In search for the missing arc root of the Southern California Batholith: P-T-t evolution of upper mantle xenoliths of the Colorado Plateau Transition Zone

Xenolith and seismic studies provide evidence for tectonic erosion and eastward displacement of lower crust-subcontinental mantle lithosphere (LC-SCML) underlying the Mojave Desert Region (i.e. southern California batholith (SCB)). Intensified traction associated with the Late Cretaceous flattening of the subducting Farallon plate, responsible for deforming the SW U.S., likely played a key role in “bulldozing” the tectonically eroded LC-SCML ∼500 km eastwards, to underneath the Colorado Plateau Transition Zone (CPTZ) and further inboard. The garnet clinopyroxenite xenoliths from two CPTZ localities, Chino Valley and Camp Creek (central Arizona), provide a rare glimpse of the material underlying the CPTZ. Thermodynamic modeling, in addition to major and trace element thermobarometry, suggests that the xenoliths experienced peak conditions of equilibration at 600-900 °C and 12-28 kbar. These peak conditions, along with the composition of the xenoliths (type “B” garnet and diopsidic clinopyroxene) strongly suggest a continental arc residue (“arclogite”), rather than a lower plate subduction (“eclogite”), origin. A bimodal zircon U-Pb age distribution with peaks at ca. 75 and 150 Ma, and a Jurassic Sm-Nd garnet age (154 ± 16 Ma, with initial εNd value of +8) overlaps eastern SCB pluton ages and suggests a consanguineous relationship. Cenozoic zircon U-Pb ages, REE geochemistry of zircon grains, and partially re-equilibrated Sm-Nd garnet ages indicate that displaced arclogite remained at elevated PT conditions (>700 °C) for 10s of Myr following its dispersal until late Oligocene entrainment in host latite. With a ∼100 Myr long thermal history overlapping that of the SCB and the CPTZ, these assemblages also contain evidence for late-stage hydration (e.g. secondary amphibole), potentially driven by de-watering of the Laramide slab. In light of these results, we suggest that the CPTZ arclogite originates from beneath the eastern half of the SCB, where it began forming in Late Jurassic time as mafic keel to continental arc magmas. The displacement and re-affixation of the arclogites further inboard during the Late Cretaceous flat slab subduction, might have contributed to the tectonic stability of the Colorado Plateau relative to adjacent geologic provinces through Laramide time and likely preconditioned the region to Cenozoic tectonism, e.g. present-day delamination beneath the plateau, high-magnitude extension and formation of metamorphic core complexes

The North American Cordilleran Anatectic Belt

The North American Cordilleran Anatectic Belt (CAB) is a ~3,000 km long region in the hinterland of the Cordillera that comprises numerous exposures of Late Cretaceous to Eocene intrusive rocks and anatectic rocks associated with crustal melting. As such, it is comparable in size and volume to major anatectic provinces including the Himalayan leucogranite belt. The CAB rocks are chiefly peraluminous, muscovite-bearing leucogranite produced primarily by anatexis of Proterozoic to Archean metasedimentary rocks. The CAB rocks lack extrusive equivalents and were typically emplaced as thick sheets, laccoliths, and dike/sill complexes. The extent, location, and age of the CAB suggests that it is integral to understanding the tectonic evolution of North America, however, the belt is rarely considered as a whole. This paper reviews localities associated with crustal melting in the CAB and compiles geochemical, geochronologic, and isotopic data to evaluate the melt conditions and processes that generated these rocks. The geochemistry and partial melting temperatures (ca. 675–775 °C) support water-absent muscovite dehydration melting and/or water-deficient melting as the primary melt reactions and are generally inconsistent with water-excess melting and high-temperature (biotite to amphibole) dehydration melting. The CAB rocks are oldest in the central U.S. Cordillera and become younger towards both the north and south. At any single location, partial melting appears to have been a protracted process (≥10 Myr) and evidence for re-melting and remobilization of magmas is common. End-member hypotheses for the origin of the CAB include decompression, crustal thickening, fluid-flux melting, and increased heat flux from the mantle. Different parts of the CAB support different hypotheses and no single model may be able to explain the entirety of the anatectic event. Regardless, the CAB is a distinct component of the Cordilleran orogenic system

Inshore and offshore marine migration pathways of Atlantic salmon post-smolts from multiple rivers in Scotland, England, Northern Ireland and Ireland

The migratory behavior of Atlantic salmon (Salmo salar) post-smolts in coastal waters is poorly understood. In this collaborative study, 1914 smolts, from 25 rivers, in four countries were tagged with acoustic transmitters during a single seasonal migration. In total, 1105 post-smolts entered the marine study areas and 438 (39.6%) were detected on a network of 414 marine acoustic receivers and an autonomous underwater vehicle. Migration pathways (defined as the shortest distance between two detections) of up to 575 km and over 100 days at sea were described for all 25 populations. Post-smolts from different rivers, as well as individuals from the same river, used different pathways in coastal waters. Although difficult to generalize to all rivers, at least during the year of this study, no tagged post-smolts from rivers draining into the Irish Sea were detected entering the areas of sea between the Hebrides and mainland Scotland, which is associated with a high density of finfish aquaculture. An important outcome of this study is that a high proportion of post-smolts crossed through multiple legislative jurisdictions and boundaries during their migration. This study provides the basis for spatially explicit assessment of the impact risk of coastal pressures on salmon during their first migration to sea

Inshore and offshore marine migration pathways of Atlantic salmon post-smolts from multiple rivers in Scotland, England, Northern Ireland, and Ireland

The migratory behavior of Atlantic salmon (Salmo salar) post-smolts in coastal waters is poorly understood. In this collaborative study, 1914 smolts, from 25 rivers, in four countries were tagged with acoustic transmitters during a single seasonal migration. In total, 1105 post-smolts entered the marine study areas and 438 (39.6%) were detected on a network of 414 marine acoustic receivers and an autonomous underwater vehicle. Migration pathways (defined as the shortest distance between two detections) of up to 575 km and over 100 days at sea were described for all 25 populations. Post-smolts from different rivers, as well as individuals from the same river, used different pathways in coastal waters. Although difficult to generalize to all rivers, at least during the year of this study, no tagged post-smolts from rivers draining into the Irish Sea were detected entering the areas of sea between the Hebrides and mainland Scotland, which is associated with a high density of finfish aquaculture. An important outcome of this study is that a high proportion of post-smolts crossed through multiple legislative jurisdictions and boundaries during their migration. This study provides the basis for spatially explicit assessment of the impact risk of coastal pressures on salmon during their first migration to sea

In search for the missing arc root of the Southern California Batholith: P-T-t evolution of upper mantle xenoliths of the Colorado Plateau Transition Zone

Xenolith and seismic studies provide evidence for tectonic erosion and eastward displacement of lower crust-subcontinental mantle lithosphere (LC-SCML) underlying the Mojave Desert Region (i.e. southern California batholith (SCB)). Intensified traction associated with the Late Cretaceous flattening of the subducting Farallon plate, responsible for deforming the SW U.S., likely played a key role in “bulldozing” the tectonically eroded LC-SCML ∼500 km eastwards, to underneath the Colorado Plateau Transition Zone (CPTZ) and further inboard. The garnet clinopyroxenite xenoliths from two CPTZ localities, Chino Valley and Camp Creek (central Arizona), provide a rare glimpse of the material underlying the CPTZ. Thermodynamic modeling, in addition to major and trace element thermobarometry, suggests that the xenoliths experienced peak conditions of equilibration at 600-900 °C and 12-28 kbar. These peak conditions, along with the composition of the xenoliths (type “B” garnet and diopsidic clinopyroxene) strongly suggest a continental arc residue (“arclogite”), rather than a lower plate subduction (“eclogite”), origin. A bimodal zircon U-Pb age distribution with peaks at ca. 75 and 150 Ma, and a Jurassic Sm-Nd garnet age (154 ± 16 Ma, with initial εNd value of +8) overlaps eastern SCB pluton ages and suggests a consanguineous relationship. Cenozoic zircon U-Pb ages, REE geochemistry of zircon grains, and partially re-equilibrated Sm-Nd garnet ages indicate that displaced arclogite remained at elevated PT conditions (>700 °C) for 10s of Myr following its dispersal until late Oligocene entrainment in host latite. With a ∼100 Myr long thermal history overlapping that of the SCB and the CPTZ, these assemblages also contain evidence for late-stage hydration (e.g. secondary amphibole), potentially driven by de-watering of the Laramide slab. In light of these results, we suggest that the CPTZ arclogite originates from beneath the eastern half of the SCB, where it began forming in Late Jurassic time as mafic keel to continental arc magmas. The displacement and re-affixation of the arclogites further inboard during the Late Cretaceous flat slab subduction, might have contributed to the tectonic stability of the Colorado Plateau relative to adjacent geologic provinces through Laramide time and likely preconditioned the region to Cenozoic tectonism, e.g. present-day delamination beneath the plateau, high-magnitude extension and formation of metamorphic core complexes

Migration patterns and navigation cues of Atlantic salmon post‐smolts migrating from 12 rivers through the coastal zones around the Irish Sea

The freshwater phase of the first seaward migration of juvenile Atlantic salmon (Salmo salar) is relatively well understood when compared with our understanding of the marine phase of their migration. In 2021, 1008 wild and 60 ranched Atlantic salmon smolts were tagged with acoustic transmitters in 12 rivers in England, Scotland, Northern Ireland and Ireland. Large marine receiver arrays were deployed in the Irish Sea at two locations: at the transition of the Irish Sea into the North Atlantic between Ireland and Scotland, and between southern Scotland and Northern Ireland, to examine the early phase of the marine migration of Atlantic salmon smolts. After leaving their natal rivers’ post-smolt migration through the Irish Sea was rapid with minimum speeds ranging from 14.03 to 38.56 km.day-1 for Atlantic salmon smolts that entered the Irish Sea directly from their natal river, to 9.69 to 39.94 km.day-1 for Atlantic salmon smolts that entered the Irish Sea directly from their natal estuary. Population minimum migration success through the study area was strongly correlated with the distance of travel, populations further away from the point of entry to the open North Atlantic exhibited lower migration success. Post-smolts from different populations experienced different water temperatures on entering the North Atlantic. This was largely driven by the timing of their migration and may have significant consequences for feeding and ultimately survivorship. The influence of water currents on post-smolt movement was investigated using data from previously constructed numerical hydrodynamic models. Modelled water current data in the northern Irish Sea showed that post-smolts had a strong preference for migrating when the current direction was at around 283° (west-north-west) but did not migrate when exposed to strong currents in other directions. This is the most favourable direction for onward passage from the Irish Sea to the continental shelf edge current, a known accumulation point for migrating post-smolts. These results strongly indicate that post-smolts migrating through the coastal marine environment are: 1) not simply migrating by current following 2) engage in active directional swimming 3) have an intrinsic sense of their migration direction and 4) can use cues other than water current direction to orientate during this part of their migration