Composite Foliation Research Articles

Structural evolution and related implications for uranium mineralization in the Patterson Lake corridor, southwestern Athabasca Basin, Saskatchewan, Canada

The Patterson Lake corridor is situated along the SW margin of the Athabasca Basin and contains several basement-hosted uranium deposits and prospects. Drill core investigations during this study have determined that granite, granodiorite, mafic and alkali intrusive basement rocks are entrained in a deep-seated NE-striking subvertical heterogeneous high-strain zone defined by anastomosing ductile to semi-brittle shears and brittle faults. The earliest phases of ductile deformation (D 1 /D 2 ), linked with Taltson (1.94–1.92 Ga) orogenesis, involved interference between early fold sets (F 1 /F 2 ) and development of an associated ductile transposition foliation (S 1 /S 2 ). During subsequent Snowbird ( c. 1.91–1.90 Ga) tectonism, this composite foliation was re-folded (D 3 ) by NE-trending buckle-style folds (F 3 ), including a regional fold centred on the Clearwater aeromagnetic high. In continuum with D 3 , a network of dextral-reverse chloritic-graphitic shears, with C–S geometry, formed initially (D 4a ) and progressed to more discrete, spaced semi-brittle structures (D 4b ; c. 1.900–1.819 Ga). Basin development (D 5a ; < c. 1.819 Ga) was marked by a set of north-striking normal faults and related east- and NE-striking transfer faults that accommodated subsidence. Primary uranium mineralization (D 5b ; c . 1.45 Ga) was facilitated by brittle reactivation of NE-striking basement shears in response to WSW–ENE-directed compressional stress ( σ 1 ). Uraninite was emplaced along σ 1 -parallel extension fractures and dilational zones formed at linkages between NE- and ENE-striking dextral strike-slip faults. Uranium remobilization (D 5c ) occurred after σ 1 shifted to WNW–ESE, giving rise to regional east- and SE-striking conjugate faults, along which mafic dykes (1.27 and 1.16 Ga) intruded. Thematic collection: This article is part of the Uranium Fluid Pathways collection available at: https://www.lyellcollection.org/cc/uranium-fluid-pathways

Structure and Metamorphism of Markam Gneiss Dome From the Eastern Tibetan Plateau and Its Implications for Crustal Thickening, Metamorphism, and Exhumation

AbstractGneiss domes developed in the eastern Songpan‐Ganzi Flysch (SGF) belt, located in the hinterland of the Longmen Shan Thrust belt of the eastern Tibetan Plateau. These domes have similar pressure‐temperature (denoted as P‐T below) paths and formed during Late Triassic‐Early Jurassic. One of them, the Markam Gneiss Dome (MGD), is cored by granite and is mantled by a metaturbidite aureole. Our structural observations show top‐to‐the‐south shearing that transposed an upright S1 foliation into a flat‐lying NW‐SE striking S2 composite foliation in the Triassic metaturbidites. Thin sections reveal that the metamorphic index minerals biotite, garnet, and staurolite grew prior to the transposition into S2, whereas kyanite and sillimanite crystallized syn‐S2 development. P‐T pseudosections for the metaturbidites indicate a temperature peak of approximately 618–632 °C and a pressure peak of approximately 6–8 kbar. The highest P‐T condition was immediately followed by isothermal decompression, which is represented by the sillimanite overgrowing andalusite and biotite. Granite was emplaced at ~10‐ to 15‐km depth, during isothermal decompression. Zircon U‐Pb geochronology of metaturbidites, granite, and pegmatite of the MGD indicates that (1) the metaturbidites are younger than 240 Ma, (2) the granite was mainly sourced from the metaturbidites, and (3) deformation leading to the transposition of S1 into S2 mainly occurred ca. 215–170 Ma. Our new P‐T‐D‐t results from the MGD confirm those of other gneiss domes in the SGF and indicate Late Triassic‐Early Jurassic crustal shortening and thickening that probably reflects coeval low‐viscosity crustal flow of the SGF.

Gneiss domes of the Danba Metamorphic Complex, Songpan Ganze, eastern Tibet

In this paper we address the formation and exhumation of the Danba Metamorphic Complex (DMC) that represents the deepest structural level of the Songpan Ganze terrane situated along the eastern margin of the Tibetan plateau. The DMC comprises a variety of gneiss domes and offers a unique opportunity to decipher their development during orogenic evolution. For that purpose, PTtD paths of metamorphic rocks sampled at different structural levels have been reconstructed. The DMC is composed of Triassic metaturbidites of the Xikang group, Paleozoic metasedimentary cover and basement of the Yangtze craton. The DMC is structurally marked by transposition of the upright S1 foliation of the Triassic metaturbidites into a NW-SE trending S2 composite foliation dipping to the NE. Transposition is associated with a localized top-to-the-northeast shear zone along the northeastern edge of the DMC and with pervasive top-to-the-southwest shearing from the core to the border of the complex. These structures are consistent with extrusion of the core of the DMC relative to the lower grade Triassic metaturbidites. The position of the biotite isograd overlapping the structural boundary of the DMC suggests that the Triassic metaturbidites have been affected by an increase in temperature as a result of extrusion. Within the DMC, the position of the metamorphic index minerals relative to the composite S2 foliation reveals that biotite, garnet, staurolite and kyanite grew before the transposition into S2, in contrast with sillimanite which crystallizes in the hinge of F2 folds and along the axial planar S2 schistosity. The sillimanite isograd delineates regional-scale overturned F2 folds and cross-cuts the staurolite and kyanite isograds consistent with an increase in temperature during D2. The melt-in isograd characterizes the deepest structural level of the DMC. PT conditions for these metamorphic rocks, determined using pseudosections and conventional thermometry, indicate a temperature increase from 400°C to more than 600°C from the edge to the core of the DMC for a relatively homogeneous pressure ranging from 5 to 6.5kbar suggesting that isograds and isotherms represent the syn-D2 thermal structure of the orogenic crust. Migmatites exposed in the deepest structural level of the DMC yield a pressure significantly lower than the surrounding metamorphic rock suggesting that they crystallized after D2 and after some exhumation of their hosts. Three different types of gneiss domes are distinguished on the basis of their position relative to the isograds, their structural characteristics, and their position relative to the margin of the Yangtze craton. Close to the craton and at the highest structural level, the Gezong dome represents a basement-rooted tectonic slice, in an intermediate position, the Gongcai dome corresponds to a basement-cored nappe, and further away and at the deepest structural level, the Bawang, Cunuchan and Qingaling domes are migmatite-cored domes. The presence at the current-day surface of this variety of gneiss domes reflects the difference in burial of the margin during the Mesozoic Indosinian orogeny.

화개지역에서 영남육괴 지리산 변성암복합체의 지질구조

영남육괴 지리산지구의 남동부에 위치하는 화개지역은 주로 선캠브리아기 지리산 변성암복합체로 구성되어 있으며, 선캠브리아기 구성암류는 남-북 방향의 밀착습곡과 동-서 방향의 개방습곡 형태의 암상분포를 보인다. 이 논문에서는 하동 회장암복합체의 서부에 위치하는 화개지역에서 지리산 변성암복합체의 변형된 암석구조에 대한 운동학적 기하학적 특징과 중첩된 변형구조들의 선후관계로부터 연구지역의 지질구조를 연구하였다. 그 결과, 연구지역의 지질구조는 적어도 세 번의 연성 변형단계를 거쳐 형성되었음을 알게 되었다. (1) <TEX>$D_1$</TEX> 변형과 관련된 구조요소는 대부분 <TEX>$D_2$</TEX> 변형에 의해 전위되어 <TEX>$S_{0-1-2}$</TEX> 복합엽리로 인지된다. (2) <TEX>$D_2$</TEX> 변형은 동-서 방향의 압축 지구조환경 하에서 남-북 방향의 <TEX>$F_2$</TEX> 습곡과 <TEX>$F_2$</TEX> 습곡축면에 (준)평행하게 발달하는 <TEX>$D_2$</TEX> 연성전단대를 형성시켰다. (3) <TEX>$D_3$</TEX> 변형은 남-북 방향의 압축 지구조환경 하에서 발생하여 그 이전 형성된 구조요소들의 방향성을 국부적으로 동북동 내지 서북서 방향으로 재배열시켰다. 이는 선캠브리아기 구성암류의 광역적인 남-북 방향의 밀착습곡과 동-서 방향의 개방습곡 형태의 암상분포는 각각 <TEX>$D_2$</TEX> 변형, <TEX>$D_3$</TEX> 변형과 밀접한 관련이 있음을 의미한다. Hwagae area, which is situated in the southeastern part of the Jirisan province, Yeongnam massif, Korea, is mainly composed of Precambrian Jirisan metamorphic rock complex (JMRC). Lithofacies distribution of the Precambrian constituent rocks mainly shows NS-trending tight fold and EW-trending open fold. This paper researched deformational phased structural characteristics of JMRC based on the geometric and kinematic features and the forming sequence of multi-deformed rock structures, and suggests that the geological structure of this area was formed through at least three phases of ductile deformation. (1) Most of structural elements related to the <TEX>$D_1$</TEX> deformation were recognized as <TEX>$S_{0-1-2}$</TEX> composite foliation which was transposed by the <TEX>$D_2$</TEX> deformation. (2) The <TEX>$D_2$</TEX> deformation occurred under the EW-directed tectonic compression, and formed the NS-trending <TEX>$F_2$</TEX> fold and <TEX>$D_2$</TEX> ductile shear zone which is (sub)parallel to the axial plane of <TEX>$F_2$</TEX> fold. (3) The <TEX>$D_3$</TEX> deformation occurred under the NS-directed tectonic compression, and partially reoriented the pre-<TEX>$D_3$</TEX> structural elements into ENE or WNW direction. It indicates that the distribution of Precambrian lithofacies showing NS and EW-trending folds in the Hwagae area is closely associated with the <TEX>$D_2$</TEX> and <TEX>$D_3$</TEX> deformations, respectively.

영남육괴 지리산지구에서 하동 북부 회장암복합체와 그 주변지역의 지질구조

영남육괴 지리산지구의 남동부에 위치하는 연구지역은 선캠브리아기 지리산 변성암복합체와 하동 북부 회장암복합체 그리고 이들을 관입하는 중생대 화성암류 등으로 주로 구성되어 있고, 지체구조는 한반도의 일반적인 방향인 북동-남서 방향이 아닌 남북 방향으로 발달한다. 본 논문은 하동 북부 회장암복합체와 그 주변부의 지리산 변성암복합체에 발달하는 남북 방향의 지체구조 형성과 관련된 지질구조를 규명하기 위하여 이들 구성암류에 대한 변형단계별 구조적 특성을 연구하였다. 그 결과 연구지역의 지질구조는 적어도 세 번의 변형단계를 거쳐 형성되었음을 알게 되었다. (1) <TEX>$D_1$</TEX> 변형은 <TEX>$S_0$</TEX> 엽리의 상부가 남동쪽으로 이동하는 대규모 연성전단작용으로 지리산 변성암복합체와 하동 북부 회장암복합체에 <TEX>$F_1$</TEX> 칼집 내지 "A"형 습곡과 <TEX>$F_1$</TEX> 습곡축면에 (준)평행한 <TEX>$S_{0-1}$</TEX> 복합엽리 내지 <TEX>$S_1$</TEX> 엽리와 <TEX>$D_1$</TEX> 연성전단대 그리고 <TEX>$F_1$</TEX> 습곡축에 평행한 <TEX>$L_1$</TEX> 신장선구조를 형성시켰다. (2) <TEX>$D_2$</TEX> 변형은 동서 방향의 압축 지구조환경 하에서 <TEX>$D_1$</TEX> 구조요소를 (재)습곡시키는 남북 방향의 <TEX>$F_2$</TEX> 개방, 밀착, 등사, 층간 습곡과 <TEX>$F_2$</TEX> 습곡축면에 (준)평행하게 발달하는 <TEX>$D_2$</TEX> 연성전단대와 <TEX>$S_{0-1-2}$</TEX> 복합엽리 내지 <TEX>$S_2$</TEX> 엽리와 <TEX>$D_2$</TEX> 연성전단대에 수반된 S-C-C' 구조와 <TEX>$L_2$</TEX> 신장선구조를 형성시켰다. 지질도 규모의 <TEX>$F_2$</TEX> 습곡의 날개부에 해당하는 하동 북부 회장암복합체와 지리산 변성암복합체의 동쪽 경계부를 따라서는 <TEX>$F_2$</TEX> 수동적 습곡작용 동안에 입도 세립화와 함께 압쇄구조와 신장선구조를 형성시키는 압쇄암화과정에서 적어도 1.4 km 이상의 폭을 갖고 연장성 있는 남북 방향의 <TEX>$D_2$</TEX> 연성전단대(하동전단대)가 형성되었다. (3) <TEX>$D_3$</TEX> 변형은 남북 방향의 압축 지구조환경 하에서 동서 방향의 <TEX>$F_3$</TEX> 킹크 내지 개방 습곡을 형성시켰고, <TEX>$D_3$</TEX> 변형 이전의 남북 방향 구조요소들을 국부적으로 (동)북동 내지 (서)북서 방향으로 재배열시켰다. <TEX>$D_2$</TEX> 변형 이전의 <TEX>$D_1$</TEX> 광역 지체구조의 방향성은 현재와 달리 북동-남서 방향이었다. 하동 북부 회장암복합체와 그 주변부의 지리산 변성암체에 발달하는 광역적인 남북 방향의 지체구조는 북동-남서 방향의 <TEX>$D_1$</TEX> 지체구조가 <TEX>$F_2$</TEX> 능동 및 수동적 습곡작용에 의해 남북 방향으로 재배열되어 형성되었다. 연구지역의 세 번의 변형작용은 (북)북동 방향의 염기성 암맥군의 관입 집중기로 알려진 고생대 말 이전에 발생하였다. The study area, which is located in the southeastern part of the Jirisan province of the Yeongnam massif, Korea, consists mainly of the Precambrian Hadong northern anorthosite complex (HNAC) and the Jirisan metamorphic rock complex (JMRC) and the Mesozoic granitoids which intrude them. Its tectonic frame is built into NS trend, unlike the general NE-trending tectonic frame of Korean Peninsula. This paper researched the structural characteristics at each deformation phase to clarify the geological structures associated with the NS-trending tectonic frame which was built in the HNAC and JMRC. The result indicates that the geological structures of this area were formed at least through three phases of deformation. (1) The <TEX>$D_1$</TEX> deformation formed the <TEX>$F_1$</TEX> sheath or "A"-type folds in the HNAC and JMRC, and the <TEX>$S_{0-1}$</TEX> composite foliation and the <TEX>$S_1$</TEX> foliation and the <TEX>$D_1$</TEX> ductile shear zone which are (sub)parallel to the axial plane of <TEX>$F_1$</TEX> fold, and the <TEX>$L_1$</TEX> stretching lineation which is parallel to the <TEX>$F_1$</TEX> fold axis owing to the large-scale top-to-the SE shearing on the <TEX>$S_0$</TEX> foliation. (2) The <TEX>$D_2$</TEX> deformation (re)folded the <TEX>$D_1$</TEX> structural elements under the EW-trending tectonic compression environment, and formed the NS-trending <TEX>$F_2$</TEX> open, tight, isoclinal, intrafolial folds with the <TEX>$S_{0-1-2}$</TEX> composite foliation and the <TEX>$S_2$</TEX> foliation and the <TEX>$D_2$</TEX> ductile shear zone with S-C-C' structure and the <TEX>$L_2$</TEX> stretching lineation which is (sub)parallel to the axial plane of <TEX>$F_2$</TEX> fold. The extensive <TEX>$D_2$</TEX> ductile shear zone (Hadong shear zone) of NS trend was persistently developed along the eastern boundary of HNAC and JMRC which would be to the limb of <TEX>$F_2$</TEX> fold on a geological map scale. The Hadong shear zone is no less than 1.4 km width, and was formed in the mylonitization process which produced the mylonitic structure and the stretching lineation with the reduction of grain size during the <TEX>$F_2$</TEX> passive folding. (3) The <TEX>$D_3$</TEX> deformation formed the EW-trending <TEX>$F_3$</TEX> kink or open fold under the NS-trending tectonic compression environment and partially rearranged the NS-trending pre-<TEX>$D_3$</TEX> structural elements into (E)NE or (W)NW direction. The regional trend of <TEX>$D_1$</TEX> tectonic frame before the <TEX>$D_2$</TEX> deformation would be NE-SW unlike the present, and the NS-trending tectonic frame in the HNAC and JMRC like the present was formed by the rearrangement of the <TEX>$D_1$</TEX> tectonic frame owing to the <TEX>$F_2$</TEX> active and passive folding. Based on the main intrusion age of (N)NE-trending basic dyke in the study area, these three deformation events are interpreted to have occurred before the Late Paleozoic.

신보광산 주변지역에서 변성광물의 성장과 변형작용 사이의 상대적인 시간관계: 우라늄 광화대의 상대적인 광화시기

옥천변성대의 남서부에 위치하는 전라북도 진안군 신보광산과 그 동부지역에서 고품위 우라늄 지화학 이상대가 보고된 바가 있다. 본 논문은 선캠브리아기 변성퇴적암류(규암, 변성이질암, 변성사질암)와 이를 관입하는 시대미상의 페그마타이트와 백악기 반암류 등이 분포하는 신보광산 동부지역에서 이들 구성암류가 경험한 변형작용과 변성광물의 성장 사이의 상대적인 시간관계에 대한 미구조의 연구결과를 보고하고, 기존 연구결과를 종합하여 광화작용의 상대적인 시기를 고찰하였다. D1 변형작용은 홍주석 반상변정 내에서 주로 신장된 석영, 흑운모, 불투명광물 등으로 정의되는 직선 형태의 내부엽리 Si 를 형성시켰다. Si 엽리를 미습곡시키고 S2 파랑엽리를 형성시키는 D2 변형작용은 전기 파랑습곡작용과 후기 파랑습곡작용으로 구분된다. 전자는 홍주석의 맨틀부에서 곡선 형태의 Si 를 형성시켰다. 후자는 홍주석을 포획하며 그 외부에서 S1-2 복합엽리를 형성시켰다. 홍주석 반상변정은 직선 Si 엽리에 해당하는 S1 엽리가 형성된 이후의 비변형작용 조건하에서 출현하여 후기 파랑습곡작용 이전까지 성장하였다. 시대미상의 페그마타이트는 D2 변형 이후에 관입하여 S1-2 복합엽리를 무질서하게 덮는 섬유상 규선석을 성장시켰다. D3 변형작용은 S1-2 복합 엽리와 D2 파랑습곡 그리고 섬유상 규선석을 미습곡시키는 F3 습곡을 형성시켰다. 이는 섬유상 규선석의 성장과 관련된 페그마타이트의 관입은 D2 변형과 D3 변형 사이의 비변형조건하에서 발생하였음을 지시한다. 흑운모의 녹니석화작용과 홍주석 반상변정 내부에서 S1-2 복합엽리와 F3 습곡축면에 평행한 두 방향의 벽개 라멜라에 의해 인지되는 후퇴변성작용은 각각 D2 후기변형과 D3 변형과 함께 발생하였다. 따라서 우라늄의 초생적 근원암을 페그마타이트로 간주하는 기존 연구결과를 고려해 볼 때, 시대미상의 페그마타이트는 지각이 상승하는 후퇴변성작용 동안에 해당하는 D2 변형과 D3 변형 사이의 비변형조건하에서 관입하여 신보광산 주변지역에 우라늄 광화대를 형성시킨 것으로 고찰된다. The geochemical high-grade uranium anormal zone has been reported in the Shinbo mine and its eastern areas, Jinan-gun, Jeollabuk-do located in the southwestern part of Ogcheon metamorphic zone, Korea. In this paper is reported the time-relationship between deformation and growth of metamorphic minerals in the eastern area of Shinbo mine, which consists of the Precambrian metasedimentary rocks (quartzite, metapelite, metapsammite) and the age-unknown pegmatite and Cretaceous porphyry which intrude them, and is considered the relative mineralization time on the basis of the previous research's result. The D1 deformation formed the straight-type Si internal foliation which is defined mainly as the arrangement of elongate quartz, biotite, opaque mineral in andalusite porphyroblast. The D2 deformation, which is defined by the microfolding of Si foliation, formed S2 crenulation cleavage. It can be divided into two sub-phases, early crenulation and late crenulation. The former occurs as the curvetype Si foliation in the mantle part of andalusite. The latter occurs as S1-2 composite foliation which warps around the andalusite. The andalusite porphyroblast began to grow under non-deformation condition after the formation of S1 foliation which corresponds to the straight-type Si foliation. It continued to grow before the late crenulation phase. The age-unknown pegmatite intruded after the D2 deformation and grew the fibrous sillimanite which random masks the S1-2 composite foliation. The D3 deformation formed F3 fold which folded the S1-2 composite foliation, D2 crenulation, fibrous sillimanite. It means that the intrusion of pegmatite related to the growth of the fibrous sillimanite took place during the inter-tectonic phase of D2 and D3 deformations. The retrograde metamorphism is recognized by the chloritization of biotite and two-way cleavage lamellae which is parallel to the S1-2 composite foliation and the F3 fold axial surface in the andalusite porphyroblast. It occurred during the D2 late crenulation phase and D3 deformation. In considering of the previous research's result inferring the most likely candidate for the uranium source rock as pegamatite, it indicates that the age-unknown pegmatite intruded during the inter-tectonic phase of D2 and D3 deformations, i.e. during the retrograde metamorphism related to the uplifting of crust, and formed the uranium ore zone around the Shinbo mine.

Tectono-metamorphic evolution of the siliciclastic units in the Middle Tuscan Range (inner Northern Apennines): Mg–carpholite bearing quartz veins related to syn-metamorphic syn-orogenic foliation

This paper accounts for a new finding of High Pressure–Low Temperature (HP-LT) metamorphism documented in continental-derived rocks exposed in the central part of the Middle Tuscan Range, representing the easternmost location, in the Tyrrhenian Region, where blueschist facies metamorphism has been documented. It describes the development mechanism of Mg–carpholite bearing quartz veins in the framework of the deformational events described for the inner Northern Apennines. The HP-LT peak metamorphism can be estimated in P≥1.1GPa and T=370°–420°C as supported by the mineralogical assemblage consisting of quartz–muscovite–chloritoid–pyrophylite–hematite±chlorite±paragonite±Ti-oxides, documented in the aluminous metasediments (pelites and quartzose sandstones) of the Triassic Verrucano succession. This paragenesis defines the S1 schistosity, an axial planar tectonic foliation associated to map- and mesoscale isoclinals folds (F1); S1 schistosity is a composite foliation resulted from a non-coaxial progressive deformation associated to the DE1 collisional event (Late Oligocene–Early Miocene). Mg–carpholite bearing quartz veins developed as tension gashes in en-échelon arrays within brittle/ductile shear zones. The development of quartz veins was guaranteed by the high volumes of Si-rich fluids produced by the dynamic recrystallization process on the siliciclastic rocks.

Progressive deformation across a ductile shear zone: an example from the Singhbhum Shear Zone, eastern India

Structural investigations in the Precambrian Singhbhum Shear Zone of eastern India document an intimate relationship between micro- to meso-scale structures and the deformation history. Shear zone rocks are characterized by composite foliation, a well-developed stretching lineation, folds, shear planes, and quartz veins. These structures reflect thrusting of the Proterozoic north Singhbhum hanging wall block over the Archaean south Singhbhum footwall block. Microstructural analysis of multiple foliation and mylonitic rocks within the shear zone helps to define its progressive evolution. During progressive deformation, overprinting of microstructures resulted in incomplete transposition or complete erasing of previously formed structures and mineral assemblages, allowing room for new dynamic equilibrium structures to form. The dominant deformation mechanism was dissolution–recrystallization, with locally important fluid circulation responsible for transformation of the quartzo-feldspathic mass into phyllonite, and quartzites and schists into mylonite. Textural features suggest that the bulk deformation was non-coaxial, evolving from dominant pure shear in the early stage followed by simple shear in a single progressive strain history of the Singhbhum Shear Zone.

Fabric development in cm-scale shear zones in ultramafic rocks, Red Hills, New Zealand

The Red Hills ultramafic massif, the South Island, New Zealand, contains a suite of cm-scale shear zones that are composed of dunite, pyroxenite, and olivine websterite. Offset on these shear zones was measured using displacement of cross-cutting dikes or distinct compositional foliation layers in the host rock, and is interpreted to occur by dominantly simple shear. Shear zones contain microstructures similar to the host rock, including coarse grain sizes (> 1 mm) and dominantly polygonal grains. The olivine lattice preferred orientation (LPO) in the host rocks is consistent with the (010)[100] slip system, active at high-temperature, dry upper mantle conditions. Within the shear zones, the olivine LPO, when plotted relative to the shear zone foliation and lineation, suggests (010)[001] slip. The shear zone LPO is typically more poorly clustered than in host rock samples and double maxima are observed in some samples, with the second maxima suggesting (010)[100] slip. When olivine LPO from both host rock and shear zone samples is plotted within the same (geographic) reference frame, it is apparent that the shear zone LPO retains evidence for the dominant LPO of the host rocks. The LPO in the shear zone rocks reflects the pre-existing fabric; the poorly clustered data and double maxima reflect the changes that developed during shear zone deformation. These rocks demonstrate the influence of pre-existing LPO on textures formed during deformation and the need to consider the LPO of deformed rocks within different reference frames.

Plane-confined magnetic lineations in mingled mafic and felsic magmas, the Sázava pluton, Bohemian Massif

Multiple magnetic lineations were revealed using the anisotropy of magnetic susceptibility (AMS) and anisotropy of magnetic remanence (AMR) methods in a domain of mafic–felsic magma mingling at the northwestern margin of the Sázava pluton, Central Bohemian Plutonic Complex, Bohemian Massif. Gabbrodioritic enclave swarms and sheets are steeply oriented and exhibit magnetic lineations plunging at steep to moderate angles (mostly 40–70°) whereas in their tonalitic host lineations plunge from vertical to horizontal angles, but mostly less than 40°. The magnetic lineations in both rock types spread along a ~ NNE-SSW steep plane that could be simplified as representing an “average” margin-parallel magmatic foliation in the pluton concordant with an “average” regional cleavage in the wall-rock. The plane-confined lineations are interpreted as having recorded the heterogeneous superposition of two processes: (1) vertical stretching during emplacement and magma mingling which left behind the steep lineations; and (2) regional tectonic stretching, which progressively rotated the mineral grains in rheologically weaker domains (chiefly in the host tonalite) to form the sub-horizontal lineation. The average foliation bearing the multiple lineations is interpreted as a composite foliation that recorded both margin-perpendicular shortening during emplacement, overprinted by coaxial regional tectonic shortening. This example reaffirms that (1) magmatic fabrics in crystallizing magmas can record accumulated strain resulting from both emplacement and regional tectonic deformation; and that (2) separating magmatic (and also magnetic) fabrics related exclusively to the internal chamber processes from fabrics caused by regional tectonic deformation is problematic or even impossible in cases where composite fabrics are recognized.

Ordovician and Late Paleozoic–Early Mesozoic tectonothermal history of the La Noria area, northern Acatlán Complex, southern Mexico: Record of convergence in the Rheic and paleo-Pacific Oceans

The La Noria area lies in the northern part of the polydeformed Acatlán Complex, (southern Mexican Mixteca terrane), and the rocks record the following sequence of events: (i) Early–Middle Ordovician deposition of the volcaniclastic El Epazote and Las Calaveras units; (ii) late Middle Ordovician intrusion of the 467 ± 16 Ma megacrystic, peraluminous, rift-related granitoids; (iii) late Devonian, D 1, greenschist facies deformation; (iv) intrusion of the Los Malpasos leucogranite and associated minor intrusions; (v) Middle Mississippian, D 2, dextral N–S deformation also under greenschist facies metamorphic conditions; and (vi) undated D 3 kink band development. U–Pb LA-ICPMS detrital zircon ages: (a) the El Epazote unit yielded: a mean 206Pb/ 238U age from the youngest five concordant 206Pb/ 238Pb age of 488 ± 10 Ma, with other age peaks at ca. 506, ca. 1077, and ca. 1779 Ma and a few concordant Neoproterozoic ages: and (b) the Las Calaveras unit yielded a mean 206Pb/ 238U age from the sixteen youngest detrital zircons of 466 ± 10 Ma with other population age peaks ca. 1111, and ca. 1753 Ma. These data imply that granitoid intrusion was roughly synchronous with deposition of some of the host rocks. Whereas sources for most of the detrital zircons may be found in either the Acatlán and Oaxacan complexes, Laurentia or Gondwana, a Neoproterozoic source is most likely in Amazonia. The rocks record three low-grade deformational episodes: (i) D 1 produced a weak compositional banding and/or schistosity (S 1) under greenschist facies conditions; (ii) D 2, also occurred under greenschist facies conditions, and developed tight to isoclinal folds (F 2) in S 1 and an axial planar spaced-cleavage (S 2) that is co-planar with S 1; and (iii) D 3 produced reverse and conjugate kink bands (F 3) that deform the S 1/S 2 composite foliation. The leucogranite and related dikes that intrude the complex record only the latter two deformational events. Ca. 330 Ma 40Ar/ 39Ar muscovite plateau ages probably closely post-date the D 2 event. D 1 may be correlated with early Carboniferous deformation elsewhere in the Acatlán Complex. On the other hand, the initial 40Ar/ 39Ar steps at ca. 300, 220 and 172 Ma probably indicate thermal disturbances below 300 °C during the Permo-Carboniferous, Triassic and Jurassic, respectively. Whereas the Ordovician history of the plutons and volcano-sedimentary units coincides with the lifespan of both the Iapetus and Rheic oceans, the Late Paleozoic–Early Mesozoic deformation better reflects closure of the Rheic Ocean and convergence tectonics on the paleo-Pacific margin following the amalgamation of Pangea.

Characteristics of a kilometer-scale high strain zone in the lower continental crust: Mt. Hay block, central Australia

A granulite facies high strain zone was studied on Capricorn Ridge, part of the Mt. Hay block granulites in the Arunta inlier, central Australia. Deformation occurred in these primarily mafic composition granulites at lower crustal conditions (∼8 kb, ∼800 °C) at 1740–1690 Ma. Pervasive and consistently oriented foliation (striking SSW and dipping steeply) and lineations (pitching steeply eastward) occur within the shear zone. The high strain zone is recognized by the presence of SL-tectonites, parallelism of grain shape fabrics and lithologic boundaries, transposition of lithologic boundaries into parallelism with the foliation, and the local occurrence of sheath-like folds. Field-based fabric mapping uses compositional foliation intensity as a proxy for finite strain to document strain variations within the Capricorn Ridge high strain zone. There is no observed microstructural grain size reduction that corresponds to these strain variations. Fabric intensity increases from the centers of the major lithologic units towards the contacts. 100 m-scale strain localization within Capricorn ridge occurs adjacent to major lithologic contacts, indicating competency contrast during lower crustal deformation.

Combining compositional zoning and foliation intersection axes (FIAs) in garnet to quantitatively determine early P-T-t paths in multiply deformed and metamorphosed schists: north central Massachusetts, USA

Quantitative compositional and microstructural analysis of garnet porphyroblasts in kyanite–staurolite–garnet grade rocks from the northeastern flank of the Pelham dome, north central Massachusetts, distinguishes the effects of Acadian deformation and metamorphism from extensive overprinting Alleghanian shearing. The P–T conditions and the metamorphic path during the Acadian were determined using samples preserving well defined stages in a lengthy tectonic history revealed by a succession of five foliation intersection axis trends preserved within porphyroblasts (FIAs). This Acadian succession extends at least 120 km to the north into rocks where no evidence has been found of an Alleghanian overprint. For each sample where garnet first nucleated during one of these stages in the tectonic history, the PT of core growth was determined by plotting the intersection of the Mn, Fe and Ca isopleths calculated for the core composition on a P–T pseudosection for that sample using THERMOCALC. Combining the PT data from all these samples indicates that the temperature and pressure increased throughout Acadian orogenesis, causing episodic garnet growth. During the Alleghanian, locally intense shearing, especially against the margin of the Pelham dome, formed the dominant schistosity, which truncated all foliations defined by inclusion trails in porphyroblasts and obliterated all remains of Acadian deformation and metamorphism in the rock matrix. Shearing was accompanied by near complete homogenization of the compositional zoning in garnet porphyroblasts and an associated apparent increase in the temperature of the matrix to 700°C in those rocks lying directly adjacent to the Pelham dome, and resulted from the rocks of the Northfield syncline being thrust a large distance southwards over the gneisses in the dome.

Mesoproterozoic tectonic evolution of the western Llano uplift, central Texas: The story in an outcrop

A small, but perfectly exposed outcrop of Mesoproterozoic rocks in the western Llano uplift displays lithologic and structural relations from which a sequence of geologic events has been determined. Detailed geologic mapping and petrologic studies combined with new U-Pb ages of key units are used to constrain the tectonic evolution of this little-studied region and provide a basis for comparison with the previously studied southeastern uplift. The outcrop consists of layered mafic and felsic gneiss cross-cut by three generations of granitic dikes and sills. The sequence of events as deduced from cross-cutting relations and U-Pb geochronology is as follows: (1) generation of compositional layering (S 0 ) and parallel biotite foliation (S 1 ) in the gneisses at 1256 Ma; (2) emplacement of the first set of granitic sills and dikes at 1253 Ma; (3) transposition, isoclinal folding, amphibolite facies metamorphism, and fabric formation (S 2 ) between 1253 Ma and 1126 Ma; (4) emplacement of pegmatitic granitic dikes at 1126 Ma; (5) open folding and boudinage between 1126 Ma and 1076 Ma; and (6) emplacement of aplitic granitic dikes at 1076 Ma. Four U-Pb ages of titanite, two each from the gneiss and pegmatitic granite, are concordant at ∼ 1114 Ma indicating that the region had been sufficiently hot to reset titanite ages in the gneiss and that the region had cooled below the titanite blocking temperature by this time. The tectonic evolution of this outcrop is remarkably similar to that of other areas of the Llano uplift, suggesting that the entire uplift experienced a similar evolution.

Subduction- and exhumation-related fabrics in the Paleozoic high-pressure–low-temperature Maksyutov Complex, Antingan area, southern Urals, Russia

We use structural and petrologic data from a cross section through the high-pressure‐lowtemperature Maksyutov Complex to develop a new model for its tectonometamorphic evolution. The Maksyutov Complex is located within the southern Urals, the only Paleozoic orogen that apparently preserved its collisional architecture without overprinting by late orogenic extensional deformation. The high-pressure complex constitutes a large antiform in the footwall of the east-dipping Main Uralian fault and is composed of two tectonometamorphic units. The core of the antiform exposes wellpreserved eclogites and blueschists in the structurally lower unit 1 that underwent peak metamorphic conditions of ~17 kbar and ~570 °C. In contrast, the structurally overlying unit 2 contains lawsonite-bearing assemblages indicating both lower peak pressure (<8 kbar) and temperature (<450 °C). Both units exhibit a composite foliation S 1 affected by northwestvergent F 2 folds. F 2 fold axes and S 1 /S 2 intersection lineations trend northeast-southwest, oblique to the present north-south trend of the Maksyutov antiform. The D 1 /D 2 fabrics record a progressive northwest-directed shearing under prograde metamorphic conditions and are interpreted as the result of eastward subduction beneath the Irendyk island arc during oblique northwest-southeast‐directed plate convergence at 370‐380 Ma. After their subduction to different depths, the structurally lower unit 1 was tectonically juxtaposed against the upper unit 2 by a ductile, top-to-the-northeast extensional D 3 shear zone associated with the retrograde metamorphic evolution. The exhumation of unit 1 occurred in Late Devonian‐Early Carboniferous time, during continuous plate convergence that was accommodated by a thrust that imbricates the basement of the East European platform and is situated below the high-pressure rocks. Further exhumation of the Maksyutov Complex to a shallow crustal level was accomplished by ductile D 4 shear zones exhibiting east-west‐trending stretching lineations present at the margins of the complex. Large-scale folding of the Maksyutov antiform and minor top-to-the-east backthrusting on the western limb took place during a late stage of the Uralian orogeny, coeval with formation of the foreland thrust-and-fold belt in Permian time.

Geometry and kinematics of a major crustal shear zone segment in the Appalachians of southern New Brunswick

In southern New Brunswick the Kennebecasis Fault follows the northern boundary of a crystalline portion of the late Precambrian – Cambrian Avalon terrane. This low-grade crystalline complex forms the basement to a series of Carboniferous through Triassic basins. This complex also contains a major shear zone relic that is largely flat lying but upturned adjacent to the fault, and that with it defines the Pocologan–Kennebecasis fault zone. The orientation of the main composite foliation (S1) and the included mineral (stretching) lineation (L1) indicate that this geometry is a primary feature of the shear zone, representing a linked pair of horizontal and vertical detachments bounding an allochthonous unit. Kinematic indicators show that this allochthon moved parallel to the strike of the north Appalachian orogen, with top towards the west or west-southwest. The bounding shear zone is not uniform, but consists of a mylonite–phyllonite adjacent to the detachment. The upturned segment of the shear zone has been the site of later, brittle reactivation, one episode of which is represented by the Kennebecasis Fault. The main shear zone relic relates to more fundamental events, such as the accretion of the Avalon terrane.

Superposed structures in the Mona Complex at Rhoscolyi Ynys Gybi, North Wales

AbstractThe Bedded Series of the Mona Complex at Rhoscolyn comprises two groups of clastic metasediments: the Holy Island Group, consistingof quartzites, impure psammites and pelites, with well-preserved bedding, is overlain conformably by the New Harbour Group, which is for the most parthomogeneously semi-pelitic without surviving bedding. Both groups have undergone the same two major tectono-metamorphic episodes, but with differing response. In the Holy Island Group the first episode (Dx) produced nearly upright and upward-facing folds (Fx) with an axial planar foliation (Sx), which varies from an anastomosing or rough-spaced cleavage in quartzites to a penetrative phyllitic schistosity in pelites. In the New Harbour Group Dx has generally obliterated original bedding surfaces, replacing them with a composite foliation (Sx) of fine compositional banding and a penetrative schistosity, together with a stretching lineation (Lx), the latter being at a high angle to the Fx axial direction. The Dx structures are attributed to a major episode of compressional tectonics.The structures attributed to the second deformation (Dy) includestrata-bound sets of quartz-filled tension fractures (attributed by most previous authors to an earlier episode), abundant NNW-verging asymmetric folds (Fy) of Sx, and a sporadically developed set of shear fractures which constitute a crenulation cleavage (Sy) axial planar to the folds. It is suggested that all these structures were produced by a single agency. One interpretation is that the observed shear fractures and folded tension fractures correspond fairly closely to and provide a natural analogy of those obtained in the classical simple shear experimentsof Riedel. In this case all the Dy structures can be accounted for by the action of a large-scale simple shear couple (Cy), whose vergence and shallow dip were both towards the NNW. Such a mechanism may imply a gravity-dominated regime of net horizontal extension in a NNW-SSE direction, with extension being less constrained to the north than to the south. J. W. Cosgrove has suggested an alternative interpretation, that all the Dy structures can be explained as reverse kink bands; the simple shear interpretation is here preferred because the angle between Sy and the estimated direction of Pmax during Dy was &lt; 45°; the kink band model would require an angle &gt; 45°.The fact that cleavage vergence boundaries for both Sx and Sy occur close to the hinge zone of the Rhoscolyn Antiform is consistent with either Dx or Dy age for the initiation of this fold. However, when fold limb length (or limb rotation) vergence is considered, the presence of an Fx0 vergence boundary but absence of an Fxy vergence boundary (and by implication of an Fy0 boundary) is consistent with a Dx age but difficult to reconcile with a Dy age.

Forethrusting, backfolding, and lateral gravitational escape in the northern part of the Western Alps (Monte Rosa region)

Research Article| July 01, 1992 Forethrusting, backfolding, and lateral gravitational escape in the northern part of the Western Alps (Monte Rosa region) UWE RING; UWE RING 1Institut für Geologie, Universität Tübingen, Sigwartstrasse 10, D-7400 Tübingen, Germany Search for other works by this author on: GSW Google Scholar OLIVIER MERLE OLIVIER MERLE 2Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar Author and Article Information UWE RING 1Institut für Geologie, Universität Tübingen, Sigwartstrasse 10, D-7400 Tübingen, Germany OLIVIER MERLE 2Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1992) 104 (7): 901–914. https://doi.org/10.1130/0016-7606(1992)104<0901:FBALGE>2.3.CO;2 Article history First Online: 01 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 UWE RING, OLIVIER MERLE; Forethrusting, backfolding, and lateral gravitational escape in the northern part of the Western Alps (Monte Rosa region). GSA Bulletin 1992;; 104 (7): 901–914. doi: https://doi.org/10.1130/0016-7606(1992)104<0901:FBALGE>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 SocietyGSA Bulletin Search Advanced Search Abstract A penetrative composite foliation and stretching lineation in the ophiolite of the Zermatt-Saas-Vanzone region developed under eclogite-facies conditions in mid-Cretaceous time. Eclogite-facies mineral parageneses in this foliation were progressively replaced by blueschist-facies and greenschist- to amphibolite-facies mineral assemblages during Late Cretaceous and mid-Tertiary time and were subsequently completely retrogressed. Kinematic indicators associated with the various stretching lineations of the different metamorphic facies reveal that shearing was initially approximately top-to-west (high-P phase) and changed during the greenschist/amphibolite-facies stage into a top-to-west/northwest orientation. Forethrusting was followed by low-angle normal shearing during the final retrograde phase. This sense of shear was upper block-to-southwest/west-southwest. Unambiguous structural relics of the high-pressure event were not recognized in the gneiss of the Monte Rosa nappe, but the close similarity of the metamorphic evolution of the Zermatt-Saas and Monte Rosa units (including the high-P event) makes a common tectonic history probable.In late Oligocene and Miocene time, the Zermatt-Saas-Vanzone region was affected by an initial phase of backfolding (Mischabel phase, predating top-southwest/west-southwest-directed, low-angle normal shearing) toward the south/southeast followed by a second backfolding event (Vanzone phase). The main geometrical and structural features suggest that the Mischabel phase resulted from vertical extrusion and horizontal intrusion of the ductile basement (Monte Rosa) into shallower units (Antrona and Zermatt-Seas ophiolites) and is in accord with a recently proposed kinematic model for the Central Alps. No clear-cut relations between the second phase of backfolding and the retrograde phase were observed; this suggests that both events took place largely simultaneously. Backfolding northeast of the Simplon line postdated retrograde normal shearing and is of late Miocene age. It is concluded that backfolding in the Zermatt-Saas-Vanzone region cannot be causally linked with backfolding northeast of the Simplon line. 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.

Collapse of the thickened Variscan crust in the French Massif Central: Mont Pilat extensional shear zone and St. Etienne Late Carboniferous basin

A large-scale extensional shear zone is described from the Mont Pilat area in the northeastern part of the French Massif Central. This shear zone is about 2 km thick and is characterized by well-developed N-dipping foliation and stretching lineations. Deformation occurred under conditions of retrograde metamorphism from high-temperature conditions to greenschist facies. It is superimposed on a composite foliation related to the earlier Variscan (380-350 Ma) compressional events associated with southward emplacement of nappes and HP-MT metamorphism. Regional N-S lineations and shear criteria observed at a regional scale indicate a northward displacement of the hanging wall during extension. Large strains locally recorded in thick mylonitic zones suggest that displacement was substantial (some tens of kilometres). Late superimposed extensional brittle deformation and pseudotachylites show that northward displacement on normal faults continued at a shallower depth. A large granitic pluton (Granite du Velay), intruded late in the the Variscan orogeny, was associated with extension. Some small leucogranitic syntectonic melts were emplaced within the shear zone and are strongly deformed. Geochronological (Rb/Sr) data on these leucogranite bodies give an age of 320 Ma and 40Ar 39Ar dating of synkinematic biotites from gneissose rocks in the shear zone yields a plateau age of 313 Ma. These data show that extensional ductile deformation and magmatism were still active during the latest Late Carboniferous. Thus, extension occurred immediately after the well-known late Palaeozoic collision-type compressional tectonics which had significantly thickened the crust. We propose that the extensional tectonism described here is due to the gravitational collapse of the initially thick Variscan crust. The Stephanian intracontinental sedimentary basin of St. Etienne was formed along the hanging wall of the normal shear zone during the late stages of this post-orogenic crustal extension.

Nature and timing of deformation in the Foothills terrane, central Sierra Nevada, California: Its bearing on orogenesis

Detailed mapping and structural analysis combined with new age dates, using U-Pb and ^(40)Ar/^(39)Ar techniques, have allowed us to constrain the timing of pre-ductile and ductile deformation in the Foothills terrane of the central Sierra Nevada. By using strain and other data, it can be shown that rigid rotation of beds (folding/faulting) predated the onset of ductile deformation and probably occurred between 160 and 151 Ma. Ductile structures, consisting of continuous and secondary cleavages and associated folds and lineations, started forming ca. 151 Ma in the Bear Mountains fault zone and then ca. 145 Ma, began moving away from the fault zone, forming diachronously over an ∼30-m.y. period. The last documented ductile structure formed ca. 123 Ma, although some secondary structures may be younger. Metamorphism of these rocks is generally upper greenschist facies, although higher-grade belts (one bearing staurolite, andalusite, and sillimanite) are present. Strain was preferentially partitioned into one of these belts of higher metamorphic grade (and sporadically elsewhere). The structural history here is much more complex, and at least one and locally two complete transpositions of the original cleavage have occurred. In these zones of complex deformation, it is in most cases possible only to identify a composite foliation consisting of new continuous cleavage and relicts of earlier phases, all lying mutually parallel. Timing constraints indicate that the pre-ductile structures may correspond to a very late stage of the Nevadan orogeny (that is, 155 ± 3 Ma), but the ductile structures postdate that orogeny (as defined) by as much as about 25-30 m.y. Models which relate the ductile structures in the central Foothills terrane to Nevadan plate-tectonic events are untenable. In addition, recent work indicates that Late Cretaceous ductile deformation in the central and southern Sierra Nevada may be relatively widespread, indicating that tectonic models for the Sierra Nevada need to be reassessed.