A FOSSIL IRONOMYIID FLY FROM CANADIAN AMBER (DIPTERA: IRONOMYIIDAE)

North America-Eurasia and South America-Africa were certainly joined in the classic reconstruction of Pangaea by Middle Triassic time. The line of collision and suture included the Appalachian Quachita-Marathon orogenic trend in the United States extending southwestward into what is now northeastern and southeastern Mexico and into Guatemala. Widespread continentality prevailed and there was no Gulf of Mexico or Caribbean Sea. In Late Triassic time and continuing into Early Jurassic time this construct began to founder by initial rifting between South America-Africa and North America. No oceanic crust was formed, however, thus Africa-South America were still completely connected by land or shallow sea to North America until mid-Jurassic time. During this same uppermost Triassic to Middle Jurassic period a largely continental magmatic arc was draped across the Pacific margin of southwestern North America and apparently continued unbroken into northwestern South America. Sometime in the Middle Jurassic oceanic crust began to form by seafloor spreading in the central Atlantic and Gulf of Mexico as separation of South America-Africa from North America accelerated. Once this dense crust began to form the trailing margins of the continents subsided below sea-level and construction of the Atlantic and Gulf coast continental shelves began. Evidence is quite conclusive that this ocean floor spreading did not reach the Pacific Ocean, but was transformed from the southwestern corner of the newly opened Gulf of Mexico northwestward across Mexico via a complex left-slip transform fault system that reached the Pacific margin near Los Angeles. In Early Cretaceous time spreading continued in the central Atlantic but extended southward into the southern Atlantic. As the main axis of spreading extended into the south Atlantic, spreading ceased in the Gulf of Mexico. The south Atlantic spreading initiated separation of South America from Africa, but they probably remained in partial contact via ridge-ridge transform faults until Late Cretaceous time. South America must have finally completely separated from North America in Early Cretaceous time, probably via a rift along the eastern edge of Yucatan and the Nicaraguan rise. By Late Jurassic time the Pacific continental margin arc had waned and was replaced by a complex, largely oceanic, magmatic arc whose position relative to southwestern North America and northwestern South America is not known. What we do know is that by Late Cretaceous-Early Tertiary time it had accreted against the Pacific margins of both. Connections between the continents are also not known but could have included a largely submarine magmatic arc, parts of which may have subsequently dispersed eastward as the Greater Antilles. Much of what is now Middle America is apparently underlain by oceanic crust at least as young as Late Cretaceous in age. By Late Cretaceous time the Greater Antilles magmatic arc seems to have fully formed and subsequently moved northeastward as a northeast-facing subduction system during Late CretaceousEarly Tertiary Laramide time. The Greater Antilles arc-trench system ceased activity in Late Eocene time as it collided with Florida and the Bahama platform and as Laramide orogeny waned throughout western North America. This was followed by a major plate reorganization in the Caribbean-Middle America region nearly 40 m.y. B.P. which established the Caribbean plate more or less as we know it today. The principal change was initiation of the Lesser Antilles magmatic arc as an east-facing subduction system that began to consume Atlantic ocean floor. Also, a west-facing subduction system may have formed about this time along a proto-Central American western margin of the Caribbean plate. However, much of what is now Central America may have initially been off southern Mexico. The northern and southern margins of the Caribbean plate evolved into complex transform and transpressive systems as North and South America moved westward past a nearly stationary Caribbean plate. These motions significantly fragmented the Greater Antilles into their present array. There is no evidence for any complete land connection between North and South America via the Greater and/or Lesser Antilles throughout later Mesozoic or Tertiary time. Nor is there any evidence for complete land connection via Central America and the Isthmus of Panama before Neogene time.

A plate-tectonic model for the evolution of Middle America and the Gulf of Mexico-Caribbean Sea region is presented. The model, which is based upon the existence of the Mojave-Sonora megashear, incorporates into the Triassic Pangea reconstruction three microplates between North and South America, thus avoiding the overlap of the Bullard fit. These plates are the Yaqui, bounded on the north by the Mojave-Sonora megashear; the east and west Maya plates, bounded on the north by the Mexican volcanic zone and on the south by a predecessor of the Motagua fault zone; and the Chortis plate (parts of Guatemala and Honduras). During Late Jurassic time, as North America split away from Europe, Africa, and South America, shear, with left-lateral sense of displacement, occurred along the transform faults that bounded the micro-plates. If ∼800 km of left-lateral displacement along the Mojave-Sonora megashear, ∼300 km along the Mexican volcanic belt, and ∼1,300 km along a proto-Motagua megashear are restored, and if Yucatan and Cuba are rotated to fit against northern South America, then (1) a curvilinear belt of late Paleozoic rocks that show lithologic as well as paleontologic similarities extends across the reconstruction and links outcrops in Texas, eastern Mexico, nuclear Central America, and Colombia; (2) a Mediterranean-like sea is delineated that was a precursor of most of the present Gulf of Mexico; (3) correlation is implied between the distinctive quartzose San Cayetano Formation of Cuba and the Caracas and Juan Griego Groups of Venezuela. Geometric constraints suggest that probably shear initially occurred along the Mexican volcanic zone near the end of the Middle Jurassic. Subsequently, probably about 160 m.y. ago, displacements that total ∼800 km began along the Mojave-Sonora megashear. Contemporaneously, Yucatan and fragments of pre-Cretaceous rocks that compose parts of central and western Cuba migrated northward toward their present positions. Rotation of Yucatan was facilitated by considerable displacement along the proto-Motagua zone and along a zone that is probably coincident with the modern Salina Cruz fault. Accumulation of widespread major salt units of Late Jurassic (Callovian to early Oxfordian) age in the Gulf Basin probably occurred contemporaneously with the arrival of these blocks at their present positions. Clastic units that interfinger with some of the youngest salt units and rim the Gulf of Mexico have not recorded major recognized translations since their accumulation. Clockwise rotation of South America and the Chortis plate occurred during Early Cretaceous time. This movement, which was manifested by subduction of Jurassic ocean floor against the previously rifted precursor of the island of Cuba and under parts of Hispaniola and Puerto Rico, is recorded by circum-Caribbean orogeny. Abrupt changes in the relative motions between North and South America during Late Cretaceous time may have resulted in extension and outpourings of basalt upon the Jurassic rocks of the ocean floor of the Venezuelan Basin. West of Beata Ridge, sea-floor spreading formed the Colombian Basin. Related subduction occurred as the Chortis plate (including part of Central America, the Nicaraguan Rise, and southeastern Cuba) was sutured against the Maya East plate along the present Motagua fault and Cayman Trench. Our model is constrained by published geologic data, the relative positions of North and South America from Atlantic sea-floor magnetic anomalies, and the requirement that the major transform faults be compatible with the poles of rotation for the appropriate relative motions between North and South America. Paleomagnetic data from Middle America are sparse but do not conflict with the predicted motions of some of the microplates, especially Chortis.

Reply to Comment on GSA Today science article, Subduction Polarity in Ancient Arcs: A Call to Integrate Geology and Geophysics to Decipher the Mesozoic Tectonic History of the Northern Cordillera of North America, by Pavlis et al.

An 40 Ar/ 39 Ar study of metamorphic and volcanic rocks from Magdalena and Santa Margarita Islands, Baja California Sur, Mexico, provides evidence for at least four events that can be related to the tectonic evolution of this portion of the southern Baja California continental borderland prior to, and during, the transition from a convergent to a transform plate boundary system. (1) Metamorphism of amphibolites and quartz-mica schist from upper plate units occurred in Late Jurassic and Early Cretaceous time (∼158–138 Ma). Analogous metamorphic rocks from upper plate rocks in the Cedros–Vizcaino–San Benito terranes of west-central Baja California have not yet been identified. However, the Santa Margarita amphibolites are similar in age to amphibolite blocks from melange of the Puerto Nuevo terrane. (2) Metabasite and metapelitic float blocks associated with serpentinized ultramafic rocks were metamorphosed and subsequently cooled in Early Cretaceous time (129–115 Ma). Their corresponding 40 Ar/ 39 Ar age spectra do not show pronounced age gradients, and they are dissimilar in age, as compared to Late Jurassic (160–170 Ma) amphibolite and late Early Cretaceous (100–115 Ma) blueschist blocks from melange in the Cedros–San Benito terranes. (3) A 30.7 Ma lamprophyre dike that crosscuts normal faults within ophiolitic rocks indicates that synsubduction extension of the upper plate on Santa Margarita Island occurred prior to mid-Oligocene time. (4) Latest Miocene–early Pliocene age (5–6 Ma) adakites were erupted into serpentinite-matrix melange on Santa Margarita Island and indicate derivation by partial melting of a young subducted slab prior to collision of ancestral East Pacific Rise ridge segments. The discovery of adakites in the continental borderland of Baja California Sur provides additional constraints for the Neogene thermal evolution of this convergent margin.

Geochronological, structural, and sedimentological data provide the basis for a regional synthesis of the evolution of the Cordilleran retroarc thrust belt and foreland basin system in the western U.S.A. In this region, the Cordilleran orogenic belt became tectonically consolidated during Late Jurassic time (∼155 Ma) with the closure of marginal oceanic basins and accretion of fringing arcs along the western edge of the North American plate. Over the ensuing 100 Myr, contractile deformation propagated approximately 1000 kilometers eastward, culminating in the formation of the Laramide Rocky Mountain ranges. At the peak of its development, the retroarc side of the Cordillera was divided into five tectonomorphic zones, including from west to east the Luning-Fencemaker thrust belt; the central Nevada (or Eureka) thrust belt; a high-elevation plateau (the "Nevadaplano"); the topographically rugged Sevier fold-thrust belt; and the Laramide zone of intraforeland basement uplifts and basins. Mid-crustal rocks beneath the Nevadaplano experienced high-grade metamorphism and shortening during Late Jurassic and mid- to Late Cretaceous time, and the locus of major, upper crustal thrust faulting migrated sporadically eastward. By Late Cretaceous time, the middle crust beneath the Nevadaplano was experiencing decompression and cooling, perhaps in response to large-magnitude ductile extension and isostatic exhumation, concurrent with ongoing thrusting in the frontal Sevier belt. The tectonic history of the Sevier belt was remarkably consistent along strike of the orogenic belt, with emplacement of regional-scale Proterozoic and Paleozoic megathrust sheets during Early Cretaceous time and multiple, more closely spaced, Paleozoic and Mesozoic thrust sheets during Late Cretaceous--Paleocene time. Coeval with emplacement of the frontal thrust sheets, large structural culminations in Archean-Proterozoic crystalline basement developed along the basement step formed by Neoproterozoic rifting. A complex foreland basin system evolved in concert with the orogenic wedge. During its early and late history (∼155 - 110 Ma and ∼70 - 55 Ma) the basin was dominated by nonmarine deposition, whereas marine waters inundated the basin during its midlife (∼110 - 70 Ma). Late Jurassic basin development was controlled by both flexural and dynamic subsidence. From Early Cretaceous through early Late Cretaceous time the basin was dominated by flexural subsidence. From Late Cretaceous to mid-Cenozoic time the basin was increasingly partitioned by basement-involved Laramide structures. Linkages between Late Jurassic and Late Cretaceous Cordilleran arc-magmatism and westward underthrusting of North American continental lithosphere beneath the arc are not plainly demonstrable from the geological record in the Cordilleran thrust belt. A significant lag-time (∼20 Myr) between shortening and coeval underthrusting, on the one hand, and generation of arc melts, on the other, is required for any linkage to exist. However, inferred Late Jurassic lithospheric delamination may have provided a necessary precondition to allow relatively rapid Early Cretaceous continental underthrusting, which in turn could have catalyzed the Late Cretaceous arc flare-up.

Seismic and field data show that the Fundy rift basin of southeastern Canada experienced two distinct episodes of deformation during Mesozoic time. The first episode, during Middle Triassic to Early Jurassic time, was extensional. Rifting associated with NW–SE extension reactivated NE trending Paleozoic compressional structures as normal faults, forming the northwestern boundary faults of the Fundy basin. Displacements on the low‐angle boundary faults locally exceeded 10 km. Rifting also reactivated east trending Paleozoic compressional structures as oblique‐slip faults with normal and sinistral strike‐slip components, forming the northern boundary faults of the Fundy basin. Several kilometers of sediments and lava flows filled the basin during rifting. The second deformational episode occurred during or after Early Jurassic time and probably before or during Early Cretaceous time. Inversion associated with NW–SE shortening occurred along all faulted margins of the Fundy basin. The northwestern boundary faults experienced several kilometers of reverse displacement, broad anticlines developed within their hanging walls, and the Fundy basin acquired its synclinal form. The northern boundary faults of the Fundy basin became oblique‐slip faults with reverse and dextral strike‐slip components. Gentle synclines, tight anticlines, and faults with reverse separation deformed the synrift strata near the northern margin of the Fundy basin. Neither collision nor subduction zones existed near the Fundy basin during Mesozoic time. Hence we believe that tectonic processes associated with seafloor spreading (e.g., incipient ridge push forces, continental resistance to plate motion) produced the shortening in the Fundy basin. Shortening occurred during the transition from rifting to drifting as North America separated from northern Africa and/or during the early stages of drifting as the seafloor‐spreading centers of the North Atlantic propagated northward.

Geometric data from more than 1,100 localities and 13 radiometric ages have been compiled and measured for post-metamorphic dikes in eastern New York, Vermont, New Hampshire, western Maine, and southern Quebec. Alkalic lamprophyres (monchiquite and camptonite) in Vermont and Quebec form three westward-trending lobate swarms. The northernmost lobe envelops the Monteregian Hills of Quebec, the central lobe crosses the central Lake Champlain Valley into the eastern Adirondacks, and the southernmost lobe crosses the northern Taconic Mountains into New York. These lobes extend from central New England, where numerous alkalic lamprophyre, spessartite, and diabase dikes were intruded in several episodes and orientations during Mesozoic time. Most of the monchiquite dikes are found in the Monteregian and Champlain areas, and with associated camptonite dikes intruded west-northwest– to east-trending regional fractures during Early Cretaceous time. Alkalic lamprophyres of this episode can also be found in New Hampshire and Maine. Most diabase dikes are found east of central Vermont, where they intruded northeast-trending fractures during Late Triassic to Early Jurassic time. Many camptonite and some spessartite dikes in eastern New England and the Taconics lobe have northeast trends and may also be of Early Jurassic age. Extensional stress directions in the New England crust shifted from northwest to north or north-northeast between Early Jurassic and Early Cretaceous time, as inferred from dike trends. The dikes may delineate extensional fracture zones that also controlled the emplacement of Mesozoic plutons in Quebec and New England.

Kinematic reconstruction of the Caribbean region since the Early Jurassic

Thermochronologic data from the Fosdick, Phillips and Chester mountains of Marie Byrd Land, West Antarctica, have been obtained through U‐Pb analysis of monazite, 40Ar/39Ar analysis of hornblende, muscovite, biotite and K‐feldspar, and apatite fission track methods. These data were collected to test the hypothesis that high‐grade metamorphic rocks in the Fosdick Mountains occupy the footwall of a Cordilleran‐style metamorphic core complex, exhumed during the breakup of this sector of Gondwana in early Late Cretaceous time. High‐grade metamorphism of rocks exposed in the Fosdick Mountains was followed by rapid cooling starting at ∼105 Ma, during the transition from convergence to extension in the adjacent continental margin of Gondwana. Monazite, hornblende, muscovite, biotite, and K‐feldspar from the Fosdick Mountains record rapid cooling (70±30°C/m.y.) from peak metamorphic conditions of 725°–780°C at 4.3–5.6 kbar to below 165°C between 105 and 94 Ma. Subsequent slow cooling was apparently punctuated by a short period of accelerated cooling through the apatite partial annealing zone (∼110°–60°C) between ∼80 and 75 Ma. Cooling rates decreased to an average of ∼l°C/m.y. after 70 Ma. Cooling ages become progressively older to the south; metamorphic grade decreases in concert with the increasing cooling ages. The southernmost samples, from the Chester Mountains, probably cooled to below K‐feldspar closure temperature (∼165°C) before inferred reheating associated with metamorphism in the Fosdick Mountains. North of the Fosdick Mountains, Devonian Ford granodiorite in the Phillips Mountains was below K‐feldspar closure temperature by early Cretaceous time. Byrd Coast granite intrusions in the eastern Phillips Mountains and east of the Fosdick and Chester mountains were emplaced between 100 and 105 Ma, and these plutons cooled very rapidly (>100°C/m.y.) to below biotite closure temperature, consistent with their epizonal character. The relationship of these granitoids to metamorphic rocks in the Fosdick Mountains is uncertain. We hypothesized the following sequence of events during the transition from convergence to extension along the Pacific margin of Gondwana. Voluminous intrusion into the lower and middle crust led to increased heat flow and high‐tem‐perature, low‐to moderate‐pressure metamorphism, forming the Fosdick metamorphic complex (FMC) exposed in the Fosdick Mountains. Decrease in strength due to intrusion and partial melting resulted in large‐scale flow, probably driven by extension‐related differential stresses. This deformation ended before the onset of rapid cooling of the FMC at ∼105 Ma. Cooling rates determined for the FMC can be modeled by decreasing the heat flux into the crust and exhuming the complex at a rate of 1.5 mm/yr. The decrease in cooling rate between closure of K‐feldspar 40Ar/39Ar at ∼94 Ma and cooling into the apatite fission track partial annealing zone by ∼80 Ma is interpreted to indicate that exhumation was at least a two‐stage process. Our observations indicate that the Fosdick and Chester mountains are part of a coherent block that was tilted ∼20° to the south during the exhumation of the FMC, probably by movement along an east trending, north dipping, normal fault between the Fosdick and Phillips mountains. The Fosdick Mountains are not a Cordilleran‐style metamorphic core complex, but the FMC provides a record of middle‐crustal processes related to the rifting of New Zealand from Gondwana in the Late Cretaceous.

The Mount Morrison roof pendant, the only roof pendant in the central Sierra Nevada containing Paleozoic fossils, is complexly deformed and contains three generations of structures, including folds, reverse faults, schistosities, and lineations. All three generations of structures occur in the Ordovician-Silurian(?) metasedimentary rocks, whereas only the younger two are recorded in the Pennsylvanian-Permian(?) metasedimentary rocks and the Permian(?)-Jurassic(?) metavolcanic rocks. The average strike directions of axial planes of folds are north-south, N25°W, and N60°W in the first, second, and third generations, respectively. Generations of structures having similar styles, orientations, and relative age relations occur in other pendants of the central Sierra Nevada. The pendant is interpreted as a thin sequence with tight isoclinal folds instead of a thick homoclinal sequence. The first deformation occurred during Devonian or Mississippian time, perhaps during the Antler orogeny. Uplift, erosion, and volcanism occurred in Late Permian time between the first and second deformations, perhaps as an expression of the Sonoma orogeny. The second generation structures formed in several pulses between Early Triassic and Early Cretaceous time, as indicated by temporal relations between deformed wall rocks and younger, crosscutting granitic plutons. The third generation structures formed between Early and Late Cretaceous time, during which these structures were crosscut by granitic rocks. The wall rocks of the batholith may form an anticlinorium instead of a synclinorium. Other roof pendants in the axial portion of the batholith may be relatively old, because they contain the same three sets of structures as found in the Ordovician-Silurian(?) rocks of the Mount Morrison roof pendant. Locally, various age belts of granitic rocks have shielded roof pendants from subsequent deformation.

The North American Cordillera has been shaped by a long history of accretion of arcs and other buoyant crustal fragments to the western margin of the North American plate since early Mesozoic time. The southernmost accreted terrane is the terrane of southwestern Mexico, a latest Jurassic-Cretaceous volcanic arc built on a Triassic accretionary prism. Interpretations of the origin of the terrane vary: Some authors consider it a far-traveled, exotic intra-oceanic island arc, while others view it as the (par)autochthonous, extended North American continental margin. We present new paleomagnetic and U-Pb zircon data from Lower Cretaceous sedimentary rocks of the terrane. These data show that the terrane has a latitudinal plate-motion history equal to that of the North America plate, both before and after accretion. This confirms paleogeographic models in which the arc successions formed on North American crust that rifted away from the Mexican mainland by approximately east-west opening of a back-arc basin above an eastward-dipping subduction zone. Additionally, it renders alternative paleogeographical models in which the terrane is considered to be exotic to the North American continent unlikely. The phase of back-arc spreading resulted in the short-lived existence of an additional Guerrero tectonic plate between the North American and Farallon plates and, upon closure of the back-arc basin, the growth of the North American continent.

Approximately 3,000 Ar, Sr, and Pb isotopic age determinations for Canadian Cordilleran rocks have been cataloged, categorized as to reliability and significance, and plotted on histograms, distribution maps for different time intervals, and space-time plots to show the magmatic evolution in this 2,300-km portion of the Circum-Pacific Mobile Belt. The history revealed is episodic, with stable distribution patterns within episodes and distinct lulls and changes in distribution between the episodes. From 230 to 214 Ma (during Late Triassic time), extensive mafic volcanism occurred in the Wrangell, Quesnel, and Stikine terranes. Volcanic-related ultramafic complexes are found scattered through the two latter terranes. Large calc-alkaline granitic plutons are known only in a belt crossing Stikinia in northern British Columbia. At the same time, blueschists formed in the Cache Creek accretion wedge. From 214 to 200 Ma (end of Triassic and part of Early Jurassic time), Early to Middle Jurassic arc magmatism began in Wrangellia and in the northern Quesnel, Stikine, and Yukon terranes. A distinct magmatic event is recognizable only in southern Quesnellia. Magmatism was absent on the North American craton. The Cache Creek and Quesnel terranes were definitely linked, Stikine and Cache Creek terranes were probably linked, and a regional metamorphic episode was completed in the Yukon Terrane by this time. From 200 to 155 Ma (late Early to early Late Jurassic time), magmatism was extensive in the Wrangell, Quesnel, Stikine, and Yukon terranes. Magmatism over-lapped into North America onlyeast of southern Quesnellia afterabout 180 Ma. By the middle of this time interval, the southern Quesnel-Slide Mountain-North America linkage was complete, and major deformation and metamorphism had affected the Omineca Belt in British Columbia. Early to Middle Jurassic magmatism in southern Wrangellia (Vancouver Island) is distinctly older than the Middle to Late Jurassic magmatism that occurred in central Wrangellia (Queen Charlotte Islands). From 155 to 140 Ma (during Late Jurassic time), a few last-gasp plutons of the late Early to early Late Jurassic episode and other rocks with partially reset 155- to 145-Ma dates occur in the Wrangell, Quesnel, and Stikine terranes. Late Jurassic magmatism (160 to 140 Ma) occurred in the Alexander Terrane (Saint Elias region). From 145 to 138 Ma (latest Jurassic and beginning of Early Cretaceous time), plutonism occurred in the Endako area of central British Columbia (Francois Lake suite) but is virtually unknown elsewhere. From 135 to 125 Ma (during Early Cretaceous time), there was a magmatic lull of major significance present throughout western North America. From 110 to 90 Ma (middle Cretaceous time), widespread plutonism occurred across all terranes. Dual culminations are evident: the Coast Plutonic and Ominica belts. Beforethis time all sutures except those outboard of Wrangellia had been closed. From 80 to 70 Ma (during Late Cretaceous time), a narrow, sinuous belt of magmatism persisted, mostly in the southeastern Coast Plutonic Belt, southwestern Yukon Territory, and scattered across the Skeena and Stikine arches. From 70 to 60 Ma (latest Cretaceous to Paleocene time), a distinct lull in magmatism occurred. Rare plutons of this time interval are known in the Coast Plutonic Belt, on the Skeena Arch, and in the southern Intermontane Belt. From 55 to 45 Ma (latest Paleocene to Middle Eocene time), widespread and voluminous magmatism occurred in all terranes. The early Cenozoic volcanic front crossed the Coast Plutonic Complex from its east side in the south to its west side in the north. Associated thermal and tectonic effects were strong even into the Omineca Belt, producing large reset metamorphic areas in the Coast and Omineca belts. This was a short-lived event, synchronous from southern British Columbia through the Yukon Territory. West of the volcanic front, offshore of Wrangellia, Metchosin volcano growth was underway at this time. Late in this time interval, the 50?–45–36-Ma Catface–Leech River event(s) of southern Wrangellia occurred. There is also time overlap with a diffuse Massett magmatic event in the Queen Charlotte Islands, and with Baranoff Island and Yakutat–Saint Elias region magmatism. Initial 87Sr/ 86Sr ratios and petrographic characteristics of Canadian Cordilleran igneous rocks are reviewed in the time frame just described. These reflect the nature of underlying crust, contemporaneous lithosphere thickness, and distance from the subduction zone. Comparisons with other parts of the Circum-Pacific Magmatic Belt shows both out-of-phase magmatism (Japan and southwestern Alaska) and perfect matching of some episodes (Sierra Nevada). Major magmatic episodes correspond to times of increased westward motion of North America with respect to hot spots or to times of increased convergence between western North America and the Farallon Plate.

Fil: Tapia Silva, Felipe Fernando. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Oficina de Coordinacion Administrativa Ciudad Universitaria. Instituto de Estudios Andinos Don Pablo Groeber. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Estudios Andinos Don Pablo Groeber; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Geologia; Argentina

<p>The Wrangellia composite terrane is one of the largest fragments of juvenile crust added to the North American continent since Mesozoic time, and refining its accretionary history has important implications for understanding how continents grow. New U-Pb geochronology and Hf isotopes of detrital zircons from Late Jurassic-Late Cretaceous strata from the forearc of the Wrangellia composite terrane allows more insight on the tectonic and paleogeographic history of the terrane.</p> <p>Our stratigraphically oldest samples from the Late Jurassic Naknek Formation have a detrital zircon U-Pb signature dominated by Early and Late Jurassic grains (195-190 Ma; 153-147 Ma). Hf isotopic compositions of these grains are juvenile to intermediate (εHf(t)=4.5-14.7). Disconformably above the Naknek Formation are two poorly understood units Ks and Kc. The Ks unit is dominated by Early to Late Jurassic grains (159-154 Ma) with a few Paleozoic grains (347-340 Ma). Hf isotopic compositions of Carboniferous-Jurassic grains are juvenile to intermediate (εHf(t)=6.0-18.8). The overlying Kc unit has Late to Early Jurassic zircons (198-161 Ma), and an increase in Paleozoic ages (374-323 Ma). Hf isotopic compositions of these grains are juvenile to intermediate (εHf(t)=4.5-14.7). Samples from the Matanuska Formation have major Late Cretaceous grains (90-71 Ma), and minor Early Cretaceous (137-106 Ma), Late to Early Jurassic (200-153 Ma), Paleozoic (367-277 Ma), and Precambrian grains (2597-1037 Ma). Hf compositions have a wider range from both the Late Cretaceous grains (εHf(t)=-1.5-14.9) and Paleozoic-Precambrian grains (εHf(t)=-23.7-16.3).</p> <p>Our results suggest an evolving provenance from Late Jurassic to Late Cretaceous time for the Wrangellia composite terrane forearc basin. The Late Jurassic Naknek Formation samples were dominantly derived from a juvenile to intermediate Jurassic igneous sediment source. During Early Cretaceous time, there is a slight increase in the number of Paleozoic grains in the Ks and Kc unit samples. The Early Cretaceous sediments have a mostly positive Hf isotopic compositions suggesting exhumation of Jurassic and Paleozoic juvenile igneous sediment sources. By Late Cretaceous time, our data illustrates another increase in Paleozoic grain abundances, in addition to the introduction of Precambrian grains, all with widely variable Hf isotopic compositions. We interpret this to reflect a larger sediment flux from the interior of Alaska where more evolved igneous rocks of that age are found.</p>

Paleomagnetic data suggest that a large part of British Columbia and southern Alaska was at the latitude of Baja California, Mexico, in early Late Cretaceous time. This allochthonous block (Baja BC) includes the Insular (Talkeetna in Alaska) and Intermontane composite terranes, the miniterranes between them, and possibly part of the Omineca metamorphic belt. Assuming that Baja BC was adjacent to North America by middle Cretaceous, an analysis using plate kinematics of the Pacific basin produces a model of northward movement of Baja BC which is compatible with the known geology of the reconstructed margin of western North America. Baja BC moved north 2400 km as part of the KuIa plate along a transform margin with North America from 85 to 66 Ma. This time interval is marked by the cessation of igneous activity in the Sierra Nevada, development of wrench faults and basins along coastal California, and initiation of Laramide‐style tectonics to the east. Within Baja BC, diminished igneous activity to the west was coeval with possible wrench‐tectonic basins along its eastern margin. By the end of the Cretaceous, Baja BC was positioned from Oregon to northern British Columbia. Between 66 and 56 Ma, Baja BC became attached to North America in a dextral‐transpressive stage, during which the Kula‐‐North America plate boundary expanded to encompass the whole block with a series of northwest trending strike‐slip faults, a fold and thrust belt in the east, and large‐scale uplifts and the formation of an incipient volcanic arc on the west. Southern Alaska was the site of convergence during the whole of the period of northward translation of Baja BC. A Late Cretaceous subduction zone in Alaska evolved into a major compressional belt as the basin and miniterranes on the northern margin of Baja BC collided with interior Alaska. The collision culminated in the Paleocene with formation of the McKinley granites and coeval dextral slip on the Denali fault system.