THE JURASSIC TECTONIC DEVELOPMENT OF SOUTH-CENTRAL ALASKA

The numerous accreted terranes of western North America highlight the importance of terrane accretion in continental growth. Large allochthonous terranes are composed of primitive mantle-derived material and represent significant additions of juvenile crust onto the continental margin. The accretion or amalgamation of terranes can drive regional deformation, metamorphism, and plutonism throughout the suture zone and provide clastic input into near-shore sedimentary basins. The growth and transport of allochthonous terranes therefore plays an important role in the tectonic development of continental margins.

Our new U/Pb zircon and radiogenic isotope data document the Jurassic evolution of the Peninsular terrane of southern Alaska. The Peninsular terrane is defined by the accreted Talkeetna arc and overlying sedimentary sequences. Ages from plutonic rocks in the Chugach Mountains record initial arc plutonism from 201–180 Ma. This is consonant with biochronology that constrains arc-related volcanism to Late Triassic to early Bajocian (206–169 Ma). Plutonic ages from the Alaska Peninsula (183–164 Ma) and the Talkeetna Mountains (177–156 Ma) suggest a northward shift in arc magmatism at ~180 Ma and record significant plutonism following the end of arc-related volcanism.

Samples from the Chugach Mountains record limited isotopic variation of ⁸⁷Sr/⁸⁶Sr_initial=0.7032–0.7037 and ¹⁴³Nd/¹⁴⁴Nd_initial=0.512707–0.512743. The restricted range in the isotopic ratios is similar to modern intraoceanic arcs such as the Izu-Bonin-Mariana arc and demonstrates the juvenile nature of the Peninsular terrane magmas. In contrast, age-corrected ¹⁴³Nd/¹⁴⁴Nd and ⁸⁷Sr/⁸⁶Sr ratios from the northerly part of the arc in the Talkeetna Mountains are consistent with assimilation of adjacent Wrangellia crust into primitive Peninsular terrane magmas. The isotopic data therefore provide a link between the two terranes and constrain the time of terrane amalgamation. Our current data indicate that the two terranes were amalgamated by 158 Ma.