Rise and fall of the Acadian altiplano: Evidence for a Paleozoic orogenic plateau in the northern Appalachian Orogen

<p>High elevation orogenic plateaus are formed by a complex interplay of deep and surficial processes yet understanding of the deeper processes is limited by few recognized exposures of the lower levels of plateaus. We present evidence for the existence of an orogenic plateau during and after the Devonian Acadian orogeny (<em>sensu lato</em>), the mid-crustal roots of which are exposed in the New England Appalachians. The four-dimensional crustal evolution of this paleo-plateau is constrained by the integration of petrochronology, petrologic and geochronologic databases, and geophysical imaging. Doubly thickened crust, widespread amphibolite to granulite-facies metamorphic conditions, a paleo-isobaric surface, and protracted mid-crustal anatexis all indicate the presence of a high elevation (~5 km), low relief plateau by 380 Ma. <sup>40</sup>Ar/<sup>39</sup>Ar thermochronology shows a distinct signature with very slow cooling rates of 2-4<strong>°</strong>C/m.y. following peak metamorphic conditions. Thermochronologic data, trace element and Nd isotope geochemistry, and monazite and xenotime petrochronology suggest a 50 m.y. lifespan of the plateau (380-330 Ma). Orogen parallel ductile flow and extrusion of gneiss domes resulted in plateau collapse, crustal thinning, and block-like exhumation at ca. 330-300 Ma. Thinning of the plateau crust may have led to the sharp 12-15 km step in Moho depth in western New England, possibly by reactivating the suture between Laurentia and accreted Gondwanan-derived terranes. The formation of the Acadian altiplano may have influenced Li-pegmatite genesis and Paleozoic paleoclimate, while its recognition may provide a window into the deeper processes of orogenic plateaus including partial melting, plutonism, and collapse by ductile extension.</p>

Top-down and bottom-up controls on mountain-hopping erosion: insights from detrital ¹⁰Be and river profile analysis in Central Guatemala

The presence of a mountain affects the circulation of water in the atmosphere and over the land surface. These effects are felt over some distance, beyond the extent of the mountain, controlling precipitation delivery and river incision over surrounding landmasses. The rise of a new mountain range therefore affects the erosion of pre-existing mountains located in close proximity. We document here this phenomenon in the mountains of Central Guatemala. The 40Ar-39Ar dating of lava flows shows that two parallel, closely spaced mountain ranges formed during two consecutive pulses of single-stepped uplift, one from 12 to 7 Ma, and the second one since 7 Ma. The distribution of erosion rates derived from the analysis of detrital cosmogenic 10Be in river sediments shows that the younger range erodes faster (~300 m/My) than the older one (20–150 m/My), and that erosion correlates with the amount of precipitation. Moisture tracking form the Caribbean Sea is intercepted by the younger range, which casts a rain shadow over the older one. The analysis of river long-profiles provides a record of longer-term interactions between the two ranges. The rivers that drain the older range were diverted by the younger range during the early stages of its rise. A few rivers were able to maintain their course across the young range, through profile steepening, but incision completely stalled along their upper reaches, upstream of the younger range. As a result, the older range has been passively uplifted, and entered a phase of a slow topographic decay: pediments have formed along its base, while ancient upstream-migrating waves of erosion, located farther up the mountain flanks, have almost stopped migrating. Aridification and cessation of river incision together explain the slowing down of erosion over the older range. They represent top-down and bottom-up processes whereby the younger range controls erosion over the older one. These controls are regarded as instrumental in the nucleation and enlargement of orogenic plateaus forming above continental accretionary wedges.

Constraints on the Timing of Surface Uplift of the Iranian Plateau (Arabia-Eurasian Collision Zone) from Clumped Isotope Thermometry on Pedogenic Carbonates

<p>Orogenic plateaus are extensive, elevated, arid, generally internally drained, morphotectonic provinces of low internal topographic relief that represent a striking and enigmatic feature of Earth’s continental landscapes. They are located along convergent plate boundaries and have a profound impact on regional and global climate, erosional processes, local- to far-field deformation mechanisms and the long-term distribution of biomes and biodiversity. Although the paramount role of large orogenic plateaus in shaping our planet is widely appreciated, the question of why, where, and how some orogenic systems develop large plateaus remains a first-order problem in our understanding of lithospheric evolution and orogenic processes.</p><p>Here, we present a clumped isotope paleoaltimetry study to document the elevation history of the Iranian Plateau, with the goal of understanding the rates and mechanisms of orogenic plateau rise. This plateau is in the Arabia-Eurasia collision zone, has a mean elevation of ~ 1.8 km, steep margins with mountain peaks higher than 4 km, and experienced surface uplift sometime after the middle Miocene as documented by the occurrence of ca. 17-My-old marine deposits in the plateau interior.</p><p>Preliminary results from Early Miocene to Quaternary pedogenic carbonates on the plateau interior and the adjacent, less elevated, intermontane Tarom basin suggest that surface uplift must have occurred sometime between 12-11 and 8 Ma. The lack of significant crustal shortening and thickening during this time interval and the occurrence of a renewed phase of adakitic volcanism by ca. 11 Ma suggests that surface uplift may have been driven by deep-seated processes associated with asthenospheric flow.</p>

The rise of high mountain peaks: Feedbacks between orographic precipitation, fluvial erosion and flexural isostasy

<p>The majority of the highest mountain peaks on Earth is located at the dissected rim of large orogenic plateaus such as the Tibetan Plateau or the Altiplano. The striking spatial coexistence of deep, incised valleys and extraordinary high peaks located at the interfluves led to the idea of a common formation even a hundred years ago: focused erosion in valleys triggers the rise of mountain peaks due to erosional unloading and isostatically driven uplift. Ridgelines rise at the interfluves parallel to major rivers, but an additional ridgeline forms perpendicular to the principal flow direction separating the dissected rim from the undissected center of the plateau. As major rivers originate within the plateau and bypass the highest peaks, the latter rigdeline does not form a principal drainage divide. However, it forms a strong orographic barrier with wet conditions at the windward and dry conditions towards the plateau center at the leeward side. The height of the ridgeline is controlled by valley incision via erosional unloading and isostatic uplift.  If the precipitation pattern responsible for localized valley incision is controlled by the geometry of orographic barriers, a series of complex feedbacks between precipitation, erosion and ridgeline uplift (including the evolution of the highest peaks) occurs.</p><p>In this study, we present first results of a novel numerical model, which couples (a) fluvial erosion based on the stream power law, (b) flexural isostasy including viscous relaxation and (c) orographic precipitation based on the advection and diffusion of moisture and its reaction on topographic barriers. Originating from a simple model setup with a plateau in the center of the model domain and moisture transported along a predominant wind direction, we explore the co-formation of valleys and the rise of ridgelines including the growth of extraordinary high peaks. As the evolving topography controls the precipitation pattern, erosion rates are high at the wet windward side of the ridgeline, which parallels the plateau rim, while the leeward side towards the plateau center is characterized by low precipitation and very low erosion rates. As it prevents elevated low-relief areas from dissection, we suggest that this mechanism is a principal cause for the longevity of orogenic plateaus.</p>

Evolution of Orogenic Plateaus at Subduction Zones - Sinking and raising the southern margin of the Central Anatolian Plateau

Orogenic plateaus have raised abundant attention amongst geoscientists during the last decades, offering unique opportunities to better understand the relationships between tectonics and climate, and their expression on the Earth’s surface.Orogenic plateau margins are key areas for understanding the mechanisms behind plateau (de)formation. Plateau margins are transitional areas between domains with contrasting relief and characteristics; the roughly flat elevated plateau interior, often with internally drained endorheic basins, and the external steep areas, deeply incised by high-discharge rivers. This thesis uses a wide range of structural and tectonic approaches to investigate the evolution of the southern margin of the Central Anatolian Plateau (CAP), studying an area between the plateau interior and the Cyprus arc. Several findings are presented here that constrain the evolution, timing and possible causes behind the development of this area, and thus that of the CAP. After peneplanation of the regional orogeny, abroad regional subsidence took place in Miocene times in the absence of major extensional faults, which led to the formation of a large basin in the northeast Mediterranean. Late Tortonian and younger contractional structures developed in the interior of the plateau, in its margin and offshore, and forced the inversion tectonics that fragmented the early Miocene basin into the different present-day domains. The tectonic evolution of the southern margin of the CAP can be explained based on the initiation of subduction in south Cyprus and subsequent thermo-mechanical behavior of this subduction zone and the evolving rheology of the Anatolian plate. The Cyprus slab retreat and posterior pull drove subsidence first by relatively minor stretching of the crust and then by its flexure. The growth by accretion and thickening of the upper plate, and that of the associated forearc basins system, caused by accreting sediments, led to rheological changes at the base of the crust that allowed thermal weakening, viscous deformation, driving subsequent surface uplift and raising the modern Taurus Mountains. This mechanism could be responsible for the uplifted plateau-like areas seen in other accretionary margins. ISBN: 978-90-9028673-0