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Oblique convergence along strike-slip faults can lead to both distributed and localized deformation. How focused transpressive deformation is both localized and maintained along sub-vertical wrench structures to create high topography and deep exhumation warrants further investigation. The high peak region of the Hayes Range, central Alaska, USA, is bound by two lithospheric scale vertical faults: the Denali fault to the south and Hines Creek fault to the north. The high topography area has peaks over 4000 m and locally has experienced more than 14 km of Neogene exhumation, yet the mountain range is located on the convex side of the Denali fault Mount Hayes restraining bend, where slip partitioning alone cannot account for this zone of extreme exhumation. Through the application of U-Pb zircon, 40Ar/39Ar (hornblende, muscovite, biotite, and K-feldspar), apatite fission-track, and (U-Th)/He geo-thermochronology, we test whether these two parallel, reactivated suture zone structures are working in tandem to vertically extrude the Between the Hines Creek and Denali faults block on the convex side of the Mount Hayes restraining bend. We document that since at least 45 Ma, the Denali fault has been bent and localized in a narrow fault zone (<160 m) with a significant dip-slip component, the Mount Hayes restraining bend has been fixed to the north side of the Denali fault, and that the Between the Hines Creek and Denali faults block has been undergoing vertical extrusion as a relatively coherent block along the displacement “free faces” of two lithospheric scale suture zone faults. A bent Denali fault by ca. 45 Ma supports the long-standing Alaska orocline hypothesis that has Alaska bent by ca. 44 Ma. Southern Alaska is currently converging at ~4 mm/yr to the north against the Denali fault and driving vertical extrusion of the Between the Hines Creek and Denali faults block and deformation north of the Hines Creek fault. We apply insights ascertained from the Between the Hines Creek and Denali faults block to another region in southern Alaska, the Fairweather Range, where extreme topography and persistent exhumation is also located between two sub-parallel faults, and propose that this region has likely undergone vertical extrusion along the free faces of those faults. [ABSTRACT FROM AUTHOR]

The [sup.40]Ar/[sup.39]Ar hornblende crystallization ages from the Las Pirquitas thrust of the Sierra de Pie de Palo, Argentina, indicate that imbrication of the proto-Andean margin had initiated by ca. 515 Ma. The ages do not permit the Pie de Palo block to represent basement of the adjacent Argentine Precordillera terrane, but rather require it to be a separate fragment not juxtaposed with the Precordillera prior to the Ordovician. The Pie de Palo block was likely autochthonous to the Famatina arc of the proto-Andean margin in the Early Cambrian. When combined with existing regional ages of magmatism, metamorphism, and deformation, the ages suggest that east-dipping subduction beneath the Famatina arc initiated by the Early to Middle Cambrian. The newly established convergent margin resulted from a westward shift of the plate boundary at the end of the Pampean orogeny in the eastern Sierras Pampeanas. The Famatina arc persisted as an Andean-type active margin until the collision and accretion of the Precordillera terrane in the Middle Ordovician. Keywords: [sup.40]Ar/[sup.39]Ar, Argentine Precordillera, Sierras Pampeanas, Las Pirquitas thrust, Cambrian, Ordovician, proto-Andean margin, convergent margin, terrane accretion.

We identify two piercing point pairs along a ~500 km transect of the arcuate strike‐slip Denali fault to document long‐term slip partitioning. Geochemical and isotopic similarity between Foraker and Panorama‐Schist Creek‐Nenana Plutons suggest ~155 km of right‐lateral displacement on the western Denali fault since 37 Ma at a rate of ~4.2 mm/year. The eastern Denali fault Maclaren‐Cottonwood Terrane geochronology correlation establishes ~305 km of displacement on the eastern Denali fault since 33 Ma at a rate of ~9.2 mm/year. The ratio of Pleistocene‐Holocene slip rates between the western (5.3 mm/year) and eastern (12.9 mm/year) Denali fault is 0.41 and our new constraints yield a Late Eocene‐Holocene ratio of 0.46. Hence, we interpret that the overall arcuate geometry of the Denali fault master strand was established by 33 Ma. We infer that the persistent long‐wave geometric stability of the Denali fault and other highly slip partitioned fault systems are related to long‐term highly oblique transpressive environs. [ABSTRACT FROM AUTHOR]

Structures and kinematics ineastern Prince William Sound near Valdez show deformation associated with dextral strike-slip post-dating the initial accretion. Moreover, the faults in eastern Prince William Sound compose a dextral transform system that representsa major crustal discontinuity. Lastly, the convergence in eastern Prince William Sound changed from nearly orthogonal to highly oblique during Eocene.

Terrane accretion forms lithospheric-scale fault systems that commonly experience long and complex slip histories. Unraveling the evolution of these suture zone fault systems yields valuable information regarding the relative importance of various upper crustal structures and their linkage through the lithosphere. We present new bedrock geologic mapping and geochronology data documenting the geologic evolution of reactivated shortening structures and adjacent metamorphic rocks in the Alaska Range suture zone at the inboard margin of the Wrangellia composite terrane in the eastern Alaska Range, Alaska, USA. Detrital zircon uranium-lead (U-Pb) age spectra from metamorphic rocks in our study area reveal two distinct metasedimentary belts. The Maclaren schist occupies the inboard (northern) belt, which was derived from terranes along the western margin of North America during the mid- to Late Cretaceous. In contrast, the Clearwater metasediments occupy the outboard (southern) belt, which was derived from arcs built on the Wrangellia composite terrane during the Late Jurassic to Early Cretaceous. A newly discovered locality of Alaska-type zoned ultramafic bodies within the Clearwater metasediments provides an additional link to the Wrangellia composite terrane. The Maclaren and Clearwater metasedimentary belts are presently juxtaposed by the newly identified Valdez Creek fault, which is an upper crustal reactivation of the Valdez Creek shear zone, the Late Cretaceous plate boundary that initially brought them together. 40Ar/39Ar mica ages reveal independent post-collisional thermal histories of hanging wall and footwall rocks until reactivation localized on the Valdez Creek fault after ca. 32 Ma. Slip on the Valdez Creek fault expanded into a thrust system that progressed southward to the Broxson Gulch fault at the southern margin of the suture zone and eventually into the Wrangellia terrane. Detrital zircon U-Pb age spectra and clast assemblages from fault-bounded Cenozoic gravel deposits indicate that the thrust system was active during the Oligocene and into the Pliocene, likely as a far-field result of ongoing flat-slab subduction and accretion of the Yakutat microplate. The Valdez Creek fault was the primary reactivated structure in the suture zone, likely due to its linkage with the reactivated boundary zone between the Wrangellia composite terrane and North America in the lithospheric mantle. [ABSTRACT FROM AUTHOR]

Along the northern margin of the Arabia‐Eurasia collision zone in the western Greater Caucasus, the Main Caucasus Thrust (MCT) juxtaposes Paleozoic crystalline basement to the north against Mesozoic metasedimentary and volcaniclastic rocks to the south. The MCT is commonly assumed to be the trace of an active plate‐boundary scale structure that accommodates Arabia‐Eurasia convergence, but field data supporting this interpretation are equivocal. Here we investigate the deformation history of the rocks juxtaposed across the MCT in Georgia using field observations, microstructural analysis, U‐Pb and 40Ar/39Ar geochronology, and 40Ar/39Ar and (U‐Th)/He thermochronology. Zircon U‐Pb analyses show that Greater Caucasus crystalline rocks formed in the Early Paleozoic on the margin of Gondwana. Low‐pressure/temperature amphibolite‐facies metamorphism of these metasedimentary rocks and associated plutonism likely took place during Carboniferous accretion onto the Laurussian margin, as indicated by igneous and metamorphic zircon U‐Pb ages of ~330–310 Ma. 40Ar/39Ar ages of ~190–135 Ma from muscovite in a greenschist‐facies shear zone indicate that the MCT likely developed during Mesozoic inversion and/or rifting of the Caucasus Basin. A Mesozoic 40Ar/39Ar biotite age with release spectra indicating partial resetting and Cenozoic (<40 Ma) apatite and zircon (U‐Th)/He ages imply at least ~5–8 km of Greater Caucasus basement exhumation since ~10 Ma in response to Arabia‐Eurasia collision. Cenozoic reactivation of the MCT may have accommodated a fraction of this exhumation. However, Cenozoic zircon (U‐Th)/He ages in both the hanging wall and footwall of the MCT require partitioning a substantial component of this deformation onto structures to the south. Plain Language Summary: Collisions between continents cause deformation of the Earth's crust and the uplift of large mountain ranges like the Himalayas. Large faults often form to accommodate this deformation and may help bring rocks once buried at great depths up to the surface of the Earth. The Greater Caucasus Mountains form the northernmost part of a zone of deformation due to the ongoing collision between the Arabian and Eurasian continents. The Main Caucasus Thrust (MCT) is a fault juxtaposing old igneous and metamorphic (crystalline) rocks against younger rocks that has often been assumed to be a major means of accommodating Arabia‐Eurasia collision. This study examines the history of rocks along the MCT with a combination of field work, study of microscopic deformation in rocks, and dating of rock formation and cooling. The crystalline rocks were added to the margins of present‐day Eurasia about 330–310 million years ago, and the MCT first formed about 190–135 million years ago. The MCT is likely at most one of many structures accommodating present‐day Arabia‐Eurasia collision. Key Points: Amphibolite‐facies metamorphism and plutonism in the Greater Caucasus basement took place ~330–310 MaThe Main Caucasus Thrust formed as a greenschist‐facies shear zone during Caucasus Basin inversion and/or rifting (~190–135 Ma)The Main Caucasus Thrust may have helped facilitate a portion of at least 5–8 km of basement exhumation during Arabia‐Eurasia collision [ABSTRACT FROM AUTHOR]

Thrust systems are a primary mechanism for accommodating the convergent component of oblique plate motion and are therefore key players in the structural development of transpressional orogens. In southern Alaska, the Denali fault system is a highly partitioned dextral-convergent fault system spatially coincident with Alaska Range topography and thus offers an opportunity to evaluate the evolution of range-bounding thrust systems in orogens resulting from oblique plate motion. Our analysis is focused on the late Miocene- Present McCallum Creek thrust system, which consists of the McCallum Creek reverse fault and a kinematically linked foreland thrust system south of the Denali fault in the eastern Alaska Range. Apatite fission-track cooling ages, tephrachronology, and balanced cross sections indicate that convergence partitioned to the McCallum Creek thrust system has accommodated ~4 km of rock exhumation and ~5.5 km of south-vergent shortening since hanging wall rocks passed through the apatite fission-track partial annealing zone at ca. 6 Ma. A blind foreland thrust system developed after ca. 3.8 Ma and was subsequently overtaken by out-of-sequence slip on the main McCallum Creek fault. Incised segments of modern streams, perched terraces, and tilted Quaternary deposits suggest that foreland structures are active in the Quaternary. Shortening on the McCallum Creek thrust system is oriented at a high angle to the Denali fault, making the McCallum Creek thrust system one of the only known structures south of the Denali fault in the Alaska Range to accommodate the collisional mode of active deformation in southern Alaska. The late Miocene reactivation of faults in the McCallum Creek area likely records evolution of the Denali fault system in response to modification of the southern Alaska convergent plate boundary. [ABSTRACT FROM AUTHOR]

New U-Pb zircon ages and initial Nd isotope data from mafic and ultramafic rocks that have been tectonically imbricated on the southwest margin of the Precordillera terrane, western Argentina, indicate that these rocks do not constitute a single ophiolite pseudostratigraphy as previously thought. Rock units include Late Proterozoic (576 [+ or -] 17 Ma) microgabbro and Silurian (418 [+ or -] 10 Ma) sills, both of which have initial Nd isotope values of +6.0 to +7.3 and identical trace element geochemistry indicating formation in extensional settings. Late Ordovician (450 [+ or -] 20 Ma) layered gabbro has a distinct initial Nd isotope value of +0.6. It intrudes quartzofeldspathic gneiss and tectonized ultramafic rocks; this assemblage represents the deep part of a continental margin volcanic arc that formed on the eastern margin of Chilenia, as the ocean basin west of the Precordillera terrane began to close in the Middle Ordovician. The new tectonic interpretations generally support existing hypotheses that the Precordillera terrane rifted and drifted from southeastern Laurentia in the Cambrian to Early Ordovician and that the western margin of the Precordillera terrane represents a suture between two fragments of continental crust. Keywords: Precordillera, Laurentia, ophiolite, U-Pb geochronology, geochemistry.

Elongate fault-bounded blocks of an Early Jurassic subduction complex are preserved along a major tectonic boundary in southern Alaska, the Border Ranges fault system (BRFS). The high-P/low-T metamorphic rocks of the subduction complex are juxtaposed along one strand of the BRFS with an approximately coeval pluton and its associated thermal aureole. New U-Pb and Rb-Sr isotopic dates from the subduction complex are 204 [+ or -] 8 Ma and 195 [+ or -] 10 Ma (2 sigma errors), respectively. The similarity of these dates to previously reported K-Ar dates supports petrographic and petrologic observations that the subduction complex did not experience any metamorphism related to the intrusion of the now adjacent pluton. Petrographic and petrologic data also show that the pluton did not experience a high-P/low-T metamorphism. The pluton and the metavolcanics that it intrudes are interpreted as part of a Late Triassic to Early Jurassic primitive island-arc complex. If the subduction complex is related to this coeval island arc, then the forearc and back part of the accretionary prism have been tectonically eroded, either by subduction erosion, strike-slip faulting, or a combination of these processes. If, however, the subduction complex was juxtaposed with the island arc during strike-slip faulting that accompanied and postdated the Cretaceous accretion of terranes of southern Alaska with North America, then the subduction complex could be related to an Early Jurassic island arc exposed on Vancouver Island, where it is part of Wrangellia. The new isotopic ages and petrologic data indicate that the BRFS probably began its history as a major fault zone as early as the Middle Jurassic, prior to the well-documented Cretaceous megathrust history.

The article informs about bedrock geologic mapping and geochronology data documenting the geologic evolution of reactivated shortening structures and adjacent metamorphic rocks in the Alaska Range suture zone at the inboard margin of Wrangellia composite terrane. Topics include Detrital zircon uranium-lead age spectra from metamorphic rocks in distinct metasedimentary belts; and ultramafic bodies within Clearwater metasediments provides an additional link to the Wrangellia composite terrane.

Horizontal-slip along restraining bends of strike-slip faults is often partitioned into a vertical component via splay faults. The active Susitna Glacier Thrust Fault (SGTF), as shown by its initiation of the 2002 M7.9 Denali Fault earthquake, lies south of, and intersects the dextral strike-slip Denali Fault. Geochronology and thermochronology data from samples across the SGTF constrain the region's tectonic history and the role of thrusting in the formation of the eastern Alaska Range south of the Denali fault. U-Pb zircon ages indicate intrusion of plutons in the footwall (~57 Ma) and hanging wall (~98 Ma). These U-Pb zircon ages correlate to those from the Ruby Batholith/Kluane Terrane ~400 km east along the Denali Fault, supporting geologic correlations and hence constraints on long-term slip rates. 40Ar/39Ar mica and K-feldspar data from footwall and hanging wall samples (~54 to ~46 Ma) reflect cooling following magmatism and/or regional Eocene metamorphism related to ridge subduction. Combined with apatite fission track data (ages 43-28 Ma) and thermal models, both sides of the SGTF acted as a coherent block during the Eocene and early Oligocene. Contrasting apatite (U-Th)/He ages across the Susitna Glacier (~25 Ma footwall, ~15 Ma hanging wall) suggest initiation of faulting during the middle Miocene. Episodic cooling and exhumation is related to thrusting on known or hypothesized faults that progressively activate due to varying partition of strain along the Denali Fault associated with changing kinematics and plate interaction (Yakutat microplate collision, flat-slab subduction and relative plate motion change) at the southern Alaskan plate margin. [ABSTRACT FROM AUTHOR]

Contrasting lithospheric strength between terranes often results in the concentration of strain and deformation within the weaker material. Dramatic alternating asymmetric topography of the central and eastern Alaska Range along the active Denali fault is due to contrasting lithospheric strength between terranes and a suture zone, controlled by fault location with respect to the irregular boundary of a relatively stronger terrane backstop. Highest topography and greatest Neogene exhumation in the central Alaska Range occur on the concave side of the arcuate Denali fault, yet to the north and on the convex side of the fault in the eastern Alaska Range. The Denali fault largely lies along a Mesozoic suture zone between two large composite terranes (Yukon and Wrangellia composite terranes: YCT and WCT), but the McKinley strand of the fault cuts across an embayment of weaker suture-zone rocks (Alaska Range suture-zone, ARSZ) within the irregular southern boundary of the YCT (Hines Creek fault). Deformation (and uplift of the Alaska Range) is driven by slip and partitioning of strain along the Denali fault, occurring preferentially in weaker rocks of the ARSZ against the stronger YCT. Where the YCT lies well north of the McKinley strand, deformation is primarily to the north of the fault (eastern Alaska Range). Where the YCT is close to the fault, deformation is primarily to the south (central Alaska Range). While the trace of the McKinley strand approximates a small circle, two restraining bends (McKinley and Hayes) pinned equidistant from the ends of this strand localize uplift and exhumation. [ABSTRACT FROM AUTHOR]

The Bajo Pequeño Shear Zone (BPSZ) is a lower-crustal shear zone that records shortening and exhumation associated with the establishment of a new plate boundary, and its placement in a regional structural context suggests that local- to regional-scale strain localization occurred with progressive deformation. A kilometer-scale field and analytical cross section through the ~80 m thick BPSZ and its adjacent rocks indicates an early Devonian (405-400 Ma) phase of deformation on the western margin of Gondwanan continental crust. The earliest stages of the BPSZ, recorded by metamorphic and microstructural data, involved thrusting of a hotter orthogneiss over a relatively cool pelitic unit, which resulted in footwall garnet growth and reset footwall white mica 40Ar/39Ar ages in proximity to the shear zone. Later stages of BPSZ activity, as recorded by additional microstructures and quartz c-axis opening angles, were characterized by strain localization to the center of the shear zone coincident with cooling and exhumation. These and other data suggest that significant regional tectonism persisted in the Famatinian orogenic system for 60-70 million years after one microplate collision (the Precordillera) but ceased 5-10 million years prior to another (Chilenia). A survey of other synchronous structures shows that strain was accommodated on progressively narrower structures with time, indicating a regional pattern of strain localization and broad thermal relaxation as the Precordillera collision evolved. [ABSTRACT FROM AUTHOR]

The Famatina margin records an orogenic cycle of convergence, metamorphism, magmatism, and extension related to the accretion of the allochthonous Precordillera terrane. New structural, petrologic, and geochronologic data from the Loma de Las Chacras region demonstrate two distinct episodes of lower crustal migmatization. The first event preserves a counterclockwise pressure-temperature path in kyanite-K-feldspar pelitic migmatites that resulted in lower crustal migmatization via muscovite dehydration melting at ∼12 kbar and 868°C at 461 ±1.7 Ma. The shape of the pressure temperature path and timing of metamorphism are similar to those of regional midcrustal granulites and suggest pervasive Ordovician migmatization throughout the Famatina margin. One-dimensional thermal modeling coupled with regional isotopic data suggests Ordovician melts remained at temperatures above their solidus for 20-30 Ma following peak granulite facies metamorphism, throughout a time period marked by regional oblique convergence. The onset of synconvergent extension occurred only after regional migmatites cooled beneath their solidus and was synchronous with the cessation of Precordillera terrane accretion at ∼436 Ma. The second migmatite event was regionally localized and occurred at ∼700°C and 12 kbar between 411 and 407 Ma via vapor saturated melting of muscovite. Migmatization was synchronous with extension, exhumation, and strike-slip deformation that likely resulted from a change in the plate boundary configuration related to the convergence and collision of the Chilenia terrane. [ABSTRACT FROM AUTHOR]

The Alaska Range, the topographic signature of the Denali fault, has an unusual physiography, with the Nenana River sourced from the south side of the divide and traversing along the range front some distance before heading north across the mountain range. Previous researchers suggested that a change from south-flowing to north-flowing drainage occurred at ca. 6 Ma, or early-middle Miocene, during initial phases of Alaska Range uplift. We applied 40Ar/39Ar dating of detrital micas from modern river sediment (proxy for basinwide source) and strata of the Neogene Tanana Basin (sink for the paleo-Nenana River), located along the northern front of the Alaska Range, to further investigate this hypothesis. In addition, we acquired 40Ar/39Ar muscovite ages from bedrock for additional source constraints and compared our results to regional geochronology data sets and geological mapping. During the earliest Miocene, the paleo-Nenana River likely flowed south. By the early Miocene, the paleo-Nenana River flowed to the north. During the middle Miocene, drainage reorganization continued, suggesting a variable history of rock uplift in the Alaska Range. By the late Miocene, sediment recycling occurred as the southern extent of the Tanana Basin was uplifted and eroded. The modern Nenana River near Cantwell has a muscovite age signature different than the Tanana Basin strata, implying continued drainage reorganization after the deposition of the Pliocene Nenana Gravel. In summary, the Nenana River drainage changed direction to north-flowing by ca. 18 Ma, driven by tectonism. Drainage reorganization continues today, demonstrating that strike-slip fault transpressive orogens can have complex paleodrainage histories. [ABSTRACT FROM AUTHOR]

Abstract: Early theories of plate tectonics assumed plates were rigid with deformation limited to within a few tens of kilometers of the plate boundary. However, observations indicate most continental plates defy such rigid behavior with deformation extending over 1000 kilometers inboard. We construct three-dimensional (3D) numerical models of the boundary between the Pacific and North American plates in Alaska to investigate the relative controls of flat slab subduction, continental scale faulting, and a non-linear rheology on deformation in the overriding plate. The models incorporate a realistic slab shape based on seismicity and seismic tomography and a variable thermal structure for both the subducting and overriding plates based on geologic and geophysical observables. The inclusion of the Denali fault in the models allows for the portion of south-central Alaska between the Denali fault and the trench to partially decouple from the rest of North America, forming an independently moving region that correlates to what has been described from geologic and geodetic studies as the Wrangell block. The motion of the Wrangell block tracks the motion of the flat slab in the subsurface indicating the subducting plate is driving the motion of the Wrangell block. Models using a composite (Newtonian and non-Newtonian) viscosity predict compressional motion along the northern bend in the Denali fault, consistent with thermochronologic data that show significant late Neogene exhumation in the central Alaska Range, including at Mt. McKinley, the tallest mountain in North America. These 3D numerical models of the Pacific–North American margin in Alaska show the subducting slab is the main driver of overriding plate deformation in south-central Alaska and combined with the Denali fault can reproduce several first order tectonic features of the region including the motion of the Wrangell block, uplift in the central Alaska Range, subsidence in the Cook Inlet-Susitna Basins, and upwelling at the slab edge beneath the Wrangell volcanics. [Copyright &y& Elsevier]

The Mesoproterozoic MARA terrane of western South America is a composite igneous-metamorphic complex that is important for Paleozoic paleogeographic reconstructions and the relative positions of Laurentia and Gondwana. The magmatic and detrital records of the MARA terrane are consistent with a Laurentian origin; however, the metamorphic and deformation records lack sufficient detail to constrain the correlation of units within the MARA terrane and the timing and mechanisms of accretion to the Gondwana margin. Combined regional mapping, metamorphic petrology, and garnet and monazite geochronology from the Sierra de Maz of northwest Argentina suggest that the region preserves four distinct litho-tectonic units of varying age and metamorphic conditions that are separated by middle- to lower-crustal ductile shear zones. The Zaino and Maz Complexes preserve Barrovian metamorphism and ages that are distinct from other units within the region. The Zaino and Maz Complexes both record metamorphism ca. 430-410 Ma and show no evidence of the regional Famatinian orogeny (ca. 490-455 Ma). In addition, the Maz Complex records an earlier granulite facies event at ca. 1.2 Ga. The Taco and Ramaditas Complexes, in contrast, experienced medium- and low-pressure upper amphibolite to granulite facies metamorphism, respectively, between ca. 470-460 Ma and were later deformed at ca. 440-420 Ma. The Maz shear zone that bounds the Zaino and Maz Complexes records sinistral oblique to sinistral deformation between ca. 430-410 Ma. The data suggest that at least some units in the MARA terrane were accreted by translation, and the Gondwana margin of northwest Argentina transitioned from a dominantly convergent margin to a highly oblique margin in the Silurian. [ABSTRACT FROM AUTHOR]

Abstract: Using single crystals to trace the chemical evolution of a magmatic system has long been a goal of igneous petrology. Crystals utilized for in situ dating hold great potential to link compositional and temporal information to better understand the evolution of a magmatic system. If micron-scale zoning of trace elements within a single zircon can be directly associated with volcanic events in magmatic systems, then new insights into long-term maintenance and storage of eruptible magma can be unlocked. This study presents new data that directly links the geochemical history recorded in individual zircon crystals with over 50,000yrs of changing physical and chemical conditions within a magma reservoir. Trace elements (Hf and Y) in zircon have not diffusively equilibrated with their host melt so that they store information about the melt when that zone crystallized. Thus, different stages of growth can be associated with discrete time periods in the magmatic system. Zircon from the two most recent rhyolite eruptions (Kaharoa and Whakatane) within the Okataina Volcanic Complex (OVC), New Zealand, have both isotopic (age) and trace element signatures that correspond with distinct changes in the temperature and phase assemblage of pulses of erupted magma within the caldera system. This discrete change is associated with caldera collapse and reflects a change from cold, wet source rhyolite to a relatively hotter and drier source rhyolite and back. The Kaharoa and Whakatane eruptions are separated by 4000yrs and 15km distance, yet the zircon populations record the same distinct thermal and chemical pulse that occurred following caldera collapse, suggesting an interconnected magmatic system that houses at least small volumes of rhyolite melt for timescales of 103 to 105 yrs that is periodically extracted during eruption. [Copyright &y& Elsevier]

Evidence is presented for a previously unrecognized late Paleozoic orogeny in two parts of Alaska''s Farewell terrane, an event that has not entered into published scenarios for the assembly of Alaska. The Farewell terrane was long regarded as a piece of the early Paleozoic passive margin of western Canada, but is now thought, instead, to have lain between the Siberian and Laurentian (North American) cratons during the early Paleozoic. Evidence for a late Paleozoic orogeny comes from two belts located 100–200 km apart. In the northern belt, metamorphic rocks dated at 284–285 Ma (three 40Ar/39Ar white-mica plateau ages) provide the main evidence for orogeny. The metamorphic rocks are interpreted as part of the hinterland of a late Paleozoic mountain belt, which we name the Browns Fork orogen. In the southern belt, thick accumulations of Pennsylvanian-Permian conglomerate and sandstone provide the main evidence for orogeny. These strata are interpreted as the eroded and deformed remnants of a late Paleozoic foreland basin, which we name the Dall Basin. We suggest that the Browns Fork orogen and Dall Basin comprise a matched pair formed during collision between the Farewell terrane and rocks to the west. The colliding object is largely buried beneath Late Cretaceous flysch to the west of the Farewell terrane, but may have included parts of the so-called Innoko terrane. The late Paleozoic convergent plate boundary represented by the Browns Fork orogen likely connected with other zones of plate convergence now located in Russia, elsewhere in Alaska, and in western Canada. [Copyright &y& Elsevier]