Geology of south-central Alaska

Nokleberg, Warren J.; Plafker, George; Wilson, Frederic H.

doi:10.1130/dnag-gna-g1.311

Cited by 86 publications

(122 citation statements)

References 71 publications

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“…The southern portion is intruded by the Jurassic Bonanza arc, which some authors have argued represents a southern extension of Talkeetna arc magmatism [Plafker et al, 1989;DeBari et al, 1999], though this relationship is debated. Overlap assemblages suggest that the two terranes formed as a single tectonic block or were juxtaposed prior to accretion onto the continental margin [Plafker et al, 1989;Nokleberg et al, 1994]. The Peninsular and Wrangellia terranes lie outboard of the Kahiltna terrane (Figure 1), a thick section of flysch and turbidite deposits [Nokleberg et al, 1994] interpreted to represent back-arc sedimentary sequences that were deformed during Jurassic to Cretaceous docking of the terranes [Wallace et al, 1989;Ridgway et al, 2002].…”

Section: Geologic Settingmentioning

confidence: 99%

“…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%

“…Overlap assemblages suggest that the two terranes formed as a single tectonic block or were juxtaposed prior to accretion onto the continental margin [Plafker et al, 1989;Nokleberg et al, 1994]. The Peninsular and Wrangellia terranes lie outboard of the Kahiltna terrane (Figure 1), a thick section of flysch and turbidite deposits [Nokleberg et al, 1994] interpreted to represent back-arc sedimentary sequences that were deformed during Jurassic to Cretaceous docking of the terranes [Wallace et al, 1989;Ridgway et al, 2002]. To the south, the Talkeetna section is juxtaposed with coeval and younger accretionary complexes of the Chugach terrane across the Border Ranges fault system (Figure 1) [Sisson and Onstott, 1986;Pavlis et al, 1988;Roeske et al, 1989;Clift et al, 2005b].…”

Section: Geologic Settingmentioning

confidence: 99%

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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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Section: Geologic Settingmentioning

confidence: 99%

Section: Geologic Settingmentioning

confidence: 99%

Section: Geologic Settingmentioning

confidence: 99%

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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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“…5), the oblique dextral-thrust Second Lake fault to the south and Taral fault (outside of map in Fig. 5) to the east (Nokleberg et al, 1994). Reconnaissance studies by Plafker and Berg (1994) and our own mapping reveal that rock types include blueschist, greenschist, muscovite and actinolite schist, siliceous schist, metachert, metabasalt, and graphitic schist.…”

Section: Liberty Creek Schistmentioning

confidence: 99%

P–T and structural constraints of lawsonite and epidote blueschists from Liberty Creek and Seldovia: Tectonic implications for early stages of subduction along the southern Alaska convergent margin

López-Carmona

Kusky

Santosh

et al. 2011

Lithos

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“…The fault extends generally westward for about 1500 km from western Canada to near the shore of the Bering Sea (Grantz, 1966;Brogan et al, 1975;Wahrhaftig et al, 1975). Geologists have long scrutinized the Denali fault because it poses a significant seismic hazard; moreover, the fault is thought to have played a prominent role in the northward translation and amalgamation of Alaskan tectonostratigraphic terranes (Csejtey et al, 1982;Plafker et al, 1989;Nokleberg et al, 1994;Ridgway et al, 2002). According to one tectonic model (Nokleberg et al, 1994), during the Cretaceous, rocks making up much of the Alaska Range orogen lay along the southern continental margin of Alaska, and oblique-convergent motion along the ancestral Denali fault resulted in the collision and suturing of numerous terranes against Alaska.…”

Section: Introductionmentioning

confidence: 99%

Geophysical Data Reveal the Crustal Structure of the Alaska Range Orogen within the Aftershock Zone of the Mw 7.9 Denali Fault Earthquake

Fisher

Ratchkovski

Nokleberg

et al. 2004

Bulletin of the Seismological Society of America

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Geophysical information, including deep-crustal seismic reflection, magnetotelluric (MT), gravity, and magnetic data, cross the aftershock zone of the 3 November 2002 M w 7.9 Denali fault earthquake. These data and aftershock seismicity, jointly interpreted, reveal the crustal structure of the right-lateral-slip Denali fault and the eastern Alaska Range orogen, as well as the relationship between this structure and seismicity. North of the Denali fault, strong seismic reflections from within the Alaska Range orogen show features that dip as steeply as 25Њ north and extend downward to depths between 20 and 25 km. These reflections reveal crustal structures, probably ductile shear zones, that most likely formed during the Late Cretaceous, but these structures appear to be inactive, having produced little seismicity during the past 20 years. Furthermore, seismic reflections mainly dip north, whereas alignments in aftershock hypocenters dip south. The Denali fault is nonreflective, but modeling of MT, gravity, and magnetic data suggests that the Denali fault dips steeply to vertically. However, in an alternative structural model, the Denali fault is defined by one of the reflection bands that dips to the north and flattens into the middle crust of the Alaska Range orogen. Modeling of MT data indicates a rock body, having low electrical resistivity (Ͼ10 X•m), that lies mainly at depths greater than 10 km, directly beneath aftershocks of the Denali fault earthquake. The maximum depth of aftershocks along the Denali fault is 10 km. This shallow depth may arise from a higher-than-normal geothermal gradient. Alternatively, the low electrical resistivity of deep rocks along the Denali fault may be associated with fluids that have weakened the lower crust and helped determine the depth extent of the aftershock zone.

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Geology of south-central Alaska

Cited by 86 publications

References 71 publications

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

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

P–T and structural constraints of lawsonite and epidote blueschists from Liberty Creek and Seldovia: Tectonic implications for early stages of subduction along the southern Alaska convergent margin

Geophysical Data Reveal the Crustal Structure of the Alaska Range Orogen within the Aftershock Zone of the Mw 7.9 Denali Fault Earthquake

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