U32A-01 10:20h
Fast Apparent Polar Wander in the Phanerozoic: True PW or Fast Drift Rates?
It is well known that in Mesozoic and younger times, continental drift velocities have been generally less than 8 cm/a, although the northward drift of India just before its collision with Tibet was an exception at more than 15 cm/a. In the Paleozoic, however, fast episodes of APW seem more frequent, and in this talk we report on Ordovician rapid south-to-north drift of the North Tien Shan area in Kyrgyzstan. While radiometric age dating of the rocks involved will not be completed until mid-2005, the presently existing age constraints and the observed changes in paleolatitude imply velocities approaching 20 cm/a, given that paleolatitudes change from 71 degrees in the Arenig to 9 degrees in the Caradocian. Fold- and intraformational conglomerate tests allow us to ascertain the (near-?)primary nature of the remanences. This fast rate resembles the significant paleolatitude changes of Baltica in Ordovician times, and raises the question whether it is related to an episode of large-scale true polar wander (TPW) or whether plate velocities were very high for only these continental blocks. TPW can be distinguished from fast plate movements because every APWP from all continents should show it and it must occur for all continents during the same interval of time. It turns out that for Gondwana and Laurentia, the rate of APW is not unusually high during the Ordovician, and that for this time interval we can therefore conclude that TPW is an unlikely explanation. Previous suggestions that fast TPW occurred in the Cambrian are, similarly, not borne out by global synchroneity of fast APW. For the Neoproterozoic, on the other hand, the database is still too scarce to reach definitive solutions.
U32A-02 10:35h
Apparent polar wander paths and relative paleolongitude in supercontinents
A popular misconception of the paleomagnetic method is that it provides no constraints on relative paleolongitude. Whilst it is indeed true that axisymmetry of the time-averaged geomagnetic field allows only paleolatitude comparisons between two or more continents when a set of paleomagnetic poles from a single geological age are compared, it is also valid that in the special cases of continents travelling together as part of a supercontinent, or during episodes of rapid true polar wander, then in these instances reconstruction of the continents to their correct paleogeography will superimpose their apparent polar wander (APW) paths. Conversely, if superposition of APW paths results in continental juxtapositions that are geologically reasonable for the time interval under consideration, then paleomagnetism can thereby generate precise constraints on relative paleolongitude. In 1956, Irving and Runcorn independently utilized this method to restore the north Atlantic Ocean, whose opening, after all, is primarily a separation of longitude. Pre-Pangean continental reconstructions rely solely upon continental paleomagnetism for quantitative precision. Recent methodologies have adopted the "closest-approach" technique for reconstructing Proterozoic supercontinents, but this ignores the power of relative paleolongitude constraints provided by the APW-superposition method. Examples from the Paleoproterozoic supercontinent Nuna and Neoproterozoic Rodinia, as well as hypothesized episodes of rapid true polar wander within the Proterozoic-Cambrian transition, will illustrate the superiority of APW manipulation in deep-time reconstructions.
U32A-03 10:50h
Paleomagnetism of the 765 Ma Luakela Volcanics in NW Zambia and Implications for Neoproterozoic Positions of the Congo Craton
Owing to the scarcity of reliable paleopoles, the Neoproterozoic position of the Congo craton (incorporating the Sao Francisco, Tanzania, and Bangweulu blocks) is very poorly known. We report new paleomagnetic data for the 765 $\pm$ 5 Ma Luakela volcanics, a NE-trending belt of basaltic to andesitic flows in NW Zambia (Key et al., 2001, J. Afr. Earth Sci., 33, 503-528). The volcanics are up to 0.8 km thick and occur within a 2 km thick succession of siliciclastic rocks that unconformably overlies Neoarchean and Paleoproterozoic rocks of the Congo craton margin, and is correlated with the Roan and Mwashia Groups of the Katanga Supergroup (Key et al., 2001). The strata are essentially undeformed, and either subhorizontal or dip shallowly to the SE. Although no metamorphic mineral growth is observed in fine-grained sedimentary rocks, alteration has strongly affected plagioclase and pyroxene in the volcanic rocks, and magnetite has been partially altered to hematite (Key et al., 2001). AF and thermal analysis of 65 samples from nine sites isolated three magnetisation components. Component A, carried mainly by SD magnetite, is directed very shallowly to the SE. Component B, carried mainly by hematite, is oriented shallowly SW-up. A low stability component C is directed very steeply downward. Some samples contain only component A, others only component B, and some contain both A and B. Component A is likely to be primary, because it is carried by SD magnetite (which petrography indicates is primary), does not resemble younger magnetisations from the Congo craton, and because the rocks have not been thermally metamorphosed. Component B, carried by hematite, we consider to be an overprint, possibly acquired during Pan-African deformation in the Lufilian Arc. Component C is similar to Permo-Carboniferous paleodirections from the region, and may have been acquired at that time. Paleopoles for components A and B (LVA and LVB) are about $90\deg$ apart, and similar to those from the Tanzania block. LVA coincides with a reliable pole for the Mbozi complex (Meert et al., 1995, Precamb. Res., 69, 113-131), for which several K-Ar results are within uncertainty of a U-Pb age of 748 $\pm$ 6 Ma (Mbede et al., 2004, ICESA Conf: The East African Rift, Addis Ababa, PDF abst.). The LVA and Mbozi poles place Congo at the equator at 765-750 Ma. LVB falls within uncertainty of a pole for the Gagwe lavas (Meert et al., 1995), which have an Ar-Ar cooling age of 795 $\pm$ 7 Ma (Deblond et al., 2001, J. Afr. Earth Sci., 32, 435-449). LVB cannot be older than 765 Ma, however. If the Tanzania block has not moved significantly relative to the Congo craton since at least 800 Ma, either the 795 Ma age is incorrect, or the Gagwe pole represents a younger overprint. The latter possibly implies that models which invoke a $90\deg$ CCW rotation of Congo between 800 and 750 Ma are no longer supported. Instead, the Congo craton rotated in the opposite direction between 750 Ma and the time of component B acquisition. The Luakela volcanics are overlain by 200 m of siltstones, followed by an unknown thickness of poorly-exposed diamictite, correlated with the `Grand Conglomerat', a widespread glaciogenic unit of Sturtian age at the base of the Kundelungu Group. The diamictite is younger than the 765 Ma volcanics, and older than volcanic pods, dated at 735 Ma, in contact with the diamictite (Key et al., 2001). The Congo craton occupied equatorial latitudes at 765-750 Ma, suggesting that the diamictite, the `Grand Conglomerat', and other Sturtian glaciogenic rocks in the Congo craton, represent a low-latitude glaciation.
U32A-04 11:05h
Evolution of the APWP for Gondwana: constraints based on the geology of eastern Australia
The many iterations of the APWP for Gondwana over the past 40 years will be reviewed. They involve all the uncertainties that challenge the determination of the correct record: nature of NRM; age and correlation; geography and history of tectonic units etc. Most arguments about the Paleozoic section of this path depend upon the interpretation of results from the various terranes of eastern Australia. The two extreme views are either that none of the results from eastern Australia can be used for APWP definition or that all of them can be used. The terrane geology of eastern Australia is now reasonably well known and the paleomagnetic results can be placed in an appropriate terrane concept. This suggests that the Molong-Monaro terrane, where most results come from, was certainly accreted to the main craton by the Middle Devonian and probably by the Early Devonian. Early Devonian palaemagnetric results from the north and south of eastern Australia confirm this to be the case. However, the often used Late Carboniferous results from glacial horizons in eastern Australia are from the New England Fold Belt, where accretion to the main craton may not have been completed until the Middle Triassic. Results from this region now also confirm this to be the case. Both the geological setting and paleomagnetic results now confirm that the South Pole APWP makes a rapid transition from North Africa to south of South Africa between the Late Ordovician (455 Ma) and the Early Devonian (405 Ma). This places Bolivia and adjacent regions of South America near the south pole in Silurian times, a position supported by sedimentological evidence for glaciation in Bolivia at that time. The pole then loops back across southwest Gondwana to reach central Africa by the Early Carboniferous.
U32A-05 11:20h
Cretaceous Apparent Polar Wander Relative to the Major Cratons and Displacement Estimates of Baja British Columbia
When paleogeographic interpretations derived from independent observations conflict, the methods and results from each discipline come under careful scrutiny, as illustrated by the Baja British Columbia controversy. Cretaceous paleomagnetic data from a large region of the Canadian Cordillera render paleopoles which are far-sided with respect to cratonic North American poles, suggesting this region, designated Baja British Columbia, translated northward during Late Cretaceous - Paleogene time. Criticism of this interpretation based on other geological reasoning prompted me to perform new reviews of Cretaceous to Eocene paleomagnetic results from the Cordillera and from the major cratons of the globe. The global review follows the method of Besse and Courtillot (1991; 2002). One difference between our methods is that I compiled paleomagnetic results from highly studied rock units to single results to balance data weightings spatially and temporally, thus reducing the number of individual results. For the period 160 to 40 Ma, 51 poles were included compared to 92 poles by Besse and Courtillot (2002). Differences between apparent polar wander paths in their and my analyses are never significant at 95% confidence, however mean pole positions differ by up to 500 km, which is important for paleogeographic analysis. The global distribution of sampling localities and the tight clustering of the paleomagnetic poles after plate reconstruction provide invaluable confirmation of plate tectonically derived Euler rotations, the reliability of paleomagnetic remanence directions, and the geocentric dipole geometry of the geomagnetic field. My Cordilleran review shows that paleolatitudes derived from plutons and remagnetized rocks are significantly more scattered than those derived from bedded rocks. Using bedded rocks only, the paleomagnetic record shows that Baja British Columbia sat $2100 \pm 500$ km south of its present position with respect to cratonic North America during the Late Cretaceous (90 to 70 Ma) with accretion finishing by the mid-Eocene (50 Ma).
U32A-06 11:35h
The Cordilleran Cretaceous Apparent Polar Wander Path : Implications for the paleogeographic evolution of Saybia and Laurentia
Late Cretaceous paleopoles from stratified sequences in the Canadian Cordillera are discordant with respect to autochthonous North America and require 3000 km of dextral translation of much of the Cordillera between 85 and 55 Ma. The region characterized by discordant paleopoles extends, in Yukon, as far east as the Omineca Crystalline belt (OCB) where 70 Ma volcanic rocks of the Carmacks Group overlie the OCB and yield paleopoles requiring 2000 km of dextral offset. The OCB is stitched together and to more westerly geological belts by mid-Cretaceous plutons. These data are consistent with a model of Laramide orogeny resulting from a transpressional collision between a northward translating ribbon continent (Saybia) that included the OCB and more westerly portions of the Cordillera. Northward translation in the north was accommodated by buckling of Saybia giving rise to the Alaskan salient. Some mid-Cretaceous stratified sequences in Saybia yield paleopoles that require smaller degrees of northward translation relative to North America and imply southward translation of portions of the ribbon continent between 120 and 90 Ma. Southward translation was coeval with voluminous, eastward-younging granitoid magmatism including minor S-type granites, and large scale dextral shearing within the OCB. These data are consistent with oblique southwestward-directed subduction of oceanic crust that lay between Saybia and North America beneath the east (inboard) margin of Saybia in the mid-Cretaceous. Oblique subduction explains the voluminous OCB arc magmatism. Southward (dextral) translation resulted from entraining of orogenic float along Saybia's inboard margin. S-type granites and younging of the arc toward the east are attributable to the progressive accretion of sedimentary orogenic float to the upper plate, continually displacing the active arc to the west and placing accreted sedimentary rocks above upward-rising gabbroic arc magmas that underplated and then melted the sediments. Doubling up of the Quesnellia-Stikinia magmatic arc may have occured at this time by southward translation of Quesnellia. The current inability to resolve the geometry of the Meso- to Neoproterozoic Rodinian supercontinent may be in part the result of the false assumption that the OCB is autochthonous North America. Proterozoic paleopoles from the OCB should not be used to constrain the North American - Laurentian APWP, and neither should OCB stratigraphy and structures be used to aid in constraining Rodinian reconstructions.
U32A-07 11:50h
Paleomagnetic Rotations in the Northwest U.S. - Setting the Stage for Contemporary Deformation Models of the Cascadia Convergent Margin
In 1957 Allan Cox reported in Nature that the paleopole for the Eocene Siletz River Volcanics (SRV) of the Oregon Coast Range lay far to the east of the geocentric axial dipole, an oddity that defied adequate explanation for nearly two decades. Subsequent paleomagnetic studies by Myrl Beck, Allan Cox, their armies of students, and colleagues from the USGS, GSC, UCSC, and other institutions demonstrated that much of the central Cascadia forearc and arc had undergone large clockwise rotations during Cenozoic time. Between 1976 and 1989, paleomagnetists assembled a remarkable data set from more than 30 studies comprising approximately 1500 sites, that showed that rotation was largest in the Oregon Coast Range (OCR), and decreased northward into Canada, southward into California, and eastward into the backarc. Three competing models were proposed to explain the rotations, in part influenced by the oceanic basaltic origin of the SRV: 1) rotation during accretion of an allochthonous oceanic terrane; 2) rotation during Basin and Range extension; and 3) rotation due to dextral shear between N. America (NA) and northward-moving oceanic plates to the west. Onlap relations determined from geologic mapping of the rotated terranes showed that rotation during accretion was insignificant, and that the argument between those preferring microplate rotation (e.g., Willamette plate of Magill et al.) and those preferring distributed shear (e.g., Columbia River area, S. Sheriff) depended on the presence or absence of accommodating strike slip faults. In 1988, members of the Northwest paleomagnetic community attended the NATO meeting on Paleomagnetic Rotations and Continental Deformation, where McKenzie, Kiessel, Laj, and others argued that in the Aegean region, block rotations could be linked to contemporary deformation and seismicity. Subsequently England and Wells successfully modeled Cascadia's westward increase in rotation along the Columbia River as due to deformation of a thin, quasi-viscous lithospheric sheet by oblique subduction. At about the same time, Beck and Christiansen noted that regions of high moment release in the great Rat Islands earthquake in the western Aleutians (Mw 8.7) coincided with the centers of large rotating fore-arc blocks. Recognizing that block rotations could be related to seismicity in Cascadia, Wells, Weaver and Blakely linked the rotation of the OCR to the geodetically determined motion of the Sierra Nevada block (SN) to calculate a pole of rotation and velocity field for the OCR. The model combined plate-like behavior for the SN, OCR, and stable NA with intervening zones of internal deformation in the Klamath Mts., northern Cascadia forearc, and the arc. The rotation pole lies nearby in northeastern Oregon. Calculated forearc velocities are consistent with the rate and direction of extension in the Basin and Range province, the change from extension in the southern Cascade arc to compression in the northern arc, and the northward shortening and crustal seismicity in western Washington as it is compressed against the Canadian Coast Mountains buttress. The model provides an independent measure of the secular motion of crustal blocks that is useful for comparison with new GPS-derived models incorporating elastic deformation of the subduction zone.
U32A-08 INVITED 12:05h
A Comparison of Geodetic and Paleomagnetic Estimates of Block Rotation Rates in Deforming Zones
Rotations of crustal blocks estimated by paleomagnetic declination anomalies and by modern geodetic (GPS) data sample vastly different time spans. We are estimating modern-day vertical axis rotations of crustal blocks from several deforming zones by inversion of GPS velocities. In general these decade-scale rotation rates compare very well to several-million-year-scale paleomagnetic rotation rates. In the US Pacific Northwest, even a fine scale feature such as the rapid landward decrease in rotation rate evident in 12-15Ma basalt flows is recorded in 10-years of GPS data. However, in some cases, notably in the California Transverse Ranges, rapid rotations evident in paleomagnetic data do not appear to be continuing today. We will discuss our results in New Zealand, Papua New Guinea, the western US, and other parts of the world.