Rheological transitions in the middle crust:insights from Cordilleran metamorphic core complexes

High-strain mylonitic rocks in Cordilleran metamorphic core complexes reflect ductile deformation in the middle crust, but in many examples it is unclear how these mylonites relate to the brittle detachments that overlie them. Field observations, microstructural analyses, and thermobarometric data from the footwalls of three metamorphic core complexes in the Basin and Range Province, USA (the Whipple Mountains, California; the northern Snake Range, Nevada; and Ruby Mountains–East Humboldt Range, Nevada), suggest the presence of two distinct rheological transitions in the middle crust: (1) the brittle–ductile transition (BDT), which depends on thermal gradient and tectonic regime, and marks the switch from discrete brittle faulting and cataclasis to continuous, but still localized, ductile shear, and (2) the localized–distributed transition, or LDT, a deeper, dominantly temperature-dependent transition, which marks the switch from localized ductile shear to distributed ductile flow. In this model, brittle normal faults in the upper crust persist as ductile shear zones below the BDT in the middle crust, and sole into the subhorizontal LDT at greater depths.<br><br>In metamorphic core complexes, the presence of these two distinct rheological transitions results in the development of two zones of ductile deformation: a relatively narrow zone of high-stress mylonite that is spatially and genetically related to the brittle detachment, underlain by a broader zone of high-strain, relatively low-stress rock that formed in the middle crust below the LDT, and in some cases before the detachment was initiated. The two zones show distinct microstructural assemblages, reflecting different conditions of temperature and stress during deformation, and contain superposed sequences of microstructures reflecting progressive exhumation, cooling, and strain localization. The LDT is not always exhumed, or it may be obscured by later deformation, but in the Whipple Mountains, it can be directly observed where high-strain mylonites captured from the middle crust depart from the brittle detachment along a mylonitic front

Opposing shear senses in a subdetachment mylonite zone: Implications for core complex mechanics

[1] Global studies of metamorphic core complexes and low‐angle detachment faults have highlighted a fundamental problem: Since detachments excise crustal section, the relationship between the mylonitic rocks in their footwalls and the brittle deformation in their hanging walls is commonly unclear. Mylonites could either reflect ductile deformation related to exhumation along the detachment fault, or they could be a more general feature of the extending middle crust that has been “captured ” by the detachment. In the first case we would expect the kinematics of the mylonite zone to mirror the sense of movement on the detachment; in the second case both the direction and sense of shear in the mylonites could be different. The northern Snake Range décollement (NSRD) is a classic Basin and Range detachment fault with a well‐documented top‐east of displacement. We present structural, paleo-magnetic, geochronological, and geothermometric evidence to suggest that the mylonite zone below the NSRD locally experienced phases of both east ‐ and west‐directed shear, inconsistent with movement along a single detachment fault. We therefore propose that the footwall mylonites represent a predetachment dis-continuity in the middle crust that separated localized deformation above from distributed crustal flow below (localized‐distributed transition (LDT)). The mylonites were subsequently captured by a moderately dipping brittle detachment that soled down to the middle crust and exhumed them around a rolling hinge into a subhorizontal orientation at the surface, produc-ing the present‐day NSRD. In this interpretation the brittle hanging wall represents a series of rotated upper crustal normal faults, whereas the mylonitic footwall represents one or more exhumed middl

The Benefits of Early Book Sharing (BEBS) for child cognitive and socio-emotional development in South Africa: study protocol for a randomised controlled trial

Background:Children in Low and Middle-Income Countries (LMICs) are at risk for problems in their cognitive, social and behavioural development. Factors such as a lack of cognitive stimulation, harsh parenting practices, and severe and persistent aggression in early childhood are central to the genesis of these problems. Interventions that target the intersection between early childhood development, parenting, and early violence prevention are required in order to meaningfully address these problems. Methods:We are conducting a randomized controlled trial to evaluate a parenting intervention for caregivers of children between 23-27 months, designed to promote child cognitive and socio-emotional development in Khayelitsha, a low income peri-urban township in South Africa. Families are randomly allocated to book sharing or to a wait-list control group. In the intervention, we train caregivers in supportive book-sharing with young children. Training is carried out in small groups over a period of eight weeks. Data are collected at baseline, post-intervention and at six months post-intervention. In addition to targeting child cognitive development, the intervention aims to improve child socio-emotional functioning. Discussion:The Benefits of Early Book Sharing (BEBS) trial aims to evaluate the impact of an early parenting intervention on several key risk factors for the development of violence, including aspects of parenting and child cognition, prosocial behaviour, aggression, and socio-emotional functioning. The study is being carried out in a LMIC where violence constitutes a major social and health burden. Since the intervention is brief and, with modest levels of training, readily deliverable in LMIC contexts, a demonstration that it is of benefit to both child cognitive and socio-emotional development would be of significance

A rusty record of weathering and groundwater movement in the hyperarid Central Andes

The Atacama Desert, on the western margin of the Central Andes, hosts some of the world's largest porphyry copper deposits (PCDs). Despite a hyperarid climate, many of these PCDs have undergone secondary “supergene” enrichment, whereby copper has been concentrated via groundwater-driven leaching and reprecipitation, yielding supergene profiles containing valuable records of weathering and landscape evolution. We combine hematite (U-Th-Sm)/He geochronology and oxygen isotope analysis to compare the weathering histories of two Andean PCDs and test the relative importance of climate and tectonics in controlling both enrichment and water table movement. At Cerro Colorado, in the Precordillera, hematite precipitation records prolonged weathering from ∼31 to ∼2 Ma, tracking water table descent following aridity-induced canyon incision from the late Miocene onward. By contrast, hematite at Spence, within the Central Depression, is mostly younger than ∼10.5 Ma, suggesting exhumation ended much later. A heavy oxygen isotopic signature for Spence hematite suggests that upwelling formation water has been an important source of groundwater, accounting for a high modern water table despite persistent hyperaridity, whereas isotopically light hematite at Cerro Colorado formed in the presence of meteoric water. Compared with published paleo-environmental and sedimentological records, our data show that weathering can persist beneath appreciable post-exhumation cover, under hyperarid conditions unconducive to enrichment. The susceptibility of each deposit to aridity-induced water table descent, canyon incision and deep weathering has been controlled by recharge characteristics and morphotectonic setting. Erosional exhumation, rather than aridity-induced water table decay, appears to be more important for the development of supergene enrichment

A Community-Based Study of Enduring Eating Features in Young Women

We conducted a prospective exploration of the temporal course of eating disorder (ED) symptoms in two cohorts of community women. One hundred and twenty-two young women (Cohort 1) identified in a general population based survey with ED symptoms of clinical severity agreed to participate in a 5-year follow-up study. A comparative sample (Cohort 2) of 706 similar aged self-selected college women (221 with disordered eating) was recruited one year later. Both ED groups were given a health literacy package in the first year. ED symptoms, health related quality of life, and psychological distress were assessed annually with the Eating Disorder Examination Questionnaire, the Short Form—12 Health Survey and the Kessler Psychological Distress Scale, respectively. Forty percent (Cohort 1) and 30.3% (Cohort 2) completed questionnaires at each year of follow-up. In both groups, there was early improvement in ED symptoms which plateaued after the first year, and participants retained high EDE-Q scores at 5 years. BMI increased as expected. Mental health related quality of life scores did not change but there were small improvements in psychological distress scores. The findings suggest little likelihood of spontaneous remission of ED problems in community women

Post-collisional Cenozoic extension in the northern Aegean: the high-K to shoshonitic intrusive rocks of the Maronia Magmatic Corridor, northeastern Greece

The Maronia Magmatic Corridor is a NE-trending belt of Oligocene plutons that intrudes the Kechros Dome of the northern Rhodope Core Complex in northeastern Greece. The post-collisional magmatism transitions from early high-K calc-alkaline magmatism in the NE to a younger, shoshonitic phase in the SW. We use a full suite of whole-rock geochemical analyses, including rare earth elements, to show a shared metasomatized mantle source of the magmatism. Evidence of plagioclase saturation from the onset of crystallization and amphibole-pyroxene-controlled fractionation in the high-K calc-alkaline magmatism suggest a drier (<4.75 wt% H2O) parental magma than is typical of subduction-related magmatism. Continued H2O depletion of the metasomatized source mantle resulted in the transition to a shoshonitic trend where deep crustal fractionation of an H2O-poor (< ~2 wt% H2O) magma in the absence of major olivine resulted in incompatible enrichment over a small range of SiO2. High-precision U-Pb zircon geochronology is presented here for the first time to provide chronological markers for the transition in the magmatic evolution of the Kechros dome. A 2.2 Myr break in magmatism separates the intrusion of the shoshonitic Maronia pluton at 29.8 Ma from the emplacement of the rest of the high-K calc-alkaline Maronia Magmatic Corridor between 32.9–32.0 Ma. The Maronia pluton is the hottest, driest, and youngest episode of post-collisional magmatism in the Kechros dome; we suggest that the emplacement of Maronia marks the cessation of magmatism in the northern Rhodope Core Complex as asthenospheric mantle upwelling migrated southward