Evolving heavy mineral assemblages reveal changing exhumation and trench tectonics in the Mesozoic Chugach accretionary complex, south-central Alaska

Abstract

The Gulf of Alaska is one of the largest accretionary complexes on Earth. In this study, we examined the earliest phase of accretion in the Mesozoic McHugh Complex and Valdez Groups, exposed in SE Alaska. The oldest preserved fragment, the Mesomélange assemblage, is Jurassic (ca. 160-140 Ma) and consists of an ~3-km-thick structural package of strongly deformed shaley materials with slices of oceanic cherts and basalts. Heavy minerals indicate dominant erosion from a magmatic arc source uplifted after the collision of the Wrangellia and the Talkeetna oceanic arc. A tectonic erosion event affected the forearc just prior to ca. 120 Ma and was likely caused by seamount collision, ridge subduction, or both. This was followed at 105 Ma by mass wasting of sandstone and conglomerates, preserved as the Graywacke-Conglomerate assemblage (ca. 105-83 Ma). Heavy minerals indicate continued fl ux from arc sources, but with signifi cant changes suggesting a larger, more diverse catchment area. Erosion of deeper crustal sources provided high-Mg diopside and garnets to the trench. Faster sediment fl ux was caused by rock uplift triggered by fi nal accretion of the Wrangellia-Peninsula terrane to North America. The start of large-scale accretion in Alaska roughly coincided with the initiation of Shimanto Complex accretion in Japan and can be understood as primarily linked to sediment supply driven by plate-margin tectonics rather than climatically induced erosion onshore.

Interpretation of gravity and magnetic data and development of two-dimensional cross-sectional models for the Border Ranges fault system, south-central Alaska

“…The late Mesozoic sedimentary rocks extend well east of the present CIB across the trailing edge of the Peninsular terrane. At least 3000 m of clastic, volcanic, and volcaniclastic rocks of Late Triassic to Early Jurassic Talkeetna Formation are underneath this succession, and compose the volcanic cover coeval with the Early Jurassic intrusive assemblages of the Peninsular terrane basement (Trop et al, 2005;Saltus et al, 2007;Clift et al, 2012). In the Anchorage area the Mesozoic rocks, as well as their late Mesozoic-Paleogene cover, emerge from beneath Neogene sediments and are well exposed to the east in the Matanuska Valley and northern Chugach Mountains (Pavlis and Roeske, 2007).…”

Mantle and geological evidence for a Late Jurassic−Cretaceous suture spanning North America

“…Blueschists are important markers of fossil subduction zones, but in the Chugach terrane, they predate the assembly of their supposedly associated accretionary complex by tens of millions of years and are, therefore, clearly reworked from a much earlier high-pressure-high-temperature event. Even the inner McHugh complex, presumably the oldest part of the Chugach accretionary complex (with individual structural panels of sandstone having maximum depositional ages as old as 169 ± 2 Ma; Amato et al, 2013), is constrained by detrital zircons to have been assembled after 146 ± 5 Ma (Amato and Pavlis, 2010;Clift et al, 2012), and hence >50-60 m.y. after the oldest blueschist metamorphism.…”

Zircon and apatite thermochronology of the Nankai Trough accretionary prism and trench, Japan: Sediment transport in an active and collisional margin setting

“…As a result, sediment that is used to build an accretionary complex is sensitive to the development of drainage systems and thus also to the tectonics of the continental interior. For example, sedimentary rocks within an accretionary complex in South Central Alaska has been shown to change in composition as the arc collided with continental North America, reflecting the expansion of what were initially limited coastal drainage systems [ Clift et al ., ]. In contrast, the opening of marginal seas might be an effective mechanism for isolating a trench system from large continental drainages by providing accommodation space for the sediment between the river mouth and the trench.…”