Thrust Block Research Articles

Surficial geology and geomorphology of the Brúarjökull surging glacier landsystem

Abstract Please click here to download the map associated with this article. A 1:30,000 scale map poster of the snout and proglacial landscape of the surging Icelandic glacier Bruarjokull, provides a spatial and temporal assessment of the geomorphic impacts of surging in glaciated terrains. Based upon aerial photography from 1998, 1999 and 2000, the map identifies the major landforms that are regarded as diagnostic of glacier surging when viewed in a landsystem framework; specifically, thrust block and push moraines, overridden thrust block moraines, zig-zag eskers, crevasse squeeze ridges, long utings, hummocky moraine and ice-cored, pitted outwash. This landscape imprint, when identi_ed in ancient glaciated terrains, can therefore be used as indicative of surge activity in palaeoglaciological reconstructions.

Arrangement for damping of structural resonance

A hull (11) has mounted thereon a number of accelerometers (20) arranged to provide a data signal corresponding to movement or vibration of the hull (11) to which each is fixed. The hull (11) and/or a thrust block (18) has mounted thereon a number of mass dampers (21) that are controlled by a detector (23) arranged between the accelerometers (20) and mass dampers (21). The detector (23) calculates from the data signal generated by the accelerometers (20) different modes of resonance so as to determine the onset of the excitation of a particular resonance mode and controls the mass dampers (21) so as to generate forces to selectively dampen the resonance mode.

Structural curiosity of the northern Istria

In northern Istria certain structural particularities exist that till now were not explained, and some of them not even known. They are all associated with underthrusting of the Adriatic microplate (Istra) under the Dinarides. The Rokava fault of cross-dinaric direction was discovered that exerts an important influence on hydrographic pattern in this part of Istria. The cross-dinaric sinistral strike-slip faults above Glinščica stream (Rosandra) are of local importance only, since at Kastelec and Črni Kal the dextral strikeslip faults of NNW-SSE direction occur as their conjugated equivalents. This particular feature has been named the Gročana (Grozzana) structure. The reason for such a structure is the echelon folding (complete folding) in the Istrian hinterland (the Kras-Notranjsko folded structure). In the Tinjan area a wedge shaped block occurs between two thrust faults that are turned up and quite steep owing to secondary folding. The wedge is open northwestward, and the flysch beds within it are folded in the cross-dinaric direction. They represent an indication of lateral squeezing in a limited area, since in the close-by roadcut of the Ankaran-Trieste motorway no trace of these folds remains (the Tinjan structure). In the Izola and Strunjan area a complex overthrust structure is present that rapidly wedges out toward central Istria. Indirect data suggest that it extends northwestward below the Bay of Trieste. The elements of this structure are the Izola thrust fault with Izola anticline in the thrust block, and folds at Strunjan that were formed in the belt between the reverse overthrusts in the Strunjan bay and Izola thrust fault area (the Strunjan structure).

The Central Southern Alps (N. Italy) paleoseismic zone: a comparison between field observations and predictions of fault mechanics

The internal sectors of the Orobic Alps (Northern Italy) are characterised by Alpine age regional shortening showing a transition, through time, from plastic to brittle deformation. Thrust faults cut Alpine ductile folds and are marked by cataclasites and, locally, by pseudotachylytes, suggesting that motion was accommodated by seismic frictional slip. In the Eastern Orobic Alps the thrusting initiated at depths deeper than 10 km (the emplacement depth of the Adamello pluton) and possibly continued at shallower depths. This demonstrates that thrust motion occurred between 10 km depth and the brittle-ductile transition, i.e., at mid-crustal depths. The Orobic Alps exhumed paleoseismic zone shows different geometries along strike. In the central sectors of the Orobic Alps, thrust faults, associated with pseudotachylytes, have average dips around 40° and show no pervasive veining. Much steeper thrusts (dips up to about 85°) occur in the eastern Orobic Alps. In this area, faults are not associated with pervasive veining, i.e., fluid circulation was relatively scarce. This suggests that faulting did not occur with supralithostatic fluid pressure conditions. These reverse faults are severely misoriented (far too steep) for fault reactivation in a sublithostatic fluid pressure regime. We suggest that thrust motion likely started when the faults were less steep and that the faults were progressively rotated up to the present day dips. Domino tilting is probably responsible for this subsequent fault steepening, as suggested by a decrease of the steepness of thrust faults from north to south and by systematic rotations of previous structures consistently with tilting of thrust blocks. When the faults became inclined beyond the fault lock-up angle, no further thrusting was accommodated along them. At later stages regional shortening was accommodated by newly formed lower angle shear planes (dipping around 30–40°), consistently with predictions from fault mechanics.

Minimization of the vibration transmission through the propeller-shafting system in a submarine

Ships and submarines are efficient sources of underwater radiated noise. In the low-frequency range, the main sources of ship vibration and noise are the working machinery such as the diesel engines, gearboxes and generators, and the propeller/propulsion system. Rotation of the propeller results in discrete tonals at the blade passing frequency and its harmonics. In addition, in the case of a submarine, hydrodynamic forces acting on the propeller due to the fluctuating pressure field are transmitted through the propeller-shaft and thrust bearings, resulting in axial excitation of the submarine hull. A hydraulic dynamic absorber, also known as a resonant changer, is used to minimize the vibration transmission through the propeller-shafting system, in order to prevent excitation of the hull axial resonances. The present work is concerned with minimizing the vibration transmission to the hull by optimizing the virtual spring, mass, and damper parameters associated with the resonant changer in a submarine. The dynamic response of the propeller-shafting system, including the propeller shaft, thrust bearing, thrust block foundation, and resonant changer, are characterized using four pole parameters.

Stratigraphy and correlation of Carboniferous ignimbrites, Rocky Creek region, Tamworth Belt, Southern New England Orogen, New South Wales

Carboniferous (Visean to Westphalian) pyroclastics and lava flows in the Rocky Creek region, used to redefine the base of the Kiaman reversal, are formally defined or redefined and the status of the main formations clarified. These units include the Caroda Formation, containing the Kooringal Dacite, Boomi Rhyolite and Barney Springs Andesite Members; the Clifden Formation with the Wanganui Andesite, Glen Idle Rhyolite, Appleogue Dacite, Bexley Rhyolite, Pine Cliffs Rhyolite and Downs Rhyodacite Members; Rocky Creek Conglomerate with the Hazelvale Rhyodacite, Mt Hook Rhyolite, Darthula Rhyodacite and Pound Rock Rhyodacite Members; and Lark Hill Formation with the Eulowrie Pyroclastic, Tycannah Rhyodacite and The Tops Rhyolite Members; a number of informal units are also described. The restriction of most volcanic units to one of the three thrust blocks (Boomi, Kathrose and Darthula blocks) of the Rocky Creek region, suggests their current relationships reflect either shortening due to overthrusting or an original distribution affected by depositional or erosional processes. A westerly increase in the proportion of ignimbrites indicates nearness to sources in that direction. Intermediate volcanism, largely confined to southern and central parts of the Boomi block in the east, began in the Visean and ended in the early Namurian. Acid volcanism also began in the Visean in the northern Boomi block but, with the exception of the Peri Rhyolite Member of the Clifden Formation, did not become widespread until later in the Namurian and Westphalian. In contrast, only acid volcanism took place during the early Namurian to Westphalian in the Kathrose and Darthula blocks. Correlations based on AS3 and SL13 SHRIMP dates illustrate a discordance of about 3% when compared with the most likely location for the base of the Kiaman reversal. The bases of both the Rocky Creek Conglomerate and Lark Hill Formation appear to be slightly diachronous.

Duplex style and triangle zone formation: insights from physical modeling

Duplexes are a common feature in thrust belts at many scales. Their geometries vary significantly from antiformal stacks with significant forethrusting in the cover (e.g. southern Pyrennes, Spain) to triangle zones (e.g. foreland Canadian Rockies) to low-displacement individually spaced ramp-anticlines (e.g. Sub-Andean thrust belt, Bolivia). We present a series of physical experiments demonstrating that the strength of the décollements relative to that of the intervening and overlying rock layers plays a significant role in controlling the duplex style. The models comprise brittle layers made of dry quartz sand and décollements made of two types of viscous silicone polymers. The strength of the décollements in the models is a function of the shortening rate applied to the model. The relative strength of the décollements and surrounding rocks affects the development of active- or passive-roof duplexes (triangle zones). It also affects the amount of translation of individual thrust blocks and the spacing of thrust ramps, which in turn determine if a duplex evolves into an antiformal stack or into individually spaced ramp-anticlines. Model results indicate that specific associations of structural features form systematically under similar rheological and boundary conditions. The presence of relatively strong décollements promotes local underthrusting of the cover, individual ramp-anticlines, internal deformation of thrust sheets, low early layer-parallel shortening, and sequential towards-the-foreland propagation of structures. Weak décollements promote forethrusting of the cover, antiformal stacks, coeval growth of structures, and low internal strain, with the exception of significant early layer-parallel shortening. No underthrusting at a regional scale occurred in any model.

Hydrotectonic regime at soles of overthrust sheets: textural and fluid inclusion evidence from basal cataclasites of the Muráň nappe (Western Carpathians, Slovakia)

Newly formed, synkinematically grown minerals in basal cataclasites of the thin-skinned Muráň nappe contain primary fluid inclusions which represent samples of a hydraulic lubricant of the overthrusting nappe block. The inclusion fluid corresponds to a high-salinity brine (37–58 wt% NaCl eq.) of the system Na-K-Ca-Mg-Cl-SO 4-CO 2. Trapping temperatures between 213 and 471 °C are substantially higher than weak metamorphic imprint of the overlying thrust body. Strongly fluctuating inclusion fluid densities and trapping pressures between 0.2 and 5.4 kbar are interpreted to reflect locally supralithostatic overpressures induced by frictional heating beneath the moving nappe. The results of this study corroborate the hypothesis of Hubert and Rubey [Bull. Geol. Soc. Am. 70 (1959) 115–206], who envisaged a profound effect of pore fluid pressure at the base of moving thrust blocks.

Source Parameters of Regional Earthquakes in Taiwan: 1999-2000 Including the Ch-Chi Earthquake Sequence

We report source parameters of 277 earthquakes that occurred during 99.2000 in the Taiwan region, a time period dominated by the Chi-Chi earthquake sequence. We used an improved CMT inversion algorithm to handle high background noise and structural heterogeneity from complex tectonic settings near Taiwan. We also searched the best fitting solutions at an increment of 1 km of focal depths for all events that occurred after the :hi-Chi main shock on September 20, 1999. In general, central and northern Taiwan was seismically quiet before the Chi-Chi main shock. Aftershocks of this sequence dominated the regional seismicity for approximately 3 months afterward. The Chi-Chi sequence consists of a wide range of focal mechanisms, but the overall pattern is a west-verging thrust block bound strike-slip tear faults near the northern and southern termini of the surface rupture – the Chelungpu fault. Seismicity outside of the source zone of the Chi-Chi sequence generally agrees with known active tectonics from previous studies. Nonetheless, under high elevations of the eastern Central Ranges, there are several intriguing cases of changing focal mechanisms from normal faulting near the surface to thrust at depth. To facilitate further research of the Chi-Chi sequence, results of our inversion and original waveforms from the Broadband Array in Taiwan for Seismology (BATS) of 170 events within this sequence are ready for distribution on CD-ROM. The dataset is also online at the BATS web site (http://bats.earth.sinica.edu.tw/Chi-Chi_CMT).

Crustal section based on CROP seismic data across the North Tyrrhenian–Northern Apennines–Adriatic Sea

Using deep seismic reflection data from the Italian lithospheric exploration project CROP in the Central Mediterranean region, a 400-km-long section, composed of three different profiles crossing the Northern Tyrrhenian Sea (CROP M-12A profile), the Northern Apennines (CROP-03) and the Adriatic Sea (CROP M-16) is reconstructed and discussed. New data allow us to outline a seismically consistent tectono-stratigraphic setting for the crust and upper mantle of the Northern Apennines thrust–belt system and its Adriatic foreland. Time–space analysis of the deformation of the investigated chain and identification of existing macrostratigraphic crustal intervals and tectonic units allow a reasonably controlled interpretation of the geodynamic evolution and of the main orogenic stages. Careful seismic reprocessing and application of advanced techniques to key zones of the explored area (such as the Tuscan Archipelago) were determinants in obtaining fundamental information for understanding of the complex lithospheric structures and their evolution. Profile interpretation supports that the Northern Apennine chain is dominated by a compressive thrust system. Crustal extension, assumed by some authors as the dominating tectonic process for the whole Tuscan Apennine area, represents a subordinate geodynamic event of the last stage (Tyrrhenian). In the Early Cretaceous–Late Jurassic, the paleogeographic framework consisted of the Europe and Adria plates separated by the Alpine Tethys Ocean. During the Late Cretaceous–Early Eocene, Adria–Europe convergence (eo-Alpine stage) and subduction beneath the Adria plate closed the Alpine Tethys Sea, with the Tethyan slab being clearly seismically imaged. The first Apenninic geodynamic stage occurred in the Late Oligocene–Early Miocene with the opening of the Balearic Basin, which generated a first “lithospheric root” of the Apenninic chain in the Tuscan Archipelago area. This root is represented by Adria-verging thrust faults that progressively flatten eastward. Upper parts of the west-verging eo-Alpine thrust blocks were truncated by the east-verging thrust faults of the Balearic stage. A deeper seismic reflector, attributed to the top of the asthenosphere, forms a mantle high below the Elba Island. From the Late Miocene to Present, the Corsica basin and western hinterland area were affected by extensional tectonics related to the Tyrrhenian opening, whereas compressional tectonics continued in the eastern hinterland and mostly on the eastward migrating foreland, with development of a second “lithospheric root” constituted by high-angle thrust faults. These faults give rise to a huge basement culmination below the main Apennines watershed. Impressive E-directed gravity-sliding of sedimentary blocks over their sloping basement occur, generating the Umbria–Marche shallow seismicity. Crustal shortening of the Apennines system amounts to 170 km, 14 km of which are due to the eo-Alpine stage, 71 km to the Balearic and 85 km to the Tyrrhenian one. In the frame of Africa–Europe convergence, the Tyrrhenian–Apennines tectonodynamics were mainly conditioned by the Mesozoic paleogeography.

Fluid transients and fluid-structure interaction in flexible liquid-filled piping

Fluid-structure interaction in piping systems (FSI) consists of the transfer of momentum and forces between piping and the contained liquid during unsteady flow. Excitation mechanisms may be caused by rapid changes in flow and pressure or may be initiated by mechanical action of the piping. The interaction is manifested in pipe vibration and perturbations in velocity and pressure of the liquid. The resulting loads imparted on the piping are transferred to the support mechanisms such as hangers, thrust blocks, etc. The phenomenon has recently received increased attention because of safety and reliability concerns in power generation stations, environmental issues in pipeline delivery systems, and questions related to stringent industrial piping design performance guidelines. Furthermore, numerical advances have allowed practitioners to revisit the manner in which the interaction between piping and contained liquid is modeled, resulting in improved techniques that are now readily available to predict FSI. This review attempts to succinctly summarize the essential mechanisms that cause FSI, and present relevant data that describe the phenomenon. In addition, the various numerical and analytical methods that have been developed to successfully predict FSI will be described. Several earlier reviews regarding FSI in piping have been published; this review is intended to update the reader on developments that have taken place over the last approximately ten years, and to enhance the understanding of various aspects of FSI. There are 123 references cited in this review article.

Neotethyan evolution of eastern Greece (Pagondas Mélange, Evia island) inferred from radiolarian biostratigraphy and the geochemistry of associated extrusive rocks

This paper presents new radiolarian biostratigraphic and igneous/metamorphic geochemical data for a Mesozoic volcanic–sedimentary mélange on the island of Evia (Euboea or Evvoia), eastern Greece. This mélange includes dismembered thrust sheets and blocks of radiolarian chert and basalt. Biostratigraphic age data show that radiolarites interbedded with basalt-derived, coarse clastic sediments near the base of a coherent succession were deposited in Middle and Late Triassic time (Late Ladinian–Carnian, Norian?). Geochemical evidence shows that associated extrusive rocks, of inferred Triassic age, range from ‘enriched’ alkaline basalts, to ‘transitional’ basalts, and more ‘depleted’ mid-ocean ridge-type basalts. Amphibolite facies meta-basalts from the metamorphic sole of the over-riding Evia ophiolite exhibit similar chemical compositions. Both the basalts and the meta-basalts commonly show an apparent subduction-related influence (e.g. relative Nb depletion) that may have been inherited from a previous subduction event in the region. The basalts are interpreted to have erupted during Middle–Late Triassic time (Late Ladinian–Carnian), related to initial opening of a Neotethyan ocean basin adjacent to a rifted continental margin. Radiolarites located stratigraphically higher in the coherent succession studied are dated as Middle Jurassic (Late Bathonian–Early Callovian). Similar-aged radiolarites are depositionally associated with ophiolitic rocks (including boninites), in some other areas of Greece and Albania. During initial ocean basin closure (Bajocian–Bathonian) the adjacent shallow-water carbonate platform (Pelagonian zone) disintegrated to form basins in which siliceous sediments were deposited and highs on which shallow-water carbonates continued to accumulate. This facies differentiation is seen as a response to crustal flexure as the Neotethyan ocean began to close. The over-riding Pagondas Mélange and other similar units in the region are interpreted as accretionary prisms related to subduction of Neotethyan oceanic crust in Middle–Late Jurassic time. These mélanges were emplaced, probably diachronously during Oxfordian–Kimmeridgian time, when the passive margin collapsed, creating a foredeep ahead of advancing thrust sheets of mélange and ophiolites.

Neotectonics of the southeastern Reelfoot rift zone margin, central United States, and implications for regional strain accommodation

Research Article| May 01, 2001 Neotectonics of the southeastern Reelfoot rift zone margin, central United States, and implications for regional strain accommodation Randel Tom Cox; Randel Tom Cox 1Department of Geological Sciences, University of Memphis, Memphis, Tennessee 38152, USA Search for other works by this author on: GSW Google Scholar Roy B. Van Arsdale; Roy B. Van Arsdale 1Department of Geological Sciences, University of Memphis, Memphis, Tennessee 38152, USA Search for other works by this author on: GSW Google Scholar James B. Harris; James B. Harris 2Department of Geology, Millsaps College, Jackson, Mississippi 39210, USA Search for other works by this author on: GSW Google Scholar Daniel Larsen Daniel Larsen 3Department of Geological Sciences, University of Memphis, Memphis, Tennessee 38152, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Randel Tom Cox 1Department of Geological Sciences, University of Memphis, Memphis, Tennessee 38152, USA Roy B. Van Arsdale 1Department of Geological Sciences, University of Memphis, Memphis, Tennessee 38152, USA James B. Harris 2Department of Geology, Millsaps College, Jackson, Mississippi 39210, USA Daniel Larsen 3Department of Geological Sciences, University of Memphis, Memphis, Tennessee 38152, USA Publisher: Geological Society of America Received: 26 Sep 2000 Revision Received: 23 Jan 2001 Accepted: 29 Jan 2001 First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2001) 29 (5): 419–422. https://doi.org/10.1130/0091-7613(2001)029<0419:NOTSRR>2.0.CO;2 Article history Received: 26 Sep 2000 Revision Received: 23 Jan 2001 Accepted: 29 Jan 2001 First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Randel Tom Cox, Roy B. Van Arsdale, James B. Harris, Daniel Larsen; Neotectonics of the southeastern Reelfoot rift zone margin, central United States, and implications for regional strain accommodation. Geology 2001;; 29 (5): 419–422. doi: https://doi.org/10.1130/0091-7613(2001)029<0419:NOTSRR>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The northeast-striking New Madrid fault system of central North America has been described as a right-lateral strike-slip system with a left-stepping restraining-bend thrust. The fault system has one of the highest rates of seismic energy release in an intraplate setting, and it has been regarded as a zone of significant earthquake hazard. The New Madrid fault system is the central part of the wider Reelfoot rift fault system, a northeast- striking basement fault zone. Although component faults of the New Madrid system are well defined by microseismicity, recent geodetic surveys suggest that little if any strain is accumulating on the principal (southern) strike-slip arm of the fault system. Seismicity and geologic data show that the “restraining-bend” thrust continues past the southern strike-slip arm to the southeastern Reelfoot rift margin. Thus, we suggest that earthquakes defining the southern arm of the New Madrid fault system are primarily aftershocks of an earthquake on that arm during the last sequence of great earthquakes (A.D. 1811– 1812), and it is the southeastern rift-margin fault system that is currently accommodating right-lateral strain along the boundary of the thrust block. This interpretation is consistent with recent geodetic results. The southeastern rift margin coincides with a 150-km-long linear topographic scarp from near Memphis to the Tennessee-Kentucky line, and S-wave reflection profiles, auger data, and a trench excavation reveal late Wisconsin–early Holocene surface faulting and late Holocene liquefaction associated with this fault-line scarp. Variation in sense of throw along strike and flower-structure geometry suggest that this is a strike-slip fault. Recognition of this rift margin as an important element of active tectonism in the Mississippi embayment has broad implications for assessment of the seismic hazard of this and similar intraplate settings. Temporal shifts in strain accommodation may give rise to short-term seismicity patterns and/or geodetic velocities that do not reveal long-term tectonic patterns. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Genetic aspects of the Katangan megabreccias: Neoproterozoic of Central Africa

The structure of the external, fold-and-thrust region of the Neoproterozoic Katangan orogenic belt of Central Africa is characterised by thrust sheets, gravitationally transported towards the foreland region in the north, and associated with prominent megabreccias. The thrust sheets contain a more or less complete succession of the Katangan sedimentary rocks. The megabreccias form tabular and wedge-shaped, mostly chaotic bodies, underlying the allochthonous sheets and composed of subangular to rounded fragments of Katangan provenance. Fragments range from millimetres to many kilometres in size and are set in an abundant clay matrix with dolomitic cement. It is commonly accepted in the literature that the Katangan megabreccias are of entirely tectonic origin; friction acting beneath the advancing thrust sheets is thought to be the main process responsible for fragmentation and abrasion of clasts. However, the analysis of stratigraphical, textural and structural features presented in this paper implies a sedimentary genesis of the Katangan megabreccias. It is shown that they resulted from subaqueous re-sedimentation by mass gravity flows of clastic materials eroded from the emergent tops and fronts of the advancing thrust sheets, in front of which olistostrome wedges were deposited. The deposition of each individual olistostrome body stopped when the approaching thrust block overrode it due to the acceleration of gravity gliding. These processes, and their products, define the Katangan foreland region. The olistostrome sedimentary model has significant implications for our understanding of the structural and tectonostratigraphic evolution of this sector of the Katangan belt.

Eddy current loss in a moving-coil tubular permanent magnet motor

The eddy current loss in the thrust rod of an iron-cored, 2-phase, tubular permanent magnet brushless motor is investigated by time-stepped transient finite element analysis. It is shown that the dominant cause of the loss is the permeance modulation which results from the teeth in the laminated thrust block, whose spacing is the same as the magnet pole-pitch. Loss predictions are validated by measurements.

Origin of mountains on Io by thrust faulting and large-scale mass movements

Voyager stereoimages of Euboea Montes, Io, indicate that this mountain formed when a large crustal block was uplifted 10.5 kilometers and tilted by approximately 6 degrees. Uplift triggered a massive slope failure on the northwest flank, forming one of the largest debris aprons in the solar system. This slope failure probably involved relatively unconsolidated layers totaling approximately 2 kilometers in thickness, overlying a rigid crust (or lithosphere) at least 11 kilometers thick. Mountain formation on Io may involve localized deep-rooted thrust faulting and block rotation, due to compression at depth induced during vertical recycling of Io's crust.

Evolution of the Sierras de Córdoba, Argentina

Located in the southeastern part of the Sierras Pampeanas (Argentina), the Pampean Ranges of Córdoba show the evolution at the western margin of the Rio de la Plata craton, from extensional stages during Neoproterozoic times to compressive stages during the Pampean orogeny. The Cumbre de Gaspar Group represents distal deep-sea clastic sedimentation deposited in a passive margin environment, very possibly during a time of incipient oceanization. The Santa Sabina Group, representing shelf sedimentation, rests conformably on the Cumbre de Gaspar Group. Stocks of granites and granodiorites of calc-alkaline affinity intruded the Cumbre de Gaspar Group during the early Pampean or earlier. The polyphase Pampean orogeny involves several distinct metamorphic and orogenic events. Metamorphism is discussed using the chemical composition of minerals. The major effect of the Pampean orogeny was the formation of a pile of westward thrust blocks. The Rio de la Plata craton, thrust above its marginal basin, forms the uppermost unit. The intrusion of the El Pilon granite (dated at 520 ± 5 Ma) post-dates these tectonic and metamorphic events. After this main orogenic event, the domain remained permanently located on the cratonic side of younger Famatinian and Andean orogenic events. The Mermela Group probably corresponds to the molasse of the Pampean orogeny and to the base of a new sedimentary cycle. It is difficult to distinguish Famatinian tectonics in the Sierras de Córdoba because the main orogeny occurred further west. However, this orogenic cycle is marked by the intrusion of the large Achala batholith. During the late Palaeozoic and Mesozoic the Sierras de Córdoba remained a stable platform. The whole region was rejuvenated by the Andean orogeny, which induced tilted blocks in its foreland and the Pocho volcanics of shoshonitic affinities.

Thermal perturbation during charnockitization and granulite facies metamorphism in southern India

Abstract We have deduced the steady‐state lithospheric geotherm at c. 1 Ga in the south Indian shield area using the available data on the concentration of radioactive elements, and the P‐T conditions of Proterozoic mantle xenoliths in the south Indian kimberlites as constraints. The geotherm was adjusted back to 2.5 Ga by keeping the surface temperature constant and calculating the temperature change at the top of convecting upper mantle. The reduced or mantle heat flux, which was treated as an adjustable parameter, was 20.9–21.3 mW/m2 at 1–2.5 Ga. Comparison of the calculated steady‐state geotherm with the available P‐T data of the Archaean (c. 2.5 Ga) charnockites and granulites from southern India suggests that the granulite facies metamorphism in this region had resulted from a major thermal perturbation, which was c. 400° C at 25 km.Seismic tomographic and gravity data essentially preclude any significant magma underplating of the granulitic crust in southern India. Previous workers have suggested that the formation of charnockites in this region was associated with copious CO2 influx from a deep‐seated source, possibly the mantle. In this work, we have evaluated both the transient and steady‐state thermal effects of the heat convected by CO2 outgassing from upper mantle. It is shown that the thermobarometric array of charnockites and granulites can be produced by the convective perturbation of the steady‐state geotherm, and that a flux of CO2 of ±90 mol/m2 yr (corresponding to Darcy velocity of ±0.30 cm/yr) for a period of ±30 Ma was needed to produce the required perturbation. This is c. 150 times the average CO2 flux through the tectonically active area of the Earth's crust at the present time. There is, however, an uncertainty of a factor of 3 in this value.Seismic tomographic and gravity data independently suggest thickening of the crust beneath the granulite terrane compared with the adjacent Dharwar craton. This suggests thermal perturbation due to overthrusting as a major potential cause for the granulite facies metamorphism in south India. Overthrusting of a 30–35‐km‐thick thrust block was needed to produce the required thermal effect. The estimated thickness of the original crust from geobarometric and seismic tomographic data south of the orthopyroxene isograd or ‘transition zone’is compatible with the emplacement of a thrust block of this magnitude. However, the latter fails to match the estimated pre‐uplift crustal thickness at the transition zone, if it is assumed that the crust has not thinned by non‐erosional processes since the Archaean. Thus, we propose a combination of overthrusting and CO2 fluxing from a deep‐seated source as the cause for the formation of charnockites in this zone. The required focusing of CO2 in this case is c. 40% of that estimated in the model where CO2 fluxing was considered to be the sole reason for thermal perturbation. This combined thrusting—CO2 fluxing model also helps explain the development of patchy charnockites in the transition zone from amphibolite facies rocks.

Constraints on the nature of the Kapuskasing structural zone from the study of Proterozoic dyke swarms

Petrochemical, paleomagnetic, and structural studies on Early Proterozoic dyke swarms show that the rocks of the Kapuskasing structural zone (KSZ), central Superior Province, were upthrust about 10–15 km along the Ivanhoe Lake fault after intrusion of 2.04 Ga Kapuskasing dykes. This uplift was part of a more widespread deformation in flanking terranes that involved sinistral motion along north–north-northwest-trending faults, dextral displacement along east-northeast–northeast-trending faults, and a distortion of the 2.45 Ga Matachewan swarm to the west and northwest of the Ivanhoe Lake fault. The most spectacular demonstration that the KSZ is largely a product of Proterozoic deformation is that Matachewan dykes change their magnetic polarity on crossing the zone, a consequence of remanence acquisition at deep crustal levels being delayed until after a reversal of the earth's magnetic field and prior to uplift of the dykes.Matachewan and Kapuskasing dykes within the amphibolite- to granulite-grade rocks of the KSZ are relatively fresh and contain a high-alumina green amphibole and feldspar laths that exhibit tea-coloured clouding due to the presence of submicroscopic inclusions of magnetite. These features may be diagnostic of crystallization at deeper crustal levels because they are found in regions where Rb–Sr biotite ages from tonalites suggest broad-scale uplift of the crust after about 2 Ga. About 50 km west of the KSZ, dykes exhibiting clouded feldspar and high-alumina amphiboles occur on the upthrown side of the Budd Lake fault zone, which has a trend similar to that of the Ivanhoe Lake fault. Therefore a second thrust block may exist, raising the possibility that the KSZ represents the basal member of a series of imbricate thrust slices that extends westwards from the Ivanhoe Lake fault. Radiometric age data suggest that the region affected by this Early Proterozoic deformation continues to the northwest for a further 300 km.

Palaeomagnetism of the (early devonian) lower old red sandstones of South Wales: implications to variscan overprinting and differential regional rotations

Component structures of magnetic remanence in the Old Red Sandstones of South Wales are related to tectonic history by detailed study of three contrasting areas: (1) remote from, (2) bordering, and (3) within the Variscan orogenic belt. A uniform southerly shallow-directed component dominates remanence in each sector. In area 1, where it is considered most representative of the regional field, this magnetisation has a mean D I = 191.1/− 8.0° (151 samples in situ, α 95 = 2°) equivalent to a pole position at 341.1°E, 41.2°S. Fold tests show that it was acquired at an intermediate stage of orogenesis (and during the Carboniferous-Permian Reversed Superchron). It is interpreted as a pervasive magnetic overprint imparted by northward fluid migration consequent upon loading of the orogen to the south. Remote from the orogenic front, a higher blocking temperature dual-polarity remanence of probable Lower Devonian age has sporadically survived ( D I = 246/38° , 27 samples tilt adjusted, α 95 = 7.2°) corresponding to a pole position at 297°E, 3°N. This remanence is entirely replaced close to the frontal zone of the orogen. Within the orogen, low blocking temperature components are more prominent and chemical alteration obscures the highest blocking temperature portion of the remanence spectrum. This region was deformed into upright folds before the magnetising event. Following remanence acquisition, continuing deformation produced asymmetry in fold structure producing a plunge and steepening of the southern limbs. Structures in the south were already more deformed than those in the north at the time of magnetisation, implying that the causative fluid front moved northwards with time. The palaeomagnetic vectors are rotated clockwise to the west in South Wales and, in part, record the effects of regional dextral transpression; the largest rotation however, is observed within the same thrust block and apparent differential rotation here is probably a consequence of strain.