Talkeetna Mountains Research Articles

Macroflora from Lower Jurassic (Pliensbachian) of Hicks Creek, southern Talkeetna Mountains, south‐central Alaska

AbstractA recently discovered Early Jurassic locality at Hicks Creek, Talkeetna Mountains, Alaska, yielded a macrofossil plant assemblage comprising predominantly bennettitaleans and ferns, accompanied by horsetails, seed ferns, cycads and conifers. Single species from different plant groups dominate the assemblage (e.g. Cladophlebis alata, Otozamites pterophylloides; less frequent Rhaphidopteris sp. and Pagiophyllum falcatum; sporadic Thallites sp., Klukia sp., Todites williamsonii, Coniopteris bella, Sagenopteris sp., Zamites sp., Nilssoniopteris pristis, Pseudoctenis sp., Eretmophyllum sp. and Elatides sp.). We also encountered a new foliage of cycadophyte type, Hanophyllum varioserratum gen. et sp. nov. Hicks Creek is one of four Alaskan areas with fossil Mesozoic plants that have been investigated. Along with rocks of Puale Bay and the East Fork of Boulder Creek exposures, this locality is of Early Jurassic age, while Cape Lisburne is Early Cretaceous (Albian). Based on a comparison of the floral composition of Hicks Creek with some other localities (Alaska and beyond), environmental conditions for the Peninsular terrane are inferred. Floristic differences between localities are explained by the varied topography, interpreted as disturbed coastal–lagoonal or as undisturbed, moist and warm inland. Some taxa (Todites williamsonii, Coniopteris bella, Nilssoniopteris pristis, Otozamites tenuatus, O. mimetes, Brachyphyllum crucis) common to the Alaska Peninsula and Middle Jurassic of Yorkshire may suggest a possible pathway of plant migration during the movement of the Peninsular terrane. The occurrence of Cladophlebis alata in the Lower Jurassic of south‐central Alaska and the Lower Cretaceous of northwestern Alaska may also shed light on the posited spread of this species.

There is more Wrangellia — magnetic characterization of southern Alaska crust

In southern Alaska, Wrangellia-type magnetic crustal character extends from the Talkeetna Mountains southwest through the Alaska Range to the Bristol Bay region. Magnetic data analyses in the Talkeetna Mountains showed that there are mid-crustal differences in the magnetic properties of Wrangellia and the Peninsular terrane. After converting total field magnetic anomaly data to magnetic potential, we applied Fourier filtering techniques to remove magnetic responses from deep and shallow sources. The resulting mid-crustal magnetic characterization delineates the regional magnetic potential domains that correspond to the Wrangellia and Peninsular terranes throughout southern Alaska. These magnetic potential domains show that Wrangellia-type crust extends southwest to the Iliamna Lake region and that it overlaps the mapped Peninsular terrane. Upon reconsidering geologic ties between Wrangellia, Peninsular, and Alexander terranes, we conclude that Peninsular terrane is part of what we here call Western Wrangellia. Western Wrangellia contains the Lower Jurassic Talkeetna volcanic arc and is similar to Wrangellia of the Vancouver Island area, Canada (Southern Wrangellia), which contains the Lower Jurassic Bonanza volcanic arc. Others have previously made this correlation and proposed that the Talkeetna arc-bearing part of southern Alaska was displaced from the Bonanza arc-bearing part of Canada. We generally agree and propose that about 1000 km of dextral displacement along ancestral Border Ranges fault segments and other faults of south-central Alaska separated Western Wrangellia from Southern Wrangellia. We think this displacement was mostly in the Late Jurassic and earliest Cretaceous, perhaps between about 160 and 130 Ma.

Three-dimensional shape and structure of the Susitna basin, south-central Alaska, from geophysical data

AbstractWe use gravity, magnetic, seismic reflection, well, and outcrop data to determine the three-dimensional shape and structural features of south-central Alaska’s Susitna basin. This basin is located within the Aleutian-Alaskan convergent margin region and is expected to show effects of regional subduction zone processes. Aeromagnetic data, when filtered to highlight anomalies associated with sources within the upper few kilometers, show numerous linear northeast-trending highs and some linear north-trending highs. Comparisons to seismic reflection and well data show that these highs correspond to areas where late Paleocene to early Eocene volcanic layers have been locally uplifted due to folding and/or faulting. The combined magnetic and seismic reflection data suggest that the linear highs represent northeast-trending folds and north-striking faults. Several lines of evidence suggest that the northeast-trending folds formed during the middle Eocene to early Miocene and may have continued to be active in the Pliocene. The north-striking faults, which in some areas appear to cut the northeast-trending folds, show evidence of Neogene and probable modern movement. Gravity data facilitate estimates of the shape and depth of the basin. This was accomplished by separating the observed gravity anomaly into two components—one representing low-density sedimentary fill within the basin and one representing density heterogeneities within the underlying crystalline basement. We then used the basin anomaly, seismic reflection data, and well data to estimate the depth of the basin. Together, the magnetic, gravity, and reflection seismic analyses reveal an asymmetric basin comprising sedimentary rock over 4 km thick with steep, fault-bounded sides to the southwest, west, and north and a mostly gentle rise toward the east. Relations to the broader tectonic regime are suggested by fold axis orientations within the Susitna basin and neighboring Cook Inlet basin, which are roughly parallel to the easternmost part of the Alaska-Aleutian trench and associated Wadati-Benioff zone as it trends from northeast to north-northeast to northeast. An alignment between forearc basin folds and the subduction zone trench has been observed at other convergent margins, attributed to strain partitioning generated by regional rheologic variations that are associated with the subducting plate and arc magmatism. The asymmetric shape of the basin, especially its gentle rise to the east, may reflect uplift associated with flat-slab subduction of the Yakutat microplate, consistent with previous work that suggested Yakutat influence on the nearby Talkeetna Mountains and western Alaska Range. Yakutat subduction may also have contributed to Neogene and later reverse slip along north-striking faults within the Susitna basin.

Partitioning Pervasive Detrital Geochronologic Age Distributions in the Southern Alaskan Forearc

The extensive detrital zircon U-Pb geochronologic dataset presented here includes new and compiled data (N=38; n=8,006) from modern rivers that together comprehensively characterizes the geographic distribution of pervasive Mesozoic—Cenozoic igneous belts across mountainous regions in south-central Alaska, including the northern Chugach Mountains, Talkeetna Mountains, and western, central, and eastern Alaska Range. These data are compared to an extensive detrital zircon U-Pb dataset from Lower Cretaceous to Pliocene strata in the forearc basin (N=29; n=8,678) using a recently developed unmixing approach to investigate the variations in long-term provenance and sediment dispersal patterns in the basin in response to tectonic events. During the Early Cretaceous, the primary sediment source was an exhumed Jurassic arc located north of the basin, but new sediment derived from accretionary prism strata in the northern Chugach Mountains during the Late Cretaceous coincides with final suturing of the Insular terranes with North America and a change in plate kinematics. Eocene strata record major sediment derivation from the western Alaska Range after passage of a subducting spreading ridge. By the Oligocene, shallow subduction of the Yakutat microplate triggered a rejuvenation of exhumation in the northern Chugach Mountains that continued through the Early-Middle Miocene. And overall inboard shift of dominant source regions to the Talkeetna Mountains and central Alaska Range likely reflects the continued insertion of the shallow slab beneath south-central Alaska. The integrated approach of strategic modern river sampling and comprehensive basin strata characterization in conjunction with an inverse Monte Carlo approach of mixture modeling demonstrates a useful approach for partitioning of widespread and pervasive ages in sediment source terranes.

Cenozoic tectono-thermal history of the southern Talkeetna Mountains, Alaska: Insights into a potentially alternating convergent and transform plate margin

AbstractThe Mesozoic–Cenozoic convergent margin history of southern Alaska has been dominated by arc magmatism, terrane accretion, strike-slip fault systems, and possible spreading-ridge subduction. We apply 40Ar/39Ar, apatite fission-track (AFT), and apatite (U-Th)/He (AHe) geochronology and thermochronology to plutonic and volcanic rocks in the southern Talkeetna Mountains of Alaska to document regional magmatism, rock cooling, and inferred exhumation patterns as proxies for the region’s deformation history and to better delineate the overall tectonic history of southern Alaska. High-temperature 40Ar/39Ar thermochronology on muscovite, biotite, and K-feldspar from Jurassic granitoids indicates postemplacement (ca. 158–125 Ma) cooling and Paleocene (ca. 61 Ma) thermal resetting. 40Ar/39Ar whole-rock volcanic ages and 45 AFT cooling ages in the southern Talkeetna Mountains are predominantly Paleocene–Eocene, suggesting that the mountain range has a component of paleotopography that formed during an earlier tectonic setting. Miocene AHe cooling ages within ∼10 km of the Castle Mountain fault suggest ∼2–3 km of vertical displacement and that the Castle Mountain fault also contributed to topographic development in the Talkeetna Mountains, likely in response to the flat-slab subduction of the Yakutat microplate. Paleocene–Eocene volcanic and exhumation-related cooling ages across southern Alaska north of the Border Ranges fault system are similar and show no S-N or W-E progressions, suggesting a broadly synchronous and widespread volcanic and exhumation event that conflicts with the proposed diachronous subduction of an active west-east–sweeping spreading ridge beneath south-central Alaska. To reconcile this, we propose a new model for the Cenozoic tectonic evolution of southern Alaska. We infer that subparallel to the trench slab breakoff initiated at ca. 60 Ma and led to exhumation, and rock cooling synchronously across south-central Alaska, played a primary role in the development of the southern Talkeetna Mountains, and was potentially followed by a period of southern Alaska transform margin tectonics.

Early Bajocian belemnites of Southcentral Alaska: new data and new perspectives on mid-Middle Jurassic Megateuthididae and Belemnopseidae biogeography

In this study, Lower Bajocian belemnites from Southcentral Alaska are formally investigated and described for the first time. Representatives of the families Megateuthididae (Belemnitina) and Belemnopseidae (Belemnopseina) occur in the Parabigotites crassicostatus to Sphaeroceras oblatum ammonite zones (Propinquans to Humphriesianum chronozones) within the Tuxedni Group of the Cook Inlet region and the Talkeetna Mountains. Among Megateuthididae, we introduce the first records of Brevibelus from Alaska, and the first records of Brevibelus gingensis and Eocylindroteuthis from outside Europe. The new taxa Pseudosimobelus tuxedniensis Dzyuba & C. D. Schraer gen. et sp. nov., Eocylindroteuthis gracilenta Dzyuba & C. D. Schraer sp. nov. and Eocylindroteuthis riegrafi Dzyuba & C. D. Schraer sp. nov. are also described. We conclude from our morphological analysis that Pseudosimobelus gen. nov. represents a transitional form between Megateuthididae and Cylindroteuthididae or may even have been ancestral to the latter. The single rostrum of Belemnopseidae identified herein as Hibolithes? sp. juv. is the earliest record of the family from Alaska. Biogeographically, the belemnites represent pan-Tethyan (Brevibelus and Hibolithes?), typically west Tethyan (Brevibelus gingensis, Brevibelus cf. breviformis and Eocylindroteuthis) and endemic north-east Pacific (Pseudosimobelus gen. nov. and Eocylindroteuthis spp. nov.) elements. These palaeontological data suggest a location of the Peninsular terrane in the Early Bajocian south of the present location. We postulate that the appearance of Brevibelus, Eocylindroteuthis and possibly Hibolithes near the north-western shores of North America in the late Early Bajocian was due to their westward migration via the Hispanic Corridor, the migratory seaway between the western Tethys and the eastern Pacific.http://zoobank.org/urn:lsid:zoobank.org:pub:BCF66B22-795B-46EC-9C86-2595433E22EF

Geology and Timing of Ore Formation in the Willow Creek Gold District, Talkeetna Mountains, Southern Alaska

The Willow Creek mining district was the third-largest lode gold district in Alaska in the 20 th century, having produced 19 metric tons (t) of gold from vein deposits and another 2 t of gold from associated placer deposits. The district is located in the southern Talkeetna Mountains, north of the Castle Mountain strike-slip fault system. Most gold occurs in widespread mineralized quartz veins hosted by granitic rocks of the Willow Creek pluton. A geochronologic study of granitic rocks, alteration minerals, and adjacent metamorphic rocks was conducted in order to better understand the timing of magmatism, thermochronologic evolution, and age of gold mineralization in the district. New U-Pb zircon dates from the Talkeetna batholith, including the Willow Creek pluton, range from 72.1 to 71.3 Ma. The U-Pb dates are similar to reported U-Late Cretaceous Pb and K-Ar dates from elsewhere in the study area, and tightly constrain the age of plutonic rocks that host the Willow Creek lode gold deposits. Granitic rocks that intrude the Willow Creek pluton along its northwestern margin yield U-Pb dates of 75.7 and 70.8 Ma. The 40 Ar/ 39 Ar thermochronologic data for the Willow Creek pluton and other granitoids reveal rapid postemplacement cooling through hornblende, muscovite, and biotite closure temperatures. Thermochronologic data from K-feldspars document slower cooling rates through lower temperatures, which may be related to slow uplift and exhumation during the Paleocene-Eocene. The data are compatible with a simple tectonothermal history following emplacement of the plutonic rocks, with essentially no evidence of significant thermal disturbance of the rocks after about 64 Ma. Uranium-thorium-lead dating of hydrothermal phosphate minerals (monazite and xenotime) and 40 Ar/ 39 Ar dates from hydrothermal sericite from wall rocks and veins indicate that the main stage of gold mineralization occurred at about 67 to 65 Ma. Combining U-Th-Pb data from monazites from the Independence, Talkeetna, Gold Bullion, and Fern deposits gave an average of 65.4 ± 2.3 Ma; hydrothermal xenotime from the Independence vein also yields a statistically indistinguishable age of 64.5 ± 2.3 Ma. These ages overlap 40 Ar/ 39 Ar dates of 67 to 66 Ma obtained from hydrothermal sericite associated with formation of the gold-bearing veins, and the age of a muscovite from a weakly mineralized pegamatite that cuts the Willow Creek pluton. In contrast to previous studies that suggested that at least some mineralization was Eocene in age, we found no compelling evidence for significant gold mineralization within the district at this time. The 40 Ar/ 39 Ar and K-Ar dates from graphitic white micas from schist south of the Hatcher Pass fault zone yield ages of 61 to 50 Ma that are probably related to uplift and cooling of these Late Cretaceous metasedimentary rocks. The absence of evidence for postemplacement thermal events affecting the plutonic rocks of the Willow Creek district to the north indicates that the schists and granitoids do not share a similar tectonothermal history, and that they have been juxtaposed by subsequent faulting along the Hatcher Pass fault zone.

Late Oligocene to present contractional structure in and around the Susitna basin, Alaska—Geophysical evidence and geological implications

The Cenozoic Susitna basin lies within an enigmatic lowland surrounded by the Central Alaska Range, Western Alaska Range (including the Tordrillo Mountains), and Talkeetna Mountains in south-central Alaska. Some previous interpretations show normal faults as the defining structures of the basin (e.g., Kirschner, 1994). However, analysis of new and existing geophysical data shows predominantly (Late Oligocene to present) thrust and reverse fault geometries in the region, as previously proposed by Hackett (1978). A key example is the Beluga Mountain fault where a 50-mGal gravity gradient, caused by the density transition from the igneous bedrock of Beluga Mountain to the >4-km-thick Cenozoic sedimentary section of Susitna basin, spans a horizontal distance of ∼40 km and straddles the topographic front. The location and shape of the gravity gradient preclude a normal fault geometry; instead, it is best explained by a southwest-dipping thrust fault, with its leading edge located several kilometers to the northeast of the mountain front, concealed beneath the shallow glacial and fluvial cover deposits. Similar contractional fault relationships are observed for other basin-bounding and regional faults as well. Contractional structures are consistent with a regional shortening strain field inferred from differential offsets on the Denali and Castle Mountain right-lateral strike-slip fault systems.

Rhyolite characterization and distribution in central Alaska

Fine grained volcanic rocks are common in lithic assemblages of interior Alaska and are amenable to geochemical characterization using a variety of analytical techniques. Our study focuses on rhyolite with the intent of identifying and delineating geochemical groups that may correlate to specific geological source areas. PXRF technology was used to analyze 676 rhyolite artifacts from 123 sites in interior Alaska. Our preliminary results recognize ten distinct geochemical groups that appear to correlate with distinct geological sources. While geological origins of eight of the ten groups identified remain unknown, two geological sources have been pinpointed, one (represented by Group H) is located in the central Alaska Range and the second (Group G) is in the Talkeetna Mountains. The provisional framework of geochemical variation among tool quality rhyolite sources in this region is an important first step toward a more robust understanding of prehistoric landuse in interior Alaska.

Paleoenvironment and paleoecology of a Late Paleocene high-latitude terrestrial succession, Arkose Ridge Formation at Box Canyon, southern Talkeetna Mountains, Alaska

Paleogene sedimentary rocks of the Arkose Ridge Formation (Talkeetna Mountains, Alaska) preserve a record of a fluvial–lacustrine depositional environment and its forested ecosystem in an active basin among the convergent margin tectonic processes that shaped southern Alaska. An ~800m measured succession at Box Canyon indicates braid-plain deposition with predominantly gravelly deposits low in the exposure to sandy and muddy facies associations below an overlying lava flow sequence. U–Pb geochronology on zircons from a tuff and a sandstone within the measured section, as well as an Ar/Ar date from the overlying lava constrain the age of the sedimentary succession to between ~59Ma and 48Ma. Fossil plant remains occur throughout the Arkose Ridge Formation as poorly-preserved coalified woody debris and fragmentary leaf impressions. At Box Canyon, however, a thin lacustrine depositional lens of rhythmically laminated mudrocks yielded fish fossils and a well-preserved floral assemblage including foliage and reproductive organs representing conifers, sphenopsids, monocots, and dicots. Leaf physiognomic methods to estimate paleoclimate were applied to the dicot leaf collection and indicate warm temperate paleotemperatures (~11–15±~4°C MAT) and elevated paleoprecipitation (~120cm/yr MAP) estimates as compared to modern conditions; results that are parallel with previously published estimates from the partly coeval Chickaloon Formation deposited in more distal depositional environments in the same basin. The low abundance of leaf herbivory in the Box Canyon dicot assemblage (~9% of leaves damaged) is also similar to the results from assemblages in the meander-plain depositional systems of the Chickaloon. This new suite of data informs models of the tectonostratigraphic evolution of southern Alaska and the developing understanding of terrestrial paleoecology and paleoclimate at high latitudes during the Late Paleocene–Early Eocene greenhouse climate phase.

Integrated provenance record of a forearc basin modified by slab‐window magmatism: detrital‐ zircon geochronology and sandstone compositions of the Paleogene Arkose Ridge Formation, south‐central Alaska

AbstractThis study presents an integrated provenance record for ancient forearc strata in southern Alaska. Paleocene–Eocene sedimentary and volcanic strata >2000 m thick in the southern Talkeetna Mountains record nonmarine sediment accumulation in a remnant forearc basin. In these strata, igneous detritus dominates conglomerate and sandstone detrital modes, including plutonic and volcanic clasts, plagioclase feldspar, and monocrystalline quartz. Volcanic detritus is more abundant and increases upsection in eastern sandstone and conglomerate. U‐Pb ages of >1600 detrital zircons from 19 sandstone samples document three main populations: 60–48 Ma (late Paleocene–Eocene; 14% of all grains), 85–60 Ma (late Cretaceous–early Paleocene; 64%) and 200–100 Ma (Jurassic–Early Cretaceous; 11%). Eastern sections exhibit the broadest distribution of detrital ages, including a principal population of late Paleocene–Eocene ages. In contrast, central and western sections yield mainly late Cretaceous–early Paleocene detrital ages. Collectively, our results permit reconstruction of individual fluvial drainages oriented transverse to a dissected arc. Specifically, new data suggest: (1) Detritus was eroded from volcanic‐plutonic sources exposed along the arcward margin of the sampled forearc basin fill, primarily Jurassic–Paleocene magmatic‐arc plutons and spatially limited late Paleocene–Eocene volcanic centers; (2) Eastern deposystems received higher proportions of juvenile volcanic detritus through time from late Paleocene–Eocene volcanic centers, consistent with emplacement of a slab window beneath the northeastern part of the basin during spreading‐ridge subduction; (3) Western deposystems transported volcanic‐plutonic detritus from Jurassic–Paleocene remnant arc plutons and local eruptive centers that flanked the northwestern part of the basin; (4) Diagnostic evidence of sediment derivation from accretionary‐prism strata exposed trenchward of the basin fill is lacking. Our results provide geologic evidence for latest Cretaceous–early Paleocene exhumation of arc plutons and marine forearc strata followed by nonmarine sediment accumulation and slab‐window magmatism. This inferred history supports models that invoke spreading‐ridge subduction beneath southern Alaska during Paleogene time, providing a framework for understanding a mature continental‐arc/forearc‐basin system modified by ridge subduction. Conventional provenance models predict reduced input of volcanic detritus to forearc basins during progressive exhumation of the volcanic edifice and increasing exposure of subvolcanic plutons. In contrast, our results show that forearc basins influenced by ridge subduction may record localized increases in juvenile volcanic detritus during late‐stage evolution in response to accumulation of volcanic sequences formed from slab‐window eruptive centers.

Focused exhumation in the syntaxis of the western Chugach Mountains and Prince William Sound, Alaska

The western Chugach Mountains and Prince William Sound are located in a syntaxial bend, which lies above flat-slab subduction of the Yakutat microplate and inboard of the Yakutat collision zone of southern Alaska. The syntaxis is characterized by arcuate fault systems and steep, high topography, which suggest focused uplift and exhumation of the accretionary prism. We examined the exhumation history with low-temperature thermochronometry of 42 samples collected across the region. These new apatite (U-Th)/He, apatite fission-track, zircon (U-Th)/He, and zircon fission-track ages, combined with ages from surrounding regions, show a bull’s-eye pattern, with the youngest ages focused on the western Chugach syntaxis. The ages have ranges of ca. 10–4 Ma, ca. 35–11 Ma, ca. 33–25 Ma, and ca. 44–27 Ma, respectively. The youngest ages are located on the south (windward) side of the Chugach Mountains and just north of the Contact fault. Sequentially higher closure temperature systems are nested across Prince William Sound in the south, the Chugach Mountains, and the Talkeetna Mountains to the north. Computed exhumation rates typically are 0.2 mm/yr across Prince William Sound, increase abruptly to ∼0.7 mm/yr across and adjacent to the Contact fault system, and decrease to ∼0.4 mm/yr north of the core of the Chugach Mountains. The abrupt age and exhumation rate changes centered on the Contact fault system suggest that it may be a critical structural system for facilitating rock uplift. Our data are most consistent with Yakutat flat-slab subduction starting in the Oligocene, and since then ∼11 km of rock uplift north of the Contact fault and ∼4–5 km of rock uplift in Prince William Sound to the south. These data are consistent with a deformation model where the western Chugach core has approached long-term exhumational steady state, though exhumation rates have probably increased in the last ∼5 m.y. We interpret that rock uplift in the overriding wedge has been driven dominantly by underplating, with long-term vertical displacement concentrated at the southern edge of the Chugach Mountains and centered on the Contact fault system. Though our data do not unequivocally differentiate between Pliocene tectonic- or climate-related causes for increased exhumation in the last ∼5 m.y., we interpret the increased rates to be due to increased influx of underplated sediments that are derived from erosion in the Saint Elias orogen collision zone.

Thermochronology of the Talkeetna intraoceanic arc of Alaska: Ar/Ar, U‐Th/He, Sm‐Nd, and Lu‐Hf dating

As one of two well‐exposed intraoceanic arcs, the Talkeetna arc of Alaska affords an opportunity to understand processes deep within arcs. This study reports new Lu‐Hf and Sm‐Nd garnet ages, 40Ar/39Ar hornblende, mica and whole‐rock ages, and U‐Th/He zircon and apatite ages from the Chugach Mountains, Talkeetna Mountains, and Alaska Peninsula, which, in conjunction with existing geochronology, constrain the thermal history of the arc. Zircon U‐Pb ages establish the main period of arc magmatism as 202–181 Ma in the Chugach Mountains and 183–153 Ma in the eastern Talkeetna Mountains and Alaska Peninsula. Approximately 184 Ma Lu‐Hf and ∼182 Ma Sm‐Nd garnet ages indicate that 25–35 km deep sections of the arc remained above ∼700°C for as much as 15 Myr. The 40Ar/39Ar hornblende ages are chiefly 194–170 Ma in the Chugach Mountains and 175–150 Ma in the Talkeetna Mountains and Alaska Peninsula but differ from zircon U‐Pb ages in the same samples by as little as 0 Myr and as much as 33 Myr, documenting a spatially variable thermal history. Mica ages have a broader distribution, from ∼180 Ma to 130 Ma, suggesting local cooling and/or reheating. The oldest U‐Th/He zircon ages are ∼137 to 129 Ma, indicating no Cenozoic regional heating above ∼180°C. Although the signal is likely complicated by Cretaceous and Oligocene postarc magmatism, the aggregate thermochronology record indicates that the thermal history of the Talkeetna arc was spatially variable. One‐dimensional finite difference thermal models show that this kind of spatial variability is inherent to intraoceanic arcs with simple construction histories.

Gaudryceras izumienseMatsumoto and Morozumi, a Maastrichtian Ammonoid from Hokkaido and Alaska and Its Biostratigraphic Implications

Gaudryceras izumiense Matsumoto and Morozumi, 1980, an ammonoid of early Maastrichtian age, occurs frequently in the Izumi Group in the Izumi Mountains, Southwest Japan. Described herein are two specimens referable to this species that were recently collected from the Hakobuchi Formation of the Yezo Group in the Hobetsu area, Hokkaido and from the Matanuska Formation in the southern Talkeetna Mountains, Alaska. In the past, a precise biostratigraphic correlation of the uppermost Cretaceous in the North Pacific realm has been difficult because of the lack of common zonal-index taxa, but the discovery of G. izumiense in Hokkaido and Alaska has made it possible to correlate lower Maastrichtian strata in these localities with coeval strata in Southwest Japan. Gaudryceratid ammonoids are fairly abundant in the uppermost Cretaceous of the North Pacific realm, and consequently, they are ideal taxa for the establishment of a precise biostratigraphic correlation framework for the uppermost Cretaceous in this vast area.

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

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.

Latest Cretaceous forearc basin development along an accretionary convergent margin: South-central Alaska

Upper Cretaceous sedimentary strata exposed in south-central Alaska provide insight on tectonic processes that shaped the northern Pacific margin following accretion of the Wrangellia composite terrane, the largest addition of crust to North America over the past 100 m.y. Sedimentologic, geochronologic, biostratigraphic, and petrographic data from the Matanuska Formation permit reconstruction of the tectono-sedimentary history of strata in a forearc basin constructed upon accreted oceanic-arc crust. The Matanuska Formation consists of >3 km of sedimentary strata exposed in the northern Chugach Mountains, Matanuska Valley, and southern Talkeetna Mountains of interior south-central Alaska. Measured stratigraphic sections and lithofacies analyses demonstrate that mass slumps and slides, debris flows, and turbidity currents deposited Campanian–Maastrichtian sandstone, conglomerate, and mudstone on a gullied, trenchward-dipping submarine ramp. Benthic foraminifera, inoceramid bivalves, and Nereites ichnogenera indicate deposition mainly at bathyal water depths. Sandstone and conglomerate petrofacies are characterized by monocrystalline quartz, plagioclase feldspar, and volcanic lithic fragments (Q39F40L21, Qm29F40Lt32, Lm25Lv42Ls32, and Qm42P54K4). Jurassic–Cretaceous arc plutons exposed north of the basin were an important sediment source, based on U-Pb zircon ages of granitoid clasts from conglomerate and detrital zircons from sandstone. Coeval arc plutons were unroofed relatively quickly, judging by the presence of 77–71 Ma detrital zircons in sandstone and 79–77 Ma granitic clasts in conglomerate, together with Maastrichtian (71–65 Ma) ammonite and foraminifera fossils. Sparse Paleozoic–Triassic detrital zircons indicate minor sediment contribution from inboard sources, including the Yukon-Tanana composite terrane and recycled Jurassic–Cretaceous sedimentary strata (Kahiltna assemblage). New data from the upper Matanuska Formation, together with recent studies from age-equivalent strata exposed in the Alaska Range and Wrangell Mountains, provide an exceptional example of basin development along a subduction margin shortly following accretion of an oceanic arc. Forearc basin development was dominated by subsidence and sediment gravity flow deposits enriched in plutonic and volcanic clasts eroded from both remnant- and coeval-arc plutons. Within the arc, newly recognized conglomerate in the northern Talkeetna Mountains records erosion of coeval- and remnant-arc source terranes to the south and Precambrian–Paleozoic sources to the north. Farther inboard, syndepositional shortening prompted thrust-top basin development and accumulation of alluvial-lacustrine strata derived from both the former continental margin to the north and accreted oceanic rocks to the south. Regional subsidence and basin development terminated during late Maastrichtian–early Paleocene time, coincident with subduction of progressively younger oceanic lithosphere inboard of an oceanic spreading center.

Magmatic development of an intra-oceanic arc: High-precision U-Pb zircon and whole-rock isotopic analyses from the accreted Talkeetna arc, south-central Alaska

The accreted Talkeetna arc, south-central Alaska, is an archetypal example of an intra-oceanic arc crustal section. Arc-related units include all levels of a lithospheric column, from residual mantle harzburgites to sub-aerial volcanic rocks, and provide a rare opportunity to study intrusive arc processes directly. We present the first high-precision U-Pb zircon ages and an extensive new data set of 143Nd/144Nd and 87Sr/86Sr isotopic analyses from Talkeetna arc plutonic rocks. These data provide new insight into the timing and extent of Talkeetna arc magmatism, the tectonic development of the arc, and the role of preexisting crustal material in the generation of arc magmas. New analyses from the exposed arc crustal section in the Chugach Mountains indicate that the Talkeetna arc began to develop as a juvenile [ϵNd(t) = 6.0–7.8 and 87Sr/86Srint = 0.703379–0.703951] intra-oceanic arc between 202.1 and 181.4 Ma. This initial arc plutonism was followed ca. 180 Ma by a northward shift in the arc magmatic axis and generation of a large plutonic suite in the Talkeetna Mountains. Plutons from the eastern Talkeetna Mountains yield U-Pb zircon ages of 177.5–168.9 Ma and are isotopically similar to the Chugach Mountains intrusions [ϵNd(t) = 5.6–7.2 and 87Sr/86Srint = 0.703383–0.703624]. However, plutons from the western Talkeetna Mountains batholith have more evolved initial isotopic ratios [ϵNd(t) = 4.0–5.5 and 87Sr/86Srint = 0.703656–0.706252] and contain inherited xenocrystic Carboniferous–Triassic zircons. These data are interpreted to represent assimilation of adjacent Wrangellia crust into arc magmas and require amalgamation of the Talkeetna arc with the Wrangellia terrane by ca. 153 Ma. As a whole, the combined U-Pb zircon and isotopic data from the Chugach and Talkeetna Mountains indicate that the main volume of Talkeetna arc magmas formed with little or no involvement of preexisting crustal material. These observations justify the use of the Talkeetna arc as a type section for intrusive intra-oceanic arc crust.

The recent warming of permafrost in Alaska

This paper reports results of an experiment initiated in 1977 to determine the effects of climate on permafrost in Alaska. Permafrost observatories with boreholes were established along a north–south transect of Alaska in undisturbed permafrost terrain. The analysis and interpretation of annual temperature measurements in the boreholes and daily temperature measurements of the air, ground and permafrost surfaces made with automated temperature loggers are reported. Permafrost temperatures warmed along this transect coincident with a statewide warming of air temperatures that began in 1977. At two sites on the Arctic Coastal Plain, the warming was seasonal, greatest during “winter” months (October through May) and least during “summer” months (June through September). Permafrost temperatures peaked in the early 1980s and then decreased in response to slightly cooler air temperatures and thinner snow covers. Arctic sites began warming again typically about 1986 and Interior Alaska sites about 1988. Gulkana, the southernmost site, has been warming slowly since it was drilled in 1983. Air temperatures were relatively warm and snow covers were thicker-than-normal from the late 1980s into the late 1990s allowing permafrost temperatures to continue to warm. Temperatures at some sites leveled off or cooled slightly at the turn of the century. Two sites (Yukon River Bridge and Livengood) cooled during the period of observations. The magnitude of the total warming at the surface of the permafrost (through 2003) was 3 to 4 °C for the Arctic Coastal Plain, 1 to 2 °C for the Brooks Range including its northern and southern foothills, and 0.3 to 1 °C south of the Yukon River. While the data are sparse, permafrost is warming throughout the region north of the Brooks Range, southward along the transect from the Brooks Range to the Chugach Mountains (except for Yukon River and Livengood), in Interior Alaska throughout the Tanana River region, and in the region south of the Alaska Range from Tok westward to Gulkana (in the Copper River Valley) and beyond to the Talkeetna Mountains. Thermal offset allows permafrost to survive in the presence of positive annual mean ground surface temperatures and was observed repeatedly since 1987 at two sites. The observed warming has not produced an increasing trend in maximum active layer thicknesses due to its seasonality. Near Healy, permafrost has been thawing at the top since the late 1980s at about 10 cm/yr. At Gulkana, permafrost was thawing from the bottom at a rate of 4 cm/yr that accelerated to 9 cm/yr after 2000.

Sedimentology and provenance of the Upper Jurassic Naknek Formation, Talkeetna Mountains, Alaska: Bearings on the accretionary tectonic history of the Wrangellia composite terrane

Research Article| May 01, 2005 Sedimentology and provenance of the Upper Jurassic Naknek Formation, Talkeetna Mountains, Alaska: Bearings on the accretionary tectonic history of the Wrangellia composite terrane Jeffrey M. Tropaff; Jeffrey M. Tropaff 1Department of Geology, Bucknell University, Lewisburg, Pennsylvania 17837, USA Search for other works by this author on: GSW Google Scholar Darren A. Szuch; Darren A. Szuch 1Department of Geology, Bucknell University, Lewisburg, Pennsylvania 17837, USA Search for other works by this author on: GSW Google Scholar Matthew Rioux; Matthew Rioux 2Department of Geological Sciences, University of California, Santa Barbara, California 93106, USA Search for other works by this author on: GSW Google Scholar Robert B. Blodgett Robert B. Blodgett 3U.S. Geological Survey, 4200 University Drive, Anchorage, Alaska 99508, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Jeffrey M. Tropaff 1Department of Geology, Bucknell University, Lewisburg, Pennsylvania 17837, USA Darren A. Szuch 1Department of Geology, Bucknell University, Lewisburg, Pennsylvania 17837, USA Matthew Rioux 2Department of Geological Sciences, University of California, Santa Barbara, California 93106, USA Robert B. Blodgett 3U.S. Geological Survey, 4200 University Drive, Anchorage, Alaska 99508, USA Publisher: Geological Society of America Received: 24 Jan 2004 Revision Received: 30 Jun 2004 Accepted: 10 Sep 2004 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (2005) 117 (5-6): 570–588. https://doi.org/10.1130/B25575.1 Article history Received: 24 Jan 2004 Revision Received: 30 Jun 2004 Accepted: 10 Sep 2004 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Jeffrey M. Tropaff, Darren A. Szuch, Matthew Rioux, Robert B. Blodgett; Sedimentology and provenance of the Upper Jurassic Naknek Formation, Talkeetna Mountains, Alaska: Bearings on the accretionary tectonic history of the Wrangellia composite terrane. GSA Bulletin 2005;; 117 (5-6): 570–588. doi: https://doi.org/10.1130/B25575.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Analysis of the Upper Jurassic Naknek Formation in the Talkeetna Mountains, Alaska, documents synorogenic sedimentation in a forearc basin along the outboard (southern) margin of the allochthonous Peninsular terrane during accretion to the western North American continental margin. New geochronologic, sedimentologic, and compositional data define a two-part stratigraphy for the Naknek Formation. Microfossil, megafossil, and U-Pb clast ages document early Oxfordian to early Kimmeridgian deposition of the lower 690 m of the Naknek Formation and early Kimmeridgian to early Tithonian deposition of the upper 225 m of the Naknek Formation. Lithofacies and paleocurrent data from the lower Naknek Formation demonstrate initial deposition on a high-gradient, southward-dipping basin floor. Submarine mass flows deposited poorly sorted, cobbl-boulder conglomerate in proximal fan-delta environments. Gravelly mass flows transformed downslope into sandy turbidity currents on a muddy pro-delta slope. During early Kimmeridgian to early Tithonian time, fan-delta environments were replaced by lower gradient marine shelf environments characterized by deposition of cross-stratified sandstone and bioturbated mudstone. Sourc-diagnostic clasts, feldspathic sandstone compositions, southward-directed paleocurrent indicators, and U-Pb zircon ages of plutonic clasts (167.6 ± 0.3 Ma; 166.5 ± 0.2 Ma, 164–159 Ma, 156.2 ± 0.4 Ma) indicate that the Naknek Formation was derived primarily from volcanic and plutonic source terranes exposed along the northern basin margin in the southern Talkeetna Mountains. Geologic mapping documents the Little Oshetna fault, a newly identified northward-dipping reverse fault that bounds the northern margin of the Naknek Formation in the Talkeetna Mountains. The concentration of boulder-rich mass-flow deposits in the foot-wall of the fault in combination with geochronologic and compositional data suggest that sedimentation was coeval with Late Jurassic shortening along the fault and exhumation of plutonic source terranes exposed in the hanging wall of the fault. From a regional perspective, coars-grained forearc sedimentation and pluton exhumation along the outboard (southern) segment of the Peninsular terrane were coeval with crustal-scale shortening and synorogenic sedimentation in retroarc basins along the inboard (northern) margin of the Wrangellia terrane (Kahiltna, Nutzotin, and Wrangell Mountains basins). We interpret the regional and synchronous nature of Late Jurassic crustal-scale deformation and synorogenic sedimentation in south-central Alaska as reflecting either initial collision of the Wrangellia and Peninsular terranes with the former continental margin of western North America or amalgamation of the two terranes prior to collision. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Subduction erosion of the Jurassic Talkeetna-Bonanza arc and the Mesozoic accretionary tectonics of western North America

The Jurassic Talkeetna volcanic arc of south-central Alaska is an oceanic island arc that formed far from the North American margin. Geochronological, geochemical, and structural data indicate that the arc formed above a north-dipping subduction zone after ca. 201 Ma. Magmatism migrated northward into the region of the Talkeetna Mountains ca. 180 Ma. We interpret this magmatism as the product of removal of the original forearc while the arc was active, mainly by tectonic erosion. Rapid exhumation of the arc after ca. 160 Ma coincided with the sedimentation of the coarse clastic Naknek Formation. This exhumation event is interpreted to reflect collision of the Talkeetna arc with either the active margin of North America or the Wrangellia composite terrane to the north along a second north-dipping subduction zone. The juxtaposition of accreted trench sedimentary rocks (Chugach terrane) against the base of the Talkeetna arc sequence requires a change from a state of tectonic erosion to accretion, probably during the Late Jurassic (before 150 Ma), and definitely before the Early Cretaceous (ca. 125 Ma). The change from erosion to accretion probably reflects increasing sediment flux to the trench due to collision ca. 160 Ma.