Origin and tectonic evolution of the Maclaren and Wrangellia terranes, eastern Alaska Range, Alaska

Nokleberg, Warren J.; Jones, David L.; Silberling, Norman J.

doi:10.1130/0016-7606(1985)96<1251:oateot>2.0.co;2

Cited by 150 publications

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“…During Jurassic and Cretaceous time, a Jurassic intra-oceanic arc and Triassic-Jurassic oceanic plateau were accreted to the former southern Alaska continental margin (Wrangellia composite terrane on Fig. 16.2A; Nokleberg et al, 1985;Ridgway et al, 2002;Trop et al, 2005;Rioux et al, 2007). Following this episode of significant crustal growth, northward subduction of oceanic crust (present coordinates) produced a continental-margin magmatic arc, forearc basin, and accretionary prism on the recently accreted TriassicJurassic oceanic crust (Fig.…”

Section: Geologic Configuration Of the Southern Alaska Forearc Basin mentioning

confidence: 99%

Modification of Continental Forearc Basins by Flat‐Slab Subduction Processes: A Case Study from Southern Alaska

Ridgway

Trop

Finzel

2011

Tectonics of Sedimentary Basins

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Forearc basins are large sediment repositories that develop in the upper plate of convergent margins and are a direct response to subduction. These basins are part of the magmatic arc-forearc basin-accretionary prism "trinity" that defines the tectonic configuration of the upper plate along most subduction-related convergent margins. Many previous studies of forearc basins have explored the links between construction of magmatic arcs, exhumation of accretionary prisms, and sediment deposition in adjacent forearc basins. These studies provide an important framework for understanding firstorder tectonic processes recorded in forearc basins that are characterized by long-lived subduction of "normal" oceanic crust. Many convergent margins, however, are complicated by second-order subduction processes, such as flat-slab subduction of buoyant oceanic crust in the form of seamounts, spreading and aseismic ridges, and oceanic plateaus. These second-order processes can substantially modify the tectonic configuration of the upper plate both in time and space, and produce sedimentary basins that do not easily fit into the conventional magmatic arc-forearc basin-accretionary prism trinity.In this chapter, we discuss the modification of the southern Alaska forearc basin by Paleocene-Eocene subduction of a spreading ridge followed by OligoceneHolocene subduction of thick oceanic crust. This thick oceanic crust is currently being subducted beneath south-central Alaska and has an imaged maximum thickness of 30 km at the surface and 22 km at depth. Findings from southern Alaska suggest that forearc basins modified from flat-slab subduction processes may contain a sedimentary and volcanic stratigraphic record that differs substantially from typical forearc basins. Processes and sedimentary features that characterize modified forearc basins include the following: (1) flat-slab subduction of a buoyant, topographically elevated spreading ridge oriented subparallel to the margin prompts diachronous uplift of the forearc basin floor and exhumation of older marine forearc basin strata as the ridge is subducted. Passage of the spreading ridge leads to subsidence and renewed deposition of nonmarine sedimentary and volcanic strata that locally exceeds the thickness of the underlying marine strata. (2) Insertion of a slab window beneath the forearc basin during spreading ridge subduction produces local intrabasinal topographic highs with adjacent depocenters, as well as discrete volcanic centers within and adjacent to the forearc basin. (3) Flat-slab subduction of thick oceanic crust also results in surface uplift and exhumation of forearc basin sedimentary strata. However, the insertion of thick crust throughout the flat-slab region (i.e., lack of a slab window) inhibits subduction-related magmatism adjacent to the forearc basin. In the case of subduction of a >350-km-wide fragment of thick oceanic crust beneath south-central Alaska, exhumation of forearc basin strata located above the region of flat-slab subduction has prompted enhanced T...

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Section: Geologic Configuration Of the Southern Alaska Forearc Basin mentioning

confidence: 99%

Modification of Continental Forearc Basins by Flat‐Slab Subduction Processes: A Case Study from Southern Alaska

Ridgway

Trop

Finzel

2011

Tectonics of Sedimentary Basins

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“…The Talkeetna Formation and overlying sedimentary sequences define the extent of the Peninsular terrane described by Jones and Silberling [1979] (Figure 1). To the northeast, the arc is in faulted and/or intrusive contact with the allochthonous Wrangellia terrane, an extensive Triassic flood basalt erupted through an existing Paleozoic arc sequence [Bond, 1973;Richter and Jones, 1973;Nokleberg et al, 1985Nokleberg et al, , 1994Barker et al, 1994]. The Wrangellia terrane extends from southern Alaska to Vancouver Island, British Columbia.…”

Section: Geologic Settingmentioning

confidence: 99%

Intermediate to felsic middle crust in the accreted Talkeetna arc, the Alaska Peninsula and Kodiak Island, Alaska: An analogue for low‐velocity middle crust in modern arcs

et al. 2010

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Seismic profiles of several modern arcs have identified thick, low‐velocity midcrustal layers (Vp = 6.0–6.5 km/s) that are interpreted to represent intermediate to felsic plutonic crust. The presence of this silicic crust is surprising given the mafic composition of most primitive mantle melts and could have important implications for the chemical evolution and bulk composition of arcs. However, direct studies of the middle crust are limited by the restricted plutonic exposures in modern arcs. The accreted Talkeetna arc, south central Alaska, exposes a faulted crustal section from residual subarc mantle to subaerial volcanic rocks of a Jurassic intraoceanic arc and is an ideal place to study the intrusive middle crust. Previous research on the arc, which has provided insight into a range of arc processes, has principally focused on western exposures of the arc in the Chugach Mountains. We present new U‐Pb zircon dates, radiogenic isotope data, and whole‐rock geochemical analyses that provide the first high‐precision data on large intermediate to felsic plutonic exposures on Kodiak Island and the Alaska Peninsula. A single chemical abrasion–thermal ionization mass spectrometry analysis from the Afognak pluton yielded an age of 212.87 ± 0.19 Ma, indicating that the plutonic exposures on Kodiak Island represent the earliest preserved record of Talkeetna arc magmatism. Nine new dates from the extensive Jurassic batholith on the Alaska Peninsula range from 183.5 to 164.1 Ma and require a northward shift in the Talkeetna arc magmatic axis following initial emplacement of the Kodiak plutons, paralleling the development of arc magmatism in the Chugach and Talkeetna mountains. Radiogenic isotope data from the Alaska Peninsula and the Kodiak archipelago range from ɛNd(t) = 5.2 to 9.0 and 87Sr/86Srint = 0.703515 to 0.703947 and are similar to age‐corrected data from modern intraoceanic arcs, suggesting that the evolved Alaska Peninsula plutons formed by extensive differentiation of arc basalts with little or no involvement of preexisting crustal material. The whole‐rock geochemical data and calculated seismic velocities suggest that the Alaska Peninsula represents an analogue for the low‐velocity middle crust observed in modern arcs. The continuous temporal record and extensive exposure of intermediate to felsic plutonic rocks in the Talkeetna arc indicate that evolved magmas are generated by repetitive or steady state processes and play a fundamental role in the growth and evolution of intraoceanic arcs.

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“…1). The Wrangellia terrane consists of a late Paleozoic sequence of island-arc volcanics and related sedimentary rocks overlain by 1500-4000 m of massive basalt fl ows of the Late Triassic Nikolai greenstone locally capped by limestone (Nokleberg et al, 1985;Plafker et al, 1989). Paleomagnetic inclinations for Triassic basalts indicate a near-equatorial position for Wrangellia in the Triassic (Hillhouse and Grommé, 1984) but a moderate paleolatitude by the Late Cretaceous (Stamatakos et al, 2001).…”

Section: Geology Of South-central Alaskamentioning

confidence: 98%

The Chulitna terrane of south-central Alaska: A rifted volcanic arc caught between the Wrangellia composite terrane and the Mesozoic margin of North America

2009

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The Chulitna terrane is an enigmatic, fault-bounded package of mid-Paleozoic to early Mesozoic sedimentary rocks and volcanics that are exotic to North America and show no clear affi nity with any of the other allochthonous terranes in southern Alaska. Geologic mapping and geochemical analyses of Triassic strata from the Chulitna terrane indicate Triassic rifting of a Devonian volcanic arc and development of an asymmetric basin that was later deformed in the Cretaceous within the broadly defi ned suture between the Wrangellia composite terrane and the Mesozoic margin of North America. The structure of the region consists of an overturned synclinorium that verges southeast, Triassic strata in the core of the syncline, and basement rocks exposed in the hanging walls of a series of out-of-syncline thrusts on the southeastern, upright limb. The basement of the complex consists of serpentinite, vesicular fl ows of basaltic andesite with minor pillow lavas, and volcanic debris with blocks of andesite and minor Devonian red chert. From southeast to northwest, there are systematic changes in the facies associated with Triassic strata that overlie this basement. To the southeast, the basement is unconformably overlain by bedded sedimentary breccias that consist of subangular clasts of basement lithologies, including red chert, serpentinite, and andesite. Further to the northwest, in the hanging wall of a more rearward thrust, the red-bed strata consist of conglomerate intermixed with siltstone and cross-bedded sandstone. In the northwest on the overturned limb of the syncline, the red beds are composed of siltstone and sandstone interfi ngered with limestone. There is an overall increase in the thickness of the red-bed unit from southeast to northwest. On the northwestern limb of the syncline, the red beds overlie a 1500-m-thick basalt sequence with columnar jointed fl ows and pillow basalts interbedded with limestone. Geochemical analyses of clinopyroxene phenocrysts in the basalts show similarities with island-arc basalts, suggesting that volcanism was related to early rifting of an oceanic arc. A marine sandstone unit overlies the entire red-bed package along a fl ooding surface and thins to the southeast. These results are consistent with Triassic deposition within a rifted volcanic arc, an upland erosion surface to the southeast, and increasing thickness of the Triassic section toward a basin-bounding fault in the northwest. This basin was initially ~18 km wide prior to accretion along the Mesozoic margin of North America, which resulted in southeast-vergent folding and 30% shortening.

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Origin and tectonic evolution of the Maclaren and Wrangellia terranes, eastern Alaska Range, Alaska

Cited by 150 publications

References 0 publications

Modification of Continental Forearc Basins by Flat‐Slab Subduction Processes: A Case Study from Southern Alaska

Modification of Continental Forearc Basins by Flat‐Slab Subduction Processes: A Case Study from Southern Alaska

Intermediate to felsic middle crust in the accreted Talkeetna arc, the Alaska Peninsula and Kodiak Island, Alaska: An analogue for low‐velocity middle crust in modern arcs

The Chulitna terrane of south-central Alaska: A rifted volcanic arc caught between the Wrangellia composite terrane and the Mesozoic margin of North America

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