The Variscan basement in the western shoulder of the Lusitanian Basin (West Iberian Margin): insights from detrital-zircon geochronology of Jurassic strata

There is no consensus about the geological nature of the westernmost portion of the Iberian Massif. In the present research, the detrital zircon U–Pb signatures of Jurassic strata of the Lusitanian Basin, known to be west-sourced, are combined with published U–Pb data for the Precambrian-Palaeozoic basement and other Lusitanian Basin units to better understand this poorly exposed portion of the Iberian Massif. Cryogenian to Ediacaran ages prevail in a northern Upper Jurassic unit, while Lower and Upper Jurassic rocks in southern locations yield mostly Carboniferous to upper Permian zircons. These age results, coupled with their respective U/Th ratios, suggest that the basin covers two distinct terranes of the Iberian Massif. Another noteworthy feature of west-derived deposits is the abundance of < 310 Ma ages. It is proposed that a combination of crustal thinning in the West Iberian Margin with regional eastward basement tilt, favoured the enrichment of relatively young zircon in the western shoulder of the basin relative to its eastern margin. The detrital zircon age signatures also reveal a middle to late Permian thermal event in restricted areas, which is probably associated with the oldest stages of Alpine extension in West Iberia. // Actualmente no existe consenso sobre la naturaleza geológica de la zona más occidental del Macizo Ibérico. En la presente investigación se ha llevado a cabo un análisis de la relación entre la signatura U-Pb de circones detríticos obtenidos en estratos jurásicos de la Cuenca Lusitana, provenientes del oeste, con datos de U-Pb publicados sobre el basamento Precámbrico-Paleozoico y otras formaciones de la Cuenca Lusitana, para comprender mejor esta zona poco expuesta del Macizo Ibérico. Las edades criogénicas y ediacarienses dominan en la parte norte de las unidades del Jurásico superior, mientras que en la parte sur dominan los circones de edades carboníferas y del Pérmico superior, tanto en las unidades del Jurásico inferior como en las del superior. Estos resultados geocronológicos junto con sus respectivas relaciones U/Th, sugieren que la cuenca cubre dos terrenos distintos del Macizo Ibérico. Otra característica notable de los depósitos procedentes del oeste es la abundancia de edades <310 Ma. Se propone que una combinación de adelgazamiento de la corteza en el Margen Ibérico Occidental junto con la inclinación regional del basamento hacia el este, favoreció el enriquecimiento de circones relativamente jóvenes en el flanco occidental de la cuenca en relación con su margen oriental. Las signaturas de edad del circón detrítico también revelan un evento térmico del Pérmico medio al tardío en áreas restringidas que probablemente esté asociado con las etapas más antiguas de extensión alpina en el oeste de Iberia

No modern Irrawaddy River until the late Miocene-Pliocene

The deposits of large Asian rivers with unique drainage geometries have attracted considerable attention due to their explanatory power concerning tectonism, surface uplift and upstream drainage evolution. This study presents the first petrographic, heavy mineral, Nd and Sr isotope geochemistry, and detrital zircon geochronology results from the Holocene Irrawaddy megadelta alongside modern and ancient sedimentary provenance datasets to assess the late Neogene evolution of the Irrawaddy River. Contrary to models advocating a steady post-middle Miocene river, we reveal an evolution of the Irrawaddy River more compatible with regional evidence for kinematic reorganization in Myanmar during late-stage India-Asia collision. Quaternary sediments are remarkably consistent in terms of provenance but highlight significant decoupling amongst fine and coarse fraction 87Sr/86Sr and εNd due to hydraulic sorting. Only well after the late Miocene do petrographic, heavy mineral, isotope geochemistry, and detrital zircon U–Pb results from the trunk Irrawaddy and its tributaries achieve modern-day signatures. The primary driver giving rise to the geometry and provenance signature of the modern Irrawaddy River was regional late Miocene (≤10 Ma) basin inversion coupled with uplift and cumulative displacement along the Sagaing Fault. Middle to late Miocene provenance signatures cannot be reconciled with modern river geometries, and thus require significant loss of headwaters feeding the Chindwin subbasin after ∼14 Ma and the northern Shwebo subbasin after ∼11 Ma. Large-scale reworking after ∼7 Ma is evidenced by modern Irrawaddy River provenance, by entrenchment of the nascent drainage through Plio-Pleistocene inversion structures, and in the transfer of significant sediment volumes to the Andaman Sea