A Case for Catastrophic True Polar Wander in the Cambrian

Of all the periods in the Phanerozoic time scale, the Cambrian system is the most perplexing. Several lines of evidence which together suggest that the Cambrian Earth may have experienced a large, rapid episode of true polar wander (TPW) include: (1) The Cambrian is short. Once thought to have a duration approaching 100 Ma, stratigraphic Rb/Sr and U/Pb geochronometry now suggests a time span of about 20 Ma. (2) Lithostratigraphic analyses suggest that Northern Africa went from high to low and back to high latitudes within this time interval. North America at least moved into the carbonate belt during the early Cambrian. (3) Improvements in biostratigraphy, magnetostratigraphy, and carbon isotopic stratigraphy around the Precambrian-Cambrian boundary now allow precise correlations to be made between strata in Siberia, China, Morocco, Australia, and with somewhat less precision to North America. Isochronous poles from Australia and Africa demonstrate that they were moving separately in the late Precambrian. (4) Paleomagnetic poles, derived from biostratigraphically-dated units, suggest nearly 90° of APW for many, if not all, of these continents during Cambrian time. We interpret these effects as an episode of TPW resulting from the collision of East and West Gondwana during the early Cambrian. This suturing event would stop subduction of a large equatorial plate dipping under Africa (responsible at least in part for the Pan-African metamorphic events). Thermal warming of the orphaned slab then removes its mass anomaly, causing the magnitudes of the minimum and intermediate eigenvectors of the total earth moment of inertia tensor to switch, leading to a 90° TPW reorientation of the crust. The Cambrian sea-level transgression may then be the result of moving a large ocean basin from the polar region onto the equatorial buldge. Similarly, the enormously accelerated rates of organic evolution observed during Cambrian time may be the result of paleoenvironmental dislocation driven by TPW

Paleomagnetic results from the Cambrian-Ordovician boundary section at Black Mountain, Georgina Basin, western Queensland, Australia

Zones of alternating magnetic polarity have been identified throughout the Cambrian-Ordovician sequence at Black Mountain, and can be presumed to record geomagnetic field reversals during or immediately after deposition. A strong correlation can be made to polarity zones recognized at Dayangcha, northeastern China, a candidate site for establishment of the Cambrian-Ordovician boundary Global Stratotype Section and Point. Polarity zones associated with the Hispidodontus discretus and Hirsutodontus simplex Assemblage-Zones at Black Mountain are absent at Dayangcha, suggesting hiatuses at these levels in the Dayangcha sequence. Secondary components preserved in the Black Mountain section may provide temporal constraints on Middle Paleozoic diagenetic events in the Burke River Structural Belt

Evidence for a Large-Scale Reorganization of Early Cambrian Continental Masses by Inertial Interchange True Polar Wander

Analysis of Vendian to Cambrian paleomagnetic data shows anomalously fast rotations and latitudinal drift for all of the major continents. These motions are consistent with an Early to Middle Cambrian inertial interchange true polar wander event, during which Earth's lithosphere and mantle rotated about 90 degrees in response to an unstable distribution of the planet's moment of inertia. The proposed event produces a longitudinally constrained Cambrian paleogeography and accounts for rapid rates of continental motion during that time

Paleomagnetic constraints on fault motion in the Hilina Fault System, south flank of Kilauea Volcano, Hawaii

Movement of the south flank of Kilauea Volcano in Hawaii has been associated with catastrophic landslide events. The surface expression of this former movement is the Hilina Fault System with fault scarps as high as 500 m. Paleomagnetic directions for lava flows exposed in the Hilina Fault scarps at Puu Kapukapu and Keana Bihopa on the Hilina Pali are used to determine the average rate of movement along faults (slip surfaces) separating the two sections. This paper reports results from two independent paleomagnetic studies within the Hilina Pali area. Twenty-one paleomagnetic sites (143 cores) were sampled by the Michigan Technological University group from lava flows between the Mo'o Ash and Middle Pohakaa Ash at Keana Bihopa in the footwall block of the 500-m-high Hilina Pali fault scarp. Thirty paleomagnetic sites (152 cores) were collected by the California Institute of Technology group from lava flows between the Mo'o Ash and Middle Pohakaa Ash in the 300-m-high Puu Kapukapu section (the hanging-wall block). A comparison of site-mean directions show that lava flows in the lower part of the Puu Kapukapu section have been tilted more than lava flows in the upper part with respect to the Keana Bihopa section. The systematic steepening of remanent directions downsection at Puu Kapukapu indicates that slippage of this block occurs along listric normal faults. The average amount of backward tilt of the Puu Kapukapu block, based on a comparison of mean directions from the two sections, is 7.8°±7.7°. Using slope stability methods, the average rate of movement of the Puu Kapukapu block since deposition of the Middle Pohakaa Ash is 1.7–2.4 cm/yr, and the average displacement (subsidence) is 680–740 m. Assuming that the average displacement resulted from a series of earthquakes producing subsidence equal to that observed in the 1975 Kalapana earthquake (3 m of subsidence along a 40-km segment of coastline on the south flank of Kilauea Volcano), one Kalapana-size earthquake occurring every 200 years would account for this displacement. Lastly, overall mean directions for the two sections indicate that Puu Kapukapu has rotated counterclockwise with respect to the Hilina Pali by 14.8°±8.5° about a nearby vertical axis. This also suggests that slippage between the two blocks occurs along listric normal faults

Polar Wander and the Cambrian - Response

Torsvik et al. question the reliability of the paleomagnetic data used in our report (1). In discussing the Sept Iles B pole (2), which they and other workers have previously treated as one of the most reliable Vendian pole readings from Laurentia (3, 4), they disagree with our recapitulation of its 540-Ma Rb-Sr age (1). The variance in Rb-Sr ages for the Sept Iles complex was discussed thoroughly by Higgins and Doig (5), who developed a geochronologic- petrologic model of the complex that showed emplacement occurring at about 540 Ma. Their well-defined isochron (5) seems perferable to an assignment of 575 Ma that is based on comparison of a paleomagnetic pole with the better-dated Callander complex (3, 4); this latter approach seems circular

The proposed GSSP for the base of Cambrian Stage 10 at the First Appearance Datum of the conodont Eoconodontus notchpeakensis (Miller, 1969) in the House Range, Utah, USA

The Stage 10 Working Group of the International Subcommission on Cambrian Stratigraphy is tasked with recommending a stratotype section and horizon for the base of Stage 10, the uppermost stage of the Cambrian System. We identify three sections in the House Range in western Utah, USA, for consideration as locations for defining and characterizing the base of the proposed stage. We also propose a boundary horizon at the base of the Eoconodontus conodont Zone combined with a distinctive negative carbon-isotope excursion named the HEllnmaria “Red Tops Boundary (HERB) event. These and other biological and nonbiological tools can be used for correlating the proposed stage

Latest Devonian (Famennian) global events in western Laurentia: Variations in the carbon isotopic record linked to diagenetic alteration below regionally extensive unconformities

Integrated analysis of the sedimentology, stratigraphy, and chemostratigraphy of the uppermost Devonian Chaffee Group of Colorado reveals the presence of two regionally extensive unconformity surfaces associated with globally recognized extinction/eustatic events. The contact between semi-restricted, marginal marine, mixed siliciclastic–carbonate deposits of the Parting Formation and open marine carbonate of the Dyer Formation is a major marine flooding surface across western Colorado. This flooding surface rests at the top of an ~ 5 m thick, transgressive, cross-bedded, shoreline sandstone unit that locally overlies a 2.5-m-thick paleokarst breccia. δ^(13)C values shift lighter across the formation contact, in some cases by as much as 5‰. Oxygen isotopic values are extremely variable between measured stratigraphic sections, in cases invariant across the contact, and in other cases covarying with the δ^(13)C values. At Ouray, CO, δ^(18)O covaries with δ^(13)C throughout the section, and reaches extreme values (< − 30‰) below the unconformity. An isotopic shift in rocks of this age in Utah, coined ALFIE, was previously correlated to the Parting–Dyer contact. This study demonstrates that the carbon and oxygen isotopic record of ALFIE is highly variable across western Laurentia, and that important carbonate chemostratigraphic variations result from diagenesis that is clearly linked to a regional unconformity and associated relative sea-level fall. This lowstand may be a signal of eustatic fall associated with the Dasberg Event, a late Famennian marine extinction event. Similar isotopic patterns exist for strata below and above a paleokarst breccia in the upper Dyer Formation that we link to the globally significant latest Famennian Hangenberg Event, which includes a eustatic lowstand and subsequent transgression. Similar to the Parting–Dyer contact, both carbon and oxygen isotopes in strata below this regional unconformity surface show the variable nature of diagenetic alteration of carbonate units during lowstand conditions. Our data also suggest that correlatable δ^(13)C chemostratigraphic shifts can be diagenetically produced during lowstands across a regionally widespread (western U.S.) basin, and that these δ^(13)C shifts may be expressed within outcrops that show no macroscopic sedimentological signature of subaerial exposure. This has broad implications for the evaluation of δ^(13)C data in the rock record, particularly the assumption that extensive correlatable isotopic anomalies reflect global marine signatures