TURNING POINTS DURING THE TECTONO-STRATIGRAPHIC EVOLUTION OF THE ALEXANDER TERRANE

Interpretations of the early history of the composite allochthonous Alexander terrane are based upon fossil and detrital zircon contents coupled with metamorphic events that generally contrast with characteristics of the more inboard, Laurentian continental margin. Some current tectonic models identify potential source units of Ordovician components of Alexander terrane at the northern end of the Caledonide chain based upon detrital zircon patterns, supported by isotopic data. However, Late Ordovician faunas from each region show distinctive contrasts. Even younger units such as the Silurian age Heceta Limestone in the southern part of the terrane contain unusual, distinctive, microbial reefs with common accessory aphrosalpingid sponges. The same reefs and associated fauna are found in the Farewell terrane (primarily with the Nixon Fork subterrane) of west-central and southwestern Alaska. These fossils correlate with similar faunas from the Omulevsk Mountains (Omulevka terrane) of Northeast Russia, which distinguish a faunal province that likely lay mainly west of the Caledonian belt. Correlation also may be inferred between the ca. 400 Ma Klakas orogeny in Alaska and the Scandian orogeny of Baltica, that appears to have been followed by clockwise rotation of Laurentia along the previously recognized Great Glen fault and its extension related to basins interpreted as transtensional. Clastic units such as the thick (ca. 1800m), Ludlovian-Early Lochkovian (ca. 427- 415 Ma) Karheen Formation and younger, Lower–Middle Devonian (Emsian–Eifelian, ca. 395– 405 Ma ) sandstones from the North Okhotsk active continental margin located on the Omolon terrane, the Cascaden Ridge unit near Livengood in east-central Alaska, and the Ulangarat Formation in the northeast Brooks Range are inferred to record filling of a foreland basin in response to uplift and erosion of the orogenically, thickened crust followed by accumulation of debris in structural pull-aparts during sinistral fault movement. Is it possible that Alexander terrane arrived at its present position after multiple turning points related to Late Ordovician, Early Devonian and younger episodes of plate movement?