Year: 1991
Type: book-chapter
Abstract: Of all the periods in the Phanerozoic time scale, the Cambrian system is the most \nperplexing. Several lines of evidence which together suggest that the Cambrian \nEarth may have experienced a large, rapid episode of true polar wander (TPW) \ninclude: (1) The Cambrian is short. Once thought to have a duration approaching \n100 Ma, stratigraphic Rb/Sr and U/Pb geochronometry now suggests a time span of \nabout 20 Ma. (2) Lithostratigraphic analyses suggest that Northern Africa went from \nhigh to low and back to high latitudes within this time interval. North America at \nleast moved into the carbonate belt during the early Cambrian. (3) Improvements in \nbiostratigraphy, magnetostratigraphy, and carbon isotopic stratigraphy around the \nPrecambrian-Cambrian boundary now allow precise correlations to be made between \nstrata in Siberia, China, Morocco, Australia, and with somewhat less precision to \nNorth America. Isochronous poles from Australia and Africa demonstrate that they \nwere moving separately in the late Precambrian. (4) Paleomagnetic poles, derived \nfrom biostratigraphically-dated units, suggest nearly 90° of APW for many, if not all, \nof these continents during Cambrian time. We interpret these effects as an episode \nof TPW resulting from the collision of East and West Gondwana during the early \nCambrian. This suturing event would stop subduction of a large equatorial plate \ndipping under Africa (responsible at least in part for the Pan-African metamorphic \nevents). Thermal warming of the orphaned slab then removes its mass anomaly, \ncausing the magnitudes of the minimum and intermediate eigenvectors of the total \nearth moment of inertia tensor to switch, leading to a 90° TPW reorientation of the \ncrust. The Cambrian sea-level transgression may then be the result of moving a \nlarge ocean basin from the polar region onto the equatorial buldge. Similarly, the \nenormously accelerated rates of organic evolution observed during Cambrian time \nmay be the result of paleoenvironmental dislocation driven by TPW.
Authors: Joseph L. Kirschvink, Robert L. Ripperdan
Language: English
Subfield: Paleontology
Field: Earth and Planetary Sciences
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W2597061860