Provenance of Eocene river sediments from the central northern Sierra Nevada and implications for paleotopography

Geochronology of fluvial deposits can be used to characterize provenance, the paleotopography of sediment source regions, and the development of regional drainage systems. We present U-Pb and (U-Th)/He ages of detrital zircon grains from Eocene gravels preserved in several paleoriver systems along the western flank of the central and northern Sierra Nevada. These ages allow us to trace the sourcing of detritus in paleorivers and to constrain the evolution of the Sierra Nevada range front. U-Pb zircon age distributions are bimodal, with a dominant peak between 110 and 95 Ma and smaller but significant peaks in the Middle to Late Jurassic, matching the predominant ages of the Sierra Nevada batholith. A small fraction (<6%) of grains has pre-Mesozoic ages, which consistently match ages from prebatholithic assemblages within the northern part of the range. (U-Th)/He ages of a subset of double-dated zircons cluster between 114 and 74 Ma and are consistent with batholithic (U-Th)/He cooling ages in the northern Sierra. Our results indicate that the Eocene river systems in the central northern Sierra Nevada likely had proximal headwaters and had relatively steep axial gradients, draining smaller areas than was commonly thought. This also suggests that the northern Sierra Nevada would have had an established drainage divide and would have acted as a major topographic barrier during the early to mid-Cenozoic. The data presented here support a model of the Eocene northern Sierra Nevada characterized by a western slope with a gradient broadly similar to that of today

Major Miocene exhumation by fault-propagation folding within a metamorphosed, early Paleozoic thrust belt: Northwestern Argentina

The central Andean retroarc thrust belt is characterized by a southward transition at ∼22°S in structural style (thin-skinned in Bolivia, thick-skinned in Argentina) and apparent magnitude of Cenozoic shortening (>100 km more in the north). With the aim of evaluating the abruptness and cause of this transition, we conducted a geological and geo-thermochronological study of the Cachi Range (∼24–25°S), which is a prominent topographic feature at this latitude. Our U-Pb detrital zircon results from the oldest exposed rocks (Puncoviscana Formation) constrain deposition to mainly Cambrian time, followed by major, Cambro-Ordovician shortening and ∼484 Ma magmatism. Later, Cretaceous rift faults were locally inverted during Cenozoic shortening. Coupled with previous work, our new (U-Th)/He zircon results require 8–10 km of Miocene exhumation that was likely associated with fault-propagation folding within the Cachi Range. After Miocene shortening, displacement on sinistral strike-slip faults demonstrates a change in stress state to a non-vertically orientedσ3. This change in stress state may result from an increase in gravitational potential energy in response to significant crustal thickening and/or lithospheric root removal. Our finding of localized Cenozoic shortening in the Cachi Range increases the estimate of the local magnitude of shortening, but still suggests that significantly less shortening was accommodated south of the thin-skinned Bolivian fold-thrust belt. Our results also underscore the importance of the pre-existing stratigraphic and structural architecture in orogens in influencing the style of subsequent deformation.Fil: Pearson, D. M.. University Of Arizona; Estados Unidos. University Of Idaho; Estados UnidosFil: Kapp, P.. University Of Arizona; Estados UnidosFil: Reiners, P. W.. University Of Arizona; Estados UnidosFil: Gehrels, G. E.. University Of Arizona; Estados UnidosFil: Ducea, M. N.. University Of Arizona; Estados Unidos. University of Bucharest; RumaniaFil: Pullen, A.. University Of Arizona; Estados Unidos. University of Rochester; Estados UnidosFil: Otamendi, Juan Enrique. Universidad Nacional de Rio Cuarto. Facultad de Cs.exactas Fisicoquimicas y Naturales. Departamento de Geologia; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Alonso, Ricardo Narciso. Universidad Nacional de Salta; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Neotethyan Subduction Ignited the Iran Arc and Backarc Differently

Most arcs show systematic temporal and spatial variations in magmatism with clear shifts in igneous rock compositions between those of the magmatic front (MF) and those in the backarc (BA). It is unclear if similar magmatic polarity is seen for extensional continental arcs. Herein, we use geochemical and isotopic characteristics coupled with zircon U‐Pb geochronology to identify the different magmatic style of the Iran convergent margin, an extensional system that evolved over 100 Myr. Our new and compiled U‐Pb ages indicate that major magmatic episodes for the NE Iran BA occurred at 110–80, 75–50, 50–35, 35–20, and 15–10 Ma. In contrast to NE Iran BA magmatic episodes, compiled data from MF display two main magmatic episodes at 95–75 and 55–5 Ma, indicating more continuous magmatism for the MF than for the BA. We show that Paleogene Iran serves as a useful example of a continental arc under extension. Our data also suggest that there is not a clear relationship between the subduction velocity of Neotethyan Ocean beneath Iran and magmatic activity in Iran. Our results imply that the isotopic compositions of Iran BA igneous rocks do not directly correspond to the changes in tectonic processes or geodynamics, but other parameters such as the composition of lithosphere and melt source(s) should be considered. In addition, changes in subduction zone dynamics and contractional versus extensional tectonic regimes influenced the composition of MF and BA magmatic rocks. These controls diminished the geochemical and isotopic variations between the magmatic front and backarc

The Cenozoic Subduction History of Greater Indian Lithosphere Beneath Tibet

Abstract HKT-ISTP 2013 Opening Sessio

Sulfide melts and long-term low seismic wavespeeds in lithospheric and asthenospheric mantle

Some studies of lithospheric and asthenospheric seismic structure, report mantle velocities as low as ∼4% below the reference models used. While these low wavespeeds may be attributed to thermal effects in tectonically young or actively volcanic regions, in older, tectonically stable regions low velocity anomalies apparently persist even past the decay time of any thermal perturbation, rendering such a mechanism implausible. Low volume melts can also reduce wavespeeds, but their buoyancy should drain them upward away from source regions, preventing significant accumulation if they are able to segregate. Sulfide, ubiquitous as inclusions in lithospheric mantle xenoliths, forms dense, non-segregating melts at temperatures and volatile fugacities characteristic of even old lithospheric mantle. We show that 1–5 volume percent sulfide melts can act to permanently create reductions up to 5.5% in seismic wavespeeds in areas of the lithosphere and the asthenosphere disturbed by prior melting events that carry and concentrate sulfide

Evaluating the importance of metamorphism in the foundering of continental crust

The metamorphic conditions and mechanisms required to induce foundering in deep arc crust are assessed using an example of representative lower crust in SW New Zealand. Composite plutons of Cretaceous monzodiorite and gabbro were emplaced at ~1.2 and 1.8 GPa are parts of the Western Fiordland Orthogneiss (WFO); examples of the plutons are tectonically juxtaposed along a structure that excised ~25 km of crust. The 1.8 GPa Breaksea Orthogneiss includes suitably dense minor components (e.g. eclogite) capable of foundering at peak conditions. As the eclogite facies boundary has a positive dP/dT, cooling from supra-solidus conditions (T > 950 ºC) at high-P should be accompanied by omphacite and garnet growth. However, a high monzodioritic proportion and inefficient metamorphism in the Breaksea Orthogneiss resulted in its positive buoyancy and preservation. Metamorphic inefficiency and compositional relationships in the 1.2 GPa Malaspina Pluton meant it was never likely to have developed densities sufficiently high to founder. These relationships suggest that the deep arc crust must have primarily involved significant igneous accumulation of garnet–clinopyroxene (in proportions >75%). Crustal dismemberment with or without the development of extensional shear zones is proposed to have induced foundering of excised cumulate material at P > 1.2 GPa

Crustal recycling by subduction erosion in the central Mexican Volcanic Belt

Recycling of upper plate crust in subduction zones, or ‘subduction erosion’, is a major mechanism of crustal destruction at convergent margins. However, assessing the impact of eroded crust on arc magmas is difficult owing to the compositional similarity between the eroded crust, trench sediment and arc crustal basement that may all contribute to arc magma formation. Here we compare Sr–Nd–Pb–Hf and trace element data of crustal input material to Sr–Nd–Pb–Hf–He–O isotope chemistry of a well-characterized series of olivine-phyric, high-Mg# basalts to dacites in the central Mexican Volcanic Belt (MVB). Basaltic to andesitic magmas crystallize high-Ni olivines that have high mantle-like 3He/4He = 7–8 Ra and high crustal δ18Omelt = +6.3–8.5‰ implying their host magmas to be near-primary melts from a mantle infiltrated by slab-derived crustal components. Remarkably, their Hf–Nd isotope and Nd/Hf trace element systematics rule out the trench sediment as the recycled crust end member, and imply that the coastal and offshore granodiorites are the dominant recycled crust component. Sr–Nd–Pb–Hf isotope modeling shows that the granodiorites control the highly to moderately incompatible elements in the calc-alkaline arc magmas, together with lesser additions of Pb- and Sr-rich fluids from subducted mid-oceanic ridge basalt (MORB)-type altered oceanic crust (AOC). Nd–Hf mass balance suggests that the granodiorite exceeds the flux of the trench sediment by at least 9–10 times, corresponding to a flux of ⩾79–88 km3/km/Myr into the subduction zone. At an estimated thickness of 1500–1700 m, the granodiorite may buoyantly rise as bulk ‘slab diapirs’ into the mantle melt region and impose its trace element signature (e.g., Th/La, Nb/Ta) on the prevalent calc-alkaline arc magmas. Deep slab melting and local recycling of other slab components such as oceanic seamounts further diversify the MVB magmas by producing rare, strongly fractionated high-La magmas and a minor population of high-Nb magmas, respectively. Overall, the central MVB magmas inherit their striking geochemical diversity principally from the slab, thus emphasizing the importance of continental crust recycling in modern solid Earth relative to its new formation in modern subduction zones

Carpathian-Pannonian Magmatism Database

A database containing previously published geochronologic, geochemical, and isotopic data on Mesozoic to Quaternary igneous rocks from the Carpathian-Pannonian region is presented. Georeferenced data making up this database belong mostly to rocks sampled from five magmatic arcs: (a) the South Apuseni Jurassic island arc/backarc province, (b) a small volume mid-Cretaceous arc of the northernmost South Carpathians, (c) a late Cretaceous arc, locally known as “banatitic,” marking the closure of the Neotethys, (d) a regionally extensive Miocene ignimbrite flare-up, and (e) the Miocene-Quaternary collisional arc and associated extension-related basalts of the Pannonian and Transylvanian basins. The database is anchored by location and ages of various igneous rocks, as well as geochemical and isotopic data, where available. The database is publicly available online (https://osf.io/23kdg/), as well as a Supporting Information S1 attached to this manuscript. We exemplify the utility of the database by calculating paleo crustal thicknesses in the Carpathians as a function of time using well-calibrated geochemical paleo-mohometers. © 2021. The Authors.Open access articleThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]