Search

The scarcity of observed active extrusive rhyolitic lava flows has skewed research to extensively focus on prehistoric lavas for information about their eruptive and emplacement dynamics. The first ever witnessed silicic lava eruptive events, Chaitén (2008) and Cordón Caulle (2011–2012) in Chile, were illuminating to the volcanology community because they featured a range of emplacement processes (endogenous versus exogenous), movement limiting modes, and eruptive behaviors (explosive versus effusive) that were often regarded as acting independently throughout an eruptive event. In this study, we documented evidence of a continuum of brittle and brittle-ductile deformation and fracture-induced outgassing during the emplacement of the ~600-yr-old silicic lava from Obsidian Dome, California, USA. This study focused on mapping the textural-structural relationships of the upper surface of the lava onto high-resolution (

Combined Hf-O isotopic analyses of zircons from tuffs and lavas within the Sierra Madre Occidental (SMO) silicic large igneous province are probes of petrogenetic processes in the lower and upper crust. Existing petrogenetic and tectonomagmatic models diverge, having either emphasized significant crustal reworking of hydrated continental lithosphere in an arc above the retreating Farallon slab or significant input of juvenile mantle melts through a slab window into an actively stretching continental lithosphere. New isotopic data are remarkably uniform within and between erupted units across the spatial and temporal extent of the SMO, consistent with homogeneous melt production and evolution. Isotopic values are consistent with enriched mantle magmas (80%) that assimilated Proterozoic paragneisses (~20%) from the lower crust. δ18Ozircon values are consistent with fractionation of mafic magma and not with assimilation of hydrothermally altered upper crust, suggesting that the silicic magmas evolved at depth. Isotopic data agree with previous interpretations where voluminous juvenile melts entered the lithosphere during the transition from a continental arc experiencing slab rollback (Late Eocene) to the arrival of a subducting slab window (Oligocene and Early Miocene) and failure of the upper plate leading to the opening of the Gulf of California (Late Miocene). An anomalously large heat flux and extension of the upper plate allow for the sustained fractionation of the voluminous SMO magmas and assimilation of the lower crust.

The Rosario segment of the Early Cretaceous Alisitos oceanic arc exposes the transition from upper-crustal volcanic and hypabyssal rocks to middle-crustal plutons, which formed in an extensional environment. The Rosario segment forms a structurally intact, unmetamorphosed, spectacularly well-exposed, gently tilted section that is 50 km long and 7 km deep. The top of the exposed section is unconformably overlain by flat-lying Late Cretaceous sedimentary rocks (Rosario Group, described elsewhere), and the base of the section passes downward into ductilely deformed metamorphic rocks (not mapped herein). We divided the Rosario segment into three subsegments: a central subaerial edifice, underpinned by the La Burra pluton; a southern volcano-bounded basin (dominantly shallow marine), underpinned by the San Fernando pluton; and a northern fault-bounded basin (dominantly deep marine), underpinned by the Los Martires pluton. Using a combination of published and new geochronologic data, we infer that the time span represented by the arc crustal section could be as little as 1.7 m.y., dated at ca. 111–110 Ma. Volcanic and plutonic samples show a continuum from basalt/basaltic andesite to rhyolite, are low to medium K, and are transitional tholeiite to calc-alkaline in character. Hf isotopic data from zircons indicate primitive magma, consistent with previously published whole-rock isotopic data. The volcanic stratigraphy can be correlated across all three subsegments using the tuff of Aguajito (Ki-A), a distinctive rhyolite welded ignimbrite that fills the 15-km-wide, >3.6-km-deep La Burra caldera on the central subaerial edifice. Additionally, a second caldera is preserved below the tuff of Aguajito (Ki-A) in the northern fault-bounded basin, floored by a large rhyolite sill complex, up to 700 m thick with a lateral extent of >7 km. Up section from the tuff of Aguajito (Ki-A), there is an abrupt shift to dominantly mafic volcanism that we correlated across all three subsegments of the Rosario segment, dividing the section into two distinct parts (phase 1 and phase 2). The pluton beneath the central subaerial edifice (La Burra) is associated with the caldera that produced the tuff of Aguajito (Ki-A) during phase 1. Plutons beneath the northern fault-bounded basin (Los Martires) and the southern volcano-bounded basin (San Fernando) were emplaced during phase 2. However, we infer that the La Burra pluton, which is associated with the phase 1 La Burra caldera, continued to grow incrementally during phase 2 because it intruded and tilted both phase 1 and phase 2 strata. The Rosario segment escaped postmagmatic deformation, other than gentle tilting (25°–35°) to the west as a single rigid block. The Rosario segment of the Cretaceous Alisitos arc represents an extensional oceanic arc with abundant silicic pyroclastic rocks, culminating in arc rifting with outpouring of mafic magmas. The excellent exposure and preservation provide us with the opportunity to herein describe the following: (1) caldera collapse features and the products of varying explosive eruptive styles; (2) caldera plumbing systems, including silicic sill complexes; (3) the transition from plutons through hypabyssal intrusions to eruptive products; (4) incremental pluton growth and its effects on the structure of the roof rocks; (5) the products of deep-water mafic to silicic eruptions; and (6) flow transformations that occur when hot pyroclastic flows enter marine basins on gentle slopes versus steep slopes. We also used this data set to address questions highly complementary to the work being done on understanding the growth of continental crust at subduction zones. Finally, this volume serves as a model for detailed geologic study of paleo-arcs.

The Gulf of California is an archetype of continental rupture through transtensional rifting, and exploitation of a thermally weakened arc to produce a rift. Volcanic rocks of central Baja California record the transition from calcalkaline arc magmatism, due to subduction of the Farallon plate (ca. 24–12 Ma), to rift magmatism, related to the opening of the Gulf of California (4%) and adakite (>56% SiO2 and MgO We describe 21 distinct volcanic and hypabyssal map units in the Santa Rosalía area, using field characteristics, petrographic data, and major- and trace-element geochemical data, as well as zircon isotopic data and ten new 40Ar-39Ar ages. Lithofacies include lavas and lava domes, block-and-ash-flow tuffs, ignimbrites, and hypabyssal intrusions (plugs, dikes, and peperites). Calcalkaline volcanic rocks (13.81–10.11 Ma) pass conformably upsection, with no time gap, into volcanic rocks with rift transitional chemistry (9.69–8.84 Ma). The onset of rifting was marked by explosive eruption of silicic ignimbrite (tuff of El Morro), possibly from a caldera, similar to the onset of rifting or accelerated rifting in other parts of the Gulf of California. Epsilon Hf zircon data are consistent with a rift transitional setting for the tuff of El Morro. Arc and rift volcanic rocks were then juxtaposed by normal faults and tilted eastward toward a north-south fault that lay offshore, likely related to the north-south normal faults documented for the early history of the Guaymas basin, prior to the onset of northwest-southeast transtenional faulting. Magmatism in the Santa Rosalía area resumed with emplacement of high-magnesian andesite lavas and intrusions, at 6.06 Ma ± 0.27 Ma, coeval with the onset of seafloor spreading in the Guaymas basin at ca. 6 Ma.The 9.69–8.84 Ma rift transitional volcanic rocks underlying the Santa Rosalía sedimentary basin provide a maximum age on its basal fill. Evaporites in the Santa Rosalía sedimentary basin formed on the margin of the Guaymas basin, where thicker evaporites formed. Overlying coarse-grained clastic sedimentary fill of the Santa Rosalía basin and its stratiform Cu-Co-Zn-Mn sulfides may have accumulated rapidly, coeval with emplacement of 6.06 Ma high-magnesian andesite intrusions and the ca. 6 Ma onset of seafloor spreading in the Guaymas basin.

En echelon fractures and veins are among the most common and distinctive geological structures, yet their three-dimensional forms and relationships to surrounding structures are commonly unclear. X-ray computed tomography (CT) offers an unrivaled ability to examine structures within rocks in three dimensions, and it is applied here to a sample of drill core from the Marcellus Shale of southwestern Pennsylvania (USA). CT images yield qualitative and quantitative data on the transition from a pyrite-rich planar vein to an en echelon veinlet array, and on the heterogeneity of veinlets within the array. Using a combination of three- and two-dimensional images, geometric data, and traditional petrography, we identify a range of veinlet shapes consistent with deformation during formation of an antitaxial graphite-calcite-pyrite vein system. Each of the veinlets is rooted in the underlying planar vein where it is narrowest. The transition from planar vein to en echelon array coincides with a change in bedding, suggesting that competency contrasts between adjacent beds controlled the fracture morphology. Veinlets initiated as short, lenticular fractures at ∼45° to the planar vein before lengthening, dilating, and rotating. None of the veinlets are strongly sigmoidal, nor is there measurable offset across the margins of the planar vein; therefore, finite non-shear strain was very limited, and fluid overpressure–induced fracturing during burial and diagenesis is probably the most likely process for fracturing and vein formation.

Long-lived intra-oceanic arcs of Izu-Bonin-Marianas (IBM)-type are built on thick, granodioritic crust formed in the absence of pre-existing continental crust. International Ocean Discovery Program Expedition 350, Site U1437, explored the IBM rear arc to better understand continental crust formation in arcs. Detailed petrochronological (U–Pb geochronology combined with trace elements, oxygen and hafnium isotopes) characterizations of zircon from Site U1437 were carried out, taking care to exclude potential contaminants by (1) comparison of zircon ages with ship-board palaeomagnetic and biostratigraphic ages and 40Ar/39Ar geochronology, (2) analysing zircon from drill muds for comparison, (3) selectively carrying out in situ analysis in petrographic thin sections, and (4) minimizing potential laboratory contamination through using pristine equipment during mineral separation. The youngest zircon ages in Site U1437 are consistent with 40Ar/39Ar and shipboard ages to a depth of ~1390 m below sea floo...

IODP Expedition 350 was the first to be drilled in the rear part of the Izu-Bonin, although several sites had been drilled in the arc axis to fore-arc region; the scientific objective was to understand the evolution of the Izu rear arc, by drilling a deep-water volcaniclastic section with a long temporal record (Site U1437). The Izu rear arc is dominated by a series of basaltic to dacitic seamount chains up to ~100-km long roughly perpendicular to the arc front. Dredge samples from these are geochemically distinct from arc front rocks, and drilling was undertaken to understand this arc asymmetry. Site U1437 lies in an ~20-km-wide basin between two rear arc seamount chains, ~90-km west of the arc front, and was drilled to 1804 m below the sea floor (mbsf) with excellent recovery. We expected to drill a volcaniclastic apron, but the section is much more mud-rich than expected (~60%), and the remaining fraction of the section is much finer-grained than predicted from its position within the Izu arc, composed half of ashes/tuffs, and half of lapilli tuffs of fine grain size (clasts 6.4 cm) are only sparsely scattered through the lowermost 25% of the section, and only one igneous unit was encountered, a rhyolite peperite intrusion at ~1390 mbsf. The lowest biostratigaphic datum is at 867 mbsf (~6.5 Ma), the lowest palaeomagnetic datum is at ~1300 mbsf (~9 Ma), and the rhyolite peperite at ~1390 mbsf has yielded a U–Pb zircon concordia intercept age of (13.6 + 1.6/−1.7) Ma. Both arc front and rear arc sources contributed to the fine-grained (distal) tephras of the upper 1320 m, but the coarse-grained (proximal) volcaniclastics in the lowest 25% of the section are geochemically similar to the arc front, suggesting arc asymmetry is not recorded in rocks older than ~13 Ma.