Amy E. Draut

A general model of arc-continent collision and subduction polarity reversal from Taiwan and the Irish Caledonides

Abstract The collision of the Luzon Arc with southern China represents the best example of arc-continent collision in the modern oceans, and compares closely with the Early Ordovician accretion of the Lough Nafooey arc of Connemara, Ireland, to the passive margin of Laurentia. We propose a general model for steady-state arc-continent collision in which arc crust is progressively added to a passive margin during a process of compression, metamorphism and magmatism lasting 3–10 Ma at any one location on the margin. Depending on the obliquity of the angle of collision, the timing of active collision may be diachronous and long-lived along the margin. Magmatism accompanying accretion can be more enriched in incompatible trace elements than average continental crust, contrasting with more depleted magmatism prior to collision. Accretion of a mixture of depleted and enriched arc lithologies to the continental margin allows the continental crust to grow through time by arc-passive margin collision events. During the collision the upper and middle arc crust are detached from the depleted ultramafic lower crust, which is subducted along with the mantle lithosphere on which the arc was founded. Rapid (2–3 Ma) exhumation and gravitational collapse of the collisional orogen forms the Okinawa and South Mayo Troughs in Taiwan and western Ireland, respectively. These basins are filled by detritus eroded from the adjacent collision zone. During subsequent subduction polarity reversal, continuous tearing and retreat of the oceanic lithosphere along the former continent-ocean transition provides space for the new subducting oceanic plate to descend without need for breaking of the original slab.

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

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

Geochemical evolution of arc magmatism during arc-continent collision, South Mayo, Ireland

The Lough Nafooey arc, in the western Irish Caledonides, collided with Laurentia during the Early Ordovician. This event is recorded in the stratigraphy of the South Mayo trough, the preserved forearc basin of this system. Lavas at the base of the oldest Lough Nafooey Group show intraoceanic arc composition. eNd(t) decreases and light rare earth elements (REEs) become more enriched upsection in the Lough Nafooey Group (ca. 495‐ 481 Ma), reflecting early collision with Laurentia. The subsequent Tourmakeady Group (ca. 481‐470 Ma) is rhyolitic, light REE enriched, and has strongly negative eNd(t). These rocks were erupted during the Grampian orogeny. The Rosroe and Mweelrea Formations (,470 Ma) show wide scatter of La/Sm and Nb/Zr values, suggesting mixed mantle sources. This correlates with rapid exhumation of the adjacent Connemara metamorphic terrane. The chemical evolution of the arc supports models of collision, orogeny, and tectonic unroofing within ;15 m.y., and shows that genesis of magmas more enriched than continental crust can occur during arc-continent collision, clarifying the potential role of arc volcanism in continental crust formation.

A model for continental crust genesis by arc accretion: rare earth element evidence from the Irish Caledonides

Abstract The formation of continental crust is a complex problem with a paradox at its center: continental material is believed to form by arc magmatism, a model that does not reconcile the bulk mafic and light rare earth element (LREE)-depleted composition of intra-oceanic arcs with the andesitic, LREE-enriched composition of continents. We present evidence supporting an arc origin for continental crust by demonstrating significant changes in magmatic chemistry during arc–continent collision. We use as an example the Connemara region of the western Irish Caledonides, where metamorphism and orogenesis of Dalradian sedimentary sequences record the collision of the oceanic Lough Nafooey Arc with the passive margin of Laurentia at ∼475 Ma (Arenig). This arc was mafic and LREE-depleted during its oceanic activity. During collision, rhyolites more LREE-enriched than the surrounding continental crust were erupted, while the mid-crust was intruded by similarly LREE-enriched gabbro and diorite. Crystal fractionation caused LREE enrichment to exceed that expected by assimilation of continental crust alone. We propose that subsequent loss of the corresponding lower crustal cumulates into the mantle removed this LREE-depleted material from the net crust added to the continent. The combined processes of crystal fractionation and lower crustal loss during arc–continent collision drive the average bulk arc composition toward that of average continental crust.

Stratigraphic and geochemical evolution of an oceanic arc upper crustal section: The Jurassic Talkeetna Volcanic Formation, south-central Alaska

The Early Jurassic Talkeetna Volcanic Formation forms the upper stratigraphic level of an oceanic volcanic arc complex within the Peninsular Terrane of south-central Alaska. The section comprises a series of lavas, tuffs, and volcaniclastic debris-fl ow and turbidite deposits, showing signifi cant lateral facies variability. There is a general trend toward more volcaniclastic sediment at the top of the section and more lavas and tuff breccias toward the base. Evidence for dominant submarine, mostly mid-bathyal or deeper (>500 m) emplacement is seen throughout the section, which totals ~7 km in thickness, similar to modern western Pacifi c arcs, and far more than any other known exposed section. Subaerial sedimentation was rare but occurred over short intervals in the middle of the section. The Talkeetna Volcanic Formation is dominantly calc-alkaline and shows no clear trend to increasing SiO 2 up-section. An oceanic subduction petrogenesis is shown by trace element and Nd isotope data. Rocks at the base of the section show no relative enrichment of light rare earth elements (LREEs) versus heavy rare earth elements (REEs) or in meltincompatible versus compatible high fi eld strength elements (HFSEs). Relative enrichment of LREEs and HFSEs increases slightly up-section. The Talkeetna Volcanic Formation is typically more REE depleted than average continental crust, although small volumes of light REE-enriched and heavy REE-depleted mafi c lavas are recognized low in the stratigraphy. The Talkeetna Volcanic Formation was formed in an intraoceanic arc above a northdipping subduction zone and contains no preserved record of its subsequent collisions with Wrangellia or North America.

Arc–continent collision and the formation of continental crust: a new geochemical and isotopic record from the Ordovician Tyrone Igneous Complex, Ireland

Abstract: Collisions between oceanic island-arc terranes and passive continental margins are thought to have been important in the formation of continental crust throughout much of Earths history. Magmatic evolution during this stage of the plate-tectonic cycle is evident in several areas of the Ordovician Grampian–Taconic orogen, as we demonstrate in the first detailed geochemical study of the Tyrone Igneous Complex, Ireland. New U–Pb zircon dating yields ages of 493 ± 2 Ma from a primitive mafic intrusion, indicating intra-oceanic subduction in Tremadoc time, and 475 ± 10 Ma from a light rare earth element (LREE)-enriched tonalite intrusion that incorporated Laurentian continental material by early Arenig time (Early Ordovician, Stage 2) during arc–continent collision. Notably, LREE enrichment in volcanism and silicic intrusions of the Tyrone Igneous Complex exceeds that of average Dalradian (Laurentian) continental material that would have been thrust under the colliding forearc and potentially recycled into arc magmatism. This implies that crystal fractionation, in addition to magmatic mixing and assimilation, was important to the formation of new crust in the Grampian–Taconic orogeny. Because similar super-enrichment of orogenic melts occurred elsewhere in the Caledonides in the British Isles and Newfoundland, the addition of new, highly enriched melt to this accreted arc terrane was apparently widespread spatially and temporally. Such super-enrichment of magmatism, especially if accompanied by loss of corresponding lower crustal residues, supports the theory that arc–continent collision plays an important role in altering bulk crustal composition toward typical values for ancient continental crust.

Laurentian crustal recycling in the Ordovician Grampian Orogeny: Nd isotopic evidence from western Ireland

Because magmatism associated with subduction is thought to be the principal source for continental crust generation, assessing the relative contribution of pre-existing (subducted and assimilated) continental material to arc magmatism in accreted arcs is important to understanding the origin of continental crust. We present a detailed Nd isotopic stratigraphy for volcanic and volcaniclastic formations from the South Mayo Trough, an accreted oceanic arc exposed in the western Irish Caledonides. These units span an arc-continent collision event, the Grampian (Taconic) Orogeny, in which an intra-oceanic island arc was accreted onto the passive continental margin of Laurentia starting at ∼ 475 Ma (Arenig). The stratigraphy corresponding to pre-, syn- and post- collisional volcanism reveals a progression of eNd(t) from strongly positive values, consistent with melt derivation almost exclusively from oceanic mantle beneath the arc, to strongly negative values, indicating incorporation of continental material into the melt. Using eNd(t) values of meta-sediments that represent the Laurentian passive margin and accretionary prism, we are able to quantify the relative proportions of continent-derived melt at various stages of arc formation and accretion. Mass balance calculations show that mantle-derived magmatism contributes substantially to melt production during all stages of arc-continent collision, never accounting for less than 21 % of the total. This implies that a significant addition of new, rather than recycled, continental crust can accompany arc-continent collision and continental arc magmatism.

Supply and dispersal of flood sediment from a steep, tropical watershed: Hanalei Bay, Kaua’i, Hawai’i, USA

In contrast to many small, mountainous watersheds in temperate coastal regions, where fl uvial discharge and wave energy commonly coincide, deposition and reworking of tropical fl ood sediment can be seasonally decoupled, and this has important implications for coral-reef ecosystems. An understanding of the interaction between tropical fl ood sedimentation and wave climate is essential to identifying and mitigating effects of watershed changes on coral reefs as urbanization and climate change proceed. Sedimentary facies and isotopic properties of sediment in Hanalei Bay, on the island of Kaua’i, Hawai’i, USA, were used to assess deposition and reworking of fldeposits from the Hanalei River in a case study demonstrating the potential ecosystem effects of runoff from a steep, tropical watershed. In Hanalei Bay, the youngest and thickest terrigenous sediment was consistently present near the river mouth and in a bathymetric depression that acted as at least a temporary sediment sink. During this 2 yr study, the largest fl ood events occurred in late winter and spring 2006; substantial terrestrial sediment delivered by those fl oods still remained in the bay as of June 2006 because oceanic conditions were not suffi ciently energetic to transport all of the sediment offshore. Additional sediment was deposited in the bay by a summer 2006 fl ood that coincided with seasonal low wave energy. In most years, fl ood sediment accumulating in the bay and on its fringing reefs would be remobilized and advected out of the bay during winter, when the wave climate is energetic. Turbidity and sedimentation on corals resulting from late spring and summer fl oods during low wave energy could have a greater impact on coral-reef ecosystems than fl oods in other seasons, an effect that could be exacerbated if the incidence and sediment load of tropical summer fl oods increase due to urbanization and climate change.

The origin and significance of the Delaney Dome Formation, Connemara, Ireland

Dalradian meta-sediments of the Laurentian margin and mafic intrusions thereof in SW Connemara, Ireland, tectonically overlie meta-rhyolites of the Delaney Dome Formation. The two units are separated by the Mannin Thrust. A new U–Pb age of 474.6 ± 5.5 Ma shows that the Delaney Dome Formation is a temporal equivalent of arc volcanic rocks preserved in the adjacent South Mayo Trough: the Tourmakeady Volcanic Group, erupted during the collision of an oceanic island arc with the Laurentian margin in the Grampian Orogeny. New rare earth and high field strength element data show that the Delaney Dome Formation and Tourmakeady Volcanic Group are chemically similar and arc-like in character. This suggests that the Delaney Dome Formation is an along-strike equivalent of the Tourmakeady Group, strike-slip faulted south of the South Mayo Trough during or after the Grampian Orogeny. Further correlation of these units with northern Appalachian rhyolites is also possible. The Delaney Dome Formation is an extrusive temporal equivalent of intrusions that penetrate the Connemara Dalradian. Thus, movement along the Mannin Thrust brought mid-crustal plutons and Dalradian country rocks tectonically above the extrusive volcanic sequence. The Mannin Thrust is identified as a major imbricating structure within a continental arc, but not a terrane boundary.

Short-term variability of 7Be atmospheric deposition and watershed response in a Pacific coastal stream, Monterey Bay, California, USA.

Beryllium-7 is a powerful and commonly used tracer for environmental processes such as watershed sediment provenance, soil erosion, fluvial and nearshore sediment cycling, and atmospheric fallout. However, few studies have quantified temporal or spatial variability of (7)Be accumulation from atmospheric fallout, and parameters that would better define the uses and limitations of this geochemical tracer. We investigated the abundance and variability of (7)Be in atmospheric deposition in both rain events and dry periods, and in stream surface-water samples collected over a ten-month interval at sites near northern Monterey Bay (37°N, 122°W) on the central California coast, a region characterized by a rainy winters, dry summers, and small mountainous streams with flashy hydrology. The range of (7)Be activity in rainwater samples from the main sampling site was 1.3-4.4 Bq L(-1), with a mean (±standard deviation) of 2.2 ± 0.9 Bq L(-1), and a volume-weighted average of 2.0 Bq L(-1). The range of wet atmospheric deposition was 18-188 Bq m(-2) per rain event, with a mean of 72 ± 53 Bq m(-2). Dry deposition fluxes of (7)Be ranged from less than 0.01 up to 0.45 Bq m(-2) d(-1), with an estimated dry season deposition of 7 Bq m(-2) month(-1). Annualized (7)Be atmospheric deposition was approximately 1900 Bq m(-2) yr(-1), with most deposition via rainwater (>95%) and little via dry deposition. Overall, these activities and deposition fluxes are similar to values found in other coastal locations with comparable latitude and Mediterranean-type climate. Particulate (7)Be values in the surface water of the San Lorenzo River in Santa Cruz, California, ranged from <0.01 Bq g(-1) to 0.6 Bq g(-1), with a median activity of 0.26 Bq g(-1). A large storm event in January 2010 characterized by prolonged flooding resulted in the entrainment of (7)Be-depleted sediment, presumably from substantial erosion in the watershed. There were too few particulate (7)Be data over the storm to accurately model a (7)Be load, but the results suggest enhanced watershed export of (7)Be from small, mountainous river systems compared to other watershed types.