Abundant Lithic Clasts Research Articles

The Evolution of Permian Source-to-Sink Systems and Tectonics Implications in the NW Junggar Basin, China: Evidence from Detrital Zircon Geochronology

The basin type of the Junggar Basin changed during the Permian, but the time constraint of the tectonic evolution remains unclear. Besides, the fan deltas developed in the Permian in the Mahu Sag in the northwestern of the oil-rich basin. However, the provenances of the sedimentary systems remain unclear. Based on petrology and detrital zircon U-Pb ages, this study investigates the source-to-sink systems evolution and tectonics implications. Abundant lithic clasts in sandstones with low compositional and textural maturity imply proximal sources. The dating results showed a dominant peak (310–330 Ma) and a secondary peak (400–440 Ma) in the northern Mahu Sag, only one peak at 295–325 Ma in the central Mahu Sag, several peaks at 270–350 Ma in the southern Mahu Sag, and multiple peaks at 370–450 Ma in the Zhongguai Uplift. Thus, the north-western Junggar Basin was divided into four major source-to-sink systems, with adjacent central West Junggar as the main provenance and northern and southern West Junggar as the secondary provenance. The proportion of sediment supply from the southern and northern West Junggar is higher during the Middle-Late Permian. It suggests that the source-to-sink systems show inheritance and evolve from a single provenance into a complex provenance, indicating the uplift of West Junggar. The tectonic inversion may occur early in the Middle Permian and the response to tectonic activity is stronger in the southern West Junggar than in the northern West Junggar.

Petrography and Shock Metamorphism of the Lunar Breccia Meteorite NWA 13120

Lunar meteorites are the fragments of rocks that fell on Earth because of the impacts of asteroids on the Moon. Such rocks preserve information about the composition, evolutionary process, and shock history of the lunar surface. NWA 13120 is a recently discovered lunar breccia meteorite having features of strong shock, which is composed of lithic and mineral clasts in a matrix of very fine-grained (<10 μm) and recrystallized olivine-plagioclase with a poikilitic-like texture. As the most abundant lithic clasts, the crystalline impact melt (CIM) clasts can be divided into four types according to their texture and mineral composition: (1) anorthosites or troctolitic anorthosite with a poikilitic-like texture, but the mineral content is different from that of the matrix; (2) anorthosites containing basaltic fragments and rich in vesicles; (3) troctolitic anorthosite containing metamorphic olivine mineral fragments; (4) troctolitic anorthosite containing troctolite fragments. Based on the petrology and mineralogy, NWA 13120 is a lunar meteorite that was derived from the ferrous anorthosite suite (FANs) of the lunar highlands, while its texture suggests it is a crystalline impact melt breccia. In addition, we infer that the parent rock of NWA 13120 is a lunar regolith breccia enriched in glass fragments. During the shock process, at pressures of more than 20 GPa, all plagioclase fragments were transformed into maskelynites, and olivine fragments occurred metamorphism. The post-shock temperature led to the partial melting of the basaltic fragments. Subsequently, all glass with diverse components in the parent rock were devitrified and recrystallized, forming the common olivine-plagioclase poikilitic-like texture and different CIM clasts. Meanwhile, the devitrification of maskelynite formed the accumulation of a large number of plagioclase microcrystals. Therefore, NWA 13120 is a meteorite of great significance for understanding the local shock metamorphism of lunar rocks on the lunar surface.

Lithic-rich and lithic-poor ignimbrites and their basal deposits: Sovana and Sorano formations (Latera caldera, Italy)

Conceptual models explaining the characteristics of ignimbrites and their basal deposits have mainly focused on variations in particle concentration and speed of the parent pyroclastic currents. Here, we focus on the effects of relative proportions of clasts that are well coupled to the carrier gas phase and those that are poorly coupled, i.e., the proportions of fine and/or low-density clasts compared to coarse and/or dense clasts. We document facies and macroscopic clast populations of two ignimbrite-producing eruptive sequences of similar volume and composition, both erupted from Latera caldera (central Italy). The Sovana ignimbrite has 1–15 vol% dense lithic clasts and locally up to 70 vol% in lithic-rich domains. It preserves evidence of emplacement of multiple overlapping flow units with local internal channeling. The ignimbrite is underlain by two basal deposits: (1) a radially distributed, 20–80-cm-thick vitric tuff referred to as BUS (basal unit Sovana) that thins gradually with distance and shows evidence of emplacement by lateral currents; and (2) a fines-depleted, lenticular, massive lapilli tuff, up to 20 cm thick, with abundant lithic clasts. The Sorano ignimbrite, in contrast, has < 1 vol% dense lithic clasts. It contains multiple reverse coarse-tail graded emplacement units, but we observed no internal channeling. In most places, its basal deposits are 20–100 cm of dominantly planar-parallel, 5–10 cm-thick, reverse coarse-tail graded tuff beds. However, in local depressions near the tops of paleocanyons that were filled by Sorano ignimbrite, the basal deposits are up to 2 m thick and are clearly cross stratified. We interpret the different deposit characteristics in terms of recent modeling and experimental results. The Sovana eruption involved major caldera collapse that introduced abundant dense lithic clasts into the erupting mixtures. BUS may have partly resulted from expulsion of a dilute, fine-grained mixture at the site where the pyroclastic fountain(s) impacted the ground, which then propagated outward as a dilute pyroclastic current ahead of the main ignimbrite-producing currents; the expulsion process is promoted by collapsing mixtures rich in poorly coupled clasts. The fines-depleted layer overlying BUS was deposited by rapid sedimentation of abundant dense clasts from the head of the ash-cloud surge, which accompanied the concentrated underflow that deposited the ignimbrite. Spatial and temporal variations in dense-clast abundance, and likely other parameters, during eruption resulted in multiple flow units. The Sorano eruption lacked poorly coupled clasts and did not produce an expulsion-driven current. Its ash-cloud head deposited ash and small pumice lapilli that were able to move in traction carpets prior to being overridden by a concentrated underflow. The overriding ash-cloud bodies locally deposited cross-stratified horizons in depressions high on the canyon walls, before they were inundated as the concentrated underflows filled the canyons with ignimbrite. To summarize, the differences between the ignimbrites and their basal deposits can be largely attributed to the differences in proportions of poorly coupled relative to well-coupled particles in the collapsing eruptive mixtures; the resulting conceptual model simplifies the previous range of interpretations for basal deposits, and is consistent with recent modeling and experimental results.

Felsic maar-diatreme volcanoes: a review

Felsic maar-diatreme volcanoes host major ore deposits but have been largely ignored in the volcanology literature, especially for the diatreme portion of the system. Here, we use two Mexican tuff rings as analogs for the maar ejecta ring, new observations from one diatreme, and the economic geology literature on four other mineralized felsic maar-diatremes to produce an integrated picture of this type of volcano. The ejecta rings are up to 50 m+ thick and extend laterally up to ∼1.5 km from the crater edge. In two Mexican examples, the lower part of the ejecta ring is dominated by pyroclastic surge deposits with abundant lithic clasts (up to 80% at Hoya de Estrada). These deposits display low-angle cross-bedding, dune bedforms, undulating beds, channels, bomb sags, and accretionary lapilli and are interpreted as phreatomagmatic. Rhyolitic juvenile clasts at Tepexitl have only 0–25% vesicles in this portion of the ring. The upper parts of the ejecta ring sequences in the Mexican examples have a different character: lithic clasts can be less abundant, the grain size is typically coarser, and the juvenile clasts can be different in character (with some more vesicular fragments). Fragmentation was probably shallower at this stage. The post-eruptive maar crater infill is known at Wau and consists of reworked pyroclastic deposits as well as lacustrine and other sediments. Underneath are bedded upper diatreme deposits, interpreted as pyroclastic surge and fall deposits. The upper diatreme and post-eruptive crater deposits have dips larger than 30° at Wau, with approximately centroclinal attitudes. At still lower structural levels, the diatreme pyroclastic infill is largely unbedded; Montana Tunnels and Kelian are good examples of this. At Cerro de Pasco, the pyroclastic infill seems bedded despite about 500 m of post-eruptive erosion relative to the pre-eruptive surface. The contact between the country rocks and the diatreme is sometimes characterized by country rock breccias (Kelian, Mt. Rawdon). Pyroclastic rocks in the diatreme are typically poorly sorted, and ash-rich. They contain a heterolithic mix of juvenile clasts and lithic clasts from various stratigraphic levels. Megablocks derived from the ejecta ring or the country rocks are often found in the diatremes. Evidence for multiple explosions is in the form of steep crosscutting pyroclastic bodies within some diatremes and fragments of pyroclastic rocks within other pyroclastic facies. Pyroclastic rocks are cut by coherent felsic dikes and plugs which may have been feeders to lava domes at the surface. Allowing for the difference in magma composition, felsic maar-diatreme volcanoes have many similarities with their ultramafic to mafic equivalents. Differences include a common association with felsic domes, inside the crater or just outside (Wau), although the domes within the crater may be destroyed during the eruption (Hoya de Estrada, Tepexitl); the dikes and plugs feeding and invading felsic diatremes seem larger; the processes of phreatomagmatic explosions involving felsic magmas may be different.

Paleoflow directions of a subaqueous lahar deposit around the Miocene Keserűs Hill lava dome complex (North Hungary) as constrained by photo-statistics and anisotropy of magnetic susceptibility (AMS)

A twofold fabric analysis by using photo-statistics on rock surfaces and low-field anisotropy of magnetic susceptibility (AMS) is presented, making it possible to infer paleoflow directions, which in turn helps to constrain the primary volcanic geomorphology of a deeply eroded mid-Miocene field (Keserűs Hill lava dome group, Visegrád Mountains, North Hungary). The analyses were carried out on the Rám Hill Pumiceous Sandstone (RHPS) that, based on its petrographic features, is considered as a subaqueously emplaced laharic deposit.The observed anisotropic frequencies of clast a-axis azimuths, the good agreement of fabric directions obtained for all and most elongated samples (elongation>2.5), the good clustering of the declinations of the K1 susceptibilities largely corresponding with the result of image analysis, allow to infer reliable flow directions. The obtained large-scale paleoflow paths show a quasi-radial pattern around the central part of the Visegrád Mountains, which quantitatively confirms the previous hypothesis on the volcanic structure, namely a central edifice-topped syn-eruptive topography.The RHPS is characterized by a clast fabric (revealed by photostatistics on cutted rock surfaces considering the lapilli-sized fraction) as strong as in subaerial PDC deposits, but by a significantly weaker magnetic fabric. The weak magnetic fabric is supposed to be the result of rock heterogeneity, thus the weakening effect of abundant lithic clasts with strong internal magnetic susceptibility anisotropy, different from the shape anisotropy of the clasts and the flow-related magnetic fabric of the fine-grained matrix.

Deep-bedded ultramafic diatremes in the Missouri River Breaks volcanic field, Montana, USA: 1 km of syn-eruptive subsidence

The ultramafic Eocene Missouri River Breaks volcanic field (MRBVF, Montana, USA) includes over 50 diatremes emplaced in a mostly soft substrate. The current erosion level is 1.3–1.5 km below the pre-eruptive surface, exposing the deep part of the diatreme structures and some dikes. Five representative diatremes are described here; they are 200-375 m across and have sub-vertical walls. Their infill consists mostly of 55-90 % bedded pyroclastic rocks (fine tuffs to coarse lapilli tuffs) with concave-upward bedding, and 45–10 % non-bedded pyroclastic rocks (medium lapilli tuffs to tuff breccias). The latter zones form steep columns 15–135 m in horizontal dimension, which cross-cut the bedded pyroclastic rocks. Megablocks of the host sedimentary formations are also present in the diatremes, some being found 1 km or more below their sources. The diatreme infill contains abundant lithic clasts and ash-sized particles, indicating efficient fragmentation of magma and country rocks. The spherical to sub-spherical juvenile clasts are non-vesicular. They are accompanied by minor accretionary lapilli and armored lapilli. The deposits of dilute pyroclastic density currents are locally observed. Our main interpretations are as follows: (1) the observations strongly support phreatomagmatic explosions as the energy source for fragmentation and diatreme excavation; (2) the bedded pyroclastic rocks were deposited on the crater floor, and subsided by 1.0–1.3 km to their current location, with subsidence taking place mostly during the eruption; (3) the observed non-bedded pyroclastic columns were created by debris jets that punched through the bedded pyroclastic material; the debris jets did not empty the mature diatreme, occupying only a fraction of its width, and some debris jets probably did not reach the crater floor; (4) the mature diatreme was nearly always filled and buttressed by pyroclastic debris at depth – there was never a 1.3–1.5-km-deep empty hole with sub-vertical walls, otherwise the soft substrate would have collapsed inward, which it only did near the surface, to create the megablocks. We infer that syn-eruptive subsidence shifted down bedded pyroclastic material and shallow sedimentary megablocks by 0.8–1.1 km or more, after which limited post-eruptive subsidence occurred. This makes the MRBVF diatremes an extreme end-member case of syn-eruptive subsidence in the spectrum of possibilities for maar-diatreme volcanoes worldwide.

Quantification of vesicle characteristics in some diatreme-filling deposits, and the explosivity levels of magma–water interactions within diatremes

Vesicles within juvenile fragments in mafic pyroclastic deposits contain important information about the state of the magma at the time of fragmentation. There have been few vesicle studies of juvenile pyroclasts from mafic phreatomagmatic deposits, however, and none we can find from maar–diatreme volcanoes. In this paper we document the vesicularity and vesicle-population characteristics of juvenile fragments sampled from non-bedded lithified deposits of the Coombs Hills diatreme complex, part of the Ferrar large igneous province, Antarctica. The diatreme-filling pyroclastic deposits, dominated by lapilli tuffs and tuff breccias, contain typically abundant lithic clasts derived mostly from the enclosing sedimentary sequence, and several types of juvenile clasts ranging from blocky to fluidal or “raggy”.In the samples measured, 77–80% of the juvenile pyroclasts ranging in size from 0.5mm to fine lapilli is in the ‘non-vesicular’ to ‘incipiently vesicular’ range (<20% vesicles). Such low vesicularities are expected for pyroclasts from maar–diatreme volcanoes where fragmentation takes place at depth in the diatreme or root zone due to magma–water interaction. A few juvenile clasts, however, are more vesicular, and seven of these were chosen and sectioned for 2D analysis of vesicle shapes and orientation, vesicle number densities (Nv), and vesicle volume distributions. The shapes of the vesicles in the studied sections are mostly elliptical (sometimes polylobate), with mean aspect ratios ranging between 0.67 and 0.72. Circular statistics are used to test for trends in the vesicle long-axis orientation data; non-uniformity of orientations is found in most cases, but the trends are weak. Vesicle volume distributions are often bimodal due to variable coalescence. Total Nv values range from 1.0×102 to 5.7×103mm−3; taking the effects of bubble coalescence into account, these values are similar to those found in pyroclasts from other phreatomagmatic volcanoes, although they also overlap partly with those seen in fire fountain deposits and some basaltic Plinian eruptions.Fluidal- or rag-shaped juvenile clasts, some circular vesicles, and the lack of microlites all suggest that the Coombs Hills magma had a relatively low viscosity prior to fragmentation, despite the basaltic andesite composition. This low viscosity allowed parts of the magma to be fragmented in a non-brittle fashion during phreatomagmatic explosions and to form fluidal clasts. Phreatomagmatic explosions in diatremes can therefore produce diverse types of juvenile clasts simultaneously, and the proportions of each will depend on the explosivity of the magma–water (slurry) interaction and other factors. Recycling of fragments is also thought to be an important factor in generating mixtures of different types of juvenile fragments in diatremes.

The implications of spatter, pumice and lithic clast rich proximal co-ignimbrite lag breccias on the dynamics of caldera forming eruptions: The 151 ka Sutri eruption, Vico Volcano, Central Italy

The 151 ka Sutri Formation, erupted from the Vico Volcano, Central Italy, represents a caldera forming eruptive sequence consisting of associated pyroclastic fall and ignimbrite deposits and voluminous coarse, proximal lithic, spatter and pumice-rich breccia facies. After an initial limited Plinian fallout phase (Sutri A) and a partial eruption column collapse event that generated a small volume pyroclastic flow (Sutri B), the later stages of the Sutri eruption produced a southerly-dispersed lithic clast-rich breccia (Sutri C) that grades vertically into a fines poor ignimbrite (Sutri D). Sutri C is interpreted to be a co-ignimbrite lag breccia involving an element of lateral flow produced during vent widening. Vent widening in the south presumably occurred in response to vent wall rock instabilities and increased magma discharge rate during the early Plinian phase that destroyed a large portion of the southern side of the pre-existing Vico stratovolcano/shield edifice. The vertical gradation into an overlying fines poor ignimbrite (Sutri D) reflects changing source conditions at the vent, a progressive increase in magma discharge rate and cessation of vent-widening activity. A succeeding, more extensive and complex association of contemporaneous proximal lithic-rich and spatter-rich breccias that are distributed radially around the present day caldera (Sutri E4) are also considered. No stratigraphic relationship exists between the Sutri E4 breccia facies and the lower breccia facies (Sutri C) due to limited outcrop. Three subdivisions of Sutri E4 are recognised based on field analysis, grainsize studies and petrographic characteristics. All occur at the same stratigraphic level radially around the caldera and are distinguished as separate facies from Sutri C and D based on componentry: Sutri E4 (sbx) — spatter-rich breccia (north–east); Sutri E4 (lsbx) — boulder size lithic clast-rich breccia with associated spatter component (southeast–south) and Sutri E4 (flsbx) — spatter-rich breccia with associated lithic clast component (southwest–west). Mixtures of dense, boulder size lithic clasts, spatter and pumice clasts were most likely ejected up along a widening conduit in the north as well as through newly created, radially distributed fissures formed by steadily increasing magma discharge rate in response to progressive collapse of the magma chamber roof into the chamber. Eruption column collapse occurred and lithic, spatter and pumice rich co-ignimbrite breccias were emplaced radially around the vent (Sutri E4 (sbx, lsbx) signalling the onset of caldera collapse, shortly followed after by emplacement of the Tufo rosso a scorie nere (Sutri E4 (flsbx) and E5) and final caldera collapse. This study is concerned with the mechanism responsible for mixing of large variably vesiculated spatter, highly vesiculated pumice and abundant lithic clasts preserved in both these proximal breccias (i.e. in the conduit and/or eruption column or during transport and emplacement) and considers the implications for the eruption styles produced. The juvenile clast types are mixed in each breccia facies together with voluminous quantities of conduit wall rock lithic clasts. We concluded that they were generated contemporaneously within a single volcanic conduit that likely widened over time into concentrically distributed fissures. The degree of vesiculation and resulting morphology of the juvenile clasts likely reflect variations in magma rise rate whereby fast magma rise rate or faster than bubble nucleation and coalescence produced spatter and slower magma rise rates promoted bubble growth and coalescence producing pumice.

On the Welding of Pyroclasts from Very Low-Viscosity Magmas: Examples from Kimberlite Volcanoes

Lithofacies in kimberlite pipes in southern Africa exhibit features consistent with welded pyroclastic rocks. These include flared conduit-filling geometries, abundant lithic clasts, lithic clast layers, subhorizontal clast fabrics, gradational contacts with volcaniclastic rocks and sintered and coalesced globular ash, and lapilli and melt-coated particles. The welding dynamics of kimberlite pyroclasts differ from those of glassy, vesiculated pyroclasts in silicic volcanic systems. Low melt viscosity (∼0.1 Pa s) results in the efficient separation of volatiles and melt and the breakup of magma into nonvesicular spherical droplets. The glassy state is difficult to form in silica-deficient magmas because the crystallization kinetics are fast in low-viscosity melts. Three pyroclastic lithofacies are recognized that primarily relate to the state of the initial melt phase in the pyroclasts on deposition: (1) densely welded kimberlite forms where the initial melt phase in pyroclasts remains as pure melt, coalescing almost immediately to form a degassed homogeneous melt that subsequently crystallizes and may be texturally indistinguishable from igneous kimberlite; (2) incipiently welded kimberlite forms where the melt phase in pyroclasts is mostly crystalline with some residual melt (pyroclasts are resistant to deformation but are sticky and can sinter together); and (3) nonwelded kimberlite forms where the melt phase in pyroclasts is fully crystalline on deposition. Transitions between these pyroclast conditions may be abrupt. Gradations to nonwelded deposits and overlaps in their textural features in several kimberlite pipes suggest that welded rocks may be deposited from large-scale fluidized systems. The presence of crystals and lithic clasts may inhibit compaction deformation-welding textures by the formation of hard-particle percolation networks. Similar rocks in other kimberlite pipes may turn out to be welded rocks. The processes outlined here may be generally applicable to other low-viscosity magmas.

Calcite strains, kinematic indicators, and magnetic flow fabric of a Proterozoic pseudotachylyte swarm, Minnesota River valley, USA

Near Granite Falls, Minnesota sub-parallel pseudotachylyte, mafic dikes, and calcite veins crosscut Archean granulite facies rocks in the Minnesota River valley adjacent to the north-dipping Yellow Medicine Shear Zone (YMSZ; N80°E) that separates the Montevideo and Morton tectonic terranes. The docking of these two Archean terranes occurred prior to intrusion of the 2.067 Ga Kenora-Kabetogama dike swarm as demonstrated by aeromagnetic anomalies (correlated with field exposures) that cross the YMSZ without offset. Tectonic adjustments along the YMSZ associated with the Penokean Orogeny (∼ 1.8 Ga) are likely responsible for pseudotachylyte formation. Pseudotachylyte is exposed in 22 sub-parallel veins (∼ N80°E, 90°) each less than 2 cm wide across an outcrop width of 45 m. The pseudotachylyte matrix is commonly banded, and contains crystal fragments (quartz, plagioclase, amphibole, rutile, apatite, ilmenite, ulvöspinel), magnetite microlites, flow banding swirls, amygdules (filled with calcite, ankerite and siderite), collapsed vesicles, and abundant lithic clasts. Pseudotachylyte formed in a number of phases. Kinematic reconstruction is complex, utilizing winged porphyroclasts, S-C structures in the country rock, and fault drag indicators along the pseudotachylyte zones. Dextral motion along the YMSZ is the most common observation. Mechanically twinned calcite within amygdules in the pseudotachylyte preserves horizontal shortening normal to the pseudotachylyte strike. Calcite veins are apparently contemporaneous with the pseudotachylyte; one set preserves twinning strains identical to the calcite amygdule strains, and the second set contains a horizontal, vein-parallel (N70°E) shortening strain. The pseudotachylyte contains a flow fabric, as determined by AMS techniques, that is a proxy for vertical flow ( K max is vertical). The Kenora-Kabetogama dikes, identified geochemically, are locally parallel to the pseudotachylyte and the adjacent YMSZ tectonic suture and preserve a vertical-to-horizontal, dike-parallel AMS fabric from east (Franklin) to west (Granite Falls). Hornblende andesite dikes (055°, 1.8 Ga) are not found south of the suture, are not associated with pseudotachylyte and have a different paleopole and AMS fabric.

Sedimentology of the Unicoi Formation in southern and central Virginia: Evidence for late Proterozoic to Early Cambrian rift-to-passive margin transition

Few detailed-facies analyses of the rift-to-passive margin transition have been undertaken in exhumed orogenic belts. The Unicoi and lower Hampton Formations in the central and southern Appalachians record the transition from late Proterozoic to Early Cambrian rifting to initial opening of the Iapetus Ocean. The lower and middle Unicoi Formation consists of feldspathic sandstones, conglomerates, and basalts. Sandstones and conglomerates were deposited from hyperconcentrated flows and tractional currents in distal alluvial-fan and proximal and medial braid-plain environments. Extrusion of basalts was associated with influx of coarse-grained siliciclastics. The presence of thick alluvial-fan sediments, basalts, and abundant lithic clasts and the feldspathic nature of the sandstones are suggestive of a rift setting. Paleontological and geochronological data indicate that rifting continued into Early Cambrian time. The upper Unicoi Formation is composed predominantly of transgressive, quartzose sandstones which represent the incipient phase of passive margin sedimentation related to a second-order, sea-level rise. Differences in degree of crustal attentuation probably controlled the distribution of sedimentary environments during transgression. On most attenuated crust to the east, initial transgressive facies consist of tidal sand-wave and sand-ridge deposits intercalated with proximal and medial braid-plain deposits. As transgression progressed cratonward, tidal sedimentation was supplanted by tide- and wave-influenced sedimentation characterized by sand-wave complexes, tidal inlets, and longshore-directed bedforms. Drowning at the top of the Unicoi Formation is indicated by outer-shelf black mudstones containing Tabc, Tbc, and Tc beds and gravity-flow deposits at the base of the Hampton Formation. Deepening may have been enhanced by continued movement along listric faults throughout the incipient phase of passive-margin development. Most of the Hampton Formation and the overlying Erwin Formation and Shady Dolomite record pro-gradation of the passive margin which culminated in development of a carbonate-rimmed shelf. The rift-to-passive margin phases of sedimentation in the central Appalachians reflect a continuum from fault-influenced to thermothectonic subsidence.