Scott D. Samson

Ancient continental margin assemblage in the northern Coast Mountains, southeast Alaska and northwest Canada

Geologic relations indicate that quartz-rich metasedimentary rocks in the northern Coast Mountains separate strata to the east that belong to the Stikine terrane from strata to the west of the Alexander, Wrangellia, and Taku terranes. The quartz-rich rocks structurally overlie western terranes along a mid-Cretaceous thrust fault and are overlain structurally (originally stratigraphically ) by strata of the Stikine terrace. These rocks are interpreted to be a continental margin assemblage that belongs to the Yukon Crystalline terrane. U-Pb and Nd isotopic data indicate that the metasedimentary rocks were shed from a source terrane consisting at least in part of Proterozoic rocks.

Structural and geochronologic relations along the western flank of the coast mountains batholith: Stikine river to Cape Fanshaw, central southeastern Alaska

Abstract Geologic and U-Pb geochronologic studies in central southeastern Alaska provide constraints on the mid-Cretaceous to mid-Tertiary deformation west of the Coast Mountains batholith. The NE-dipping Sumdum and Fanshaw faults record the W-directed emplacement of the Ruth assemblage (Yukon-Tanana terrane) and Taku terrane over the subjacent Gravina belt and Alexander terrane. Ductile fabrics of the Sumdum-Fanshaw fault system truncate early-formed foliation and thrust faults in Albian and older Gravina belt strata. U-Pb age data from syn- and post-tectonic plutons suggest that deformation was ongoing at 92.9 ± 3.0 Ma but had mostly ceased by 90 Ma. The Sumdum-Fanshaw fault system marks the tectonic boundary between the Alexander-Wrangellia terrane and inboard Yukon-Tanana and Stikine terranes and lies within a thrust belt that extends from southern Alaska to northern Washington. Mid-Cretaceous structures are truncated to the east by NE-dipping ductile fabrics of the Coast shear zone. Sheet-like Paleocene tonalites were emplaced into and deformed within the shear zone. Undeformed mid-Eocene dikes cross-cut the tonalites. Kinematic relations in the Coast shear zone suggest a complex displacement history that includes both east-side-up (reverse) and west-side-up (normal) shear. The shear zone probably accommodated the collapse of overthickened crust developed during mid-Cretaceous shortening.

Geology of the western flank of the Coast Mountains between Cape Fanshaw and Taku Inlet, southeastern Alaska

The western flank of the Coast Mountains batholith between Cape Fanshaw and Taku Inlet is underlain primarily by Jura-Cretaceous strata of the Gravina belt; pre-Permian(?), Permian, and Triassic strata of the Taku terrane; and mid-Proterozoic(?) to upper Paleozoic rocks of continental margin affinity. The continental margin rocks include mid-Proterozoic(?) to lower Paleozoic(?) quartzite and marble of the Tracy Arm assemblage; mid-Paleozoic metavolcanic and subordinate metasedimentary rocks of the Endicott Arm assemblage; and upper Paleozoic quartz-rich metaturbidites and metaconglomerate of the Port Houghton assemblage. We suggest that these three assemblages are correlative with components of the Yukon-Tanana terrane, which underlies a large region of Yukon and eastern Alaska. Rocks of the Gravina belt, Taku terrane, and Yukon-Tanana terrane are juxtaposed along west-vergent thrust faults of mid-Cretaceous age and are internally deformed and disrupted along latest Cretaceous to early Eocene dip-slip and possibly strike-slip shear zones. These rocks and structures, together with mid-Cretaceous to Eocene plutons of the Coast Mountains batholith, separate rocks of the Alexander-Wrangellia terrane to the west from rocks of the Stikine terrane to the east. Mid-Cretaceous thrust faults in the area belong to a system of thrusts that form the inboard margin of the Alexander-Wrangellia terrane from central Alaska to northwestern Washington. The continental margin rocks in the northern Coast Mountains may be part of the in situ North American margin that has been overthrust by the Stikine and adjacent terranes. Alternatively, these rocks may have (1) rifted from and then returned to the North American margin, (2) moved >800 km along left-lateral or right-lateral faults from elsewhere along the North American margin, or (3) originated near a continent other than North America.

Nd and Sr isotopic characterization of the Wrangellia terrane and implications for crustal growth of the Canadian Cordillera

Nd and Sr isotopic data are reported from samples of most of the major lithologic units of the Wrangellia terrane exposed on Vancouver Island and the Queen Charlotte Islands. Initial

Origin and tectonic setting of Ordovician bentonites in North America: Isotopic and age constraints

epsilon_{Nd}

U–Pb geochronology of Late Cretaceous and early Tertiary plutons in the northern Coast Mountains batholith

values range from +1.0 to + 7.3 and initial

Nd and Sr isotopic constraints on the petrogenesis of the west side of the northern Coast Mountains batholith, Alaskan and Canadian Cordillera

^{87}Sr/^{86}Sr