ALBERT

Description: A complex array of faulted arc rocks and variably metamorphosed forearc accretionary complex rocks form a mappable arc–forearc boundary in southern Alaska known as the Border Ranges fault (BRF). We use detrital U–Pb zircon dating of metasedimentary rocks within the Knik River terrane in the western Chugach Mountains to show that a belt of Early Cretaceous amphibolite-facies metamorphic rocks along the BRF was formed when older mélange rocks of the Chugach accretionary complex were reworked in a sinistral-oblique thrust reactivation of the BRF during a period of forearc plutonism. The metamorphic subterrane of the Knik River terrane has a maximum depositional age (MDA) of 156.5 ± 1.5 Ma and a detrital zircon age spectrum that is indistinguishable from the Potter Creek assemblage of the Chugach accretionary complex, supporting correlation of these units. These ages contrast strongly with new and existing data that show Triassic to earliest Jurassic detrital zircon ages from metamorphic screens in the plutonic subterrane of the Knik River terrane, a fragmented Early Jurassic plutonic assemblage generally interpreted as the basement of the Peninsular terrane. Based on these findings, we propose the following new terminology for the Knik River terrane: (1) "Carpenter Creek metamorphic complex" for the Early Cretaceous "metamorphic subterrane", (2) "western Chugach trondhjemite suite" for the Early Cretaceous forearc plutons within the belt, (3) "Friday Creek assemblage" for a transitional mélange unit that contains blocks of the Carpenter Creek complex in a chert–argillite matrix, and (4) "Knik River metamorphic complex" in reference to metamorphic rocks engulfed by Early Jurassic plutons of the Peninsular terrane that represent the roots of the Talkeetna arc. The correlation of the Carpenter Creek metamorphic complex with the Chugach mélange indicates that the trace of the BRF lies ~1–5 km north of the map trace shown on geologic maps, although, like other segments of the BRF, this boundary is blurred by local complexities within the BRF system. Ductile deformation of the mélange is sufficiently intense that few vestiges of its original mélange fabric exist, suggesting the scarcity of rocks described as mélange in the cores of many orogens may result from misidentification of rocks that have been intensely overprinted by younger, ductile deformation.

Description: 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.

Description: Cambrian sandstones of the western United States and northern Mexico record deposition above a major unconformity on the rift margin of Laurentia. Detrital zircon ages from the Cambrian-Ordovician Bliss Sandstone in southern New Mexico are used to test models for the influence and significance of the Transcontinental Arch during the early Paleozoic. We obtained U-Pb zircon ages of basement rocks to test potential local source rocks in New Mexico. An orthogneiss in the Florida Mountains has an age of 1627 ± 18 Ma (all errors 2). The San Diego Mountain granite underlying the Bliss Sandstone is ~1400 Ma. A rapakivi granite in the Little Hatchet Mountains has a U-Pb Concordia age of 1077 ± 4 Ma, coeval with the Pikes Peak granite and several granites in Sonora, Mexico. Three samples of the Florida Mountains granite underlying the Bliss Sandstone yield an average age of 510 ± 5 Ma; this is a rare and distinctive age of magmatism in western Laurentia. Zircons with these ages can be used as tracers for understanding transportation paths of sediment. The Bliss Sandstone was studied from eight localities in southern New Mexico; one overlies the Cambrian Florida Mountains granite, and three lie to the west. The sandstones are quartzarenite, subarkose, and arkose. The most abundant detrital zircon population included Grenville ages with a main peak at 1250 Ma. Other significant peaks are at 1465 Ma, 1625 Ma, 1710 Ma, and 1850 Ma, and these populations generally decrease in abundance with increasing age. Only five Archean zircons (out of 845) were discovered. Cambrian detrital zircons are present in the Florida Mountains Bliss Sandstone locality, directly overlying the Cambrian granite. They are also present in two localities 60–80 km southwest and northwest of the Florida Mountains. Maximum depositional ages from sandstones with Cambrian zircons range from 509 ± 7 Ma to 504 ± 12 Ma, overlapping with the age of the Florida Mountains granite. The four localities east of the Florida Mountains did not have any Cambrian detrital zircons. We interpret these data as indicating that the Cambrian intrusions of the Oklahoma-Colorado rift were not a source for the Cambrian zircons in the Bliss Sandstone. The Tapeats Sandstone in the Grand Canyon does not have Cambrian zircons and has a minor Grenville peak at 1070 Ma ( Gehrels et al., 2011 ), significantly younger than the 1250 Ma Grenville peak in the Bliss Sandstone. We interpret these differences as being related to different source areas on opposite sides of the Transcontinental Arch. The differences in the specific ages of Cambrian plutons between the Oklahoma rift and the Florida Mountains granite can be used to test depositional models for other Cambrian sandstones of the southwest U.S. and in Mexico, where the presence of Cambrian zircons in the Caborca Block has implications for the Mojave-Sonora megashear hypothesis.

Description: Detrital zircon U-Pb ages are presented from the Liberty Creek schist in the central Chugach Mountains that indicate two distinct periods of preservation of blueschist-facies metamorphism along the southern Alaskan margin. A maximum depositional age (MDA) of 136 Ma demonstrates that the Liberty Creek schist was deposited long after the Early Jurassic cooling ages (196–185 Ma) recorded in other western Alaskan schist bodies containing blueschist-facies rocks, thus revealing two distinct blueschist-facies preservation events: an Early Jurassic event and a post–Early Cretaceous event. This Early Cretaceous depositional age also indicates that there have been major reorganizations within this subduction complex because the Potter Creek assemblage (MDA of 169–156 Ma), directly south of the Liberty Creek schist, is an older but more shallowly exhumed assemblage. Strike-slip faulting has rearranged the accretionary complex by carrying the Potter Creek assemblage outboard and south of the Liberty Creek schist. The predominance of 140–130 Ma zircons in the Liberty Creek schist sample and a population of detrital zircons that is distinct from nearby terranes suggest a sedimentary source different from other related accretionary assemblages. Three suggested Cordilleran source terranes are the Chitina Valley batholith immediately to the east; the Firvale suite of the Coast plutonic complex, ~1500 km to the southeast near Vancouver, British Columbia; or our preferred source, the southern Mexican Guerrero terrane, ~3000 km to the southeast. The detrital zircon signature of the Liberty Creek schist and these distances to potential sources support models suggesting thousands of kilometers of strike-slip movement along the western Cordillera since Cretaceous time, consistent with the Baja–British Columbia hypothesis.

Description: Detrital zircon U-Pb ages are presented from the Liberty Creek schist in the central Chugach Mountains that indicate two distinct periods of preservation of blueschist-facies metamorphism along the southern Alaskan margin. A maximum depositional age (MDA) of 136 Ma demonstrates that the Liberty Creek schist was deposited long after the Early Jurassic cooling ages (196–185 Ma) recorded in other western Alaskan schist bodies containing blueschist-facies rocks, thus revealing two distinct blueschist-facies preservation events: an Early Jurassic event and a post–Early Cretaceous event. This Early Cretaceous depositional age also indicates that there have been major reorganizations within this subduction complex because the Potter Creek assemblage (MDA of 169–156 Ma), directly south of the Liberty Creek schist, is an older but more shallowly exhumed assemblage. Strike-slip faulting has rearranged the accretionary complex by carrying the Potter Creek assemblage outboard and south of the Liberty Creek schist. The predominance of 140–130 Ma zircons in the Liberty Creek schist sample and a population of detrital zircons that is distinct from nearby terranes suggest a sedimentary source different from other related accretionary assemblages. Three suggested Cordilleran source terranes are the Chitina Valley batholith immediately to the east; the Firvale suite of the Coast plutonic complex, ~1500 km to the southeast near Vancouver, British Columbia; or our preferred source, the southern Mexican Guerrero terrane, ~3000 km to the southeast. The detrital zircon signature of the Liberty Creek schist and these distances to potential sources support models suggesting thousands of kilometers of strike-slip movement along the western Cordillera since Cretaceous time, consistent with the Baja–British Columbia hypothesis.

Description: Late Mesoproterozoic mafic magmatism in the southwestern U.S. diabase province is expressed as diabase dikes, sills, sheets, and flows. Previous radiometric ages range from 1140 Ma to 1040 Ma. We used high-precision thermal ionization mass spectrometry to date baddeleyite in diabase from four localities in Arizona to obtain 206 Pb/ 238 U dates of 1080 ± 2 Ma, 1080 ± 3 Ma, 1088 ± 3 Ma, and 1094 ± 2 Ma. We also obtained single-crystal laser-ablation and ion microprobe ages on zircons from two localities in New Mexico that indicate a wider geographic extent of this diabase province. The samples have SiO 2 ranging from 46.6 to 50.1 wt%, Mg# from 67 to 83, and Nd ranging from +4.7 to –1.4. A compilation of previously published ages of silicic rocks in the same age range suggests that mantle-derived magma induced crustal anatexis resulting in silicic magmatism, and this bimodal event forms a large igneous province that stretches 1500 km from east to west and 500–1000 km from north to south. Because some of the ca. 1.1 Ga plutonism extends outside the United States into northern Mexico, we suggest renaming this event as the Southwestern Laurentia large igneous province (SWLLIP). Magmatism in the province from 1094 to 1080 Ma occurred largely after the end of the Grenville orogeny. Two models that are not mutually exclusive are proposed: (1) lithospheric delamination following the Grenville collision; and (2) arrival of a mantle plume beneath south-central Laurentia, which spread beneath the lithosphere, with a northward-heading portion causing Keweenawan magmatism (at the boundary with the Superior craton), and a southward-heading portion creating the Southwestern Laurentia large igneous province. Other large igneous provinces have been previously correlated to these events, but the 1075 Ma Warakurna large igneous province in Australia is too young, and the 1110 Ma events in the Amazonian Congo and Kalahari cratons are too old.

Description: Subduction erosion is an important process at convergent margins but evidence in the geologic record is scarce because the involved materials are typically lost into the mantle. Detrital zircon dating of accretionary complex sediment allows us to document episodic growth that can be linked to possible triggers. Detrital zircons (n = 2508) from the Mesozoic Chugach accretionary complex, southern Alaska, have U-Pb ages that record progressive subduction accretion punctuated by two periods of tectonic erosion. Lithology and maximum depositional ages permit division of the Chugach accretionary complex into four main units. The oldest, the blueschist-greenschist unit, represents partially subducted sediment associated with a Jurassic oceanic arc, with subduction erosion from 180 to 170 Ma. The end of this erosion period is dated by the oldest maximum depositional age of the Potter Creek assemblage, a 〈156–169 Ma unit consisting of chert, argillite, and volcanic rocks. A trondhjemite pluton that intrudes the forearc was caused by ridge subduction at 125 Ma and resulted in a second period of subduction erosion lasting until 104 Ma. The end of Aptian–Albian erosion was marked by deposition of massive sandstone and conglomerate of the 101–91 Ma McHugh Creek assemblage. This influx of clastic sediment is interpreted to have occurred in response to the collision of Wrangellia with North America. This event and the erosional event preceding it steepened the forearc region, allowing mass wasting of forearc crust into the trench, filling it by 89 Ma, the oldest maximum depositional age of the Valdez Group flysch. Clasts in conglomerate include granodiorite from the basement of the Talkeetna arc dated from 199 ± 3 Ma to 179 ± 3 Ma, and sandstone clasts with maximum depositional ages of 100 Ma. From 89 Ma to at least 72 Ma, the Valdez Group flysch was deposited via turbidite fans onto the oceanic crust beyond the trench and accreted as imbricate thrust slices that retained coherent bedding. The source for detrital zircons in the Potter Creek assemblage is likely a Middle–Late Jurassic oceanic arc, possibly the Talkeetna arc. The abundance of zircons from ash fall tuffs is consistent with easterly winds and suggests the Chugach accretionary complex was south of latitude 25°N in the Late Jurassic. The dominant source for the Albian McHugh Creek assemblage and the Upper Cretaceous Valdez Group flysch was likely the arc associated with the Coast Mountains batholith. Jurassic (ca. 165 Ma) zircons in the McHugh Creek assemblage could have been derived from exhumed plutons, or may be second-cycle zircons derived from the Potter Creek assemblage. The first appearance of Proterozoic and Archean zircons in the Valdez Group records the breakdown of topographic barriers formed by the accreted arc terranes as rivers encroached into continental North America by the middle Late Cretaceous.