Paleomagnetism of Paleozoic strata of the Alexander terrane, southeastern Alaska

Robert F. Butler, George E. Gehrels, David R. Bazard

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Paleomagnetic samples were collected from 180 sites (sedimentary horizons or igneous flows) in nine Paleozoic formations of the Alexander terrane on or near Prince of Wales Island in southeastern Alaska. Analysis of samples collected from 68 sites within 2 stratigraphic sections of red sedimentary rocks from the Lower Devonian Karheen Formation allows determination of a characteristic component with unblocking temperatures between 550 °C and 680 °C in many samples. Site-mean characteristic directions from 29 sites pass fold and reversal tests and define polarity zones correlative between the two stratigraphic sections. These paleomagnetic data indicate an Early Devonian paleolatitude of 14° ± 4° (north or south). Geochronologic analysis of detrital zircon grains from the Karheen Formation requires that the Alexander terrane was adjacent to a continent with crust distributed within 1.6-1.8 Ga and 1.45-1.6 Ga age intervals. Early Paleozoic locations as part of the Australian paleo-Pacific margin of Gondwana or the Scandinavian margin of Baltica are consistent with the paleomagnetic, geologic, and geochronologic data, and available paleontologic data favor the Baltica paleoposition. The Lower Permian Halleck Formation volcanic rocks contain a stable paleomagnetism indicating a paleolatitude of ∼25°N, consistent with the 25°N-30°N Permian paleolatitudes determined from three other formations of the Alexander- Wrangellia terranes. The 10°-20° paleolatitudes determined from numerous paleomagnetic studies of Late Triassic igneous rocks from Alexander-Wrangellia are almost certainly Northern Hemisphere paleolatitudes. Available evidence indicates: (1) early Paleozoic development of the Alexander terrane as a volcanic arc without significant incorporation of continental crust; (2) mid-Paleozoic juxtaposition with a continent containing 1.6-1.8 Ga and 1.45-1.6 Ga crust, probably the Scandinavian margin of Baltica; (3) rifting from that margin in Devonian time followed by tectonic transport to 25°-30° latitude in the northern paleo-Pacific by Permian time; and (4) southward motion to a Late Triassic paleolatitude of 10°-20° followed by accretion to North America with subsequent dispersal of fragments from northern Oregon to southern Alaska. The Ordovician Descon Formation, Silurian Heceta Limestone, Devonian Port Refugio Formation, and Pennsylvanian Ladrones Formation did not yield primary magnetizations. Upper Paleozoic carbonate rocks and siliciclastic strata of the Devonian Wadleigh Limestone and the Mississippian Peratrovich and Pennsylvanian Klawak Formations have characteristic magnetizations that are complex and, in part, demonstrably secondary. Rock-magnetic investigations of the Peratrovich Formation and the Wadleigh Limestone reveal wasp-waisted hysteresis loops with parameters closely matching those of other carbonate rocks known to have undergone chemical remagnetization.

Original languageEnglish (US)
Pages (from-to)1372-1388
Number of pages17
JournalBulletin of the Geological Society of America
Issue number10
StatePublished - Oct 1997

ASJC Scopus subject areas

  • Geology


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