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The Devonian connection between the Brooks Range of Alaska, USA, with the continental margin of Arctic Canada and its subsequent Jurassic–Cretaceous counterclockwise rotation to form the Amerasian Basin, is a highly debated topic in Arctic tectonics. This resource-rich region was assembled from terranes that formed part of Laurentia or Baltica, or were juvenile oceanic arcs in the early Paleozoic that were brought together during Caledonian Orogenesis and the subsequent collision that formed Pangea (Uralide Orogeny). Elements of these orogens, as well as older ones, are predicted to occur in the Brooks Range of Arctic Alaska. This study presents the first combined zircon U-Pb and oxygen data from six Brooks Range metasedimentary units with assumed Neoproterozoic to Devonian ages. Three distinct detrital zircon patterns are identified in these units: (1) those with Neoproterozoic maximum depositional ages characteristic of the Timanide Orogen of northern Baltica and adjacent parts of Siberia, (2) an almost unimodal Siluro–Ordovician (443.5 ± 2.3 Ma) detrital zircon population consistent with the oceanic Apoon arc believed to have existed off shore of northern Laurentia and to have accreted to the North Slope subterrane during the Caledonian event, and (3) those with Middle Devonian maximum depositional ages consistent with post-accretion extension during the final (Scandian) phase of Caledonian Orogenesis. Oxygen isotopes from the same zircons reveal minor to significant crustal contamination with approximately two thirds (n = 255/405) having δ18O values >5.9‰ (above the mantle field of 5.3 ± 0.6‰). Pattern 1 units exhibit a progressive increase in δ18O values throughout the Proterozoic (5.99 to 9.29‰) indicative of increasing crustal growth and Timanide age zircons yield average δ18O values of 7.18 ± 0.64‰ (n = 26) suggestive of more crustal influence than Caledonian age zircons, possibly reflecting northern Baltica signatures. The unimodal population in Pattern 2 yields average δ18O values of 5.49 ± 0.66‰ (n = 17) and 6.02 ± 0.27‰ (n = 23) prior to and during, respectively, the main Caledonian event and suggest derivation from Devonian juvenile arc sources possibly representing the initiation of the collision between Laurentia and Baltica. Similar to Pattern 1, the δ18O values associated with Pattern 3 show a progressive increase in δ18O values throughout the Proterozoic (5.00 to 9.39‰). However, Pattern 3 also exhibits a distinct juvenile fingerprint (6.13 ± 0.24‰, n = 51) during the main Caledonian event and a slight increase to 7.12 ± 1‰ (n = 7) in post-Caledonian zircons possibly suggest correlating with a post-accretion phase in which proximally sourced zircon-bearing detritus was deposited in extension-related basins marking the joining of Laurentia and Baltica.

The Arctic Alaska terrane of northern Alaska and Yukon is one of several exotic terranes in the North American Cordillera with putative early Paleozoic connections to the northern Caledonian-Appalachian orogen. The U-Pb and 40 Ar/ 39 Ar isotopic data from detrital minerals in pre-Mississippian sedimentary units of the northeastern Brooks Range are presented here to investigate the consequences of Caledonian orogenesis on sediment dispersal trends and the paleogeography of northern Laurentia. Neoproterozoic–Cambrian siliciclastic rocks of the informal Firth River group and the Neruokpuk Formation were most likely deposited along a passive margin that sourced Archean and Paleoproterozoic basement rocks of the Canadian shield and reworked Mesoproterozoic and younger sedimentary units. These strata are overlain by a Lower Ordovician–Lower Devonian succession of fine-grained siliciclastic turbidites, herein referred to as the Clarence River group, which records a prominent shift in provenance most likely associated with the onset of the Caledonian-Appalachian orogeny in northeast Laurentia. U-Pb detrital zircon age populations of ca. 470–420 and 990–820 Ma, along with 40 Ar/ 39 Ar detrital muscovite ages of ca. 470–430 Ma, support provenance connections with the East Greenland Caledonides, Pearya, and Svalbard. Partially reset 40 Ar/ 39 Ar ages in these sedimentary successions are linked to low-grade metamorphism associated with the Early–Middle Devonian Romanzof orogeny, a poorly understood tectonic event in the Brooks Range that is possibly associated with the emplacement of an allochthonous oceanic assemblage, herein named the Whale Mountain allochthon.

Mesozoic orogenic belts fringe the Alaska and eastern Russia portion of the Arctic Basin. From west to east, these include the fold belts of Novaya Zemlya, Taimyr Peninsula, northern Verkhoyansk–Kolyma, Chukotka and the Brooks Range, as well as their continuations onto the continental shelves. The Taimyr and Novaya Zemlya structures were traditionally interpreted as the continuation of the late Palaeozoic Uralian orogenic belt. This is probably correct for Taimyr, but not for Novaya Zemlya, where shortening post-dates Uralian deformation. The Mesozoic tectonic evolution of the Verkhoyansk–Kolyma, Chukotka and Brooks Range orogens relates to the accretion of numerous continental and arc terranes to the Siberian and North American margins starting in the Late Jurassic and driven by palaeo-Pacific dynamics. This history is complicated by the opening of the Amerasia Basin of the Arctic, which displaced the Arctic Alaska–Chukotka microplate from a position adjacent to Arctic Canada towards the palaeo-Pacific. Although the Chukotka fold belt and the Brooks Range both formed along the southern edge of Arctic Alaska–Chukotka, most shortening took place prior to Amerasia Basin opening. The remoteness of this region and the complexity of its geology has left numerous questions regarding its tectonic evolution unresolved, providing rich avenues for future research.

New U-Pb geochronology and a large and growing geochemical data base has clarified the age and composition of the extensive magmatic belts of NE Russia, allowing a better understanding of their relation to the plate tectonic evolution of the circum- Pacific margins of Russia, Alaska and the North American Cordillera, as well as to plate tectonic events that formed the Arctic Ocean. Prior to the magmatism described here, the paleo-Pacific margin of Siberia (now the Verkhoyansk-Kolyma orogen), Arctic Chukotka and the Brooks Range, Alaska, shared passive margin sedimentation from the Triassic to mid-Jurassic. Deformation along this once contiguous margin began with closure of deep-water basins and culminated with emplacement of oceanic and arc rocks onto the margin with continuing shortening of the margin into the Jura-Cretaceous. The Main Kolyma batholith belt (calc-alkaline granites to granodiorites) (158-144 Ma, peak 150±3 Ma) and the paired Uyandina-Yasachnaya volcanic arc are thought to have resulted from west-dipping subduction. Northern belt plutons (calc-alkaline) are 140-129 Ma and mostly post-date the E-W trending oroclinal bend of the northernVerkhoyansk fold belt. Calc-alkaline granites to granodiorites spanning 125-100 Ma intruded across Chukotka (Artic Russia) and Alaska during extension, right lateral shear and subduction zone retreat towards the Pacific perhaps coeval with the onset of rifting in the Amerasia Basin of the Arctic. The Uda-Murgal arc evolved independently along the Taigonos and Okhotsk part of the Russian margin, between 150-130 Ma and 107-97 Ma, and was followed by the establishment of the margin-parallel Okhotsk-Chukotka volcanic belt (OCVB), erupted in a neutral to slightly extensional tectonic regime, beginning 106 Ma, but, mostly between 98-80 Ma and ending ~ 76 Ma. Age equivalent magmatism in the Bering Strait is associated with continued extension, melting and remobilization of the deep crust. Between 80-60 Ma, subduction-related magmatism stepped south to the outer Bering Shelf and then to the Aleutians before 46 Ma. A similar process occurred in Russia with the pre-55 Ma onset of opening of the Sea of Okhotsk and step-wise migration of the subduction zone to eastern Kamchatka and Kuril Islands. These events broadly coincide with the opening of the Eurasia Basin of the Arctic. Profound differences occur in the tectonic setting of magmatism through time along strike of the subducting margin from Russia to the North American Cordillera. From 125 to 60 Ma, the NE Russian and Alaskan sectors of the margin underwent mostly neutral to extensional tectonism while the Cordillera underwent crustal shortening. Although vast amounts of paleo-Pacific oceanic lithosphere were subducted beneath both sectors of the margin, it appears that in a mantle reference frame, the Cordillera moved mostly towards its subduction zone while the overriding continental plates in NE Russia and Alaska moved parallel to or away from their subduction zones, allowing magmatism to take place in tectonically neutral or extensional settings. The coupled timing of trench-towards versus trench-away events related to motion of the overriding plates led to the observed “dance” of magmatism and deformation along the northern circum-Pacific margin.

Development of unconventional shale gas wells can generate significant quantities of drilling waste, including trace metal-rich black shale from the lateral portion of the drillhole. We carried out sequential extractions on 15 samples of dry-drilled cuttings and core material from the gas-producing Middle Devonian Marcellus Shale and surrounding units to identify the host phases and evaluate the mobility of selected trace elements during cuttings disposal. Maximum whole rock concentrations of uranium (U), arsenic (As), and barium (Ba) were 47, 90, and 3333 mg kg−1, respectively. Sequential chemical extractions suggest that although silicate minerals are the primary host for U, as much as 20% can be present in carbonate minerals. Up to 74% of the Ba in shale was extracted from exchangeable sites in the shale, while As is primarily associated with organic matter and sulfide minerals that could be mobilized by oxidation. For comparison, U and As concentrations were also measured in 43 produced water samples returned from Marcellus Shale gas wells. Low U concentrations in produced water ( Geochemical modeling to determine mobility under surface storage and disposal conditions indicates that oxidation and/or dissolution of U-bearing minerals in drill cuttings would likely be followed by immobilization of U in secondary minerals such as schoepite, uranophane, and soddyite, or uraninite as conditions become more reducing. Oxidative dissolution of arsenic containing sulfides could release soluble As in arsenate form under oxic acidic conditions. The degree to which the As is subsequently immobilized depends on the redox conditions along the landfill flow path. The results suggest that proper management of drill cuttings can minimize mobilization of these metals by monitoring and controlling Eh, pH and dissolved constituents in landfill leachates.

The South Anyui suture zone consists of late Paleozoic–Jurassic ultramafic rocks and Jurassic–Cretaceous pre-, syn-, and postcollisional sedimentary rocks. It represents the closure of a Mesozoic ocean basin that separated two microcontinents in northeastern Russia, the Kolyma-Omolon block and the Chukotka block. In order to understand the geologic history and improve our understanding of Mesozoic paleogeography of the Arctic region, we obtained U-Pb ages on pre- and postcollisional igneous rocks and detrital zircons from sandstone in the suture zone. We identified four groups of sedimentary rocks: (1) Triassic sandstone deposited on the southern margin of Chukotka; (2) Middle Jurassic volcanogenic sandstone that was derived from the Oloy arc, a continental margin arc, along the Kolyma-Omolon block, south of the Anyui Ocean, a sample of which yielded no pre-Jurassic zircons and a single peak at 164 Ma; (3) suture zone sandstone that yielded Late Jurassic maximum depositional ages and likely predated the collision; and (4) a Mid-Cretaceous syncollisional sandstone that had a maximum depositional age of 125 Ma. These rocks were intruded by postkinematic plutons and dikes with ages of 109 Ma and 101 Ma that postdate the collision. We present a seismic-reflection line through the South Anyui suture zone that indicates south-vergence of thrusting of the Chukotka block over the Kolyma-Omolon block, opposite of most existing models and opposite of the vergence in the Angayucham suture zone, the postulated along-strike equivalent in Alaska. This suggests that Chukotka and Arctic Alaska may have different pre-Cretaceous histories, which could solve space problems with existing reconstructions of the Arctic region. We combine our detrital zircon data and interpretations of the seismic line to construct a new GPlates model for the Mesozoic evolution of the region that decouples Chukotka and Arctic Alaska to solve space problems with previous Arctic reconstructions.

The Mesoproterozoic–Neoproterozoic sedimentary succession of the eastern part of the Siberian Craton consists of several unconformity-bounded, kilometer-scale siliciclastic-carbonate cycles. The overlying Lower Cambrian rocks are often compositionally similar to the uppermost units of the Neoproterozoic succession. Twenty-nine samples were collected for U–Pb detrital zircon study and 27 samples were analyzed for whole-rock Sm–Nd isotopes. In total, 1491 detrital zircon grains were dated and 1148 grains were selected for provenance interpretation. Samples from the Uchur and Aimchan groups only contain detrital zircons of Paleoproterozoic and Archean age. Samples from the Kerpyl Group located on the Siberian Craton contain Paleoproterozoic and Archean grains as well, but samples from the Kerpyl Group in the Sette-Daban Ridge have significant numbers of Mesoproterozoic detrital zircons. Mesoproterozoic detrital zircons predominate in samples from the Uy Group. In the northern part of the study area, samples from the uppermost Neoproterozoic and Lower Cambrian strata contain numerous ca. 790–590 Ma detrital zircons, whereas in the southern part of the study area only Paleoproterozoic and Archean grains have been found. The whole-rock Sm–Nd isotopic values of clastic rocks show that most samples have isotopic signatures typical for the Siberian Craton basement, whereas some samples from the Kerpyl Group and younger rock units have isotopic signatures typical of the Grenville Orogen. Most of the Archean and Paleoproterozoic detrital zircons were eroded from the basement of the Siberian Craton, although some ca. 2080–2030 Ma detrital zircons are likely to have a non-Siberian provenance. However, rocks younger than ca. 1700 Ma are not known in the Siberian Craton basement and all Mesoproterozoic and younger grains must therefore have a non-Siberian provenance. The detrital zircon age distributions and whole-rock Nd isotopic signatures of many samples from the Kerpyl Group and younger units are very close to those of the Grenville Orogen in North America, suggesting that erosion of the latter contributed to clastic deposition along the Siberian margin. Three paleocontinental restorations proposed by Sears and Price (1978, 2003), Rainbird et al. (1998) and Pisarevsky and Natapov (2003) are invoked to explain the occurrence of Grenville-age detrital zircons in the Siberian sedimentary succession. The provenance of ca. 790–590 Ma detrital zircons is most likely to be located within the Central Taimyr accretionary belt formed along the northern margin of the Siberian Craton in the Neoproterozoic.

In the Bendeleben Mountains, Seward Peninsula, mid-Cretaceous granites are exposed in an uplifted block bounded on its south side by an E-W striking normal fault. The Bendeleben fault has well-preserved scarps 4–7 m in height that offset Holocene moraines. Seismic activity, young normal faulting, and Quaternary basaltic volcanism are all evidence of active extension. South of the Bendeleben fault, there is a 3–4 km deep basin. Fifteen apatite (U-Th)/He ages from granitic samples of the footwall yield an Eocene weighted mean age of 41.3±4.8 Ma. Biotite 40Ar/39Ar ages from the country rock of the Bendeleben pluton are 81–83 Ma. In spite of the young fault scarps, HeFTy and Pecube thermal modeling results illustrate that rapid exhumation of the Bendeleben Mountains occurred in the Late Cretaceous-Eocene and slowed since the Oligocene. A weak age-elevation relationship of apatite He ages and a lack of correlation between age and distance from the fault indicate that exhumation was accomplished with minimal block rotation on a steeply dipping, long-lived normal fault. Timing of extension in the Seward Peninsula can be correlated with deformation in the offshore Hope Basin where seismic reflection lines document Early Tertiary large-magnitude normal faulting followed by minor post-Miocene reactivation. The faulting observed in the Bendeleben Mountains is part of an extensional system that spans a large portion of the Bering Strait region. The tectonic model proposed in previous studies suggests that clockwise rotation of the Bering block relative to North America is the cause of extensional deformation in western Alaska.

Detrital zircon U–Pb ages from Triassic strata exposed in the circum-Arctic, analyzed by LA-ICP-MS and SHRIMP-RG, are compared at the regional scale to better understand the paleogeography of northern Pangea and help restore rift opening of the Arctic. Data sets are compared based on their zircon age distributions, cumulative age probability plots, and the K–S test. Three major source regions are characterized. These fed clastic material to transcontinental river systems that transported material from the highlands of northwestern Pangea to its once continuous paleo-Pacific continental margin. The paleo-Lena River System was fed from sources in the Baikalian and Altay-Sayan mountainous regions of Siberia. Zircon populations are characterized by a limited number of Precambrian zircons (~ 1.8–2.0 Ga with fewer ~ 2.5–3.0 Ga), lack of 0.9–1.8 Ga zircons, and a dominant 480–500 Ma and 290–300 Ma age population. The paleo-Taimyr River System was sourced from the Uralian orogenic belt region and deposited along a rifted portion of the Siberia–Baltica margin beginning in the Permo–Triassic. Precambrian zircon populations are similar to those of the paleo-Lena system, and samples closest to Siberia have similar populations in the 480–500 Ma and 290–300 Ma age ranges. Chukotka, Wrangel Island and Lisburne Hills, Alaska, have sparse ages between 900 and 1800 Ma, Ordovician ages are younger (~ 440–450 Ma), and, along with abundant ~ 300 Ma ages, they contain ~ 250–260 Ma and lesser ~ 215–235 Ma zircons, interpreted as derived from silicic volcanic centers associated with Permo–Triassic to Triassic continental flood basalt provinces in Siberia, Taimyr and Kara Sea region. The trans-Laurentian River System was likely fed by rift-related uplift along the proto North Atlantic/Arctic margin and delivered sediment to the Cordilleran margin of Pangea. These samples have no significant upper Paleozoic zircons and have a much broader age range of Precambrian zircons.

The formation of the Amerasian Basin of the modern Arctic remains enigmatic in terms of both timing and method of formation. Most models used to describe its formation involve movement of the Arctic Alaska-Chukotka microplate across the basin’s current location. Detrital zircon U-Pb geochronology has been shown to be an inexpensive yet powerful method by which the tectonic correlation and proximity between multiple terranes over geologic time can be approximated. Five detrital zircon samples were collected from Late Jurassic sandstones from the Lower Yana River area and compared to previous results from detrital zircons collected from nearby Triassic strata. Jurassic samples had detrital zircon age populations of 147–210 Ma, 223–396 Ma, 1639–2183 Ma, and 2281–3116 Ma. Comparison of all detrital zircon ages from the Lower Yana River to those dated from Triassic and Jurassic sandstones of Chukotka, the Verkhoyansk fold-and-thrust belt, and the In’yali-Debin synclinorium supports the interpretation that Chukotka was separated from the Kular area during the Triassic. Jurassic detrital zircon age populations suggest that the Anyui Ocean had closed by the Tithonian, bringing Chukotka to a location where it could be fed by similar depositional systems as the Verkhoyansk fold-and-thrust belt and the Lower Yana River area. Sedimentological and detrital data presented here also suggest that the Yana fault does not represent a regional suture between the Kolyma-Omolon superterrane and the Siberian craton.

Large-volume granitic plutons in the Burro Mountains, southwestern New Mexico, cover an area of 1300 km 2 and include biotite leucogranite and biotite-hornblende granodiorite. These intrusions are part of the ca. 1.4 Ga granite and rhyolite province stretching across Laurentia. U-Pb zircon dating of five samples of the biotite leucogranite yielded ages ranging from 1469 ± 12 to 1455 ± 11 Ma (2σ uncertainty). Three samples of granodiorite range from 1470 ± 16 Ma to 1459 ± 14 Ma. All of the zircon ages are within error and together have a weighted mean age of 1462 ± 3 Ma, but the granodiorite is older than the granite based on crosscutting relationships. Pressure and temperature estimates from metapelitic country rock, the first from southern New Mexico, are 4–6.5 kbar and 575–650 °C, consistent with their mineralogy. Decompression textures on garnets indicate exhumation following peak conditions, interpreted to have been reached during magmatic heating at 1460 Ma. Decompression had a magnitude of 1–3 kbar. An amphibolite from the country rock has metamorphic zircons dated at 1458 ± 9 Ma, with low Th/U and soccer-ball morphology enclosed within hornblende; this is the age of amphibolite-facies metamorphism. Electron microprobe monazite U-Pb dates from metapelite country rock mostly range from 1500 to 1400 Ma, with a peak at 1474 Ma. This is interpreted as indicating that the area experienced high-temperature metamorphism associated with pluton emplacement at this time. The 40 Ar/ 39 Ar dates from granite and amphibolite form two main groups. The older group includes four hornblende dates of 1477–1467 Ma and biotite and white mica ages from 1472 to 1459 Ma. These are interpreted as reflecting cooling of the plutons and their country rock soon after intrusion. The younger group of hornblende and biotite ages is at 1228 ± 3 Ma and is close in age to the adjacent Redrock granite, which intruded at 1225 Ma. There is a dominant strong foliation in the country rock gneisses and metasedimentary rocks. Foliated granite and gabbro both yield ca. 1630 Ma U-Pb zircon ages. The Burro Mountain granite is not pervasively deformed, but the granodiorite has strong augen gneissic foliations and mylonitic shear zones with S-C fabrics. This fabric formed prior to intrusion of undeformed dikes of Burro Mountain granite and thus is synmagmatic within geochronologic uncertainty. The fabric is identical to the fabric in the country rock, and xenoliths of country rock are foliated with orientations parallel to the foliation in the granodiorite. The similar orientations and the timing of metamorphism suggest that most of the deformation in the Burro Mountains occurred at 1460 Ma and was synchronous with intrusion of the granodiorite. We interpret the 1460 Ma plutonism, deformation with steep fabrics, exhumation, and rapid cooling as having formed during gneiss dome development within an extensional tectonic setting, although a transpressional setting is also permissible.

Indications of intense deformation in the Anyui-Chukotka Fold System and the South Anyui Suture Zone have been noted for a long time [3, 5, 19, 36]. The character and age of the deformation, however, remain a matter of debate. Using structural paragenetic and deformational kinematic analyses, we establish three deformation stages in the Anyui-Chukotka Fold System. The structural assembly comprising open folds and NW-trending axial-plane cleavage was formed during the stage of regional compression (D1) related to the collision of the Chukotka-Arctic Alaska microcontinent with Eurasia. The assembly of the second stage in the Alyarmaut Rise is distinguished by isoclinal folds F2, gently dipping metamorphic schistosity, and pervasive cleavage in combination with folded quartz veins and lenses. Planar structural elements of the second stage are disturbed by low-amplitude normal and reverse faults and kink folds of stage D3. The U-Pb (SHRIMP-RG) and 40/39Ar methods were used for determination of the isotopic age of the deformations. The Aptian-Albian zircon age (117–108 Ma) has been established for six postcollision granitic plutons of the Anyui-Chukotka Fold System and the South Anyui Suture. Syncollision deformation completed 125–117 Ma ago. The extensional tectonic stage D2 accompanied by emplacement of the Lyupveem pluton occurred 120–105 Ma ago. The 40/39Ar age of the biotite from the metamorphic rocks marks the age of syndeformation metamorphism (109–103 Ma). The lower limit of brittle failure and deformation D3 is estimated at 105 Ma.

The Okhotsk terrane, located east of the South Verkhoyansk sector of the Verkhoyansk fold-and-thrust belt, has Archean crystalline basement and Riphean to Early Paleozoic sedimentary cover similar to that of the adjacent the North Asian craton. However, 2.6 Ga biotite orthogneisses of the Upper Maya uplift of the Okhotsk terrane yielded Early Devonian 40Ar/39Ar cooling ages, evidence of a Mid-Paleozoic metamorphic event not previously known. These gneisses are also intruded by 375±2 Ma (Late Devonian) calc-alkaline granodiorite plutons that we interpret as part of a continental margin volcanic arc. Therefore, Late Devonian rifting, which affected the entire eastern margin of North Asia separating the Okhotsk terrane from the North Asian craton, was probably a back-arc event. Our limited 40Ar/39Ar data from the South Verkhoyansk metamorphic belt suggests that low grade metamorphism and deformation started in the Late Jurassic due to accretion of the Okhotsk terrane to the North Asia margin along the Bilyakchan fault. Shortening and ductile strain continued in the core of the South Verkhoyansk metamorphic belt until about 120 Ma due to paleo-Pacific subduction along the Uda-Murgal continental margin arc.

The Seward Peninsula of northwestern Alaska is part of the Arctic Alaska–Chukotka terrane, a crustal fragment exotic to western Laurentia with an uncertain origin and pre-Mesozoic evolution. U-Pb zircon geochronology on deformed igneous rocks reveals a previously unknown intermediate-felsic volcanic event at 870 Ma, coeval with rift-related magmatism associated with early breakup of eastern Rodinia. Orthogneiss bodies on Seward Peninsula yielded numerous 680 Ma U-Pb ages. The Arctic Alaska–Chukotka terrane has pre-Neoproterozoic basement based on Mesoproterozoic Nd model ages from both 870 Ma and 680 Ma igneous rocks, and detrital zircon ages between 2.0 and 1.0 Ga in overlying cover rocks. Small-volume magmatism occurred in Devonian time, based on U-Pb dating of granitic rocks. U-Pb dating of detrital zircons in 12 samples of metamorphosed Paleozoic siliciclastic cover rocks to this basement indicates that the dominant zircon age populations in the 934 zircons analyzed are found in the range 700–540 Ma, with prominent peaks at 720–660 Ma, 620–590 Ma, 560–510 Ma, 485 Ma, and 440–400 Ma. Devonian- and Pennsylvanian-age peaks are present in the samples with the youngest detrital zircons. These data show that the Seward Peninsula is exotic to western Laurentia because of the abundance of Neoproterozoic detrital zircons, which are rare or absent in Lower Paleozoic Cordilleran continental shelf rocks. Maximum depositional ages inferred from the youngest detrital age peaks include latest Proterozoic–Early Cambrian, Cambrian, Ordovician, Silurian, Devonian, and Pennsylvanian. These maximum depositional ages overlap with conodont ages reported from fossiliferous carbonate rocks on Seward Peninsula. The distinctive features of the Arctic Alaska–Chukotka terrane include Neoproterozoic felsic magmatic rocks intruding 2.0–1.1 Ga crust overlain by Paleozoic carbonate rocks and Paleozoic siliciclastic rocks with Neoproterozoic detrital zircons. The Neoproterozoic ages are similar to those in the peri-Gondwanan Avalonian-Cadomian arc system, the Timanide orogen of Baltica, and other circum-Arctic terranes that were proximal to Arctic Alaska prior to the opening of the Amerasian basin in the Early Cretaceous. Our Neoproterozoic reconstruction places the Arctic Alaska–Chukotka terrane in a position near Baltica, northeast of Laurentia, in an arc system along strike with the Avalonian-Cadomian arc terranes. Previously published faunal data indicate that Seward Peninsula had Siberian and Laurentian links by Early Ordovician time. The geologic links between the Arctic Alaska–Chukotka terrane and eastern Laurentia, Baltica, peri-Gondwanan arc terranes, and Siberia from the Paleoproterozoic to the Paleozoic help to constrain paleogeographic models from the Neoproterozoic history of Rodinia to the Mesozoic opening of the Arctic basin.

U-Pb SHRIMP-dating of zircons from twenty five intrusions representative of the Main granitoid batholith belt and associated dike swarms (Yano-Kolyma gold bearing province, North East of Asia) are mostly ∼150 ± 3 Ma (Kimmeridgian-Tithonian). Two less widespread impulses of magmatism dated at 160–155 Ma and 146–143 Ma representing the full range of ages present in the Main belt. Paleoproterozoic (∼1.8 Ga) inherited zircons were found in three intrusions from the south-western part of the belt where Precambrian crust of the North-Asia craton is inferred to underlie it.

Paleozoic rocks exposed across the northern flank of the mid-Cretaceous to Late Cretaceous Koolen metamorphic dome make up two structurally superimposed tectonic units: (1) weakly deformed Ordovician to Lower Devonian shallow marine carbonates of the Chegitun unit which formed on a stable shelf and (2) strongly deformed and metamorphosed Devonian to Lower Carboniferous phyllites, limestones, and andesite tuffs of the Tanatap unit. Trace element geochemistry, Nd isotopic data, and textural evidence suggest that the Tanatap tuffs are differentiated calc-alkaline volcanic rocks possibly derived from a magmatic arc. We interpret the associated sedimentary facies as indicative of deposition in a basinal setting, probably a back arc basin. Orthogneisses in the core of the Koolen dome yielded a Devonian (between ∼369 and ∼375 Ma) U-Pb zircon age which is similar to the ages of the Tanatap tuffs as well as granitic plutons formed within a Devonian active continental margin of northern Alaska. The stratigraphy of the Chegitun unit is similar to that of the Novosibirsk carbonate platform which overlies the Late Precambrian Bennett-Barrovia block. The basement of the block is exposed in Chukotka where ortogneiss in the Chegitun River valley yielded Late Proterozoic (∼650 to 550 Ma) U-Pb ages. These two tectonic units form the shelf of the Chukchi and East Siberian Seas and may continue into northern Alaska as the Hammond subterrane. The deep-water Tanatap unit can be traced along the southern boundary of the Bennett-Barrovia block from the Novosibirsk Islands to northern Alaska This basin was paired with a Devonian magmatic arc that existed farther to the south. The northern margin of the Bennett-Barrovia block collided with North America in the Late Silurian to Early Devonian. In Chukotka, during Middle to Late Carboniferous time the reconstructed Devonian arc-trench system at the southern edge of the Bennett-Barrovia block collided with an unknown continental object, fragments of which now occur to the south of the South Anyui suture. Triassic to Cretaceous deformation strongly modified the Paleozoic units. Our results provide new constraints on the geometry and Paleozoic history of the Chukotka-Arctic Alaska block, the essential element involved in the opening of the Canada basin.

We present a kinematic model for the sequence of deformation and sedimentation in the frontal Brooks Range and adjacent Colville Basin in the Etivluk River region, 250 km west of the Trans-Alaska Crustal Transect (TACT). The model is based on a tectonic subsidence analysis of the foreland basin, combined with structural, stratigraphic, and thermal studies of the northern edge of the Brooks Range thrust belt. We interpret six discrete tectonic events that led to the present-day configuration of the thrust belt in this area: (1) emplacement of ophiolitic allochthons over the distal continental margin rocks in Valanginian time, hundreds of kilometers south of this study, (2) Hauterivian uplift of the Barrow Arch rift margin, affecting the northern part of the Colville Basin, (3) Barremian contraction involving emplacement of distal continental margin and ophiolitic allochthons onto the Endicott Mountains allochthon and creation of a southward dipping flexural basin on the North Slope autochthon, (4) mid-Cretaceous exhumation of imbricated rocks in the Brooks Range during northward propagation of the thrust front into the foreland, (5) minor thrusting in Late Cretaceous-Paleocene in the northern foreland to the northern limit of contractional structures, and (6) regional exhumation of the orogen and the foreland in Paleocene-Eocene time. This sequence of deformation agrees well with a simple model of a forward propagating thrust system.

[1] To test existing models for the formation of the Amerasian Basin, detrital zircon suites from 12 samples of Triassic sandstone from the circum-Arctic region were dated by laser ablation-inductively coupled plasma-mass spectrometry (ICP-MS). The northern Verkhoyansk (NE Russia) has Permo-Carboniferous (265–320 Ma) and Cambro-Silurian (410–505 Ma) zircon populations derived via river systems from the active Baikal Mountain region along the southern Siberian craton. Chukotka, Wrangel Island (Russia), and the Lisburne Hills (western Alaska) also have Permo-Carboniferous (280–330 Ma) and late Precambrian-Silurian (420–580 Ma) zircons in addition to Permo-Triassic (235–265 Ma), Devonian (340–390 Ma), and late Precambrian (1000–1300 Ma) zircons. These ages suggest at least partial derivation from the Taimyr, Siberian Trap, and/or east Urals regions of Arctic Russia. The northerly derived Ivishak Formation (Sadlerochit Mountains, Alaska) and Pat Bay Formation (Sverdrup Basin, Canada) are dominated by Cambrian–latest Precambrian (500–600 Ma) and 445–490 Ma zircons. Permo-Carboniferous and Permo-Triassic zircons are absent. The Bjorne Formation (Sverdrup Basin), derived from the south, differs from other samples studied with mostly 1130–1240 Ma and older Precambrian zircons in addition to 430–470 Ma zircons. The most popular plate tectonic model for the origin of the Amerasian Basin involves counterclockwise rotation of the Arctic Alaska–Chukotka microplate away from the Canadian Arctic margin. The detrital zircon data suggest that the Chukotka part of the microplate originated closer to the Taimyr and Verkhoyansk, east of the Polar Urals of Russia, and not from the Canadian Arctic.

Today, the major challenge in reservoir characterization is integrating data coming from different sources in varying scales, in order to obtain an accurate and high-resolution reservoir model. The role of seismic data in this integration is often limited to providing a structural model for the reservoir. Its relatively low resolution usually limits its further use. However, its areal coverage and availability suggest that it has the potential of providing valuable data for more detailed reservoir characterization studies through the process of seismic inversion. In this paper, a novel intelligent seismic inversion methodology is presented to achieve a desirable correlation between relatively low-frequency seismic signals, and the much higher frequency wireline-log data. Vertical seismic profile (VSP) is used as an intermediate step between the well logs and the surface seismic. A synthetic seismic model is developed by using real data and seismic interpretation. In the example presented here, the model represents the Atoka and Morrow formations, and the overlying Pennsylvanian sequence of the Buffalo Valley Field in New Mexico. Generalized regression neural network (GRNN) is used to build two independent correlation models between; 1) Surface seismic and VSP, 2) VSP and well logs. After generating virtual VSP's from the surface seismic, well logs are predicted by using the correlation between VSP and well logs. The values of the density log, which is a surrogate for reservoir porosity, are predicted for each seismic trace through the seismic line with a classification approach having a correlation coefficient of 0.81. The same methodology is then applied to real data taken from the Buffalo Valley Field, to predict interwell gamma ray and neutron porosity logs through the seismic line of interest. The same procedure can be applied to a complete 3D seismic block to obtain 3D distributions of reservoir properties with less uncertainty than the geostatistical estimation methods. The intelligent seismic inversion method should help to increase the success of drilling new wells during field development.

Today, the major challenge in reservoir characterization is integrating data coming from different sources in varying scales, in order to obtain an accurate and high-resolution reservoir model. The role of seismic data in this integration is often limited to providing a structural model for the reservoir. Its relatively low resolution usually limits its further use. However, its areal coverage and availability suggest that it has the potential of providing valuable data for more detailed reservoir characterization studies through the process of seismic inversion. In this paper, a novel intelligent seismic inversion methodology is presented to achieve a desirable correlation between relatively low-frequency seismic signals, and the much higher frequency wireline-log data. Vertical seismic profile (VSP) is used as an intermediate step between the well logs and the surface seismic. A synthetic seismic model is developed by using real data and seismic interpretation. In the example presented here, the model represents the Atoka and Morrow formations, and the overlying Pennsylvanian sequence of the Buffalo Valley Field in New Mexico. Generalized regression neural network (GRNN) is used to build two independent correlation models between; (1) Surface seismic and VSP, (2) VSP and well logs. After generating virtual VSP's from the surface seismic,more » well logs are predicted by using the correlation between VSP and well logs. The values of the density log, which is a surrogate for reservoir porosity, are predicted for each seismic trace through the seismic line with a classification approach having a correlation coefficient of 0.81. The same methodology is then applied to real data taken from the Buffalo Valley Field, to predict inter-well gamma ray and neutron porosity logs through the seismic line of interest. The same procedure can be applied to a complete 3D seismic block to obtain 3D distributions of reservoir properties with less uncertainty than the geostatistical estimation methods. The intelligent seismic inversion method should help to increase the success of drilling new wells during field development.« less

[1] The Koolen metamorphic dome of Chukotka Peninsula has been interpreted as a mid-Cretaceous extensional core complex. On the NW flank of the dome, three lithotectonic units are exposed: (1) High-Grade unit, composed of sillimanite-grade rocks; (2) Tanatap unit, composed of polydeformed greenschist-grade phyllites and marbles; (3) Chegitun unit composed of unmetamorphosed early Paleozoic limestones. Four Early Cretaceous to Tertiary deformational events are documented. D1 is attributed to the collisional closure of the South Anyui suture. The 40Ar/39Ar ages from the Tanatap unit constrain this event at 117–124 Ma. Relict kyanite-bearing assemblages in the High-Grade unit probably formed during this event under moderately high pressure conditions. D2 is related to extensional deformation in the Koolen dome. In the High-Grade unit, sillimanite-grade metamorphism was followed by rapid cooling and exhumation at 109–104 Ma. In the Tanatap unit, early fabrics underwent isoclinal folding, and a new cleavage (S2t) was developed. The kinematics of the High-Grade unit indicates ductile deformation by top-to-the-south shear, as is the case in the dome core. This suggests that the Koolen dome is a unidirectional core complex formed by south directed extension. We believe collapse of overthickened crust caused extension in the Koolen dome. The crust was thickened during Early Cretaceous collision and during subsequent oroclinal bending of the structures in the Bering Strait region. The youngest structures in the Chegitun Valley are NW striking normal faults kinematically linked to NE striking right-lateral strike-slip faults. They are probably the continuation of structures found offshore in the Hope Basin.

Major oil discoveries in the foothills of the Venezuelan and Colombian Andes have recently focused the interest of exploration companies toward sub-Andean basins. Seismic, well, and core data from the El Furrial (Venezuela) and Cusiana (Colombia) productive fields have been integrated herein with other regional information to document the evolution of the thrust belt and the history of the petroleum systems, and to propose practical guidelines for prediction of sandstone reservoir quality in such a complex geodynamic environment. Although timing of deformation is slightly different in these areas of eastern Venezuela and Colombia, sedimentary and tectonic burial of the foreland autochthon in both regions led to the maturation of prolific Cretaceous marine source rocks, resulting in successive and diachronous hydrocarbon migration and trapping episodes. Early sedimentary burial at the current location of the Serrana del Interior (Venezuela) and the Eastern Cordillera (Colombia) resulted in long-range migration of early-generated hydrocarbons toward the foreland, forming the large accumulation of hydrocarbon along the Faja Petrolifera (Eastern Venezuela). Early entrapped hydrocarbons also have been preserved in pre-Andean prospects of the Andean foothills, as evidenced by the complex charge history of the Cusiana field. However, wide areas of source rocks in the Andean foothills and adjacent foreland reached the oil window only during the late Neogene and Pliocene-Quaternary, when maximum burial was attained. This produced a second migration episode, coeval with the growth of frontal anticlinal prospects. The main reservoir in Cusiana is fluvial sandstone of the Mirador Formation (Eocene); in El Furrial, it is sandstone of the Naricual-Merecure Formation (Oligocene). Pressure solution and quartz cementation decreased permeability of these sandstones. Results of studies of the anisotropy of the magnetic susceptibility (AMS), coupled with studies of fluid inclusions in quartz overgrowths and thermal modeling, demonstrate that sandstone reservoirs of these oil fields were compacted both vertically, by the load of the synflexural sequence, and horizontally, by tectonic stress (layer-parallel shortening) prior to being tectonically emplaced into the allochthon. Layer-parallel shortening by pressure solution is a major source of silica in the underthrust foreland. Venezuelan and Colombian sandstones still have reasonably good reservoir characteristics, although they have been buried to great depths. Overpressure that developed in these reservoirs as a result of rapid foredeep sedimentation probably caused a delay in compaction. Early carbonate cements also may have contributed locally to prevent compaction until secondary porosity developed as a result of dissolution of this early diagenetic phase. Finally, development of structural closures and hydrocarbon trapping has resulted progressively in the shutting down of the hydraulic system, preventing the transport of exotic silica by regional fluid flow.

The MMVB was developed through different tectonic stages related with the interaction of the tectonic plates at the Northwestern corner of South America. During Jurassic and early Cretaceous the MMVB went through rift stage that evolved to an aulacogen. Many of the structures related with the extensional phase of the MMVB were modified after the Tertiary tectonics, however the rift structures in the northern portion of the basin are still well preserved. The northern part of the MMVB is a monocline dipping toward the southeast that represents a half graben inside the rift. The monocline structure is dipping south-eastern, as is the direction, where isopach contours thicken. Most of the structures present at the area are normal and reverse faults of variable vergence, oriented in northeast-southwest direction. Kinematics of the rift seemed to be rotational during the syn-rift accumulation and non-rotational during post-rift sedimentation. Most of the faults remaining the rift structure were inverted after compressional tectonics that started at Late Cretaceous. According the model proposed by Hayward and Graham (1989), inversion seemed to be mild in the northern MMVB area. Transpresion caused by the oblique collision of the Western Cordillera, added a strike-slip component to the reactivation process, through the clockwise rotation of the blocks.

An immense wedge of Carboniferous to Jurassic siliciclastic strata accumulated on the Verkhoyansk passive margin of the Siberian craton. U-Pb ages of detrital zircons from Pennsylvanian to Middle Jurassic sandstones are remarkably consistent and show a systematic change in the proportion of age populations. Most zircons originated from the southern margin of Siberia, which was tectonically active in the Paleozoic and early Mesozoic, and were transported to the Verkhoyansk margin by a major transcontinental river system that existed for ~200 m.y., the paleo–Lena River. Specific sources are the Angara-Vitim batholith of Transbaikalia (315 and 291–288 Ma age peaks), plutons of the Altay-Sayan region of the Central Asia fold belt (494–482 Ma), Proterozoic granitoids of northern Transbaikalia and the East Sayan Range (1888–1832 Ma), and minor contributions from the Siberian Platform and Aldan Shield (2900–2300 Ma). The paleo–Lena River met its demise when the Verkhoy-ansk margin was deformed in the Late Jurassic and Cretaceous, and sediment was diverted north to the Arctic Ocean. Thus, the life span of major transcontinental drainage systems can be comparable to that of the plate boundaries that surround them.

During mid- to Late Cretaceous time, the northern paleo–Pacific Ocean was characterized by subduction for >6000 km along strike. The associated magmatic and orogenic belt now comprises parts of the U.S. and Canadian Cordillera, Alaska, and the Russian Far East. Crustal shortening in the northwestern Cordillera sector of the belt was contemporaneous with extension in northern and interior Alaska and the Bering Strait region, while extensional to neutral tectonics prevailed in the Sea of Okhotsk region, Russian Far East. The along-strike continuity of magmatism during the ≈120–80 Ma time interval resulted from subduction beneath the northern circum-Pacific margin, even beneath the broadest parts of the belt in the Alaska and Bering Strait region, which we suggest were modified by large-magnitude crustal extension. Variations in deformational style along strike of the orogenic belt may be related to changes in the nature of the mechanical coupling between the subducting oceanic and overriding continental plates, where the central extensional part of the belt developed above a part of the slab characterized by southward retreat or rollback of the subduction zone.