although the total reorientation and the TPW acceleration do depend on this timescale. We find that the maximum TPW speed is (9$\deg$ per million years). ($\Delta$C$_{dyn}$/0.003$\Delta$C$_{rot}$).(1000 yrs/$\tau$$_{relax}$). For a model where $\tau$$_{force}$=10$^{8}$ years, and an average mantle viscosity of 10$^{22}$ Pa.s (higher than standard upper mantle estimates), and $\Delta$C$_{dyn}$/ $\Delta$C$_{rot}$ = 0.003, we obtain a maximum TPW angle of 89.6 degrees. This simple approach allows us to assess whether multiple TPW events are possible but the major uncertainty continues to be the mantle viscosity structure and layering.
U33A-0026 1340h
Rapid, Coherent Apparent Polar Wander in the Terminal Neoproterozoic - Early Cambrian?
Rapid apparent polar wander (APW) has been documented for several continents in the Precambrian-Cambrian transition period. Recent paleomagnetic results from Laurentia and Baltica confirm that rapid and coherent APW occurred for these continents, taking them from the polar region at ca. 580 Ma to near-equatorial latitudes by the start of the Cambrian. Similarly large (~90 degree) Vendian-Cambrian APW tracks for Siberia and West Gondwana are evident, although they are not as well constrained, especially in age. Coeval results from Australia show no APW, however. The rapid latitudinal shift for Baltica, Laurentia and perhaps Amazonia occurred at rates in excess of 20 cm/yr, suggesting that these continents were drifting on the same fast-moving plate and/or were recording significant true polar wander (TPW) during the terminal Neoproterozoic. In the Quebec-New England region of Laurentia, 570-550 Ma rift magmatism shows evidence of having had a plume source, which, assuming the fixity of plumes may indicate that TPW was the major contributor to the observed large and rapid motion of Laurentia during that period. Conversely, no record of TPW is present in the Australian data. Regardless of the magnitude of TPW contribution, the long APW tracks of Laurentia, Baltica, Siberia and perhaps West Gondwana appear to be a robust feature of the terminal Neoproterozoic.
U33A-0027 1340h
Bending or buckling of a Brasiliano belt: a paleomagnetic investigation of the curvature of the Paraguai belt, SW Amazon craton, central Brazil, with implications for Neoproterozoic/Cambrian tectonics of West Gondwana
The Paraguai belt (SW Brazil), which marks the Neoproterozoic limit of the SE Amazon craton, displays ca. 100 degrees of curvature along its ca. 1200 km extent. The origin of this arcuate shape could be primary, reflecting the original geometry of the post-Rodinia cratonic margin (reentrant); or secondary, resulting from regional-scale, crustal rotations (orocline) during late Neoproterozoic/Cambrian deformation. A preliminary paleomagnetic study of the Neoproterozoic Araras Fm, which lies in the middle section of the cap carbonate sequence, was undertaken along different strike domains in order to ascertain the primary or secondary origin of the curvature. Thermal demagnetization reveals a stable magnetic component with a moderately steep, downward direction, carried by pyrrhotite and/or magnetite. A negative result for a fold test, carried out on flat-lying rocks of the undeformed craton and steeply tilted beds of the adjacent fold-and-thrust belt, demonstrates that this magnetization is of a secondary origin. However, the covariance of the declination of this magnetic component with regional strike is noteworthy. Although this post-folding direction may have been acquired at different times by different portions of the belt during rotation of the Amazon craton as a whole, we favor a simpler explanation. We propose that the magnetization was acquired synchronously along the belt between two discrete episodes of deformation. In this scenario, the second deformational phase gave rise to the arcuate structure of the Paraguai belt, accompanied by the passive rotation of the observed paleomagnetic direction.. Thus, we suggest that the curvature of the Paraguai belt is the result of either: a) buckling of an initally straight belt during the accretionary history of the Panthalassan margin of W. Gondwana, or b) localized bending due to the indentation of the Paraguai belt during the assembly of W. Gondwana. In addition, paleomagnetic poles calculated from individual sites constrain the age of deformation in this portion of W. Gondwana to be younger than middle Cambrian (ca. 520 Ma).
U33A-0028 1340h
Paleomagnetically Testing the Integrity of South America
The Falkland Plateau is, physiographically, one of the most unusual pieces of continental crust extending some 1500 miles eastward from South America into the South Atlantic. The Falkland Islands/Malvinas have been shown palaeomagnetically to have undergone a very substantial (~120$^{o}$) Jurassic-Early Cretaceous clockwise rotation (Mitchell et al. 1986; Taylor & Shaw 1989; Randall pers comm. 1999) and this was part of a more general rotation of microplates during the break-up of Gondwana. More recently paleomagnetic studies of Jurassic and Cretaceous units in the North Patagonian Massif of Argentina, associated with the Gastre Fault system, the postulated onshore continuation of the Falkland Fracture zone, have shown 25-30$^{o}$ of clockwise rotation occurred in latest Jurassic to earliest Cretaceous times (Geuna et al. 2000). This led us to investigate the pre-Jurassic paleomagnetic record of the Deseado Massif, the closest continental region to the Malvinas/Falkland Islands, to determine whether it too records a Jurassic-Cretaceous rotation history. We present results from a range of Late Triassic-Jurassic units, both sedimentary and intrusive, which suggest the occurrence of clockwise vertical-axis rotations in Mesozoic localities from the Deseado Massif comparable in both magnitude and sense to those in the North Patagonian Massif but considerably smaller than those in the Falklands/Malvinas. This latter contrast suggests that the intervening region between the Deseado Massif and the islands, in the present Argentine shelf, would have been a zone of major structural discontinuity although offshore exploration has yet to identify any such zone. Alternatively it may be a zone of distributed deformation with rotation decreasing eastward. In either case the observed rotations in all these areas were roughly coeval. The continuity of rotations within Patagonia requires further clarification and investigation to determine whether there was a distinct southern Patagonian microplate.
U33A-0029 1340h
New Tertiary Paleomagnetic Poles at 13 and 30 Ma from Mongolia: Clues on the Inclination Shallowing Problem in Central Asia
Construction of APWPs of different Asian blocks has been particularly active in the last past 20 years, in order to unravel the complex history of the India-Asia collision. The APWPs are valid only inasmuch they represent true primary magnetization. Cenozoic samples from Asia have revealed potential problem of inclination shallowing in sediments. It is therefore important to attempt to estimate paleodirections from lava flows. We present new paleomagnetic results obtained at 24 sampling sites (171 cores) from Tertiary basaltic and trachy-basaltic lava flows collected in 1999 in Taatsyn Gol (TG) region, Mongolia (45.3$^{o}$N, 101.1$^{o}$E). Rock magnetism experiments and thermal demagnetizations allowed us to isolate a high temperature component (HTC) carried by magnetite. HTC directions of the eight sites corresponding to eight distinct flows from TG1 locality (13 Ma) cluster after bedding correction (ks/kg=19.5/10.1), but the fold test remains inconclusive. Sixteen sites from the TG2-3 locality (30 Ma) occur as distinct horizontal flows. Assuming that HTCs from the two localities represent the primary magnetization of the basaltic lava flows, we computed the corresponding Tertiary paleopoles from the two distinct HTC mean directions. These poles lie at $\lambda$=74.3$^{o}$N, $\phi$=161.0$^{o}$E, dp/dm=14.4/17.1 at 13 Ma, and $\lambda$=83.7$^{o}$N, $\phi$=271.3$^{o}$E, dp/dm=3.7/5.0 at 30 Ma. Whereas the pole at 13 Ma is fairly consistent with that of the reference APWP for Eurasia (Besse and Courtillot, 2002), the 30 Ma pole appears far-sided with respect to the corresponding reference pole, arising from a shallowing of paleomagnetic inclination with respect to the predicted one. The discrepancy amounts to 9.8$^{o} \pm 4.2^{o}$ in paleolatitude, which would imply a post Oligocene (30 Ma) convergence of more than 500 km between Mongolia and Siberia. This is unrealistic, as there is no deformed zone north of Mongolia that could have absorbed such motion. Another possibility is that the 30 Ma pole may not have fully averaged paleosecular variation. In order to try and fix this problem, we have performed a complementary sampling of 13 sites (101 cores) of 30Ma basalts from Taatsyn Gol in the summer of 2004; the first results will be presented.
U33A-0030 1340h
The Position of Baltica During 700-450 Ma Ago: New Data from the Janisjarvi Impact Structure
Paleomagnetic reconstructions of continents are often hampered by the lack of dated poles for key intervals. Impact rocks can occasionally provide a solution to the problem since they can be accurately dated and they often yield reliable paleomagnetic data. We present new paleomagnetic and rock magnetic data for Baltica derived from the Janisjarvi impact structure, Russian Karelia, the southeastern part of Fennoscandian Shield. Isotopic datings (40Ar-39Ar, 40K-40Ar) yield an age $\sim$ 700 Ma for this impact event. Some 50 oriented hand samples were collected from the Janisjarvi impact structure. The a.f. demagnetization is able to isolate a hard and stable remanence component in the impactites (tagamites, suevites and breccias). The same component was also identified in several Svecofennian ($\sim$ 1880 Ma) target rocks on and near the shoreline of Lake Janisjarvi. The target rocks retained also a primary Svecofennian component thus providing a fully positive impact test. Demagnetization data, coupled with rock magnetic results, suggest that the impact component, of dual polarity, is primary and either of thermal or thermochemical origin. The remanent magnetization direction from 16 sites is D = 74\deg, I = 72\deg (alfa95 = 2.1\deg). This yields a pole position Plat. = 54\deg N, Plon. = 96\deg E (A95 = 3.5\deg) which places Baltica to 60°S paleolatitude at 700 Ma. When plotting the pole on the calibrated APWP of Baltica a magnetization age of 500-478 Ma is obtained. Five possible reasons may explain the conflicting isotopic and paleomagnetic ages: 1. post-impact tilting of the impactites, 2. remagnetization, 3. error in the isotopic ages, 4. error in the APWP, or 5. inaccurate pole. The Late Precambrian to Lower Ordovician reconstructions of Baltica based on the new data from the Janisjarvi impact structure will be discussed.
U33A-0031 1340h
Toward Defining Archean Plate Motion
Paleomagnetic data from previous paleomagnetic studies of mid-Archean rocks ($\sim$3.0-3.5 Ga) have been interpreted as reflecting a plate tectonic style similar to that of the Cretaceous to recent. Times of rather slow motion (a few centimeters per year) are punctuated by very rapid motion (over 14 cm/yr). However, prior results have been based on whole rock samples, which have been subjected to prolonged low grade metamorphic conditions. In some rocks (mafic lavas and sedimentary rocks) this has resulted in a crystallization remanence of unknown age. In others (plutonic rocks) it has the potential to produce a complex history of overprinting. Here we revisit these issues by approaching the paleomagnetic study at a mineral scale, following our prior work and studies in the 1960's and 1970's indicating the presence of magnetic inclusions in silicate minerals. In rock magnetic investigations we find that hornblende carries a multidomain signal and therefore we expect it to record magnetic overprints. In contrast, quartz and microcline yield single domain to pseudosingle domain behavior, suggesting that the magnetic inclusions they contain could preserve primary magnetizations. To explore the paleomagnetic signals of these crystals, we are developing a CO$_{2}$ laser/SQUID magnetometer experimental approach which allows us to obtain directional and paleointensity information from oriented single mineral grains. Preliminary data from 3.2 Ga plutons of the Kaapvaal craton (southern Africa) suggest that prior indications of very high plate velocities may be premature. We will discuss alternative interpretations, including overprinting of whole rock samples and the possibility of mid-Archean non-dipolar fields.
U33A-0032 1340h
Paleomagnetic Data From Ontong Java Plateau are Anomalous $\sim$ Did the Plateau Form on Another Plate?
A recent study of Ocean Drilling Program basalt core paleomagnetic data from Ontong Java Plateau (OJP) found paleolatitudes that disagree with previous estimates of the Early Cretaceous Pacific APWP, a result attributed to poor quality of data used in prior pole calculations [Riisager, P., S. Hall, M. Antretter, X. Zhao, Earth Planet. Sci. Lett., 208, p. 235, 2003]. My compilation of paleomagnetic data from Cretaceous Pacific basalt cores drilled by the Deep Sea Drilling Project and Ocean Drilling Program shows that paleocolatitude data in of ages 118-129 Ma display greater scatter than other age bins. The only factor that allows this data group to be coherently subdivided is whether or not the coring site is located on OJP. Without OJP data, paleocolatitude scatter is much less and gives a similar pole position (48.9$\deg$N, 327.1$\deg$E, N=40) to data in the 110-118 Ma interval. Data from the plateau give a pole that is 15$\deg$ farther north (64.9$\deg$N, 323.4$\deg$E; N=37), indistinguishable from late Jurassic and earliest Cretaceous skewness poles. The OJP and non-OJP poles are distinct at the 95$%$ confidence level despite having indistinguishable mean ages of 121.6 $\pm$1.1 Ma (OJP) and 123.4 $\pm$4.1 (non-OJP). Because Ontong Java Plateau data come from 6 different sites spread over the northern plateau, tectonic tilting is not a likely explanation for the difference. Also unlikely are systematic errors such as incomplete averaging of secular variation (large number of independent magnetic units sampled), inaccurate radiometric dates (many high quality dates), or inadequate paleomagnetic techniques (detailed studies by several different investigators). Rapid true polar wander does not seem a plausible explanation because global true polar wander curves have a different trend. Perhaps the simplest explanation is the one often used when anomalous data are found within a plate: the anomalous region had a different history of tectonic drift. In this scenario, OJP formed on a separate plate that drifted southward relative to the Pacific plate before becoming attached. This finding is similar to the Stealth Plate hypothesis [Larson, R. L., and W. W. Sager, Proc. ODP, Sci. Res., 129, p. 471, 1992], which was formulated to explain a similar angular difference between skewness-derived paleolatitudes from the Hawaiian and Japanese magnetic lineations relative to the Phoenix lineations. The most significant challenge for this hypothesis, however, is to account for the space and plate boundaries. If true, this hypothesis implies either that much of the Jurassic Quiet Zone seafloor was created by Pacific-Stealth spreading or that an unknown plate boundary near OJP separated the plateau from the rest of the Pacific plate.
U33A-0033 1340h
Remagnetized Paleopoles: Problems and Applications
Since remagnetized poles were first suspected by Creer and his coworkers in the 1960s, paleomagnetists have made considerable advances in our knowledge of how rocks become remagnetized. Creer published the "remagnetization hypothesis" in 1968, proposing that the formation of secondary iron oxides in red beds as a result of tropical weathering can explain why some paleopoles from early Paleozoic units in North American and Europe were acquired in the late Paleozoic. Subsequent studies have demonstrated that remagnetization is widespread and can be caused by many diagenetic processes such as weathering fluids, basinal/mineralizing fluids, orogenic fluids, hydrocarbons, burial diagenetic processes (e.g., clay diagenesis and maturation of organic matter), as well as other processes. Several geologic controversies such as the origin of magnetization in red beds, the Laurentia/Northern Appalachians megashear hypothesis, and the Cambrian loop hypothesis have also emphasized the importance of recognizing remagnetization. Although some of these geologic hypotheses have been shown to be incorrect because of remagnetization, they were ultimately very useful because they drove research and led to a much better understanding of how rocks become remagnetized. Whereas remagnetization may complicate the interpretation of depositional magnetizations, we now understand that secondary magnetizations can be tangible evidence of chemical and/or thermal events in the diagenetic history of a rock. Paleopoles for such magnetizations, when combined with other information, can provide valuable information on the nature and timing of diagenetic events. Although considerable progress has been made in understanding remagnetization, more work is needed to test different mechanisms and several major issues (e.g., origin of synfolding magnetizations) remain to be completely resolved.
U33A-0034 1340h
The Precambrian of the Kalahari Craton: Apparent Polar Wander Segments, not a Path
The existing Precambrian paleomagnetic data from the Kalahari craton do not define a coherent apparent polar wander path but only delineate disconnected segments, mainly due to the lack of age control. From 2.0 to 1.8 Ga, the APWP crosses northern Africa from east to west, from 1.1 to 1.0 Ga the path crosses northern Africa from north to south, and at the closing of the Precambrian the path tracks across northern Africa from south to north. Thus knowledge of the time when the magnetization was acquired is particularly critical. The Kalahari craton is a key element of the Rodinia supercontinent. Its position relative to Laurentia is very well defined at 1109 Ma, but there are no poles for about 100 Ma prior to and after this date rendering all Rodinia reconstructions ambiguous
U33A-0035 1340h
Testing Inheritance of Nuna Paleogeography Into the Supercontinent Rodinia
Apparent Polar Wander paths based on high quality paleomagnetic poles are critical tools for evaluating paleogeographic reconstructions of Precambrian supercontinents. Recently such data have presented challenges to various reconstructions for the Neoproterozoic supercontinent, Rodinia. In light of these challenges it has become increasingly important to refine our understanding of the positions of two of the largest landmasses making up Rodinia, Laurentia and Australia. Laurentia and Australia, connected in various reconstructions of Rodinia without an intervening Grenville-aged orogenic belt, should have inherited such a connection from an earlier supercontinent, Nuna. Although existing Paleoproterozoic APWPs for Laurentia and Australia have appeared to support this inherited connection, revised ages for key poles in the North American path and lack of field tests on key poles from Australia warrant a recomparison of these APWPs. New data from the Wharton Group of the Dubawnt Supergroup in the Western Churchill Province, Canada, help contribute to a better understanding of the Laurentian APWP. Results from the ca. 1755-Ma Pitz rhyolites and associated conglomerates lend preliminary support to the recently published pole by Irving et al. from the Cleaver dykes in northwest Canada. Additional data from the Thelon sandstone, overlying the Wharton Group unconformably, can provideconstraints on this pole position and the succeeding APWP segment. Future work from the Lochness formation of the northern Australian Lawn Hill platform should refine the comparison between Laurentian and Australian Paleoproterozoic APWPs, and allow for better understanding of the paleogeography of Nuna and its transition to Rodinia.
U33A-0036 1340h
Climate Catastrophe, True Polar Wander, and Inclination Shallowing in the Ediacaran Period
Syntheses of global paleomagnetic data for the Ediacaran Period are difficult to reconcile with standard, uniformitarian plate-tectonic interpretations. Instead, they appear to support a substantial component of true polar wandering (TPW) contributing to each continent's apparent polar wander (APW) path. Construction of magnetostratigraphy-based APWP's has been undertaken in order to quantify the timing and magnitude of putative true polar wander events. If verified, large-scale, multi-episode TPW would establish a second axisymmetric reference frame, about Earth's equatorial minimum inertial axis, suitable for APWP superposition and longitude-controlled paleogeographic reconstruction. In addition to TPW, the terminal Proterozoic interval witnessed repeated episodes of low paleolatitude glaciation. High-resolution magnetostratigraphy of South Australia's Nuccaleena cap dolostone documents three correlatable geomagnetic reversals intimately associated with the solid cap facies. This implies a conservative estimate of 100's kyr duration for postglacial return to "normal" sedimentologic and ocean geochemical regimes. While confirming a low paleolatitude for Marinoan glacial deposits, paleomagnetic inclinations in the Nuccaleena cap dolostone are steeper than those from the underlying, glaciogenic Elatina Formation. Whether due to paleomagnetic compaction shallowing; to geomagnetic low-intensity or non-dipole field contributions; or to rapid APW, demonstration of similar behavior in correlative units across Australia could offer hope of resolving several paleogeographic and geodynamic enigmas that characterize the Ediacaran Period.