East-west Shortening Research Articles

Large-scale Hercynian West-directed tectonics in southeastern Sardinia (Italy)

Widespread Hercynian west-directed thrusting and west-facing folds, until now only reported in a limited area, are ubiquitous in the Sarrabus unit, southeastern Sardinia. Intense thrusting and imbrication in the higher tectonic levels of the Sarrabus unit potentially originated during west-directed thrusting of higher tectonic units, which have been eroded. The floor thrust of the Sarrabus unit, the Villasalto thrust, is a west- directed thrust that places the Sarrabus unit above the Gerrei and Meana Sardo units. East-west shortening in southeastern Sardinia followed north-south shortening in central Sardinia and internal deformation in the Castello Medusa-Riu Gruppa, Gerrei, Meana Sardo and Barbagia units. After restoration of movement along the Villasalto thrust, the Sarrabus unit is located east of the Gerrei and Meana Sardo unit. Therefore, it cannot be directly correlated with the Meana Sardo or the Barbagia units. © Elsevier, Paris

Basement–cover relationships and orogenic evolution in the Central East Greenland Caledonides

Recent structural, petrological and geochronological data from the Fjord Region of central East Greenland requires revision of tectonic models for this segment of the Caledonian-Appalachian orogen. The data document the existence of a regionally extensive, orogen parallel, east-dipping extensional detachment zone (Fjord Region Detachment Zone) separating high grade, infracrustal para- and orthogneisses and migmatites in the footwall or Central Metamorphic Complex (basement) from overlying Neoproterozoic to Middle Ordovician metasedimentary rocks in the hanging wall. A tectonically disturbed depositional contact exists between the basement gneisses and overlying amphibolite grade Neoproterozoic metasedimentary rocks along the western margin (towards the foreland) of the Central Metamorphic Complex. Top-to-the-east movement on the Fjord Region Detachment Zone and associated extensional faults in the hanging wall post-dates orogen-perpendicular contractional structures in the hinterland. U-Pb and 40Ar-39Ar ages from the detachment zone and underlying infracrustal gneisses indicate that extensional movement on the detachment zone started as early as in the Late Silurian and continued into the Middle Devonian. We have found no evidence in the upper crustal level in the hinterland of an Early Devonian contractional event as indicated in some previous models. Sedimentological as well as isotopic data from the foreland suggest that extension in the hinterland was contemporaneous with folding and thrusting in the foreland. To explain these relationships we envision a scenario where upper crustal extension in the hinterland triggered upwelling of a low viscosity (“fluid”) lower crust in the hinterland, represented by the eastern part of the Central Metamorphic Complex. The hinterland welt exerted enough subhorizontal stress to cause east-west shortening in the foreland. Diapiric upwelling of the inferred “fluid” lower fluid crust was facilitated by invasion of large volumes of granitic melt dated to c. 425 Ma.

Origin of the ‘Berwick Monocline’: geometrical and geophysical considerations

Synopsis Analysis of minor fold vergence and thrust directions in Carboniferous strata at Sea House, north Northumberland, has revealed the synclinal member of a major monoclinal fold pair and thus established the easterly vergence of the southern sector of the Berwick monoclinal disturbance or ‘Berwick Monocline’, a discrete NNW-SSE structure on the western shore of the North Sea. The most intense deformation within the Berwick structure is found in its central sector, where displacement on the steep east-directed Boundary reverse fault is about 0.5 km; the footwall is, however, almost undeformed, and extends offshore as the Marshall Meadows flat zone. The Berwick disturbance as a whole is arcuate in plan, the concave side facing east. The association of a weak gravity anomaly low with higher amplitude aeromagnetic anomalies over the Marshall Meadows flat zone is explained by postulating a concealed anomalous basement massif, the Marshall Meadows block, in which a low density, possibly granitoid core is associated with a locally more strongly magnetized margin. The marked attenuation of Dinantian strata in the Marshall Meadows flat zone is consistent with lower density and consequent positive buoyancy of the underlying Marshall Meadows block as compared with adjacent basement. A few tens of kilometres south of the Berwick ‘Monocline’, between Holburn and Lemmington, the Carboniferous cover rocks are affected by a pair of monoclinal structures. Their combined geometrical relationship (location, trend, extent, vergence and arcuate form) t o the adjacent granite-cored Cheviot block is comparable with the relationship of the Berwick structure to the postulated Marshall Meadows block. It is suggested that during regional east-west shortening the Berwick and Holburn/Lemmington structures formed in response to contrasts in the Caledonian basement between the stronger, less dense igneous rocks of the Cheviot and postulated Marshall Meadows blocks, and the adjacent weaker, denser Silurian shale and greywacke host rocks.

Tectono-stratigraphy of late Archaean greenstone terranes in the southern Eastern Goldfields, Western Australia

Late Archaean greenstone terranes in the southern part of the Eastern Goldfields are regionally extensive, faultbounded entities defined on the basis of small but distinct differences in stratigraphy and structure. The greenstones comprise volcano-sedimentary successions that were all deposited at the same time (c. 2720-2675 Ma) on sialic crust. Hence, the terranes are interpreted to represent contemporaneous, probably adjacent, basins. These basins were affected by the same history of granite plutonism and regional deformation (c. 2675-2620 Ma). Early subhorizontal deformation is poorly understood, but includes at least locally significant thrust stacking (D1) with a south to north movement direction. This was followed by regional east-west extension affecting all supracrustal successions above a province-wide detachment, now lying at depths between 4 and 7 km. This detachment may have evolved from the greenstone-basement contact, but only after extensive reworking. Clastic basins developed along or above major extensional faults but no volcanism is associated with this extension. Subsequent east-west (D2) shortening resulted in regional scale thrusting, folding and associated late-stage transcurrent fault movements. The D2 event is contemporaneous with a major thermal event represented by voluminous granite emplacement (c. 2660 Ma) and associated peak regional metamorphism in the greenstone succession. Predominantly monzo granite, derived from older sialic crust, was emplaced throughout the period of basin formation and deformation: early synvolcanic granite (c. 2719-2710 Ma) is a minor component, late synvolcanic granite (c. 2685-2675 Ma) is widespread, and syn-D2 granite (c. 2660 Ma) is abundant. There are a few small late tectonic intrusions (c. 2640 Ma to?2600 Ma). The lateral lithostratigraphic persistence of the contemporaneous basins, the syn-D2 low pressure regional metamorphism, and the province-wide monzo granite magmatism suggest that the greenstone basins possibly developed in a tectonic setting similar to that of failed rifts above aborted spreading centres. The basins closed in response to far-field compressional stresses.

Extensional emplacement of a high-grade granite gneiss complex into low-grade greenstones, Eastern Goldfields, Yilgarn Craton, Western Australia

A high-grade granite gneiss complex was emplaced into low-grade greenstones after an early episode of Late Archaean deformation (D1) in the Eastern Goldfields of Western Australia. The components of the granitic gneiss complex range from migmatitic granitic gneiss to foliated and massive granite. The complex is surrounded by a marginal zone of high-grade (amphibolite facies) greenstones. The oldest phases of the complex are foliated and gneissic granite with igneous emplacement ages of c. 2675 Ma, that postdate the bulk of greenstone volcanism (c. 2720-2675 Ma). The complex includes voluminous and largely undeformed monzogranite that is correlated with widespread granite plutonism at c. 2660 Ma. Fabrics in marginal high-strain zones between the granite gneiss complex and greenstones, and in faults separating high-grade from low-grade greenstones, indicate that emplacement, or uplift, of the complex was partly accommodated by normal faulting. Final emplacement occurred during the c. 2660 Ma granite event, and overlapped with regional east-west shortening (D2–D3) in the adjacent greenstones. Uplift of the granitic gneiss complex to higher crustal levels may have resulted from preferential injection of granite magma into gentle domes of the basal greenstone contacts. These domes formed during earlier regional extension of the greenstones.

Geodetic measurement of crustal motion in southwest China

Research Article| February 01, 1997 Geodetic measurement of crustal motion in southwest China Robert W. King; Robert W. King 1Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Search for other works by this author on: GSW Google Scholar Feng Shen; Feng Shen 1Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Search for other works by this author on: GSW Google Scholar B. Clark Burchfiel; B. Clark Burchfiel 1Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Search for other works by this author on: GSW Google Scholar Leigh H. Royden; Leigh H. Royden 1Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Search for other works by this author on: GSW Google Scholar Erchie Wang; Erchie Wang 1Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Search for other works by this author on: GSW Google Scholar Zhiliang Chen; Zhiliang Chen 2Chengdu Institute of Geology and Mineral Resources, Chengdu, Sichuan, China Search for other works by this author on: GSW Google Scholar Yuping Liu; Yuping Liu 2Chengdu Institute of Geology and Mineral Resources, Chengdu, Sichuan, China Search for other works by this author on: GSW Google Scholar Xuan-Yang Zhang; Xuan-Yang Zhang 2Chengdu Institute of Geology and Mineral Resources, Chengdu, Sichuan, China Search for other works by this author on: GSW Google Scholar Ji-Xiang Zhao; Ji-Xiang Zhao 2Chengdu Institute of Geology and Mineral Resources, Chengdu, Sichuan, China Search for other works by this author on: GSW Google Scholar Yulin Li Yulin Li 3Chinese Academy of Surveying, Beijing, China Search for other works by this author on: GSW Google Scholar Author and Article Information Robert W. King 1Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Feng Shen 1Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 B. Clark Burchfiel 1Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Leigh H. Royden 1Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Erchie Wang 1Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Zhiliang Chen 2Chengdu Institute of Geology and Mineral Resources, Chengdu, Sichuan, China Yuping Liu 2Chengdu Institute of Geology and Mineral Resources, Chengdu, Sichuan, China Xuan-Yang Zhang 2Chengdu Institute of Geology and Mineral Resources, Chengdu, Sichuan, China Ji-Xiang Zhao 2Chengdu Institute of Geology and Mineral Resources, Chengdu, Sichuan, China Yulin Li 3Chinese Academy of Surveying, Beijing, China Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1997) 25 (2): 179–182. https://doi.org/10.1130/0091-7613(1997)025<0179:GMOCMI>2.3.CO;2 Article history 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 Robert W. King, Feng Shen, B. Clark Burchfiel, Leigh H. Royden, Erchie Wang, Zhiliang Chen, Yuping Liu, Xuan-Yang Zhang, Ji-Xiang Zhao, Yulin Li; Geodetic measurement of crustal motion in southwest China. Geology 1997;; 25 (2): 179–182. doi: https://doi.org/10.1130/0091-7613(1997)025<0179:GMOCMI>2.3.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 Global Positioning System measurements performed over a 2–4 yr period confirm the Xianshuihe-Xiaojiang fault system as one of the primary active structures in southwest China. Stations southwest of these faults show southerly motions of 5–15 mm/yr relative to the western Sichuan basin, and stations in a 200 km geodetic network located northwest of the basin move at only 0–5 mm/yr. These results are consistent with clockwise rotation of southwestern Sichuan and western Yunnan about the eastern Tibetan syntaxis, accommodated by left-lateral slip of 12 ± 4 mm/yr on the Xianshuihe-Xiaojiang fault system. The results imply that if eastward extrusion of crustal material from the plateau occurs at present, it is not accommodated by east-west shortening along the margin of the plateau and must involve wholesale eastward motion of low-lying regions to the east. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Structural evolution and early accretion of the Archean Malartic Composite Block, southern Abitibi greenstone belt, Quebec, Canada

The Malartic Composite Block of the southern Abitibi belt underwent at least two phases of ductile deformation over a period of 25 Ma during the late Archean. It is divided into seven tectono-stratigraphic domains on the basis of their lithologicai content, structural styles, and geochemical affinities of the volcanic and plutonic rocks. The Malartic Composite Block is bounded to the south by the sedimentary rocks of the Kewagama Group that have been also affected by deformation. The crosscutting relationships between the structural elements, along with precise geochronological data, provide a basis for evaluation of the evolution of the deformation in this part of the southern Abitibi. The D1 tectonic event produced ductile thrust faults and folds in the mafic and ultramafic domains of the Malartic Composite Block. The calc-alkalic rocks of the Val-d'Or Domain (2705 Ma) were erupted on top of these deformed domains, thus establishing the minimum age for D1. The D2 event is divided into three increments that are linked to the same progressive deformation. D2.1 produced northwest-trending folds in the rocks of the Kewagama Group, but did not affect the rocks of the Malartic Composite Block. The age of the youngest detrital zircon in the Kewagama indicates that D2.1 was active after 2687 Ma. D2.2 developed a consistent east–west regional foliation that overprints all the rocks of the area. Radiometric data on related metamorphic minerals indicate an age of 2680 Ma for this event. Locally, S2.2 is modified by Fo2.3 folds of asymmetric Z shape associated with a late dextral transcurrent shearing. A minor D3 event produced small, local conjugate kink folds recording an east–west shortening.

Formation of Neoproterozoic metamorphic complex during oblique convergence (Eastern Desert, Egypt)

Major portions of the Pan-African Orogen in the Eastern Desert of Egypt were formed by island-arc accretion in the Neoproterozoic. These areas are characterized by their lack of major crustal thickening. Metamorphic core complexes occur parallel to the strike of the Eastern Desert Orogen. These domes exhibit polyphase metamorphism and deformation in contrast to the structurally overlying nappes which include ophiolitic melanges and island-arc volcanic rocks. These nappes show northwest directed, orogen-parallel thrusting in the internal parts and west to southwest directed imbrication in the external parts of the orogen. Structures related to exhumation of the metamorphic core complexes partition into different displacement paths localized within a crustal-scale wrench corridor of the Najd fault system. Northwest trending orogen-parallel, sinistral strike-slip faults define the western and eastern margins of the domes. North and south dipping low-angle normal faults developed along the northern and southern margins of the domes and form extensional bridges between them. 40 Ar 39 Ar ages obtained from syntectonic muscovites within the shear zones gave Neoproterozoic ages of 595.9±0.5 and 588.2±0.3 Ma. The synchronous activity of strike-slip and normal faults suggests a regional east-west shortening which was accomodated by deep-level basal decollement beneath the metamorphic core complexes and a coeval northwest-southeast, orogen-parallel extension. This extension was accompanied by intramontane molasse sedimentation and emplacement of calc-alkaline plutons. Since the rapid exhumation of gneisses in the core complexes cannot be explained by thickening of the crust, the authors favour a model which calls for enhanced heat flow along the Najd fault system which would have enabled the formation of syn-extensional plutonism and triggered the exhumation of the metamorphic core complexes. Lateral buoyancy forces were concentrated within the Najd wrench corridor and enabled orogen-parallel extension.

The Harz Mountains, Germany: Finite-element modelling of the evolution based on the interpretation of the gravity field

The new Bouguer anomaly map of the Harz Mountains based on 60,000 gravity measurements is presented. The interpretation is done by three dimensional (3-D) gravity modelling, mostly concentrating on the granitic intrusions. The Brocken Granite is modelled as a flattish body with maximum thickness of 2.5 km. The Ramberg Granite is up to 8.5 km thick with a northsouth extent of 35 km. The present mass distribution is an important boundary condition for geodynamic investigations using the finite-element method (FEM). It can be shown that the Harz Mountains are not isostatically compensated. Further calculations deal with the Central European Variscan Belt and reveal a horizontal shortening of 600 m for the Harz Mountains. Considering the Harz Mountains as a wrench-fault-system by taking into account an east-west shortening of 1 km results in a calculated uplift of 220 m which is much less than the vertical displacement at the northern boundary fault zone estimated from gravity and seismic studies.

Structural and metamorphic evidence of local extension along the Vivero fault coeval with bulk crustal shortening in the Variscan chain (NW Spain)

The Vivero fault is a W-dipping, N-S-striking ductile shear zone separating the Ollo de Sapo antiform in its western hangingwall and the Lugo dome in its eastern footwall. Two stages of deformation ( F 1 and F 2) produced nearly coaxial folds with sub-horizontal axes. A crenulation cleavage S 2 transposes an older S 1. Three sets of shear bands in the hangingwall define a pervasive fabric consistent with an E-W bulk shortening perpendicular to a composite S 1–2 foliation and NNE-stretching parallel to L 2. The Vivero fault zone is marked by a mylonitic foliation with a steeply NW-plunging stretching lineation and extensional crenulation cleavage (ECC) indicating normal slip. In the vicinity of the fault, sub-horizontal NNE-trending F 3 folds, with a crenulation cleavage S 3, deform earlier-formed fabrics, including a mylonitic foliation. Pressure-temperature conditions obtained from mineral assemblages on both sides of the Vivero fault yield a minimum throw of 5.5 km. Andalusite-bearing pelite in the hangingwall was infolded by an F 2 synform into the kyanite field at 450–500°C. The eastern edge of these rocks was later accreted to the footwall and heated to andalusite-staurolite conditions at ∼600°C. Slip on the Vivero and Valdoviño faults is kinematically related. East-west shortening during F 2 involved folding and sinistral strike-slip on the Valdoviño fault which induced local extension along the newly generated Vivero fault. Synkinematic emplacement of granitoids along the Vivero fault is favoured by extension. Coeval slip on both faults took place during the later stages of F 2 folding. Geometrical constraints caused northwards escape of the crustal block bounded by the Valdoviño and Vivero faults, recorded by NNE-stretching defined by L 2.

Quantitative Analysis of a Transpressional System, El Biod Arch, Ghadames Basin, Algeria: ABSTRACT

Trap definition within the northern extension of the Hassi Touareg - Rhourde El Baguel fault zone in the western Ghadames Basin of Algeria is difficult due to complex structural geometries. The fault zone consists of a narrow system of discontinuous. locally en echelon faults. Although north-trending to the south, the zone curves to a northeast trend to the north. Reserves associated with the southern portion of the system total 1500 MMBOR and 2 TCFG. Several lines of evidence support a strike-slip component of motion for the northern segment. Horizontal slickensides have been described in cores taken from wells within the fault trend. Fracture patterns measured from logs taken within the NE-SW fault trend show clusters expected for right-lateral Reidel shears. Although complicated by all evaporate sequence at mid-level in the stratigraphic section, we interpret downward converging faults imaged on recent 2D seismic as positive flower profiles. Map patterns are also interpreted as right-lateral, recognizing that the 2D grid cannot resolve all of the structural complexity. To confirm the component of strike-slip fault displacement, we applied a new quantitative method relating map view structural orientations to the shear magnitude, the degree of convergence or divergence, and the magnitudes of horizontal andmore » vertical strains. Strike-slip to convergence ratios ranging from 2:1 to 3:1 were measured in the study area. Higher ratios (10:1) measured above the salt may indicate a detachment. These ratios also fit the regional tectonic pattern: to the south, where the fault zone trends due north, structural geometries support dip-slip inversion indicative of east-west shortening. Applying the same shortening vector to the northeast-trending part of the zone suggests oblique right-lateral motion, with a strike-slip to convergence ratio of 2:1.« less

The geology and structure of the mid-crustal Wawa gneiss domain: a key to understanding tectonic variation with depth and time in the late Archean Abitibi–Wawa orogen

The amphibolite-facies central Wawa gneiss domain (CWGD) preserves structures that developed at the mid-crustal level of the ca. 2.7 Ga Abitibi–Wawa orogen in the southern Superior Province. The relative ages of these domainal structures are documented and brackets on their absolute ages established using existing U–Pb age data. Correlation of tectonic events within the CWGD, and comparison of these events with the evolution of other structural levels of the orogen, has led to subdivision of orogenesis into five stages. During stage 1 (2700–2680 Ma), 2.9 and 2.7 Ga rocks were tightly folded and (or) thrusted at all crustal levels in at least one thick-skinned compression event. During stage 2 (2680–2670 Ma), folding and thrusting of Timiskaming-age sediments at high levels of the orogen was thin-skinned and had no effect on CWGD gneisses. During stage 3 (2670–2660 Ma), while the upper crust was relatively stable, a 1 km thick package of volcanics and sediments, the Borden Lake belt, was underthrust northwards to depths of 30 km and in-folded with orthogneiss of the CWGD. During stage 4 (2660–2637 Ma), coeval east–west extension and granulite metamorphism of the middle crust produced gently dipping shear zones that overprinted earlier fold structures in the CWGD and lower structural levels of the orogen. This took place with minimal effect on the upper crust. Stage 5 (2630–2580 Ma) marks a period of east–west shortening and (or) fault reactivation in the Kapuskasing uplift and upper-crustal greenstone belts that allowed penetration of deep-crustal metamorphic fluids into the latter. In general, analysis of the structural evolution of the CWGD indicates that deformation and metamorphism in the middle crust of the Abitibi–Wawa orogen outlasted that at upper-crustal levels, resulting in the generally shallower dips of planar fabrics in the deeper structural levels of the Kapuskasing uplift crustal cross section.

Structural control of lode gold deposits in the Pony mining district, Tobacco Root Mountains, Montana

Lode gold production in the Pony mining district has come principally from veins emplaced along faults and shear zones that strike northwest, west to west-northwest, northeast, and north. Host rocks are Archcan gneisses and amphibolites, Proterozoic diabase dikes, and Late Cretaceous granitic rocks of the Tobacco Root batholith. The faults and shear zones are interpreted to be related to east-west crustal shortening during the Late Cretaceous to early Tertiary Laramide orogeny that resulted in oblique left slip on the northwest-trending Mammoth fault, the principal zone of strain in the district. The west- to west-northwest-striking fractures are mostly characterized by reverse and left slip, and are interpreted as synthetic oblique-slip faults with respect to left slip on the Mammoth fault. Northeast-striking fractures are inferred to be antithetic strike-slip faults and the north-trending fractures are interpreted to be reverse and thrust faults. The vast majority of the gold production has been from the Boss Tweed, Clipper, and Mammoth mines that were developed on west- to west-northwest-trending veins in shear zones hosted by the metamorphic rocks.Initial movement on the Mammoth fault, and probably on the westerly to west-north-westerly trending shear zones, was prior to emplacement of Middle Proterozoic dikes. Early movement on the west- to west-northwest-striking shears resulted mainly in production of fine-grained cataclasite with minor amounts of mylonite. The shear zones were silicified, pyritized, and multiply reactivated. The youngest major episode of deformation, inferred to have occurred during the Laramide orogeny, resulted in brecciation of the silicified cataclasite and mylonite and provided permeability for mineralizing solutions.The most productive veins, also the longest and widest in the district, are nearly parallel to the direction of inferred maximum horizontal compressive stress during the Laramide and thus may be partly extensional fractures as well as having undergone contemporaneous oblique slip. Local undulations in the fault surfaces also affected the mineralization inasmuch as the ore zones are thickest where the hanging-wall dip is low and/or where the strike bends to the left. In addition, the largest stopes are found on the south sides of well-developed, easterly striking fractures that form a trailing extensional imbricate fan within the Boss Tweed fault zone.Deposition of chalcopyrite, galena, sphalerite, mackinawite, pyrrhotite, hessite, tetradymite, silver telluride, and gold is interpreted to have taken place in the Late Cretaceous during or shortly after emplacement of the Tobacco Root batholith and its satellite plutons. Ore minerals are found principally as replacements and in fractures in pyrite and thus appear to be dependent on the presence of pyrite and open fractures to provide permeability for mineralizing solutions. The source of the gold is not known, but it could have come from the igneous rocks or it might have been scavenged from the metamorphic rocks.

Longitudinal fluvial drainage patterns within a foreland basin-fill: Permo-Triassic Sydney Basin, Australia

The north-south trending Permo-Triassic Sydney Basin (southern sector of the Sydney-Bowen Basin) is unique compared to many documented retro-arc foreland basins, in that considerable basin-fill was derived from a cratonic source as well as a coeval fold belt source. Quantitative analysis of up-sequence changes in sandstone petrography and palaeoflow directions, together with time-rock stratigraphy of the fluvial basin-fill, indicate two spatially and temporally separated depositional episodes of longitudinal fluvial dispersal systems. A longitudinal drainage-net similar in geometry to the modern Ganga River system (reduced to 60% original size) explains many of the palaeoflow patterns and cross-basinal petrofacies variation recorded in the basin-fill. The Late Permian to Early Triassic rocks reveal a basin-wide southerly directed fluvial drainage system, contemporaneous with east-west shortening recorded in the New England Fold Belt. In contrast, the Middle Triassic strata reveal a change to an easterly directed fluvial system, correlated to a shift in orogenic load to a NW-SE orientation in the fold belt northeast of the basin. The detailed petrofacies variation in the deposits of the second longitudinal fluvial dispersal system reveals vertical jumps in petrofacies compositions, with uniform compositions between jumps. The petrological jumps are interpreted as the result of minor fault adjustments in the fold belt, resulting in changing rates of sediment supply to the foreland basin. Uninterrupted erosion of the same terrain most likely caused the compositional uniformity between jumps. The identification of similar longitudinal fluvial systems, with transverse variation in detrital composition, is likely to help resolve the tectonic history of foreland fold belts elsewhere.

Variations in continental crustal structure in Canada from LITHOPROBE seismic reflection and other data

LITHOPROBE is Canada's national, collaborative, multidisciplinary geoscience project directed toward an enhanced understanding of how the North American continent has evolved. Research in its eight transects, which span the country and geological time, are spearheaded by seismic reflection studies. These, combined with other studies, provide new insight into the varied tectonic processes that have been active in forming the continent. The Archean Kapuskasing Structural Zone, a prominent feature which cuts obliquely across east-west Superior Province structures, has been imaged as a low-angle thrust sheet above a major décollement, along which 55–70 km of crustal shortening has occurred to bring mid-crustal rocks to the surface. The shortening was accomplished by brittle faulting and erosion at levels above 20 km and ductile shear and flow in the lower crust. The GLIMPCE transect in Lake Huron shows a spectacular series of east-dipping crustal-scale reflections that coincide with the Grenville Front Tectonic Zone. A two-stage model involves terrane collision with the cratonic margin in the Early Proterozoic followed by crustal-scale stacking and ramping associated with the Middle Proterozoic Grenvillian orogeny. In Lake Superior, GLIMPCE data reveal the structure of the Middle Proterozoic mid-continent rift that almost splits North America. The Lithoprobe East transect covering the Paleozoic Newfoundland Appalachians reveals three lower crustal. blocks, defined seismically on the basis of reflectivity characteristics imaged along the seismic profiles. Reflective upper crust can be related to geological structures at the surface; the thin-skinned tectonics implied have yet to be reconciled with the block tectonics of the lower crust. Results from the southern Canadian Cordillera show that the Mesozoic-Cenozoic east-west shortening in the eastern and central Cordillera occurred by compressional deformation forming crustal-scale antiforms that overrode a westward thinning cratonic North America; and in the western Cordillera by subduction processes that have tectonically eroded the lower lithosphere of accreted terranes and left underplated sediments and oceanic lithosphere. The reflection Moho has highly variable characteristics as imaged within transects and among different transects. Crustal and Moho reflectivity is caused by a wide range of features, including fault/shear zones, lithologie contacts, compositional layering, fluids in zones of high porosity, and anisotropy.

Thermal Maturity Patterns in Alaska: Implications for Tectonic Evolution and Hydrocarbon Potential

Nearly 10,000 vitrinite reflectance and conodont color alteration index determinations from sedimentary rocks in Alaska were used to produce a thermal maturity map of rocks exposed at the surface and to evaluate subsurface thermal maturity relations in the Colville and Cook Inlet basins. Rocks exposed at the surface of the Tertiary interior basins and in the Aleutian forearc and backarc basins uniformly are of very low thermal maturity, indicating that these basins are at or near maximum burial, have seen little uplift and exhumation, and are probably thermally immature with respect to hydrocarbon generation. In contrast, many sedimentary basins show elevated levels of thermal maturity at the surface, with the highest values at basin margins. This geometry suggests a patt rn of greater uplift along basin margins, possibly reflecting isostatic readjustments as crustal loads are removed by erosion. We investigated thermal maturity relations in three sedimentary basins (Colville, Cook Inlet, and Kandik) in more detail. Thermal maturity patterns in the Colville basin are broadly asymmetric, indicating systematic differential uplift ranging from a minimum of no uplift in the north (Point Thomson area) to 9-13 km of uplift and exhumation in the central Brooks Range; even greater uplift further to the south is indicated by the presence of greenschist facies and higher grade metamorphic rocks. This pattern may reflect the deflexing of the lithosphere subsequent to the principal episode(s) of crustal convergence and thickening. These patterns further suggest a similar thermal history for the proximal Colville basin and the northern foothills belt, suggesting the possibility of hydrocar on accumulations in the foothills. Thermal maturity isograds within the Brooks Range cut major thrust faults, indicating that maximum burial postdated the principal phases of thrusting. In contrast, isograds in the foothills belt to the north are warped broadly by local structure, indicating continued north-south shortening subsequent to maximum burial. Such deformation could have remobilized hydrocarbons in early traps. A broad southward extension of thermally immature rocks in the central portions of the foothills belt suggests relatively young east-west shortening (parallel to the strike of the orogene), a feature that to date has not been included in regional tectonic syntheses. Alternatively, the thermal maturity pattern could be explained by tectonically unrelated episodes of uplif in the eastern and western parts of the Brooks Range. In the Cook Inlet basin, vitrinite reflectance isograds also are indicative of relatively greater uplift at the basin margins than at the basin center, which appears to be presently at its maximum burial depth. Uplift in the Cook Inlet basin may reflect compression along the faults bounding the basin. Relatively high thermal maturity along the western margin of the basin also may reflect magmatic heat sources from the Alaska Peninsula-Aleutian volcanic arc. The Seldovia arch, a major structural feature of the End_Page 1874------------------------------ basin, does not appear to deform vitrinite reflectance isograds, implying that deformation on that structure ceased prior to maximum burial. In the Kandik basin, a thermal maturity anomaly (thermally mature younger rocks in fault contact with thermally immature older rocks) provides clues to the nature and timing of east-west thrusting. Mesozoic foreland basin deposits associated with thrusting buried Paleozoic rocks of the easternmost part of this fold-and-thrust belt to relatively shallow depths, driving potential hydrocarbon source rocks into the oil-generation window. The western foreland basin deposits were overridden by advancing thrusts, and tectonically buried to as deep as 10 km. These disparate thermal domains are juxtaposed along the Glenn Creek fault, which may represent a terrane boundary in east-central Alaska.

The Chaparral shear zone: Deformation partitioning and heterogeneous bulk crustal shortening during Proterozoic orogeny in central Arizona

The Proterozoic Chaparral shear zone of central Arizona is one of a network of subvertical, northeast-striking shear zones that divide the Proterozoic orogenic belt of Arizona into tectonic blocks. The zone is several kilometers wide and contains variably developed mylonitic foliation that is subparallel to the regional subvertical foliation. Stretching lineations plunge shallowly northeast, and displacement across the zone was dominantly right-lateral. Several lines of evidence constrain displacement across the zone to be greater than 5 km, but probably less than tens of kilometers: (1) stratigraphic and plutonic rocks can be correlated across the zone, (2) structural and metamorphic histories of tectonic blocks on opposite sides of the zone are similar, and (3) integration of shear strain (estimated by deflection of earlier fabric) suggests greater than 5 km of strike slip. More important than scale of movement across the zone, structural studies have clarified several aspects of the assembly history of the orogen. Early north- to northwest-striking S 1 foliation, on both sides of the Chaparral shear zone, records one or more deformational events that took place between 1.75 and 1.7 Ga. Early fabrics record northeast-southwest or east-west shortening and west-verging thrusting that we interpret to have formed during development of a primitive arc complex. In contrast, intense deformation in the Chaparral shear zone took place during regional northwest-southeast shortening, D 2 , that produced the dominant subvertical northeast-striking foliation and the present block architecture of the orogen. D 2 is interpreted to have been in part synchronous with emplacement of 1.70 Ga granitoids and to record assembly of volcanic belts to North America. Of general interest is the character of D 2 deformation partitioning in and across the shear zone. Two types of high-strain domains were generated during shortening. One type, represented by rocks southeast of the shear zone, involved extreme shortening and transposition by folding. The other type, represented by the shear zone, involved simple shear deformation in a complicated array of anastomosing and conjugate shear zones. Overprinting relationships suggest that the second type may have nucleated on the first, implying an important component of deformation partitioning in time, as well as space. Mylonitic fabric in the shear zone developed by progressive heterogeneous simple shear followed by brittle fracturing. Conjugate shear bands suggest a shallow southeast-plunging σ 1 late during strike-slip deformation. This is consistent with regional D 2 shortening but not with the incremental shortening direction expected during right-lateral simple shear. This supports a regional kinematic model in which the Chaparral shear zone (right-lateral) and a temporally related left-lateral shear zone were regional-scale conjugate shear zones that accommodated heterogeneous D 2 northwest-southeast orogenic shortening via escape-block tectonics. Thus, orogen-parallel strike-slip displacements in the Chaparral shear zone were a response to partitioned shortening in the late stages of accretion rather than transpressional accretion during oblique subduction.

Geophysical Interpretation and Modelling of Three-Dimensional Structure in the Duchess Area, Mount Isa, Australia

In most areas of polydeformed crystalline basement, interpretation of structure in the third dimension is based solely on structural observations of incomplete surface exposures. One way to alleviate this problem is to use geophysical data and rock-property data to place additional constraints on structural interpretation. Geophysical data can provide information on the surface distribution of rock units, and can be used to constrain forward models of structural interpretations. Most available geophysical modelling routines, however, do not lend themselves to specification of complex structure in a geologically meaningful way.The Duchess area in the Mount Isa Inlier has been used as a test area for a structural and geophysical modelling package which allows specification and forward modelling of a multiphase deformation history. This study combines the input of field-based structural mapping, rock-property measurements, three-dimensional structural interpretation, and interpretation of a high-resolution aeromagnetic survey. The aim of the study was to use magnetic data to place constraints on the detailed three-dimensional structural interpretation of the area.The deformation history of the Duchess area consists of an early layer-parallel foliation and associated high-strain zones, which are overprinted by a regional east-west shortening event which resulted in the formation of upright folds and a steep foliation, with north-trending axial surfaces. These structures are overprinted by a series of northwest, northeast and north-trending faults which are also associated with east-west shortening.The approach in this study was to produce a preliminary three-dimensional interpretation based on surface mapping and magnetic interpretation. The results of this stage were then used, with constraints from magnetic susceptibility data, in a forward model of the magnetic anomaly field of the interpreted structure. The results of the model were then used to modify iteratively the interpretation until it was consistent with both surface observations and magnetic data.This approach has allowed additional constraints to be placed on: a) the subsurface geometry of folded magnetic units; b) absolute sense of displacement on faults; c) the surface and subsurface geometry of granitic bodies; and c) the nature and extent of deep structural features not exposed at the surface.

The role of thrusting in the structural development of the Mount Isa Mine and its relevance to exploration in the surrounding region

Resolution and prediction of the structure underground at Mount Isa from the surface geology is now possible using evidence for north to south movement on large-scale thrusts during the first deformation at 1610 Ma. This evidence for the direction of movement during thrusting is preserved (1) macroscopically within three successive thrust imbricates as the north-south alignment of respective truncations of older stratigraphic units by the angular unconformity at the base of the Mount Isa Group; (2) macroscopically and mesoscopically as a zone of vertical north-south-striking foliation with a horizontal stretching lineation that formed on a sinistral transform fault during Dalong the Wonga-Duchess belt; (3) mesoscop- ically as a regionally distributed north-south-oriented stretching lineation (LI) that is locally well preserved on S0, surfaces in the hinges of folds with north-south axial planes that formed during the second regional deformation; and (4) microscopically as a horizontal, north-south- aligned mineral elongation lineation defined by inclusion trails preserved in porphyroblasts that grew early during the second deformation adjacent to the Wonga-Duchess belt. The swing in strike of a large-scale 70o-west-dipping lateral ramp on the western margin of the large-scale Kokkalukkanurker duplex from north-south (parallel to the direction of movement during thrusting) to north-northwest-south-southeast caused a portion of the Meerenurker roof thrust lying on this lateral ramp to be overturned for at least 50 km near Mount Isa. This overturning resulted from tilting the 70o-west-dipping lateral ramp through another 70 o as it rode obliquely (north-south) over the north-northwest-south-southeast- striking portion of the same lateral ramp on the next imbricate to the south. This generated the gross structural relationships seen in three dimensions at Mount Isa whereby the greenstone basement that truncates Mount Isa Group rocks below the mine overlies them to the west. The second deformation involved intense east-west shortening and formed macroscopic folds that conveniently expose varying levels of the thrust pile generated during the first deformation. In higher grade rocks west of Mount Isa and to either side of the Wonga belt, axial plane schistosity development and/or reactivation of bedding during this deformation was quite intense and tended to obliterate and/or rotate foliations and lineations formed during the first deformation. Only in the hinges of the folds formed during the second deformation, where the deformation was less intense or silicification had occurred, are earlier lineations preserved relatively unaffected by this event. The geology of the Mount Isa Valley and the rocks to the west of the mine is controlled by a series of north-south-striking high-angle reverse faults that slice through the Meerenurker thrust as it crosses the above-mentioned lateral ramp on the western edge of the large-scale duplex. Two of the faults, Mount Gordon and Mount Isa, dip steeply west with similar dis- placements of 4 to 6 km west side up. The other two faults, Holly and Tastoka, dip steeply east, with the latter fault having a more significant displacement of 2.5 to 3 km east side up that greatly affects the geology on the eastern side of the Mount Isa Valley. The effects of these faults on the geometry produced by thrusting during the first deformation and folding during the second allow accurate prediction of the third dimension anywhere in the Mount Isa Valley and the region immediately to the west. Consequently, they can be used to predict the location of structurally controlled sites of potential copper mineralization similar to those found in the Mount Isa mine where mineralization formed during the third deformation.

Reconciliation of two-sided thrusting, burial metamorphism, and diachronous uplift in the Cascades of Washington and British Columbia

Several recently published data sets have shed new light on many of the fundamental kinematic aspects of mid-Cretaceous orogeny in the Cascades of northwestern Washington and southwestern British Columbia. Specifically, (a) it is now clear that contractional deformation associated with this orogeny affected rocks east of the crystalline core of the range to a greater degree than had previously been suspected, and (b) geobarometric data from the crystalline core of the range now permit a quantitative reconstruction of the crustal-thickening event responsible for core metamorphism to be attempted. I propose a new kinematic model for mid-Cretaceous orogeny that reconciles these data with previously collected data sets that focused on deformation in the northwest Cascades-San Juan Islands thrust system. The model derived from this analysis suggests that the Cascades underwent a minimum of between 400 and 500 km of east-west shortening between about 115 and 85 Ma, owing to the collision of the greater Insular terrane, which probably included rocks of the Chilliwack-Nooksack terrane, with the western edge of the greater Intermontane terrane, which included the Methow basin and Bridge River-Hozameen terrane. On the basis of their relative and absolute times of uplift, I assign the deepest part of the metamorphic core, the Skagit Gneiss, to the greater Insular terrane, and the structurally higher carapace of the core, the Settler-Nason metapelites and the Cogburn Creek-Mad River metabasalt/metachert, to the westernmost Intermontane terrane. The contact between the Skagit Gneiss and the overlying assemblages must represent a fault or suture across which at least 120 km of displacement took place. This fault is also the root zone from which many of the nappes in the northwestern Cascades and San Juan Islands were derived. I concur with previous workers who suggested that the collisional event was not precipitated by the closure of a several-thousand-kilometer-wide ocean but was rather due to a short-lived margin-normal convergence event that both followed and was followed by periods during which the continental margin was highly influenced by oblique subduction and strike-slip faulting.