Late Paleozoic Orogeny Research Articles

Late Paleozoic–Cenozoic Tectonothermal Evolution of Transbaikalia: Thermochronology of the Angara–Vitim Granitoid Batholith

Abstract —The tectonothermal evolution of Transbaikalia is reconstructed using U/Pb, 40Ar/39Ar, and apatite fission track thermo-chronology of samples from the Late Paleozoic Angara–Vitim granitoid batholith (AVB). Successive closure of the zircon and amphibole isotope systems provides evidence that the AVB rocks cooled down rapidly soon after crystallization and 7–4 km of rocks were denuded subsequently during an extensive late Paleozoic orogeny in southern Siberia. The isotopic system of feldspar closed in the Middle Jurassic–Early Cretaceous (170–140 Ma) after a period of tectonic stability and slow closure of the biotite isotopic system in the early Permian–Middle Jurassic (295–170 Ma). The 170–140 Ma span was the time when the Mongol–Okhotsk orogen began its evolution, and the orogeny caused denudation of ~3 km of rocks. Denudation was slow in the Paleogene–Miocene (60–5 Ma) but accelerated over the past 5 million years (a ~3–2 km thick layer) during rapid cooling of rocks and activity under a far-field effect of the India–Eurasia collision.

THERMOCHRONOLOGY OF THE ANGARA-VITIM GRANITOID BATHOLITH AS A CHRONICLE OF THE EVOLUTION OF THE MONGOL-OKHOTSK OROGEN

The thermal history of the Late Paleozoic granitoids of the Angara-Vitim batholith (ABB) was reconstructed using a multi-system approach including U/Pb, 40Ar/39Ar, and fission track dating methods. The rocks of the Barguzinsky and Chivyrkuysky complexes were studied at sites in the southwestern and northeastern parts of the batholith, as well as on the Svyatoy Nose Peninsula and the eastern shore of Lake Baikal. The similar thermal history is reconstructed for the rocks of all sites as a whole. The steep position of the thermal history graph immediately after the formation of the ABB rocks indicates a major denudation epoch with an amplitude of about 7–5 km associated with the Late Paleozoic orogeny. After the epoch of tectonic stabilization, characterized by the time-stretched closure of the biotite isotope system, the closure of the feldspar/plagioclase isotope system occurs in the range of 170–140 Ma. This interval coincides with the closure of the Mongol-Okhotsk Ocean and the beginning of the formation of the orogen of the same name and is characterized by an amplitude of denudation of about 3 km. Further, in the period of 60–3 Ma, a slow denudation took place on the territory of Transbaikalia, which ended with a relatively sharp cooling of rocks during the last 3 Ma and denudation with an amplitude of about 3–2 km. This may be due to the long-range tectonic impact of the Indo-Eurasian collision. The obtained data on the tectonic-thermal evolution of AVB rocks can serve as a basis for identifying the relationship with the formation of the Mongol-Okhotsk orogen and sedimentation in the Jurassic-Early Cretaceous Irkutsk, Kansk-Achinsk, Tuva, Kuznetsk and West Siberian basins.

The anisotropic properties of granites – effects of tectonic emplacement mode on potential crystalline host rocks for nuclear waste deposits in Germany

Abstract. For permanent nuclear waste disposal sites, crystalline rocks, especially granitic/granodioritic batholiths, are considered an appropriate host rock. Principally, three types of granitic plutons occur in the extra-alpine crystalline basement of Germany that were consolidated during the late Paleozoic Variscan orogeny of Central Europe: (i) Pre-Variscan voluminous granodiorites that are hardly affected by the subsequent continent–continent collision; (ii) voluminous granites in various tectonic settings intruded during the late orogenic stage of the Variscides; (iii) post-orogenic granites related to vast Permian intracontinental extension. Thus, in terms of the syn-intrusive tectonic setting and post-intrusive processes there are significant differences. Although it can be expected that different tectonic environments caused significant differences in the material properties, for Germany, however, there is no systematic study regarding the fabric of such plutonites. In order to find the most suitable “granite” we investigate the primary anisotropy of granites evolved during the emplacement and crystallization of the melt. For this we sample rocks of all three principal types and various syn-intrusive tectonic settings, i.e., compression, extension, strike-slip, transtension, and transpression. By means of combined measurements of the “Anisotropy of the Magnetic Susceptibility” and the “Shape Preferred Orientation” we characterize the syn-intrusive flow pattern, i.e., the magmatic foliation and lineation. The Crystallographic Preferred Orientation is analyzed by a combination of neutron time-of-flight experiments and electron backscatter diffraction measurements at the Frank Laboratory of Neutron Physics at JINR, Dubna, Russia, and the TU Bergakademie Freiberg respectively. Furthermore, special attention is given to the systematic mapping of annealed microcracks evolved during late magmatic fluid escape and/or post-crystallization hydrothermal activity. In a second step we compare the primary anisotropy with the post-magmatic fracture pattern of the particular granites. Those fractures constitute probable fluid pathways and, thus, the first-order risk for a potential permanent nuclear waste disposal. All datasets are organized in a Geological Information System allowing for a complete traceability of the different investigation steps. The results of this study will serve as a basis for a future detailed exploration.

Phanerozoic orogeny in the South China Block traced by clastic components from Cambrian to Triassic sedimentary rocks

Systematic detrital zircon U-Pb dating was applied to Cambrian to Triassic sedimentary rocks in the Yong’an basin, southeastern South China, and yields four age groups. Cambrian and Ordovician quartz schists display prominent Grenvillian (906–1199 Ma) and minor late Neoarchean to earliest Paleoproterozoic (2418–2619 Ma) populations, showing characteristics of an East Gondwanan origin. Devonian and Carboniferous sandstones feature euhedral Neoproterozoic (733–999 Ma) and early Paleozoic (405–468 Ma) detrital zircons, suggestive of a proximal accumulation sourced from the nearby Wuyi terrane. Middle Permian to early Triassic sandstones contain abundant late Paleoproterozoic (1743–1979 Ma), Grenvillian (893–1271 Ma), and early Paleozoic (403–471 Ma) detrital zircons, together with increasing late Paleozoic (264–374 Ma) populations, indicating multiple provenances. Late Triassic sandstones show dominant proportions of late Paleoproterozoic (1805–1938 Ma) zircons, derived mainly from the Wuyi terrane. Three Phanerozoic orogenic events in the South China Block (SCB), i.e. early Paleozoic (Kwangsian or Wuyi-Yunkai), late Paleozoic, and early Mesozoic (Indosinian) orogenies, probably are responsible for the changes in clastic provenance among the four groups of sedimentary rocks. The late Paleozoic orogeny in the SCB was previously thought to be a limited minor event. This study suggests that the impact of late Paleozoic orogenesis on the SCB could be much wider. The uplift of the Wuyi Mountains probably started after the early Paleozoic orogeny, much earlier than previously proposed, and has provided long-term clastic supply to the Yong’an basin and nearby basins.

Thermochronological insights into the intracontinental orogeny of the Chinese western Tianshan orogen

The Chinese western Tianshan orogen (CWTS) experienced multiple tectono-thermal events since the late Paleozoic. In order to better understand the intracontinental orogeny, we integrate new thermochronologic data to reconstruct the long-term cooling history of the CWTS. Our new data reveal that individual tectonic units within the Tianshan orogen show spatial/temporal disparities in their cooling histories. The CWTS experienced a rapid cooling episode during a late Paleozoic orogeny. The Cimmerian collisional orogeny (i.e., the Qiangtang-Kunlun and Lhasa-Qiangtang collision) and the closure of the Mongol-Okhotsk ocean were not strictly coincident with the exhumation events in the CWTS, and thus these distal events were likely not the sole orogenic drivers. The thermal curves reflect that lower crustal rocks had exhumed to middle-upper crustal levels in the late Permian-Triassic, and a rapid cooling phase during this period records the collapse of the CWTS. Furthermore, magmatism related to the melting of subducted Paleozoic plates was probably a crucial factor in the Mesozoic thermal evolution. The age-elevation relationship produced here emphasizes the importance of faulting during intracontinental reactivation and implies that the CWTS has expanded from its central part to both the south and north sides since the late Mesozoic. Low exhumation rates and sedimentological analysis indicate that the relief gradually decreased in the Jurassic-early Cretaceous. The initial reactivation time of the CWTS could be as old as ~38 Ma based on a rapid exhumation event and sedimentary evidence. In the Cenozoic, the crust-mantle interaction, tectonic compression, and continuous northward pushing of the Indian plate rather than the India-Eurasia collision are interpreted as the driving forces that have rebuilt the CWTS. The variable tectono-thermal evolution of individual tectonic units ascribes to the discrepant thermal-rheological properties, complex tectonic boundaries, and the various components of the Tianshan orogen.

Widespread Paleoproterozoic basement in the eastern Cathaysia Block: Evidence from metasedimentary rocks of the Pingtan–Dongshan metamorphic belt, in southeastern China

The Cathaysia Block in South China Block (SCB) is famous for its polyphase granitic magmatism and world-class mineralization, especially W, Sn and REE. Although isotopic data suggest that widespread igneous rocks were derived from Precambrian basement, outcrops of the basement rocks are rare in the Cathaysia Block. Therefore, the distribution of the Archean-Paleoproterozoic basement beneath Cathaysia Block is still unclear.The northeastern-striking Pingtan–Dongshan metamorphic belt (PDMB) is located along the coastal region of southeastern China, and consists mainly of voluminous granites and volcanic rocks with minor metasedimentary rocks. These metasedimentary rocks were intruded or captured by 160–143Ma granites. U–Pb dating of detrital and metamorphic zircons from seven metasedimentary rocks indicates that their protoliths were deposited between 183Ma and 160Ma, rather than pre-Devonian as previously proposed. Dating of metamorphic zircons suggests that the most important metamorphism took place at ca 97Ma, consistent with emplacement of A-type granites and mafic magmatism in the coastal region, and implying an extensional setting.Age spectra of detrital zircons in samples from the middle and southern PDMB exhibit a bimodal distribution with abundant Late Paleoproterozoic and Early Mesozoic zircons. In contrast, age spectra of detrital zircons from the northern PDMB display pronounced Neoproterozoic and Early Paleozoic age peaks as well as Late Paleoproterozoic and Early Mesozoic ones, suggesting a different provenance. The comparisons of age and Hf-isotope with the exposed rocks of the SCB suggest that the terrigenous material in these sediments was sourced from the Wuyi terrane, in the eastern Cathaysia Block.The age spectra of detrital zircons from Early Paleozoic to Late Mesozoic sedimentary rocks in the SE SCB show that late Paleoproterozoic (1.9–1.8Ga) detritus increases markedly in Middle Permian to Early Cretaceous sedimentary rocks. Comparison with zircon U–Pb–Hf isotopic data from exposed Paleoproterozoic rocks suggests that a Paleoproterozoic basement existed extensively beneath the eastern Cathaysia Block. It has been uplifted and eroded since Middle Permian time, and then covered by Late Cretaceous volcanic rocks. The uplift of the Paleoproterozoic basement was probably caused by the Late Paleozoic orogeny in the eastern Cathaysia Block.

Influence of pre-Andean history over Cenozoic foreland deformation: Structural styles in the Malargüe fold-and-thrust belt at 35°S, Andes of Argentina

The Andes are the classic example of a subduction-related orogen. Segmentation of the orogenic belt is related to dynamics of the subduction zone and to upper plate thermomechanical properties. Understanding the controlling factors on deformation along the orogen requires studying cross sections at different latitudes and determining the respective roles of plate interactions, upper plate weakness zones, and crustal architecture. A newly constructed balanced cross section of the Argentinean Andes at 35°S, in the transition between a flat-slab and a normal subduction segment, shows tectonic inversion of Mesozoic normal faults and development of new thrusts during Andean shortening. Estimated shortening of 26.2 km, equivalent to 22% of the initial length, is lower than previous estimates obtained from partial cross sections using non-inversion structural models. Comparison of this estimate with crustal area balance constrained by geophysical data indicates that (1) crustal thickness was varied across the transect before Andean shortening, with a thick (∼45 km) crustal block to the west related to late Paleozoic orogeny, and a thinner block (∼32 km) in the east related to Mesozoic stretching; and (2) a structural model incorporating tectonic inversion is consistent with regional shortening and crustal thickness trends. Our results underscore the role of the inherited characteristics of the upper plate in subduction-related orogens, including preexisting faults and preorogenic crustal thickness variations.

Late Paleozoic magmatism in South China: Oceanic subduction or intracontinental orogeny?

A gneissic granite with an U-Pb age of 313±4 Ma was found in northeastern Fujian Province, South China. It is an S-type granite characterized by high K2O, Al2O3 and low SiO2, Na2O contents with high A/CNK ratio of 1.22 for the whole rock. Zircons with stubby morphology from the gneissic granite yield 206Pb/238U ages ranging from 326 to 301 Ma with a weighted average age of 313±4 Ma, and negative ɛHf(t) values from −8.35 to −1.74 with Hf model ages (TCDM) of 1.43 to 1.84 Ga. This S-type granite probably originated from late Paleoproterozoic crust in intracontinental orogeny. Integrated with previous results on paleogeographic reconstruction of South China, the nature of Paleozoic basins, Early Permian volcanism and U-Pb-Hf isotope of detrital zircons from the late Paleozoic to early Mesozoic sedimentary rocks, we suggest the occurrence of a late Paleozoic orogeny in the eastern Cathaysia Block, South China. This orogenic cycle includes Late Carboniferous (340–310 Ma) orogeny (compression) episode and Early Permian (287–270 Ma) post-orogenic or intraplate extension episode. Therefore, the late Paleozoic magmatism in the southeastern South China probably occurred during the intraplate orogeny rather than the arc-related process.

Ouachita, Appalachian, and Ancestral Rockies deformations recorded in mesoscale structures on the foreland Ozark plateaus

Mesoscale structures in Paleozoic rocks of the Ozark plateaus reveal four Pennsylvanian deformation episodes in midcontinent North America. The two earliest episodes can be assigned to progressive northwestward docking of the Ouachita terrane with North America. Early extensional structures (Event 1) indicate a northwest/southeast maximum horizontal stress (Hmax) during Early Pennsylvanian Ouachita terrane advance. Event 2 extensional and strike-slip structures indicate Hmax across the Ozark plateaus that varies systematically from north-northwest/south-southeast in the south to northeast/southwest in the north. This suggests development of a slip-line deformation field in response to minor northeastward lateral escape of lithospheric blocks away from the northwestward-moving Ouachita terrane's leading edge, which acted as an indenter in western Arkansas, southeastern Oklahoma, and Texas. Younger contractional and strike-slip structures of Event 3 indicate northeast/southwest Hmax across the entire Ozark plateaus, and deformation orientation and intensity are not readily assigned to Ouachita foreland deformation and may be related to Middle Pennsylvanian Ancestral Rockies contractional deformation. Finally, Event 4 contractional structures indicate northwest/southeast Hmax consistent with southern Appalachian late stage convergence. Deformation episodes are localized along basement fault zones, particularly at major bends, suggesting minor restraining-bend uplifts along strike-slip faults. Geometries of conjugate normal fault and hybrid shear joint arrays indicate localized areas of high differential stress consistent with basement block uplift at these bends. High-angle faults reactivated in a reverse sense and bedding-parallel veins suggest tensile minimum stresses and pore fluid pressures exceeding lithostatic stress, consistent with brine pulses driven into the midcontinent during Late Paleozoic orogeny (as proposed by other authors).

Kinematics and dynamics of the Mesozoic orogeny and late-orogenic extensional collapse in the Sino-Mongolian border areas

The Sino-Mongolian border areas underwent two important tectonic events during Mesozoic time after late Paleozoic orogeny: a late Triassic to earlier Jurassic contractional event that resulted in a large-scale south-vergent thrust during the orogeny and a late Jurassic-earlier Cretaceous extensional event in a north-south direction that formed a metamorphic core complex. The kinematic and dynamic analyses show that the thrust sheet moved southwards with a kinematic vorticity number of ca. −0.10 and sub-horizontal maximum compressive stress axis that belongs to a contraction-thickening shear. The upper plate of the late-orogenic detachment relatively moved in a 165° direction. The average kinematic vorticity in its earlier stage was 0.74 that belongs to simple shear dominated shearing and related to the maximum compressive stress axes dipping at ~66°, while the later average kinematic vorticity was ~0.55 that belongs to pure shear dominated shearing with sub-vertical maximum compressive stress axes. This suggests that the thrusting led to the crust thickened and the lower plate rocks that were originally located in the upper crust depressed through a brittle-ductile transition zone into the lower crust and became warmer. The heated rocks trended to uplift since their increasing volume and decreasing density while the loading of the upper-plate rocks increased due to the structural thickening. Under the combined effect of the loading and the thermal-uplifting, the ductile shear zone in between increased in its component of vertical pure shear. Once its pure-shear component ex-ceeded its simple-shear one the ductile shear zone became an extension-thinned shear zone. This progressive transitional process reflects internal and essential temporal and spatial rela-tionships: the extensional factor nucleated during the crust thickening by thrusting and increase of the extensional factor finally led to late-orogenic collapse.

Re?Os isotopic dating of molybdenite and pyrite in the Baishan Mo?Re deposit, eastern Tianshan, NW China, and its geological significance

The Baishan Mo–Re deposit is located in the eastern section of the eastern Tianshan orogenic belt, NW China. The deposit has a grade of 0.06% Mo and a high content of rhenium of 1.4 g/t. Rhenium and osmium isotopes in sulfide minerals from the Baishan deposit are used to determine the age of mineralization. Rhenium concentrations in molybdenite samples are between 74 and 250 μg/g. Analysis of eight molybdenite samples yields an isochron age of 224.8±4.5 Ma (2σ). Pyrite samples have rhenium and osmium concentrations varying in the range 33.4–330.6 ng/g and 0.08–0.81 ng/g, respectively. Isotope data on seven pyrite samples yield an isochron age of 225±12 Ma (2σ) on the 187Re/188Os versus 187Os/188Os plot and an age of 233±14 Ma (2σ) on the 187Os versus 187Re correlation diagram. The ages of molybdenite and pyrite are consistent within the analytical errors. Combined with field observations, the data indicate that Mo–Re mineralization in the Baishan deposit is produced by a magmatic-hydrothermal event in an intracontinental extensional setting after late Paleozoic orogeny. The initial 187Os/188Os ratio of pyrite is 0.3±0.07. The δ34S values of molybdenite vary from +0.5 to +3.6‰. Both data indicate that mineralization is derived mainly from a mantle source.

Late Paleozoic orogeny in Alaska's Farewell terrane

Evidence is presented for a previously unrecognized late Paleozoic orogeny in two parts of Alaska's Farewell terrane, an event that has not entered into published scenarios for the assembly of Alaska. The Farewell terrane was long regarded as a piece of the early Paleozoic passive margin of western Canada, but is now thought, instead, to have lain between the Siberian and Laurentian (North American) cratons during the early Paleozoic. Evidence for a late Paleozoic orogeny comes from two belts located 100–200 km apart. In the northern belt, metamorphic rocks dated at 284–285 Ma (three 40Ar/ 39Ar white-mica plateau ages) provide the main evidence for orogeny. The metamorphic rocks are interpreted as part of the hinterland of a late Paleozoic mountain belt, which we name the Browns Fork orogen. In the southern belt, thick accumulations of Pennsylvanian-Permian conglomerate and sandstone provide the main evidence for orogeny. These strata are interpreted as the eroded and deformed remnants of a late Paleozoic foreland basin, which we name the Dall Basin. We suggest that the Browns Fork orogen and Dall Basin comprise a matched pair formed during collision between the Farewell terrane and rocks to the west. The colliding object is largely buried beneath Late Cretaceous flysch to the west of the Farewell terrane, but may have included parts of the so-called Innoko terrane. The late Paleozoic convergent plate boundary represented by the Browns Fork orogen likely connected with other zones of plate convergence now located in Russia, elsewhere in Alaska, and in western Canada.

The Rı́o Lácteo Formation of Southern Chile. Late Paleozoic orogeny in the Andes of southernmost South America

The late Paleozoic Rı́o Lácteo Formation forms the metamorphic basement of the Mesozoic and Cenozoic Andean orogenic belt in the Lago General Carrera area (about 45°S) of southern Chile. The rocks were emplaced during a late Paleozoic orogeny on the active continental margin of southernmost South America. They form part of a SW to NE striking fold belt with dominant overthrusting towards the SE. The metamorphic grade and structural style are indicative of deformation in the crystalline core of an orogenic belt, probably resulting from microplate interactions, rather than fore-arc accretion. The Rı́o Lácteo Formation represents metamorphorphosed marine turbidites, limestones and pyroclastic rocks which accumulated in an intraplate setting. The strata were intensely deformed and subjected to medium grade, greenschist to epidote-amphibolite facies metamorphism in pre-late Carboniferous times. Four phases of deformation have been identified. The first phase produced a strong planar fabric. This was followed by subhorizontal SW to NE stretching and NW to SE compressive shortening. The third phase formed a spaced cleavage related to E-W extension. The fourth episode produced broad shear zones, characterised by chevron folding and intrusive dykes of crush breccia, related to thrusting towards the southeast.

Structural analysis of sheath folds in the Sylacauga Marble Group, Talladega slate belt, southern Appalachians

Remnant blocks of marble from the Moretti-Harrah dimension-stone quarry provide excellent exposure of meter-scale sheath folds. Tubular structures with elliptical cross-sections (4 ≤ R yz ≤ 5) are the most common expression of the folds. The tubes are elongate subparallel to stretching lineation and are defined by centimeter-scale layers of schist. Eccentrically nested elliptical patterns and opposing asymmetry of folds (‘S’ and ‘Z’) are consistent with the sheath-fold interpretation. Sheath folds are locally numerous in the Moretti-Harrah quarry but are not widely distributed in the Sylacauga Marble Group; reconnaissance in neighboring quarries provided no additional observations. The presence of sheath folds in part of the Talladega slate belt indicates a local history of plastic, non-coaxial deformation. Such a history of deformation is substantiated by petrographic study of an extracted hinge from the Moretti-Harrah quarry. The sheath folds are modeled as due to passive amplification of initial structures during simple shear, using both analytic geometry and graphic simulation. As indicated by these models, relatively large shear strains ( y ≥ 9) and longitudinal initial structures are required. The shear strain presumably relates to NW-directed displacement of overlying crystalline rocks during late Paleozoic orogeny.

Late paleozoic orogeny in the northwestern Gondwana continental margin, western Argentina and Chile

During Early Carboniferous-Early Permian time, the segment of the northwestern Gondwana continental margin (western Argentina and Chile) between present-day latitudes 27°S and 50°S was structurally organized into a marginal orogen and a foreland rising into cratonic highlands. Orogenesis and magmatism were governed by subduction of an ancestral Pacific plate under Gondwana. During the Late Devonian-Early Carboniferous, former slope-rise deposits were thickened to form the orogen and were thrust slightly onto the foreland; parts of the orogen emerged above sea level. Following this early diastrophic phase, tectonic uplift in the orogen was regionally checked by erosion until the late Early Permian when uplift rates overcame erosion. The development of this orogen is peculiar in that throughout much of this time: a) the orogenic bett was partly occupied by marine areas; b) there was no reversal of provenance, so that foreland basins were continuously fed from the interior highlands; and c) the orogenic belt did not advance further onto the foreland. Thus the orogen stabilized in position and relief. Magmatism was intense during the period of orogenic stability and was dominated by plutonism; there are no remnants of a well developed volcanic arc. Reconstructions involving Late Devonian-Early Carboniferous collision of crustal masses against Gondwana and a seaward jump of the subduction trench are not entirely substantiated by available data. An alternative suggests that from the Late Devonian to the Early Permian the trench maintained approximately the same position seaward of present-day coastal exposures.

Geology and the Wheeler Survey

The Wheeler Survey was the only one of the territorial surveys of the 1860's and 1870's to concentrate on topographic mapping rather than geology. This difference in emphasis contributed to the poor relations of the Army survey with the two Interior Department surveys and with the scientific community after 1873. Geologists' complaints with the Wheeler Survey centered on: the rapid pace of fieldwork, the publication policy, the accuracy and utility of the topographic base maps, and the subordination to the Army's young lieutenants.Ten geologists and three paleontologists served in the field with Wheeler. His survey's official publications reported reconnaissance work in Colorado, New Mexico, Utah, Arizona, Nevada, and California. The survey's geologists collected many new fossils for description and stratigraphic evaluation by F.B. Meek and by C.A. White. E.D. Cope discovered the Puercan (earliest Paleocene) faunas of New Mexico, one of his most significant scientific discoveries, while with Wheeler. Mining geologist John Church based his theory of heat generation in the Comstock Lode on field work for the Wheeler survey. While serving with Wheeler, J.J. Stevenson made controversial discoveries of late Paleozoic orogeny in the Rockies and Cretaceous coal deposits. G.K. Gilbert recognized the fault block nature of the Basin Ranges, and began his study of Lake Bonneville during field work with the Wheeler survey; he established in its publications the basis for his later career.

Ductile deformation zones in Blue Ridge basement rocks and estimation of finite strains

The deformation of basement massifs in orogenic belts presents several fundamental problems—what is their mechanical response to deformation, how does the strain vary through them, and how does one explain the “missing” basement suggested by palinspastic reconstructions? The Blue Ridge anticlinorium in northern Virginia has at its core Precambrian granitic basement deformed during the late Paleozoic orogeny; there is a major cleavage fan across this anticlinorium. The deformation in the basement is inhomogeneous, being concentrated along narrow zones of ductile deformation where rocks are mylonitized. These zones may range in size from microscopic dimensions to tens of metres wide, and they tend to develop preferentially in certain basement rock types. Narrow deformation zones in coarse granitic rocks often occur in conjugate sets whose bisector is the regional cleavage. Integration of strain across these zones added to the penetrative strain, calculated from the rotation of these zones, and the shortening due to displacement on thrust faults, give the total shortening of the basement. Such shortening must be taken into account in attempting any kind of geologic reconstructions.

Pre-Jurassic Geologic Framework, Northern Gulf Basin: ABSTRACT

The early history of the Gulf basin is conjectural. It once was believed the basin formed by late Paleozoic foundering of Llanoria, a postulated large offshore landmass occupying much of the present basin area. End_Page 1840------------------------------ Currently, there are two schools of thought: (1) the basin has existed since late Precambrian; or (2) it was formed by early Mesozoic sea-floor spreading in the Gulf, a product of the general breakup of Pangea into continental blocks. Late Paleozoic orogeny, in phase with or a part of a west-southwestward continuation of Appalachian folding, created a northern structural rim for the basin which strongly influenced subsequent sedimentation and structural trends. Postorogenic tension faulting along and south of this rim was particularly active during the Triassic. Jurassic sediments along the flank and gulfward from the structural rim overlie this faulted basin floor and are in unconformable contact with rocks ranging in age from Triassic to Mississippian. Triassic sediments are fluvial to deltaic redbeds. Paleozoic deposits include both Ouachita facies and unmetamorphosed fluvial to offshore marine clastic rocks and highly fossiliferous shallow-water carbonate rocks. Seismic data suggest Triassic and/or late Paleozoic sediments underlie the Jurassic throughout the Gulf basin. These pre-Jurassic rocks comprise a large, very sparsely tested frontier for oil and gas. End_of_Article - Last_Page 1841------------

Palinspastic Map of Devonian Strata of Alabama and Northwest Georgia

Forty-four published structural sections were used to prepare a 1:500,000-scale palinspastic map of the outcrop traces of Devonian strata in Alabama and Georgia, restored to remove the effects of the late Paleozoic Alleghany orogeny. Preparation of the palinspastic map assumed a decollement under the Valley and Ridge province at the level of the Lower Cambrian Rome Formation. Estimated foreshortening of the deformed outcrop belts ranges from 11 mi (18 km) in the northeast to 53 mi (85 km) at the southwest termination of the Appalachians where they plunge beneath the coastal plain. The foreshortening increases from 17 percent in the northeast to 64 percent in the southwest. This is interpreted as evidence that the Appalachian orogenic trend has begun a deflection toward th west before plunging beneath the coastal plain, and also supports the hypothesis that the Appalachian orogenic belt swings farther westward and merges into the Ouachita orogenic trend.

Postorogenic Development of the Appalachians

W. D. Thornbury has pointed out the need for a re-examination of the idea of a persistent Appalachia east of the Appalachian geosyncline during the Paleozoic and Mesozoic eras, as a means of resolving the problems that still obscure the postorogenic drainage history of the Appalachian Mountain system. Such an examination indicates that the late Paleozoic Appalachian orogeny raised the Piedmont–Blue Ridge–Valley and Ridge mountain elements differentially to heights that exceeded the elevation of any residual eastern oldland that may have survived Paleozoic denudation and Appalachian orogeny. Late Triassic faulting was a factor in determinining the boundary between the Blue Ridge and Piedmont, but it did not alter the general direction or pattern of the drainage. That pattern, set by Permian deformation, was one of west-flowing streams in the south and of southeast-flowing streams in the north. Superposition of drainage in the fold belt is not regional but local, with demonstrable relations to specific structural or lithologic features, or both. The fold belt served as provenance for part of the clastic sediment in all formations of the Newark Group and for a substantial fraction of the clastic content of Lower and Upper Cretaceous and early Tertiary deposits in the Atlantic Coastal Plain from New Jersey to Virginia. Late Tertiary (Miocene?) marine transgression of brief duration accounts for marginal stream diversion and stream superposition in southern New England and part of the Newark trough–Piedmont in New York and New Jersey.