Using ignimbrites to quantify structural relief growth and understand deformation processes: implications for the development of the Western Andean Slope, northernmost Chile

Large-volume ignimbrites are excellent spatial and temporal markers for local deformation and structural relief growth because they completely inundate and bury the underlying paleotopography and leave planar surfaces with relatively uniform, low-gradient slopes dipping less than 2°. Using one of these planar surfaces as a reference frame, we employed a line-balanced technique to reconstruct the original morphology of an ignimbrite that has undergone postemplacement deformation. This method allowed us to constrain both the amount of posteruptive deformation and the topography of the pre-eruptive paleolandscape. Our test case was the unwelded surface of the 21.9 Ma Cardones ignimbrite, located on the western slope of the Central Andes in northernmost Chile (18°20′S). By reconstructing the original surface slope of this ignimbrite, we demonstrate that the pre–21.9 Ma topography of the Western Andean Slope was characterized by structural relief growth and erosion in the east, and the creation of accommodation space and sedimentation in the west. The paleoslope at that time was dissected by river valleys of up to 450 ± 150 m deep that accumulated great thicknesses (>1000 m) of the Cardones ignimbrite, and likely controlled the location of the present-day Lluta Quebrada as a result of differential welding compaction of the ignimbrite. Our reconstruction suggests that growth of the Western Andean Slope had already started by ca. 23 Ma, consistent with slow and steady models for uplift of the Central Andes. Subsequent deformation in the Miocene generated up to 1725 ± 165 m of structural relief, of which more than 90% can be attributed to fault-related folding of the ∼40-km-wide Huaylillas anticline. Uplift related to regional forearc tilting is less than 10% and could have been zero. The main phase of folding likely occurred in the mid- to late Miocene and had ceased by ca. 6 Ma

The Guarda structure (Portugal): Impact structure or not? Microstructural studies of Quartz, Zircon and Monazite

The Guarda Structure in north-eastern Portugal has been proposed as a potential impact structure. We have studied the structure in detail, but no field or microscopic evidence has been found to support the impact hypothesi

Early Miocene large-volume ignimbrites of the Oxaya Formation, Central Andes

During the early Miocene ignimbrite flare-up, significant parts of the Central Andes (17–20°S) were covered by large-volume ignimbrites. High-precision 206Pb/238U zircon dates constrain the flare-up in northern Chile at c. 18°S to a 3 myr period, starting with the deposition of the Poconchile ignimbrite at 22.736 ± 0.021 Ma. Of four main pulses, the two largest occurred at 21.924 ± 0.017 and 19.711 ± 0.036 Ma, when the >1000 km3 in volume Cardones and Oxaya ignimbrites erupted, respectively. The ignimbrites are high-SiO2 rhyolites and show significant heterogeneities in crystal content, mineral proportions and trace-element compositions. The zoned Oxaya ignimbrite implies incremental extraction of a crystal-poor magma overlying a crystal-rich magma. In contrast, petrological and textural heterogeneities in pumice clasts are spread throughout the Cardones ignimbrite and we propose magma mixing caused by destabilization of multiple magma bodies within a magmatic mush system. Distal and medial deposits of the Cardones ignimbrite, with a maximum welded thickness of at least 1000 m, entirely covered the western flank of the Central Andes, which implies infill of a significant topographic relief. Both compaction and welding resulted in a maximum thickness reduction of around 30% for the Cardones ignimbrite. Supplementary material: U–Pb method and complete data tables, ICP-OES and ICP-MS method and complete data tables, and detailed stratigraphic description of the Cardones ignimbrite are available at http://doi.org/10.6084/m9.figshare.c.2858707

Timescales for pluton growth, magma-chamber formation and super-eruptions

Generation of silicic magmas leads to emplacement of granite plutons, huge explosive volcanic eruptions and physical and chemical zoning of continental and arc crust1,2,3,4,5,6,7. Whereas timescales for silicic magma generation in the deep and middle crust are prolonged8, magma transfer into the upper crust followed by eruption is episodic and can be rapid9,10,11,12. Ages of inherited zircons and sanidines from four Miocene ignimbrites in the Central Andes indicate a gap of 4.6 Myr between initiation of pluton emplacement and onset of super-eruptions, with a 1-Myr cyclicity. We show that inherited zircons and sanidine crystals were stored at temperatures <470 °C in these plutons before incorporation in ignimbrite magmas. Our observations can be explained by silicic melt segregation in a middle-crustal hot zone with episodic melt ascent from an unstable layer at the top of the zone with a timescale governed by the rheology of the upper crust. After thermal incubation of growing plutons, large upper-crustal magma chambers can form in a few thousand years or less by dike transport from the hot-zone melt layer. Instability and disruption of earlier plutonic rock occurred in a few decades or less just before or during super-eruptions