ALBERT

Description: Tectonic displacement of drainage divides and the consequent deformation of river networks during crustal shortening have been proposed for a number of mountain ranges, but never tested. In order to preserve crustal strain in surface topography, surface displacements across thrust faults must be retained without being recovered by consequent erosion. Quantification of these competing processes and the implications for catchment topography have not previously been demonstrated. Here, we use structural mapping combined with dating of terrace sediments to measure Quaternary shortening across the Indus River valley in Ladakh, NW Himalaya. We demonstrate ~0.21 m k.y. –1 of horizontal displacement since ca. 45 ka on the Stok thrust in Ladakh, which defines the southwestern margin of the Indus Valley catchment and is the major back thrust to the Tethyan Himalaya in this region. We use normalized river channel gradients of the tributaries that drain into the Indus River to show that the lateral continuation of the Stok thrust was active for at least 70 km along strike. Shortening rates combined with fault geometries yield vertical displacement rates that are compared to time-equivalent erosion rates in the hanging wall derived from published detrital 10 Be analyses. The results demonstrate that vertical displacement rates across the Stok thrust were approximately twice that of the time-equivalent erosion rates, implying a net horizontal displacement of the surface topography, and hence narrowing of the Indus Valley at ~0.1 m k.y. –1 . A fill terrace records debris-flow emplacement linked to thrust activity, resulting in damming of the valley and extensive lake development. Conglomerates beneath some of the modern alluvial fans indicate a northeastward shift of the Indus River channel since ca. 45 ka to its present course against the opposite side of the valley from the Stok thrust. The integration of structural, topographic, erosional, and sedimentological data provides the first demonstration of the tectonic convergence of drainage divides in a mountain range and yields a model of the surface processes involved.

Description: The recent discovery of ultrahigh-pressure (UHP) mineral parageneses in the far-transported (greater than 400 km) Seve Nappe Complex of the Swedish Caledonides sheds new light on the subduction system that dominated the contracting Baltoscandian margin of continental Baltica during the Ordovician and culminated in collision with Laurentia in the Silurian to Early Devonian. High-grade metamorphism of this Neoproterozoic to Cambrian rifted, extended, dike-intruded outer-margin assemblage started in the Early Ordovician and may have continued, perhaps episodically, until collision of the continents at the end of this period. The recent discovery of UHP kyanite eclogite in northern Jämtland (west-central Sweden) yields evidence of metamorphism at depths of 100 km. Although UHP rocks are only locally preserved from retrogression during the long-distance transport onto the Baltoscandian platform, these high-pressure parageneses indicate that deep subduction played an important role in the tectonothermal history of the complex. Based on existing isotopic age data, this UHP metamorphism occurred in the Late Ordovician, shortly before, or during, the initial collision between the continents (Scandian orogeny). In some central parts of the complex, migmatization and hot extrusion occurred in the Early Silurian, giving way to thrust emplacement across the Baltoscandian foreland basin and platform that continued into the Early Devonian. Identification of HP/UHP metamorphism at different levels within the Scandian allochthons, definition of their pressure-temperature-time paths, and recognition of their vast transport distances are essential for an understanding of the deeper structural levels of the orogen in the hinterland (e.g., the Western Gneiss Region), where the attenuated units were reworked together during the Early Devonian.