The age and provenance of the southern Alaskan
Campanian to Paleocene Valdez Group of the Chugach
terrane and its relationship with the younger outboard
Paleocene to Eocene Orca Group of the Prince
William terrane is poorly understood but an important
component of the Cordilleran collage (Plafker et al.,
1994). The Valdez and Orca Groups are both part of
the Chugach-Prince William terrane (CPW), which
is a thick accretionary complex that extends 2200 km
along the southern Alaskan margin (Fig. 1; Cowan,
2003). The deep-water turbidites of these terranes
are quartzofeldspathic and volcanic-lithic sandstones
and basaltic rocks (Dumoulin, 1987; Plafker et al.,
1994). The CPW is intruded by near-trench plutons of
the Sanak-Baranof belt (Davidson and Garver, 2017)
and are believed to be related to a slab window that
formed during subduction of Kula-Farallon or Kula-
Resurrection spreading ridges (Marshak and Karig,
1977; Delong et al., 1978; Moore et al., 1983; Kusky
et al., 1997a; Bradley et al., 2003; Haeussler et al.,
2003). There are two hypotheses for the formation
of the CPW along the North American Cordilleran
margin: 1) either the CPW terrane formed in situ by
subduction of the Resurrection plate (Haeussler et al.
2003); or 2) the rocks formed in the Pacific Northwest
or California and were transported at least 2000 km
along coastwise strike-slip fault systems (Cowan,
2003; Garver and Davidson, 2015).
This study is an investigation into the age and
provenance of the Valdez Group and its relationship
with the Orca Group in the central Chugach Mountains
using detrital zircon U-Pb dates. New detrital zircon
U-Pb dates and their grain-age distributions from the
Valdez and Orca Group turbidites are combined with
dates from Kochelek et al. (2011), Amato et al. (2013),
and Davidson and Garver (2017) and then synthesized
to understand the difference in age between the units
and provenance. New and existing U-Pb dates indicate
maximum depositional ages (MDA) of the Valdez
Group are concentrated in three groups: 84-78 Ma,
74-65 Ma, and 62-60 Ma. The youngest group of
MDAs are age-correlative with the Orca Group but
were collected from rocks in areas mapped as Valdez
Group, indicating that either Orca Group rocks occur
in the Valdez Group or the youngest Valdez Group
rocks are stratigraphically equivalent to those of the
oldest Orca Group. If the latter, the Valdez Group is
not Campanian to Maastrichtian in age as has been
traditionally viewed (Plafker et al., 1994) but is Upper
Cretaceous to Paleocene and in part correlative to the
lowest part of the Orca Group.
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Raising the Resurrection plate from an unfolded-slab plate tectonic reconstruction of northwestern North America since early Cenozoic time
The configuration of mid-ocean ridges subducted below North America prior to Oligocene time is unconstrained by seafloor isochrons and has been primarily inferred from upper-plate geology, including near-trench magmatism. However, many tectonic models are permitted from these constraints. We present a fully kinematic, plate tectonic reconstruction of the NW Cordillera since 60 Ma built by structurally unfolding subducted slabs, imaged by mantle tomography, back to Earth’s surface. We map in three-dimensions the attached Alaska and Cascadia slabs, and a detached slab below western Yukon (Canada) at 400−600 km depth that we call the “Yukon Slab.” Our restoration of these lower plates within a global plate model indicates the Alaska slab accounts for Pacific-Kula subduction since ca. 60 Ma below the Aleutian Islands whereas the Cascadia slab accounts for Farallon subduction since at least ca. 75 Ma below southern California, USA. However, intermediate areas show two reconstruction gaps that persist until 40 Ma. We show that these reconstruction gaps correlate spatiotemporally to published NW Cordillera near-trench magmatism, even considering possible terrane translation. We attribute these gaps to thermal erosion related to ridge subduction and model mid-ocean ridges within these reconstruction gap mid-points. Our reconstructions show two coeval ridge-trench intersections that bound an additional “Resurrection”-like plate along the NW Cordillera prior to 40 Ma. In this model, the Yukon slab represents a thermally eroded remnant of the Resurrection plate. Our reconstructions support a “northern option” Farallon ridge geometry and allow up to ∼1200 km Chugach terrane translation since Paleocene time, providing a new “tomographic piercing point” for the Baja-British Columbia debate.
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- Award ID(s):
- 1848327
- NSF-PAR ID:
- 10208320
- Date Published:
- Journal Name:
- GSA Bulletin
- ISSN:
- 0016-7606
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Southern Alaska has a long history of subduction, accretion, and coastwise transport of terranes (Coney et al., 1980; Monger et al., 1982; Plafker et al., 1994). The Chugach-Prince William (CPW) terrane is about 2200 km long and extends through much of southern Alaska (Plafker et al., 1994) (Fig. 1A). The inboard Chugach terrane can be divided into two parts, a mélange and sedimentary units that are Permian to Early Cretaceous in age and a turbidite sequence that is from the Upper Cretaceous (Plafker et al., 1994). In the Prince William Sound area, the outboard Prince William terrane is comprised of Paleocene to Eocene turbidites and associated basaltic rocks of the Orca Group (Davidson and Garver, 2017), and the turbidites of the inboard Chugach terrane are known as the Valdez Group. The turbidites are intruded by the Sanak-Baranof Belt (SBB), a group of 63-47 Ma plutons that are progressively younger to the east. The Border Ranges fault system marks the northern boundary of the CPW terrane, separating the Chugach terrane from the Wrangellia composite terrane and the Contact fault separates the Chugach and Prince William terrane (Fig. 1; Plafker et al., 1994). There are three ophiolite sequences in the Orca Group: Knight Island (KI), Resurrection Peninsula (RP), and Glacier Island (GI) (Fig. 1B). The KI ophiolite contains a sequence of massive pillow basalts, sheeted dikes, and a minor amount of ultramafic rocks (Tysdal et al, 1977; Nelson and Nelson, 1992; Crowe et al., 1992). The RP ophiolite is a typical ophiolite sequence and has interbedded Paleocene turbidites (Davidson and Garver, 2017). Paleomagnetic data gathered from the RP ophiolite indicated a mean depositional paleolatitude of 54° ± 7° which implies 13° ± 9° of poleward displacement (Bol et al., 1992). These data suggest that the RP ophiolite was translated northward to its current position after being formed in the Pacific Northwest, and thus the CPW terrane may have been originally located at 48-49° north and at 50 Ma was transferred 1100 km to the north by strike-slip faulting (Cowan, 2003). However, an opposing hypothesis suggests that the terrane has not experienced significant displacement and formed in Alaska due to a now-subducted Resurrection plate (Haeussler et al., 2003). KI and RP ophiolites have traditionally been assumed to be oceanic crust that was tectonically emplaced into the CPW terrane (Bol et al., 1992; Lytwyn et al., 1997). However, a more recent study suggests a hypothesis that the ophiolites originated in an upper plate setting and formed due to transtension (Davidson and Garver, 2017). Previous workers have used discriminant diagrams to identify the volcanic rocks of KI ophiolite and RP ophiolite as mid-ocean ridge basalts (Lytwyn et al., 1997; Miner, 2012). This project presents new geochemical and geochronological data from the GI ophiolite to determine its age and tectonic setting. The purpose of this study is to compare the data from GI with the data from KI and RP, and the comparison of the geochemical data will allow for a greater understanding of the tectonic setting of southern Alaska.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1130/B35677.1
