Plio-Pleistocene exhumation of the eastern Himalayan syntaxis and its domal ‘pop-up’

The eastern termination of the Himalayan orogen forms a structural syntaxis that is characterised by young (from 10 to < 1 Ma) mineral growth and cooling ages that document Late Miocene to Pleistocene structural, metamorphic, igneous and exhumation events. This region is a steep antiformal and in part domal structure that folds the suture zone between the Indian and Asian plates. It is dissected by the Yarlung Tsangpo, one of the major rivers of the eastern Himalayan–Tibet region, which becomes the Brahmaputra River in the Indian foreland basin before emptying into the Bay of Bengal. Exceptionally high relief and one of the deepest gorges on Earth have developed where the river's tortuous route crosses the Namche Barwa–Gyala Peri massif (> 7 km in elevation) in the core of the syntaxis. Very high erosion rates documented in sediment downstream of the gorge at the foot of the Himalaya contribute ~ 50% of total detritus to the sediment load of the Brahmaputra. The initiation of very high rates of exhumation has been attributed either to the extreme erosive power of a river flowing across a deforming indentor corner and the associated positive feedback, or to the geometry of the Indian plate indentor, with the corner being thrust beneath the Asian plate resulting in buckling which accommodates shortening; both processes may be important. The northern third of the syntaxis corresponds to a steep domal ‘pop-up’ structure bounded by the India–Asia suture on three sides and a thrust zone to the south. Within the dome, Greater Himalaya rocks equilibrated at ~ 800 °C and 25–30 km depth during the Miocene, with these conditions potentially persisting into the latest Miocene and possibly the Pliocene, with modest decompression prior to ~ 4 Ma. This domal ‘pop-up’ corresponds to the area of youngest bedrock ages on a wide variety of thermochronometers and geochronometers. In this paper we review the extensive scientific literature that has focused on the eastern syntaxis and provide new chronological data on its bedrock and erosion products to constrain the age of inception of the very rapid uplift and erosion. We then discuss its cause, with the ultimate aim to reconstruct the exhumation history of the syntaxis and discuss the tectonic context for its genesis. We use zircon and rutile U–Pb, white mica Ar–Ar and zircon fission track dating methods to extract age data from bedrock, Brahmaputra modern sediments (including an extensive compilation of modern detrital chronometry from the eastern Himalaya) and Neogene palaeo-Brahmaputra deposits of the Surma Basin (Bangladesh). Numerical modelling of heat flow and erosion is also used to model the path of rocks from peak metamorphic conditions of ~ 800 °C to < 250 °C. Our new data include U–Pb bedrock rutile ages as young as 1.4 Ma from the Namche Barwa massif and 0.4 Ma from the river downstream of the syntaxis. Combined with existing data, our new data and heat flow modelling show that: i) the detrital age signature of the modern syntaxis is unique within the eastern Himalayan region; ii) the rocks within the domal pop-up were > 575 ± 75 °C only 1–2 Myr ago; iii) the Neogene Surma Basin does not record evidence of the rise and erosion of the domal pop-up until latest Pliocene–Pleistocene time; iv) Pleistocene exhumation of the north-easternmost part of the syntaxis took place at rates of at least 4 km/Myr, with bedrock erosion of 12–21 km during the last 3 Ma; v) the inception of rapid syntaxial exhumation may have started as early as 7 Ma or as late as 3 Ma; and vi) the Yarlung Tsangpo is antecedent and subsequently distorted by the developing antiform. Together our data and modelling demonstrate that the domal pop-up with its exceptional erosion and topographic relief is likely a Pleistocene feature that overprinted earlier structural and metamorphic events typical of Himalayan evolution. Keywords: Eastern Himalayan syntaxis; Namche Barwa; Surma Basin; Yarlung Tsangpo–Brahmaputra; U–Pb rutile dating; Thermal modellin

Assimilation—On (Not) Turning White: Memory and the Narration of the Postwar History of Japanese Canadians in Southern Alberta

This essay explores understandings of “race” – specifically, what it means to be Japanese – of nisei (“second generation”) individuals who acknowledge their near complete assimilation structurally and normatively into the Canadian mainstream. In historically-contextualized analyses of memory fragments from oral-history interviews conducted between 2011-2017, it focusses on voices and experiences of southern Alberta, an area whose significance to local, national, continental, and trans-Pacific histories of people of Japanese descent is belied by a lack of dedicated scholarly attention. In this light, this essay reveals how the fact of being Japanese in the latter half of the twentieth century was strategically central to nisei lives as individuals and in their communities. In imagining a racial hierarchy whose apex they knew they could never share with the hakujin (whites), the racial heritage they nevertheless inherited and would bequeath could be so potent as to reverse the direction of the colonial gaze with empowering effects in individual engagements then and as remembered now. We see how the narration and validation of one’s life is the navigation of wider historical contexts, the shaping of the post-colonial legacy of Imperial cultures, as Britain and Japan withdrew from their erstwhile colonial projects in Canada

Population genomics of invasive rodents on islands: Genetic consequences of colonization and prospects for localized synthetic gene drive

Introduced rodent populations pose significant threats worldwide, with particularly severe impacts on islands. Advancements in genome editing have motivated interest in synthetic gene drives that could potentially provide efficient and localized suppression of invasive rodent populations. Application of such technologies will require rigorous population genomic surveys to evaluate population connectivity, taxonomic identification, and to inform design of gene drive localization mechanisms. One proposed approach leverages the predicted shifts in genetic variation that accompany island colonization, wherein founder effects, genetic drift, and island-specific selection are expected to result in locally fixed alleles (LFA) that are variable in neighboring nontarget populations. Engineering of guide RNAs that target LFA may thus yield gene drives that spread within invasive island populations, but would have limited impacts on nontarget populations in the event of an escape. Here we used pooled whole-genome sequencing of invasive mouse (Mus musculus) populations on four islands along with paired putative source populations to test genetic predictions of island colonization and characterize locally fixed Cas9 genomic targets. Patterns of variation across the genome reflected marked reductions in allelic diversity in island populations and moderate to high degrees of differentiation from nearby source populations despite relatively recent colonization. Locally fixed Cas9 sites in female fertility genes were observed in all island populations, including a small number with multiplexing potential. In practice, rigorous sampling of presumptive LFA will be essential to fully assess risk of resistance alleles. These results should serve to guide development of improved, spatially limited gene drive design in future applications

Population genomics of invasive rodents on islands: Genetic consequences of colonization and prospects for localized synthetic gene drive

Introduced rodent populations pose significant threats worldwide, with particularly severe impacts on islands. Advancements in genome editing have motivated interest in synthetic gene drives that could potentially provide efficient and localized suppression of invasive rodent populations. Application of such technologies will require rigorous population genomic surveys to evaluate population connectivity, taxonomic identification, and to inform design of gene drive localization mechanisms. One proposed approach leverages the predicted shifts in genetic variation that accompany island colonization, wherein founder effects, genetic drift, and island-specific selection are expected to result in locally fixed alleles (LFA) that are variable in neighboring nontarget populations. Engineering of guide RNAs that target LFA may thus yield gene drives that spread within invasive island populations, but would have limited impacts on nontarget populations in the event of an escape. Here we used pooled whole-genome sequencing of invasive mouse (Mus musculus) populations on four islands along with paired putative source populations to test genetic predictions of island colonization and characterize locally fixed Cas9 genomic targets. Patterns of variation across the genome reflected marked reductions in allelic diversity in island populations and moderate to high degrees of differentiation from nearby source populations despite relatively recent colonization. Locally fixed Cas9 sites in female fertility genes were observed in all island populations, including a small number with multiplexing potential. In practice, rigorous sampling of presumptive LFA will be essential to fully assess risk of resistance alleles. These results should serve to guide development of improved, spatially limited gene drive design in future applications

Middle Miocene out-of sequence thrusting and successive exhumation in the Peloritani Mountains, Sicily: Late stage evolution of an orogen unreveled by apatite fission track and (U-Th)/He thermochronometry

Apatite fission track (AFT) and (U‐Th)/He (AHe) thermochronometry are applied to constraint the thermal history of the basement rock forming the Peloritani Mountains in northeast Sicily, Italy. AFT ages range between 29.0 ± 5.5 Ma and 5.5 ± 0.9 Ma while AHe ages vary from 19.4 Ma to 3.3 Ma. Most AFT ages are younger than the overlying terrigenous sequence that, in turn, postdates the main orogenic phase. Through the coupling of the thermal modeling with the stratigraphic record, a middle Miocene thermal event is revealed. This event affected an innerintermediate portion of the Peloritani Mountains and is confined by the distribution of the AFT ages <15 Ma. The U‐shaped pattern of AFT ages along N–S transects across the mountain chain is consistent with burial below a thrust stack that is a few km thick, created by an out‐of‐sequence thrusting phase that affected the inner portion of the belt. The tectonic load was then removed initially by erosion enhanced by the high relief at 10–7 Ma. The difference between young AFT and AHe ages is used to infer a rate of exhumation of 0.3 mm/yr for this stage. The gap between the youngest AHe ages and the sedimentary record above indicate a final exhumation stage with increasing rates of denudation (1–3 mm/yr) since the Pliocene times due to extensional tectonics. Citation: Olivetti, V., M. L. Balestrieri, C. Faccenna, F. M. Stuart, and G. Vignaroli (2010), Middle Miocene out‐of‐sequence thrusting and successive exhumation in the Peloritani Mountains, Sicily: Late stage evolution of an orogen unraveled by apatite fission track and (U‐Th)/He thermochronometry