Major Structural Break Research Articles

Post-deformation, sulphide-quartz vein hosted gold ore in the footwall alteration zone of the Palaeoproterozoic Långdal VHMS deposit, Skellefte District, northern Sweden

The Palaeoproterozoic, c. 1.88 Ga old Långdal VHMS deposit is situated in the eastern part of the Skellefte District, northern Sweden. In the stratigraphic footwall to the VHMS ore a sulphide-quartz vein system with high gold grades was mined in the second half of the 1990′s. The Långdal VHMS ore is hosted by the uppermost part of the Skellefte Group volcanic rocks, close to the contact with an overlying fine-grained sedimentary unit. Regional structural studies indicate that bedding surfaces in volcanic rocks are parallel to the contact between the volcanic and the sedimentary rocks. Based on the differences in structural style on each side, the contact is interpreted as a major structural break. The Långdal ore is situated close to this break that may have focussed fluid flow during metamorphism and deformation. The orientation of the contact indicates that it either is a D2 structure or that it was at least active during D2. The structural development in the altered footwall rocks to the Långdal VHMS ore indicates that gold-bearing sulphide and sulphide-quartz veins both pre- and post-date the first deformation. Gold associated with the vein system can thus not only be attributed to syngenetic exhalative or replacement processes. The close spatial relationship with the massive sulphide deposits suggests, however, that the gold was remobilized from these syngenetic systems. It is concluded that sulphides were introduced at several stages during the geological evolution of the area as: a) syngenetic disseminations of sulphide and folded, pre-S1 stringer sulphide±quartz veins in the footwall related to the syngenetic VHMS deposit, b) syn-S1 sulphide veins in the footwall gold ore, c) main, post-S1, sulphide-quartz veins associated with the gold ore in the footwall rocks to the Långdal VHMS deposit, and d) post-S1 to pre-S2 galena and sphalerite rich veins post-dating the main, post-S1, sulphide-quartz vein system in the footwall to the Långdal ore. From these relationships it is suggested that gold was re-mobilized from the sulphide rich parts of the VHMS system into post-D1 structures during or slightly after the peak metamorphism. The timing of this event is poorly constrained to post-date the syngenetic ore emplacement by 20–80 m.y.

Alpine structural and metamorphic signature of the Sila Piccola Massif nappe stack (Calabria, Italy): Insights for the tectonic evolution of the Calabrian Arc

Combined structural and petrographical investigations, coupled with 40Ar/39Ar geochronology, were carried out in the Sila Piccola Massif of the Calabrian Arc in order to define the structural geometry and map out the major structural and metamorphic breaks within the exposed nappe sequence. On the basis of the contrasting Alpine pressure‐temperature (P‐T) and structural signatures the nappe stack can be divided in two major tectonic complexes, bounded by a flat‐lying ductile to brittle extensional shear zone. The upper complex consists of a nappe‐like structure, where a major top to the east compressional shear is recorded. The lower tectonic complex consists of an ophiolite‐bearing sequence showing typical high‐P/low‐T parageneses (Mg‐carpholite and Na‐amphibole). The 40Ar/39Ar geochronology on phengites in equilibrium with blueschist minerals provided a minimum age estimate for the blueschist event in the lower complex rocks at the Oligocene‐Eocene boundary (around 35 Ma). Ductile to brittle top to the west extensional shear accompanied the nearly isothermal retrogression and exhumation of the lower complex rocks, reworking the previous nappe contacts with shear localization along the upper/lower tectonic complex discontinuity. The 40Ar/39Ar dating indicates that this postnappe stacking tectonic evolution took place from 30 Ma onward. It is proposed that exhumation of the deep‐seated rocks occurred below a top to the west extensional detachment active during convergence and orogenic complex formation (synorogenic extension). The age of this detachment is bracketed between 30 Ma and the post‐orogenic Neogene basin sedimentation (middle‐upper Miocene). The revised structural and metamorphic scenario is here integrated into a new tectonic evolutionary reconstruction, which involves an early high‐P/low‐T top to the east crustal thickening episode during the construction of the Apennine orogenic wedge (Eocene‐Oligocene), followed and overprinted by a top to the west extensional shear, probably active from the late Oligocene.

Testing for the presence of a terrane boundary within Neoproterozoic (Dalradian) to Cambrian siliceous turbidites at Callander, Perthshire, Scotland

The Southern Highland Group (Dalradian) and Keltie Water Grit Formation, which includes the Lower Cambrian Leny Limestone, form an inverted, 1.4 km thick, largely arenaceous, sequence at Callander. The grits have the same detrital mineralogy throughout, mainly quartz, plagioclase (An 1–3 ), muscovite, and biotite. Chlorite formed from detrital biotite during low-grade regional metamorphism ( T <270°C). There are some vertical changes in major element (but not trace element) chemistry of the grits, and detrital muscovites have a wide, but comparable, range in composition throughout, apart from an influx of Na-rich micas in the Keltie Water Grits. 40 Ar/ 39 Ar laser fusion dating of detrital muscovites yields an age spectrum with a peak at 1600–1800 Ma in the Dalradian rocks; similar old ages occur in the Keltie Water Grits but are diluted by ages of 507–886 Ma. We interpret these new data as showing that the rocks were most likely deposited as a single sequence, possibly with a disconformity, in Neoproterozoic to Early Cambrian times, before the onset of Grampian orogenesis in the Early Palaeozoic. No major structural or stratigraphical breaks have been identified and there is no direct evidence for the presence of two separate terranes.

A comparison of selective extraction soil geochemistry and biogeochemistry in the Cobar area, New South Wales

In parts of the deeply weathered and semi-arid environments of the Cobar area (NSW, Australia), detection of mineralisation using conventional soil sampling and total metal analysis is impeded. This is due to the intense leaching of trace elements within the weathered profile, discontinuous coverage of transported materials and the existence of diffuse regional geochemical anomalies of ill-defined source. Selective chemical extractions, applied to various regolith components, and biogeochemistry offer a means of isolating localised geochemical patterns related to recent dispersion of trace elements through the overburden. Lag geochemical patterns across the McKinnons deposit (Au) and Mrangelli prospect (Pb–Zn–As) reflect mechanical dispersion processes and minor hydromorphic effects. Concentrations of more mobile elements tend to be higher in the non-magnetic fraction, due to higher proportions of goethite and poorly crystalline hematite than in the magnetic fraction. The subdued soil geochemical responses for metals extractable by cold 40% hydrochloric acid (CHX) and for total element concentration reflect the leached nature of the residual profile, low grade of mineralisation, dilution by aeolian components and disequilibrium of fine fractions with coarser, relict Fe-oxides. The stronger contrast for CHX for most metals, compared with total extraction, indicates surface accumulation of trace elements derived from underlying mineralisation. Enzyme leach element anomalies are intense but generally located directly over bedrock sources or major structural breaks, irrespective of the nature of the overburden. Though mechanisms for the dispersion of trace elements extracted by enzyme leaching are not well established, the lack of lateral transport suggests vertical migration of volatile metal species (atmimorphic dispersion). The strong, multi-element response to mineralisation in cypress pine needles indicates significant metal recycling during the present erosional cycle. However, a comparison of the trace element concentrations in vegetation (cypress pine needles) and selective extractions of soils indicates that recycling by the plants is not the dominant mechanism for transportation of metals through the overburden. The vegetation may be responding to hydromorphic dispersion patterns at depth. The use of selective extractions may be useful in detecting mineralisation through deeply leached profiles, but offers even greater potential when integrated with biogeochemistry to detect targets buried by significant thickness of transported cover.

Magnetic signature of upper plate structures and subducting seamounts at the convergent margin off Costa Rica

Offshore the Pacific margin of Costa Rica the Caribbean Plate converges with the subducting Cocos Plate along the Middle America Trench. The tectonics of both plates have been studied on the basis of reflection seismic, gravimetric, and magnetic data and swath‐mapping. The magnetic data were acquired by GEOMAR, Kiel, Germany, during cruise SO‐76 of R/V Sonne off Costa Rica in 1992. Two different magnetic anomaly patterns are observed. Off Nicoya Peninsula the linear magnetic anomalies trend about N50°E and can be traced from the ocean basin to the landward end of the profiles. To the southeast of a major structural break trending parallel to Fisher Seamount and the associated ridge the strike of the magnetic anomalies is N70°E. In the area southeast of Fisher Seamount the magnetic signatures of numerous seamounts are superimposed upon the linear anomalies. We interpret the magnetic lineations as chrons 5C to 6B (16–22.5 Ma). Magnetic modeling demonstrates a significant magnetization (∼1 A/m) of the margin wedge in the continental slope, a structural unit known from seismic studies. This is not compatible with the previously discussed origin of the continental slope as an accretionary wedge. A composition of the margin wedge similar to the ophiolitic rocks found onshore on Nicoya Peninsula appears likely. A number of morphological furrows observed in the landward trench slope opposite the seamount covered domain of the oceanic crust coincide with magnetic anomalies. Three‐dimensional magnetic modeling demonstrates that these anomalies can be explained well by subducting seamounts. This implies that the seamounts remain intact, at least magnetically, for a long time interval during the subduction process. Thus they may indeed be important seismogenic asperities.

Fractal geometry of the sumatra active fault system and its geodynamical implications

The Sumatra active fault system is a 1650 km long northwest trending dextral active strikeslip fault which accommodates the oblique convergence between the Indo-Australian and the Eurasian plates. It consists of 11 fault segments connected northward to the Andaman extensional back arc basin and southward to the extensional fault zone of the Sunda Strait. The geometries of the 11 segments are quantified using a fractal approach and it is found that their fractal dimensions (D) range from 1.00 ± 0.03 to 1.24 ± 0.03. Larger D values are associated with more irregular fault geometry. There are six discontinuities, reflected by sharp changes of fractal dimensions, observed along the segments. The locations of the discontinuities correspond to sites of major structural breaks in Sumatra mainland and its fore arc and back arc. The discontinuities and variations of D values suggest that the Sumatra mainland is not rigid. Instead it appears to be segmented into several blocks which may correlate to the segmentation in the Sumatra fore arc. This segmentation may explain the large discrepancy among the displacements and velocities of the Andaman Sea opening, the Sumatra fault motion and the Sunda Strait opening. This research also demonstrates the applicability of the fractal approach for analyzing the variations of fault geometry due to geodynamic processes.

The expectations theory of the term structure: a cointegration/causality analysis of US interest rates

This study analyses monthly US Treasury short and long term interest rate data over the period 1950-82. We test for Granger causality between the short and long rate series in the context of an appropriately formulated vector autoregression which takes account of the non-stationarity of the data, and possible cointegration between the series. Unlike earlier studies, an allowance is also made for the seasonality of the monthly data, and special attention is paid to the presence of a major structural break in the data. Bootstrap simulation techniques are used to obtain critical values for unit root and cointegration tests which allow for this structural change. We find strong evidence that long rates caused short rates over the sample period, and some evidence of causality in the reverse direction. This supports the expectations theory of the term structure.

Metamorphic zones, terranes, and Cenozoic faults in the Marlborough Schist, New Zealand

Abstract The northeast‐striking Picton Fault Zone is a major structural and metamorphic break that divides the Marlborough Schist into two separate blocks. The northwestern (Kaituna) block shows a regular increase in textural and metamorphic grade to the southeast from prehnite‐pumpellyite to greenschist facies and textural zone I to IV. The southeastern (Arapawa) block shows an irregular increase in grade to the southeast from prehnite‐pumpellyite to pumpellyite‐actinolite facies and textural zone I to IIB. The Kaituna block is correlated with the Caples and Torlesse Terranes and the Otago Schist of the southern South Island. The Arapawa block is correlated with the Waipapa Terrane (possibly also the Torlesse Terrane) and Kaimanawa Schist of the North Island. Isotects and the Picton Fault Zone are dextrally offset by 10–15 km across the ENE‐striking Queen Charlotte Fault Zone. The latter should be included with the late Cenozoic Marlborough faults.

A note on the stationarity of the primary commodities relative price index

The Prebisch-Singer hypothesis of a secular decline in the relative price of primary commodities is investigated searching for the presence of a deterministic rather than stochastic trend and controlling for major structural breaks. The evidence is in favour of a deterministic trend with a negative slope for the largest part of the century and a major structural break at the end of World War One.

Prosody as a cue to syntactic structure: The role of prosodic information in disambiguating phonetically ambiguous sentences

To go beyond dictation tasks, speech recognition needs to be integrated with an understanding component. Natural language understanding research, however, has typically considered text input, and has not had to face the massive phonetic ambiguity arising from speech input: Even with no phonemic errors, word boundary ambiguities can yield thousands of parses for a substantial portion of moderate length sentences. This study assesses the potential role of prosody in aiding speech understanding. Using phonetically similar sentence pairs with and without a major structural break, seven types of structural ambiguity were investigated. Professional speakers read ten sentences of each type after reading diambiguating contexts. Naive listeners selected the appropriate context for each sentence. For recording and listening, only one member of each pair was used in each session; sentences were otherwise randomized. The results indicate substantial differences across structural types: e.g., appositions and tags were nearly always correctly identified, whereas main‐main versus main‐subordinate pairs were at the chance level. This suggests that prosody can disambiguate some, but not all, structural ambiguities. [Work supported by NSF.]

Pre‐Caledonian basement in Ireland and its cover relationships

AbstractPre‐Caledonian basement is exposed in three areas within the Irish orthotectonic Caledonides: 1. In northwest County Mayo the Erris Complex comprises the Annagh Division, which is largely Grenvillian but includes older gneisses, and the Inishkea Division, which is probably equivalent to the older Moines of Scotland. 2. In the northeast Ox Mountains, Rosses Point and Lough Derg inliers there is a granulite fades, dominantly metasedimentary basement which is probably late Grenville in age. 3. Laxfordian and probably older gneisses are seen in the Inishtrahull Platform northeast of Malin Head, County Donegal.In addition, some gneisses within the belt of ‘Connemara migmatites’ in south Connemara may be fragments of allochthonous basement and some high grade metamorphic rocks in the Tyrone Central inlier have been referred to as pre‐Caledonian basement, but there is at present no conclusive support for these suggestions. The Ox Mountains succession seen in the Clew Bay region and the southwest and central Ox Mountains is probably not pre‐Caledonian basement.Basement in the paratectonic Caledonides is confined to the Cadomian and possibly pre‐Cadomian Rosslare Complex (older than 625 Ma) and the nearby late Precambrian (to early Cambrian ?) Cuilenstown Formation in southeast County Wexford. In addition, an unseen block of basement defined by magnetics immediately to the north of the Dingle peninsula of County Kerry is probably of Rosslare Complex affinity. Granulite facies xenoiiths recovered from volcanic rocks of Carboniferous age about 70 km to the west of Dublin, indicate the character of basement there, though its age is not proven.All contacts between basement and Lower Palaeozoic Caledonian cover in the orthotectonic Caledonides are synmetamorphic slide zones and faults and in the paratectonic Caledonides the contacts are either faults, which are often rooted in older mylonites, or are unexposed and presumed to be major structural breaks.

Stratigraphy of the Upper Devonian argillite successions in north Cornwall

AbstractSome 880 m of grey, green and purple argillites are estimated to be present in complexly deformed Upper Devonian successions which occur in several overlapping coastal sections. Four lithostratigraphical formations are formally defined, in order of decreasing age: the Marble Cliff Limestone; Longcarrow Cove Slate; Harbour Cove Slate; and Polzeath Slate Formations. Improved correlation and biostratigraphical control is available for the two younger formations which are studied in detail. The formations range in age from lowest Frasnian to mid Famennian. Despite the intense deformation there are no major structural breaks in the successions.

The distribution and implications of heat flow from the gregory rift in kenya

Data from geothermal mapping along the Kenyan Gregory Rift zone using ground and aerial infrared techniques indicate that an average of 11–30 MW/km are advectively emitted along the rift. The bulk of the advected heat is lost through the central part of the Kenyan dome. Two other secondary geothermal sites are at the northern and southern limits of the rift adjacent to rift bifuration zones. These three geothermal sites are separated by ≈ 300 km. Active thermal fields occur only 1 7 as frequently as rift volcanoes. Geothermal spacing could be controlled by the length of the heat source at the lithosphere-upper mantle boundary, the thickness of the crust and the length of the rift segment between major structural breaks. At faster spreading centers the heat source is probably more continuous and following Elder's models should produce upwelling domes much more frequently than along the East African Rift. Advective cooling on one section of the East Pacific Rise at 21°N indicates that domes should appear every 40–80 km. If this is the case then approximately four times more heat is given off along a kilometer of East Pacific Rise crest as it is emitted along the Gregory Rift.

Major Structural Break between Paleozoic and Mesozoic Rocks in the Eastern Sierra Nevada, California

A major structural break lies between the Paleozoic and Mesozoic rocks in the eastern Sierra Nevada. Tight folds in the Paleozoic section are overturned to the west and faulted against an apparently downthrown steeply dipping sequence of Mesozoic metavolcanic rocks on the west. This fault is a segment of a large regional high-angle fault that separates the Paleozoic and Mesozoic strata in the eastern Sierra Nevada from the Pine Creek pendant northward approximately 50 mi (80 km) to Mount Dana. The development of this fault preceded the widespread intrusions of the Upper Cretaceous magmas of the John Muir sequence of the Sierra Nevada batholith.

Geology of the region between the Matusevich and Tucker Glaciers, north Victoria Land, Antarctica

Abstract An outline is given of the geology of the region between the Matusevich and upper Tucker Glaciers in north Victoria Land, Antarctica. The Rennick Glacier lies along a major structural break between high-grade metasediments and plutonic rocks to the west and low-grade metasediments to the east. The regional strike of foliation in both groups of metasediments is north-west. West of the Rennick Glacier, the basement complex consists of high-grade, regionally metamorphosed Precambrian paragneiss and schist of the Wilson Group intruded and migmatised by granite, granodiorite, tonalite, and pegmatites of the Granite Harbour Intrusives. East of the Rennick Glacier, the Robertson Bay Group, a thick late Precambrian geosynclinal sequence of interbedded greywacke and cleaved argillite, extends throughout the Admiralty Mountains to the Robertson Bay — Tucker Glacier area. In the Bowers Mountains, the Bowers Group, an infaulted dominantly shallow-water sequence of interbedded basic volcanics, volcanic-derive...

Geology and Structure of the Tip of Baja California, Mexico

The submarine geology of the continental margin at the tip of the peninsula of Baja California, Mexico, has been mapped through the use of continuous reflection profiling. A sequence of folded and faulted sediments, correlated with the Pliocene Salada Formation, unconformably overlies the volcanics and pyroclastics of the Comondu Formation and the granitic rocks of the Baja California pluton. Thirty-five dredge hauls, collected by Shepard from submarine canyons in the region, have substantiated these correlations. The structure of the continental terrace is dominated by normal faulting, trending northeast-southwest, which suggests uplift of the southern part of the peninsula. Cabrillo Seamount, lying to the southeast, is a detached granitic fault block. The fault pattern controls the trends of the submarine canyons, which have supplied up to a kilometer of probable Quaternary sediments to the San Lucas Fan system at the foot of the continental slope. The La Paz fault is a major structural break at the western side of the Cape Region. Uplift of the region with associated normal faulting and strike-slip movement along the La Paz fault are related to formation of the Gulf as the continent overrode the East Pacific Rise since the late Miocene. The northern part of the Gulf appears to have opened in the late Miocene or early Pliocene, while the mouth did not open until late Pliocene.

THE SOUTH AUSTRALIAN CONTINENTAL SHELF AS A HABITAT FOR PETROLEUM

The Continental Shelf off South Australia is predicted to be a preferred habitat of petroleum. Non-metamorphosed sediments which are known or presumed to extend onto this shelf include Permian, Cretaceous and Cainozoic sediments. A single presumed Permian trough lying seaward of, and sub-parallel to, the Coorong is interpreted (aeromagnetic data) to extend to possibly 4,000 feet. As part of the Permian fossil valley system of central-eastern South Australia is is predicted to be filled by shales and sands with limited marine facies, and by glacigene sediments. The north-south orientation of the trough would require a landmass to the south from which the Permian glaciers originated. Some form of continental drift may be required to satisfy this requirement.An east-west graben or geosyncline (Gambier-Otway Basin) choked with Cretaceous mudstones and sub-greywackes projects across the shelf in the extreme south-east. Landward dips at depth extending well out beneath the shelf, may suggest a southerly boundary to the basin consistent with a pre-existing landmass in this direction.Extensive erosion at the close of Lower Cretaceous times accords with major geological change at least in more southerly shelf areas. Marine sedimentary onlap and overlap become conspicuous elements, and the accumulation of oceanward thickening sedimentary deposits is more in keeping with outbuilding terrace development. A major structural break accordingly is indicated at the end of the Lower Cretaceous, at which time development of the modern continental shelf began.The Murray River had its principal outlets in the extreme south-east zone during mid-Cretaceous to early Tertiary times, but shifted to its present position during late Cainozoic at which time its sediments were swept across the developing shelf south of Kangaroo Island. Thick sedimentary developments in each of these areas provide promising environment for petroleum generation. These are areas of extensive coastal bitumen activity.Principal petroleum prospects are to be expected within the Middle Cretaceous to Tertiary developments, but older Cretaceous and Permian deposits are potentially prospective.

STRATIGRAPHY AND STRUCTURE OF THE KINDERHOOK QUADRANGLE, NEW YORK, AND THE “TACONIC KLIPPE”

Detailed mapping of the predominantly argillitic Cambro-Ordovician “Taconic sequence” in the Kinderhook quadrangle, New York, indicates that the Chatham thrust divides these rocks into two distinctive sequences. To the east slates predominate and form a hilly upland, but west of this thrust less resistant shales underlie a gently rolling lowland. Stratigraphically, the eastern sequence is composed of a transgressive black Trentonian (Middle Ordovician) slate, locally with a thin basal limestone, resting upon a thick series of Lower Cambrian green slate, graywacke, quartzite, limestone, and limestone conglomerate (Rensselaer-Nassau-Schodack). Canadian carbonate rocks crop out intermittently along the Chatham thrust, and they are interpreted as slivers trapped during thrusting. The western sequence is primarily Normanskill (Middle Ordovician) and Deepkill (Lower and Middle Ordovician) shale, unconformably underlain by Lower Cambrian shale and overlain by Siluro-Devonian limestone. Though many small thrusts are present, folding is the principal mode of deformation. The Chatham thrust, however, is a major structural break and continues at least 20 miles southwest of the quadrangle and an unknown distance northward. It is a post-middle Trentonian feature, and maximum displacement is a few miles rather than tens of miles. Unconformities are very important in the geologic history of this area, but some have gone undetected because of the structural complexity and poor exposure. The following formations are believed to have basal unconformities in the Kinderhook quadrangle: the Upper Silurian Manlius limestone, the Trentonian black slate (locally with a basal limestone), the Deepkill shale, and the Canadian carbonate rocks. Evidence for the existence of the “Taconic klippe” was not found in mapping this quadrangle. Analysis of the development of the klippe hypothesis indicates that it is based principally upon stratigraphic considerations; available structural evidence weighs against this interpretation. While the klippe hypothesis seems to explain well the relations at the north end of the Taconic Range, the problem of adequately defining the boundaries of this “klippe” causes serious doubt about its existence. An alternative interpretation of the regional relations is suggested, involving unconformities and facies changes in a single indigenous sequence. Trentonian rocks lie unconformably on rocks as old as Precambrian from Vermont to Pennsylvania and pass indiscriminately in and out of the “Taconic klippe.” The Normanskill and Deepkill rocks (mainly shale) are interpreted as passing transitionally into limestone to the west; an analogous situation exists in the Middle Ordovician rocks of Virginia. The Deepkill is believed to rest unconformably on rocks of Early Cambrian to middle Canadian age. Middle Canadian formations in the Kinderhook quadrangle are carbonate rocks and appear to rest unconformably upon Lower Cambrian slates; their striking similarity to equivalent rocks in the near-by “autochthonous” series suggests they have not been displaced any great distance. The Lower Cambrian is a thick series of argillite, gray-wacke, and quartzite with some thin carbonate rocks near the top. The thick, lower part of this series is considered a southward continuation of the Mendon Series of Vermont. The upper strata are interpreted as the offshore equivalents of shallow-water quartzites and carbonate rocks deposited marginal to an eastern welt in later Early Cambrian time.

Crustal shortening and axial divergence in the Appalachians of southeastern Pennsylvania and Maryland

Calculation of crustal shortening ratios in the northern Appalachians has been undertaken on the basis of continuous exposures provided by the anthracite fields of Pennsylvania. The ratios increase from northeast to southwest with a maximum approximately in the Harrisburg section. The increase is accompanied by convergence of axes from the northeast toward the Susquehanna River, and consequently the Appalachian trends bulge northwestward. An arc is formed with Baltimore approximately at the center of curvature. In the same measure as the folds crowd northwestward thrusts appear in the crystalline axis and involve the Glenarm series and the lower Paleozoic rocks. The gneiss basement does not participate with equal intensity in the folding, and an axial divergence between basement and cover becomes visible. Most of the “Appalachian folding” seems to have taken place above the gneiss complex and the basement is only superficially involved. The Catoctin volcanic complex and the Glenarm series participate in the Appalachian trend. This seems to indicate a major structural break above the Baltimore gneiss complex.