Late Paleozoic Time Research Articles

Dispersed remnants of a northeast Pacific fringing arc: Upper Paleozoic terranes of Permian McCloud Faunal affinity, western U.S.

Two fragmentary, subparallel belts of terranes within the western North American Cordillera contain upper Paleozoic rocks and are characterized by contrasting lithotectonic assemblages and contrasting Permian faunal affinity. These two belts are (A) volcanic‐arc related successions of Permian McCloud faunal affinity (McCloud belt) and (B) subduction‐related accretionary complexes of Permian Tethy an faunal affinity (Cache Creek belt). This paper supports the hypothesis that the fragmentary terranes of the McCloud belt once constituted parts of a northeast Pacific fringing‐arc system and, in constrast to some earlier interpretations, concludes that the volcanic arc evolved above an eastward dipping subduction zone. The absolute distance between this arc and western North America during the late Paleozoic cannot be constrained, however, there is little evidence to suggest closure of a major (>10³ km) ocean basin or protracted periods of westward dipping subduction. Parts of Devonian to Permian volcanic island arc sequences of the western U.S. Cordillera are represented in the northern Sierra, eastern Klamath, Bilk Creek, Grindstone, and Chilliwack terranes. These scattered volcanic arc remnants share several fundamental characteristics: (1) The sequences were constructed across continental‐affinity basement assemblages. (2) They underwent similar tectonic evolution during late Paleozoic time, such as coeval pulses in volcanism and related depositional histories. (3) They contain Early Permian McCloud‐type fauna, of distinctive biogeographic affinity. (4) McCloud belt terranes are spatially and possibly genetically related to westward lying accretionary complexes of the Cache Creek belt which contain fragments of Upper Triassic blueschist and Permian limestone blocks bearing Tethyan Permian fusulinids and corals. Based on the presence of distinctive Early Permian McCloud fauna, the island arc remnants discussed in this paper are referred to as the McCloud belt. The similarities between McCloud belt terranes suggest that they were once parts of a long‐lived late Paleozoic volcanic arc complex. The stratigraphy of these terranes records the tectonic evolution of the volcanic arc. Volcanism within the Paleozoic arc began as early as the Early Devonian, and was succeeded or accompanied by carbonate and epiclastic sedimentation during the Middle and Late Devonian. An influx of detritus derived from sedimentary and metasedimentary rocks during the Late Devonian and Mississippian records the uplift and erosion of arc basement sequences, an event that was, at least locally, accompanied by ongoing volcanism. A pulse in volcanism during the Pennsylvanian and coeval uplift led to the establishment of shallow marine and deltaic volcanogenic and carbonate sedimentation and, locally, subaerial exposure which persisted into the early Early Permian. Elsewhere during the Early Permian, shallow‐water carbonate deposition was established, with only minor influx of volcanogenic debris. During the Late Permian, and locally as early as late Early Permian, a major pulse in volcanism occurred and is now represented by voluminous sequences of volcanogenic debris and lavas. Most of these sequences are overlain by Upper Triassic carbonate platformal sequences. Arc volcanism was recorded, at least locally, until the Middle Jurassic. Paleomagnetic data are used to constrain a schematic pre‐Cretaceous reconstruction of these volcanic arc sequences and their associated subduction assemblages. The inferred paleogeography of island arc sequences and the adjacent accretionary terranes suggests that volcanism occurred above an east dipping subduction zone, with a long‐lived paleogeographic relation to North America. The current structural position of these terranes is largely the result of Mesozoic and locally younger deformation.

Source parameters for 11 earthquakes in the Tien Shan, central Asia, determined by P and SH waveform inversion

The Tien Shan mountain belt of central Asia was formed in late Paleozoic time and reactivated in Tertiary time following the collision of India with Eurasia, more than 1500 km to the south. To study the style and distribution of faulting occurring today in the Tien Shan, we digitized long‐period World‐Wide Standard Seismograph Network P and SH waveforms of 11 of the largest Tien Shan earthquakes between 1965 and 1982 and then used a least squares inversion routine to constrain their fault plane solutions and depths. Four of the earthquakes occurred near the southern edge of the Tien Shan, and two occurred in the intermontane Fergana Basin. These earthquakes occurred at depths of 10–20 km and are associated with thrust faulting on east‐west to southwest‐northeast striking fault planes that dip moderately (35°–55°). The other five earthquakes, in the northern Tien Shan, were mostly deeper (15–44 km), and the fault plane solutions of four of them are similar to the events farther south. The exception, an event on June 2, 1973, probably occurred on a gently northward dipping, east‐west striking fault plane. All events occur within the basement at depths of 10 km or greater. Waveforms for earthquakes near the edges of the Dzungarian, Fergana, and Kucha basins are fit best using velocity structures with thick sediment layers, implying that sediments from these basins have been and are being underthrust beneath the neighboring mountains. We examined only the relatively small (mb = 5.5–6.2) events after 1965 and did not study any of the largest events (Ms > 8) that occurred in the Tien Shan earlier in this century. Still, we can conclude that north‐south shortening is presently occurring in the Tien Shan, with the formation of basement uplifts flanked by moderately dipping thrust faults. The present‐day tectonics of the Tien Shan seem to be analogous to those of the Rocky Mountains in Colorado, Wyoming, and Utah during the Laramide orogeny in Late Cretaceous and Early Tertiary time.

Geologic and geochemical controls of mineralization in the Southeast Missouri barite district

Residual barite deposits of the Southeast Missouri barite district were derived from the weathering of lower Paleozoic dolostones, chiefly the Upper Cambrian Potosi and Eminence Dolomites. Open-space, Mississippi Valley-type mineralization in the bedrock was controlled by the intersection of permeable lithologies with late Paleozoic near-vertical faults or linear zones of intense jointing. Linear trends of residual deposits are the surface expression of these bed-rock controls. A consistent mineralogical and barite textural zonation is disposed about the central structures. Sulfide mineralization is concentrated about the central structures, whereas barite is more widespread. Barite textures vary systematically from coarse bladed near the central structures to fine plumose more distally. Such zoning indicates outward ore fluid flow from the central faults and joint zones.Fluid inclusions in sphalerite indicate that warm Na-Ca-Cl brines (69 degrees -105 degrees C, 20-25.5 equiv wt % NaCl) were present during early mineralization. Salinity data for fluid inclusions in barite (0-14 equiv wt % NaCl) may be representative of the barite mineralizing fluid, but contamination by later fluids is possible. Sulfur isotope studies show that the paragenetically early sulfides are isotopically lighter than later barite. Individual minerals involved in mainstage mineralization have the following ranges of delta 34 S values: pyrite-marcasite, 8.4 to 34.7 per mil; galena, 3.2 to 11.8 per mil; sphalerite, 7.2 to 15.8 per mil; barite, 20.5 to 32.2 per mil. Barite delta 34 S values show enrichment in 34 S away from the central fault. This spatial distribution is due to the incorporation of sulfate from two reservoirs. Isotopically light sulfate, developed most strongly about the central fault, originated from the oxidation of H 2 S in the ore fluid emanating from the central fault. The heavy sulfate that prevailed distally was indigenous to the host dolostone formation water. The delta 18 O values of main-stage barite vary from 17.0 to 19.6 per mil and the range is attributed to mixing of two fluids of different oxygen isotope composition and to variable isotopic fractionation between sulfate and water due to changing temperature. The light sulfur and oxygen isotope composition of supergene barite (delta 34 S = 15.8ppm, delta 18 O = 9.0ppm) is due to incorporation of isotopically light sulfate produced locally during the oxidation of sulfide minerals in the soil environment.Barite precipitation was caused by fluid mixing, in which a Ba-Pb-Zn-H 2 S-bearing brine discharged from faults and joints into lower Paleozoic dolostones and mixed with an oxidizing, sulfate-bearing, dilute fluid. Increased supersaturation of barite with progressive mixing caused the systematic spatial variation of barite textures from coarse bladed near the central structures to fine plumose more distally. Similar barite-sulfide mineralization in the Central Missouri barite district probably formed in the same manner. Barite mineralization across the north flank of the Ozark dome postdates local Appalachian-Ouachita-Marathon orogenic deformation and suggests the existence of dilute sulfate-bearing water, probably shallow meteoric ground water, in the Paleozoic section during barite mineralization. Uplift of the Ouachitas in late Paleozoic time apparently initiated Mississippi Valley-type mineralization of the Ozark region.

Paleozoic evolution of southern margin of Permian basin

Prior to Late Pennsylvanian time, the Permian basin of West Texas was a part of the southern margin of the North American craton and was the site of shallow, warm-water carbonate deposition. As the South American margin of Gondwana gradually moved northwestward during late Paleozoic time, the oceanic floor deposits and overlying turbidite fan deposits between North America and South America became parts of large accretionary wedges of semiconsolidated sediments. In a series of steps, these wedges were folded and thrust toward the northwest against, then finally onto, the southern margin of North America. These compressive steps caused the repeated piling up of the accretionary wedges to form rapidly eroding highlands on the cratonic margins. Repeated loading on the North American margin formed a series of elongate, deep-water, depositional troughs (fore-deeps) immediately north and west of the accretionary wedges, and these received thick accumulations of turbidites. The youngest of these deep basins include the northwestern part of the Val Verde basin to the north and the Marfa basin to the west which received basinal deposits from within Desmoinesian into early Late Permian time. Loading of the cratonic margin also resulted in renewed faulting along generally northwest- to north-trending older zones of weakness. These zones are apparently of Precambrian age. On the craton, the late Paleozoic faults mainly had high-angle to vertical fault planes and commonly had large components of horizontal displacement. This fault system outlines the major uplifts and basins in the Permian basin region. The joining together of Gondwana and Euramerica across the Marathon salient of the orogenic belt was essentially completed by mid-Wolfcampian time. After that time, the southern margin of the Permian basin, represented by the Dugout and Marathon allochthons and the Diablo Platform, became a relatively stable tectonic area. The Glass Mountains expose a series of north- and northwest-dipping cuestas of Permian carbonate shelf and shelf-edge deposits that accumulated on a platform constructed from these two allochthonous accretionary wedges. These Permian deposits prograded north and northwestward into the narrow Hovey Channel, which connected the Marfa basin with the southern end of the Delaware basin. Carbonate-reef growth finally restricted the inflow of marine water into the Delaware basin near the end of late Guadalupian time.

Evidence of a microplate in the Southern Andes?

Paleomagnetic poles for Late Paleozoic-Early Mesozoic rocks exposed in stable and possible mobile zones of Western Argentina are reported. They suggest two interpretations. One of the interpretations suggests that a mobile zone situated at about 32.1°South 69.3°West, in the Central Andes, rotated about 60° clockwise after Late Paleozoic time. The other interpretation suggests that at least the Andean zone between 32.1° and 31.7° South is allochthonous and was situated in the Pacific, at tropical latitudes, in the Late Paleozoic. On this interpretation the accretion of this microplate to South America ocurred in Permo-Triassic times.

Depositional Trends In Upper Paleozoic Terrigenous Clastic Rocks, Sacramento Mountains, New Mexico

Field analysis of terrigenous elastic units lying within the upper Pennsylvanian-lower Permian cyclic elastic/ carbonate sequence (Holder and Labore/ta Formations) of the northern Sacramento Mountains, New Mexico, illustrates a marine to non-marine environmental progression as infilling of the Orogrande basin occurred. The oldest units studied, flat-based sandstone bodies A, B and C (Virgilian), coarsen upward texturally and dis­ play large-scale (up to 5m thick) foresets, hummocky stratification, bimodal or polymodal cross-stratification trends, and abundant burrows. These characteristics indicate that siliciclastic shelf bars were present on a storm­ dominated and tide-influenced shelf. Higher in the section, units D (upper Virgilian) and E (lower Wolfcampian) exhibit fluvially influenced depositional features such as concave-up erosional bases, upward-fining textural sequences and unimodal or asymmetric, bimodal paleocurrent distributions. Units D and E are interpreted as estuarine and fan deltaic deposits, respectively Several upward-coarsening fan-deltaic sequences were recognized in the study area suggesting that delta shifting , rather than sea level fluctuations, was a significant local cause of cyclicity. The sequence of units A-E may represent the relatively continuous progradation of terrigenous elastic sediments from the Pedernal uplift into the Orogrande basin during late Paleozoic time.

State of stress, permeability, and fractures in the Precambrian granite of northern Illinois

In situ fracture logging, permeability tests, and stress measurements have been conducted in UPH 3, a 1600‐m‐deep hole drilled into the Precambrian granitic basement of northern Illinois. Two major fracture zones are revealed, which cannot be discerned in UPH 2, a similarly deep hole about 1 km away. The segments of the UPH 3 core that were oriented indicate the existence of three sets of subvertical joints striking at N55°E, N40°W and E–W. These sets correspond to surface and shallow borehole joint directions in the Precambrian and Paleozoic rock of southern Wisconsin as well as other areas of the Midwest. The permeability values in UPH 3 display an overall reduction with depth from about 10−4 darcy at 700 m to 10−8–10−9 darcy at 1600 m. Permeability is highest in the zones of greatest fracturing, one of which occurs near the top of the granite and is probably related to fractures which were formed when the granite was at the surface in late Paleozoic times. Permeability reduction with depth is consistent with previous laboratory and field results in crystalline rocks. Hydrofracturing measurements in UPH 3 reveal a compressional stress field with the largest stress horizontal and oriented at N48°E (±30°). Based on linear regression of 13 test results in the depth range of 686–1449 m, the greatest horizontal stress has a magnitude of [20.5+(0.023×depth(m))] MPa. The least horizontal compression has a value of [8.7+(0.019×depth(m))] MPa. The vertical stress, based on density measurements, is given by [−1.3+(0.026xdepth(m))] MPa. Both magnitudes and directions support previous results in the technically stable Great Lakes region of the midcontinent. However, a mb = 4.4 earthquake did occur in 1972 some 90 km south of UPH 3, at a depth of 13 km. The focal mechanism solution revealed strike slip motion with the pressure axis horizontal and trending northeast, in accord with our measured stress directions and relative magnitudes but not predicted from a frictional sliding criterion based on Byerlee's law.

Tectono-stratigraphic terranes and mineral resource distributions in Mexico

About 80% of the southern part of the North American Cordillera within the Republic of Mexico is made up of suspect terranes. These terranes are suspect because their paleogeographic setting with respect to cratonic North America at various times through much of Phanerozoic time is uncertain. Much of northeastern and southeastern Mexico is underlain by basement accreted during late Paleozoic time, an extension of the Appalachian–Ouachita orogeny. This orogen has been considerably modified by Jurassic strike-slip translations related to the opening of the Gulf of Mexico. Western and southwestern Mexico is largely made up of several distinct but coeval latest Jurassic to Late Cretaceous submarine magmatic arc terranes with unknown basement that appear to have accreted against the disrupted North American margin by early Tertiary time. Only northeastern Sonora and the State of Chihuahua appear to be floored by unmoved North American cratonic basement. The combined effect of Mesozoic accretions and translations essentially eliminates the overlap of South America upon Mexico that is drived from late Paleozoic – early Mesozoic reconstructions of the closed Atlantic Ocean. This new vision of accretionary and translational tectonics in Mexico has profound implications for the study of tectogenesis in the southern Cordillera as well as for the interpretation of Mexico's vast natural resources. Preliminary analysis indicates that Mexico's gold–silver and lead–zinc deposits are directly or indirectly related to the terrane distributions discussed.

Flexure of Anadarko Basin: ABSTRACT

The Anadarko basin in Oklahoma has long been a major oil and gas producing region and contains the deepest wells drilled in North America. The region has had a long sedimentary-tectonic history reaching back to the Proterozoic and was the site of an early Paleozoic basin. The present shape of the Anadarko basin, however, was developed in late Paleozoic times as a result of the uplift of the Wichita Mountains. COCORP seismic reflection profiles show at least 8 to 9 km (5 to 5.6 mi) of overthrusting northward, and the Anadarko basin was developed as a result of flexural bending of the lithosphere due to this shortening. Downwarping of the basin can be observed to extend for over 300 km (185 mi) northward, indicating a high flexural rigidity (Te > 40 km [25 mi]). However, nearer the Wichita front, the basin steepens rapidly as the post-Mississippian sediments thicken to over 20,000 ft (6,100 m). The shape of the bending is such that it cannot be explained by the use of a constant rigidity elastic plate model. We have modeled the post-Mississippian development of the Anadarko basin as the result of flexure of an elastic-plastic plate due to vertical and horizontal loading caused by the Wichita Mountains. Implications of these results for the development of the Anadarko basin and the mechanical properties of continental lithosphere will be discussed. End_of_Article - Last_Page 552------------

Corals from a dismembered late Paleozoic paleo-Pacific plateau

A coral belonging to the rare Late Mississippian–Early Pennsylvanian Family Pseudopavonidae and a specimen of the Permian waagenophyllid coral genus Parawentzelella have been recovered from the Cache Creek assemblage in northern British Columbia and from a small limestone block in southern British Columbia, Canada, respectively. Both of these fossils are closely related to corals known from eastern Japan and western Sze-chuan, China; Parawentzelella also occurs in Indochina. These corals apparently occur in shallow-water carbonates that overlie pieces of oceanic volcanic ridges or plateaus. In both Asia and North America these corals now lie geographically close to coeval, but completely different, coral faunas that lived on shallow carbonate platforms built on continental shelves. This suggests that the circum-Pacific terranes bearing these unusual corals were displaced from a single, shallow-water oceanic region that in late Paleozoic time lay in the paleo-Pacific Ocean far from any continental margin. In Late Permian or early Mesozoic time the region colonized by these fossils was torn apart; the rock masses bearing these fossils were then carried on oceanic plates to subduction zones at continental margins on opposite sides of the Pacific Ocean, where they became lodged.

Sur la présence de Microcodium (Algue?, Incertae sedis?) dans le Paléozoïque supérieur de l'Arctique canadien

Seven localities yielding Microcodium are described from the Carboniferous and Permian of the Yukon Territory, Axel Heiberg, and Ellesmere. The Microcodium is generally perforating. However, microbreccias and reworked fragments containing Microcodium indicate that this enigmatic taxon was already present in Late Paleozoic time. Microcodium is therefore not necessarily the result of pedogenic Cenozoic alteration of older sediments. Cenozoic inorganic, bacterial, or pedogenic origins are improbable. An algal affinity is more plausible.

活断層調査の例 柳ケ瀬断層

The Yanagase fault is a remarkable active fault in central Honshu, Japan, extending north-nothwestwards from the northeast of Lake Biwa to the northeast of Tsuruga City.Both geomorphological and geological detailed studies have been carried out along the fault zone to clarify the history of its movement especially in Quaternary time.A linear fault line valley consisting of four different streams in a straight arrangment is developed along the fault zone over a distance of about 25km, which is recognizable as a lineament in a broad sense. Geomorphic features caused by left-lateral faulting such as shutter ridges, offset streams are detected along the southern part of the lineament. And the west-side block of the fault shows subsiding topographic features such as rather wide alluvial plain around Lake Yago, but the east-side uplifting such as elevated river terraces of two to five steps, composite alluvial fans, high triangular terminal facets and so on.The paleozoic system consisting of slate, sandstone, chert, greenstones is sheared along the fault in a zone of a few hundred meters wide, in which rocks are strongly crushed to be transformed into fault gouge in a maximum width 50 meters or so. Porphyrite dykes within the fault zone are also crushed at some localities.Fault displacement in late Quaternary fan deposits has been observed at several places in the southern, part of the fault zone. However no evidence of Quaternary fault movement has been found out in the northern part, although the fault zone is overlain by Quaternary fan deposits here and there. Each fault shows different sense of displacement, that is lateral-slip, east-side uplift dip slip and west-side uplift dip slip. The youngest deposit cut by the faults has been dated back to about 4, 000 years B. P. in Carbon-14 age.Judging from the west-dip imbrication of gravels observed in the highest terrace deposits along the wind gap east of Nakanogo, the Yogo River had been running once eastward from Nokanogo through the wind gap to Shimo-niyu. The fault movement with west-side subsidence and east-side uplift occured at 80, 000 to 100, 000 years ago along the southern part, seems to have changed its direction to the south as in the present.It is concluded the Yanagase fault has moved repeatedly since late Paleozoic time, and has been active in its southern half in late Quaternary time, on the contrary not or scarcely active in the northern part. Its dominant sense of movement in late Quaternary is supposed to be left-lateral, west-side subsidence and east-side uplift.

Age, origin, and significance of an unconformity that predates island-arc volcanism in the northern Sierra Nevada

The paleogeographic position of rocks of the northern Sierra Nevada relative to North America has long been the topic of debate. A detailed study of the Lakes Basin region of the northern Sierra Nevada substantiates that the Shoo Fly Complex, of early Paleozoic age, was folded and unconformably overlain by an island-arc sequence in late Paleozoic time. Microfossils recovered during the study represent the first in situ fauna ever collected from the Shoo Fly Complex and provide an Ordovician-Silurian lower limit on the early deformation. Late Jurassic (Nevadan) deformation strongly affected all rocks in the region. Regional considerations suggest that mid-Paleozoic deformation may have been widespread in the Sierra Nevada and could indicate an early phase of an eastward-prograding suture that ultimately resulted in emplacement of the Roberts Mountain allochthon of the Great Basin.

Northwestern Africa and the Avalonian plate: Relations during late Precambrian and late Paleozoic time

Morocco, and especially the Moroccan Meseta, is compared to the eastern margin of the Appalachians. The structural history of these regions is discussed for two key periods and orogenic episodes: (1) late Precambrian and the Avalonian–Pan-African orogeny, and (2) late Paleozoic and the Alleghanian-Hercynian orogeny. The observed similarities suggest that Morocco and the eastern margin of the Appalachians belong to the same continental block, the Avalonian plate.

Northwestern Africa and the Avalonian Plate: relations during Late Precambrian and Late Paleozoic time : Piqué, Alain, 1981. Geology, geol. Soc. Am., 9(7):319–322

Northwestern Africa and the Avalonian Plate: relations during Late Precambrian and Late Paleozoic time : Piqué, Alain, 1981. Geology, geol. Soc. Am., 9(7):319–322

Cooling mechanisms and effects on mantle convection beneath Antarctica

Temperature in the mantle beneath Antarctica is a function of initial temperatures established during processes of accretion, primarily subduction, of continental nuclei which formed Pangaea. Subsequent near-surface temperature, which is latitude-dependent, has determined the pattern and rate at which the continent has broken up. Subduction and conduction play primary roles in the mantle temperature regimen; whereas, ice-sheet flow systems and deep groundwater flow beneath the ice sheet play secondary roles. All of these effects tend to maintain low mantle temperatures beneath Antarctica. Because of lower mantle temperature, mantle viscosity (η) is several orders of magnitude higher than it is beneath equatorial continents. As η increases, rates of plate motion decrease since strain rate is inversely proportional to η. Since continental accretion ended in late Paleozoic time, mantle temperatures have been increasing due to radioactive decay at rates dependent upon thermal gradients, thus low-latitude fragments of Pangaea were first to separate by rifting and convection. North America was first to drift away from Africa and South America in Triassic time along a rift system which lay along the equator. Greenland rifted from Eurasia and Australia from Antarctica 50–60 m.y. ago. Both rifting episodes occurred in the same time period at the same high latitudes, only one north, the other south. The mantle underlying the remaining high southern-latitude fragment of Pangaea (Antarctica) retains a relatively high viscosity. Indirect evidence for high mantle viscosity beneath Antarctica is seen by the abnormally deep isostatically-uncompensated continental shelves, and an asymmetrically spreading circumpolar ridge. Additional evidence for this is the aseismicity south of 63°S, although aseismicity can also be explained by high, subglacial hydrostatic pressure acting upon bottom melt water. Effective stresses are lowered between adjacent tectonic blocks and accumulating stress is easily dissipated. A present zone of high mantle temperatures beneath the Transantarctic Mountains suggests a possible future spreading center from which additional fragments of the continent might drift; however, the inward-directed circumpolar ridge, high mantle viscosities, and a possible adequate heat loss by volcanism might slow or eliminate the process.

Deformation and metamorphism in the Hylas zone and adjacent parts of the eastern Piedmont in Virginia

Structural and petrographic studies of the Hylas zone northwest of Richmond, Virginia, reveal that late Paleozoic ductile shearing produced mylonites and ultramylonites from pre-existing biotite gneiss, granite gneiss, and amphibolite in the Goochland complex and from Petersburg(?) granite. Rocks in the Goochland complex west of the Hylas zone underwent at least two deformations (D 1 , D 2 ) prior to ductile shearing in the Hylas zone (D 3 ). D 3 caused a pervasive mylonitic foliation (S c ) and a later widely spaced shear cleavage (S s ) to be formed in rocks in the Hylas zone. Brittle deformation in the form of high-angle faulting and locally intense fracturing (D 4 ) was superimposed on the Hylas zone about 220 m.y. ago and is probably equivalent to the Palisades disturbance of New England. D 4 was associated throughout the Piedmont with synchronous downwarp and dominantly continental sedimentation to form a series of parallel Triassic basins represented in the study area by the Richmond Basin. Northwest-oriented high-angle faults that apparently displace Triassic sedimentary rocks are interpreted to predate the Late Cretaceous. Regional prograde metamorphism of the Goochland complex to amphibolite facies (M 1 ) was inclusive of D 1 and D 2 , because structural elements that compose these deformational events consist primarily of oriented metamorphic minerals. Well-equilibrated microstructures in the gneisses suggest that the M 1 peak persisted past D 2 . M 1 is inferred to have occurred about 340 m.y. ago. D 3 was accompanied by retrograde metamorphism (m 2 ) to greenschist facies in the Hylas zone during late Paleozoic time. The presence of laumontite, quartz, and calcite in brecciated zones suggests that brittle deformation during D 4 occurred under zeolite facies conditions. Cenozoic reverse faulting 90 km along strike of the study area in the Brandywine area of the Coastal Plain in Maryland may indicate a continuation of the Hylas zone to the north. Superposed deformation and intermittent reactivation of zones of instability similar to that of the Hylas zone are reported in the Eastern Piedmont fault system, which extends from Alabama to Virginia. Current stress release beneath the Coastal Plain may be influenced by the instability of the Hylas zone and analogous fault systems throughout the Piedmont.

Iberian-Armorican arc and Hercynian orogeny in western Europe

Recently assembled geological and geophysical evidence from southern Brittany reveals the organization of submerged structures off the southern Brittany coast. Fragments of a high-pressure–low-temperature belt are caught between a northern high-temperature zone of middle Paleozoic age, and a southern zone of intermediate metamorphism. It is not known whether the rocks in the high-pressure–low-temperature belt belong to a middle Paleozoic allochthonous slab or whether they are parautoch-thonous and the result of Carboniferous obduction. Farther to the south a major geophysical discontinuity is interpreted as a suture, the trace of which cuts across the sedimentary facies zonation established between the Armorican Massif and Spain. It is therefore suggested that the formation of the Iberian-Armorican arc predates the Hercynian orogeny. A geodynamic model is proposed that attempts to synthesize the orogenic mechanisms in Spain and southern Brittany during middle and late Paleozoic time.

Sequential development of the Appalachian orogen above a master decollement — A hypothesis

Surface geology and seismic-reflection data suggest that rather than having a massive rooted central core the southern part of the Appalachian orogen from the Appalachian Plateau to the Atlantic continental shelf is underlain by an eastward-dipping decollement zone. We infer that this decollement zone was a long-lived structural element, intermittently growing from east to west during late Proterozoic to late Paleozoic time. Onshore displacement along the detachment surface was episodic through this long period of time, so that reliable estimates of total shortening for the entire orogen are not possible. Palinspastic restoration of only the western margin of the orogen, the area west of the Brevard zone, suggests a minimum of 280 km (175 mi) of Alleghenian displacement. Total accumulated shortening through time within the entire orogen must be considerably greater, probably exceeding hundreds of additional kilometres.

The internal fabric of an alpine peridotite near Pinchi Lake, central British Columbia

Near Pinchi Lake, central British Columbia, a fault-bounded alpine peridotite occurs within the Cache Creek Group. It comprises massive harzburgite, dunite, and pyroxenite, all of which display tectonite fabrics. Near isoclinal folds (F1), outlined by dunite, pyroxenite, and spinel layers (compositional layering) constitute the earliest recognizable penetrative deformation. A second set of near-coaxial open folds (F2) refolds the earlier set; both are cut by a later fracture cleavage (F3). Flattened, elongate olivine and orthopyroxene grains within the harzburgite outline foliations and lineations parallel with axial surfaces and minor fold axes of both fold sets. Narrow zones of mylonitization within the isoclinally folded dunites transect the composition layering, are parallel with the axial surfaces of the F1 isoclines, and appear to be related to refolding of F1 by F2. Relic olivines are kinked along {OKL}:[100], have pronounced α-maxima perpendicular to F1 and γ-axes maxima parallel to fold axis L1. Orthopyroxene, kinked on (100):[001] shows weaker fabric, but is symmetric with earliest composition layering, some 15°–20° away from F1. Asymmetry of fabrics results from olivine recrystallizing under conditions where orthopyroxene only kinks.It is speculated that the peridotite is a fragment of Paleozoic oceanic lithosphere. F1 and F2 are believed to be mantle transport fabrics resulting from a northerly driven plate, whereas F3 fractures are high-level emplacement features produced during late Paleozoic time.