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1.
Structural studies of Lower Permian sequences exposed on wave‐cut platforms within the Nambucca Block, indicate that one to two ductile and two to three brittle — ductile/brittle events are recorded in the lower grade (sub‐greenschist facies) rocks; evidence for four, possibly five, ductile and at least three brittle — ductile/brittle events occurs in the higher grade (greenschist facies) rocks. Veins formed prior to the second ductile event are present in some outcrops. Further, the studies reveal a change in fold style from west‐southwest‐trending, open, south‐southeast‐verging, inclined folds (F1 0) at Grassy Head in the south, to east‐northeast‐trending, recumbent, isoclinal folds (F1 0; F2 0) at Nambucca Heads to the north, suggesting that strain increases towards the Coffs Harbour Block. A solution cleavage formed during D1 in the lower grade rocks and cleavages defined by neocrystalline white mica developed during D1 and D2 in the higher grade rocks. South‐ to south‐southwest‐directed tectonic transport and north‐south shortening operated during these earlier events. Subsequently, north‐northeast‐trending, open, upright F3 2 folds and inclined, northwest‐verging, northeast‐trending F4 2 folds developed with poorly to moderately developed axial planar, crenulation cleavage (S3 and S4) formed by solution transfer processes. These folds formed heterogeneously in S2 throughout the higher grade areas. Later northeast‐southwest shortening resulted in the formation of en échelon vein arrays and kink bands in both the lower and higher grade rocks. Shortening changed to east‐northeast‐west‐southwest during later north‐northeast to northeast, dextral, strike‐slip faulting and then to approximately northwest‐southeast during the formation of east‐southeast to southeast‐trending, strike‐slip faults. Cessation of faulting occurred prior to the emplacement of Triassic (229 Ma) granitoids. On a regional scale, S1 trends east‐west and dips moderately to the north in areas unaffected by later events. S2 has a similar trend to S1 in less‐deformed areas, but is refolded about east‐west axes during D3. S3 is folded about east‐west axes in the highest grade, multiply deformed central part of the Nambucca Block. The deformation and regional metamorphism in the Nambucca Block is believed to be the result of indenter tectonics, whereby south‐directed movement of the Coffs Harbour Block during oroclinal bending, sequentially produced the east‐west‐trending structures. The effects of the Coffs Harbour Block were greatest during D1 and D2.  相似文献   

2.
Abstract

The shape and structural development of the box-like Parrabel Dome (PD) within the Hastings Block is poorly understood because it has only been weakly cleaved, complexly folded and extensively faulted in comparison to the adjoining blocks. Better characterising this block will provide important controls on the tectonics of the southern New England Orogen. The structural development of the PD and southern Hastings Block (SHB) provides evidence of the degree of rotation, translation and deformation of the Hastings Block, a key terrane within the southern New England Orogen. A major decollement under the Hastings Block–Nambucca Block was suggested to facilitate south-directed deformation caused by the developing Coffs Harbour Orocline. The orientation of bedding and the stratigraphic facing of some fault blocks within the northern Hastings Block (NHB) are consistent with development of the PD, while other fault blocks indicate significant disruption of the NHB prior to, during and after dome development. A deep-seated fault is suggested by the gravity worm analysis consistent with the boundary zone between the PD, NHB-Yarrowitch Block and the east-dipping and younging sequences in the SHB. The eastern limb of the PD underwent clockwise rotation after formation. Fault blocks have been rotated and translated within a restraining bend as the NHB moved post-PD formation northwest along the interface between the NHB and SHB.
  1. KEY POINTS

  2. The Hastings Block was translated and rotated into its current position from the southeastern end of the Tamworth Belt.

  3. Gravity worm data indicate a boundary between northern and southern Hastings Block.

  4. The Hastings and Nambucca blocks have been detached from the basement Gondwana rocks.

  5. Fault block analysis within the Parrabel Dome, northern Hastings Block indicates relocation of some blocks by faulting.

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3.
F1 macroscopic folds in the Late Palaeozoic Coffs Harbour Beds in the SE portion of the New England Fold Belt are commonly transected by cleavage. These macroscopic folds are tight to isoclinal structures, with a consistent vergence to the NE. Axial surfaces are either steeply dipping to the SW or vertical, and are typically faulted. Anomalous bedding‐cleavage relations occur where the steeply dipping cleavage intersects overturned limbs of F1 macroscopic and some F1 mesoscopic folds. Elsewhere F1 mesoscopic folds have a well developed, axial‐surface cleavage and are rarely downward facing. Cleavage is commonly strike‐divergent from axial surfaces of F1 macroscopic folds, except adjacent to the Demon Fault System, where they are parallel. These anomalous cleavage‐folds relations possibly developed during the one deformation. D1 structures are refolded by kink‐like folds that are steeply plunging. The structural style of the D1 deformation indicates that it possibly resulted from accretionary processes at a consuming plate margin.  相似文献   

4.
Brittle failure is common in the Devonian to Permian rocks in the Northern Hastings Block (NHB) and is manifested by faults of different orientation and kinematic histories, but the timing of fault movement is not well defined. In this study, faults in the NHB were analysed with the map pattern of cross-cutting faults used to estimate the relative time of movement and relationship to other faults. We defined five episodes of faulting or fault reactivation that affected the NHB. The Yarras Fault System on the southwestern side of the NHB and the Parrabel Fault and related faults on the eastern side of the NHB are the two major fault systems responsible for transporting and rotating the NHB in the late Carboniferous. Faults on the eastern, northeastern and northern part of Parrabel Dome started and stopped moving after emplacement of the Hastings Block and before the intrusion of the Werrikimbe Triassic granitoids. We suggested that the movement on the major bounding faults is related to the accommodation of the NHB to the folding and cleavage development in the adjoining Nambucca Block, and is associated with the earliest part of the Hunter–Bowen Orogeny. Limited dextral movement on the extensions of the Taylors Arm Fault System caused minor displacements in the northeastern part of the NHB during the Late Triassic. Some small faults cut the Triassic granitoids or Triassic Lorne Basin sediments indicating tectonic activity continued post-Triassic.  相似文献   

5.
S1 cleavage in the Hawick Rocks of the Galloway area is non-axial planar, cutting obliquely across the F1 folds in a predominantly clockwise sense. Individual S1 cleavage planes within cleavage-fans in F1 folds strike clockwise, locally anti-clockwise, of axial surfaces, and the mean plane to the S1 cleavage-fans dips predominantly more steeply than the axial surface. F1 folds investigated at scattered localities in Silurian and Ordovician rocks north of the Hawick Rocks are also transected by the S1 cleavage, indicating that non-axial planar S1 cleavage is widespread in the Southern Uplands. The S1 cleavage is a composite fabric. Objects deformed within sandstones and tuffs indicate oblate strain. F1 fold plunge varies from NE to SW and fold hinges locally are markedly curvilinear. Steeply plunging and locally downward-facing F1 folds are present along the southeast margin of the Hawick Rocks. The non-axial planar S1 cleavage relationships persist in the steeply plunging F1 folds. Synchronous development of the non-axial planar S1 cleavage and the variably plunging F1 folds is proposed.  相似文献   

6.
Marble, calc-silicate rock, quartzite and mica schist of Precambrian age in the ‘main Raialo syncline’ in the Udaipur district of central Rajasthan, India, have been affected by folding of four main generations (F1–F4), the first two of which are seen in the scale of map to microsection. The very tight to isoclinal F1 folds with long limbs and thickened hinges are generally reclined or inclined, and plunge gently castward or westward where least reoriented. The axial planes of the F1 folds have been involved in upright warps on east-west axes (F1′), nearly coaxial with the F1 folds, in some sectors. These folds have been overprinted by upright F2 folding of varying tightness with the axial planes striking north to northeast, resulting in interference patterns of different types in all scales. A penetrative axial plane foliation related to F1 folding and a crenulation cleavage parallel to the F2 axial pianes are seen in the micaceous rocks. Two sets of conjugate folds and kink bands of smail scale have been superimposed on the F1–F2 folds in thinly foliated rocks. The first of these sets (F3) has its conjugate axial planes dipping gently northeast and southwest, whereas the paired axial planes of the later set (F4) are vertical with north-northwest and east-west strikes.  相似文献   

7.
Transpressional deformation has played an important role in the late Paleozoic evolution of the western Central Asian Orogenic Belt (CAOB), and understanding the structural evolution of such transpressional zones is crucial for tectonic reconstructions. Here we focus on the transpressional Irtysh Shear Zone with an aim at understanding amalgamation processes between the Chinese Altai and the West/East Junggar. We mapped macroscopic fold structures in the southern Chinese Altai and analyzed their relationships with the development of the adjacent Irtysh Shear Zone. Structural observations from these macroscopic folds show evidence for four generations of folding and associated fabrics. The earlier fabric (S1), is locally recognized in low strain areas, and is commonly isoclinally folded by F2 folds that have an axial plane orientation parallel to the dominant fabric (S2). S2 is associated with a shallowly plunging stretching lineation (L2), and defines ∼NW-SE tight-close upright macroscopic folds (F3) with the doubly plunging geometry. F3 folds are superimposed by ∼NNW-SSE gentle F4 folds. The F3 and F4 folds are kinematically compatible with sinistral transpressional deformation along the Irtysh Shear Zone and may represent strain partitioning during deformation. The sub-parallelism of F3 fold axis with the Irtysh Shear Zone may have resulted from strain partitioning associated with simple shear deformation along narrow mylonite zones and pure shear-dominant deformation (F3) in fold zones. The strain partitioning may have become less efficient in the later stage of transpressional deformation, so that a fraction of transcurrent components was partitioned into F4 folds.  相似文献   

8.
The lead-zinc bearing Proterozoic rocks of Zawar, Rajasthan, show classic development of small-scale structures resulting from superposed folding and ductile shearing. The most penetrative deformation structure noted in the rocks is a schistosity (S 1) axial planar to a phase of isoclinal folding (F 1). The lineations which parallel the hinges ofF 1 folds are deformed by a set of folds (F 2) having vertical or very steep axial planes. At many places a crenulation cleavage (S 2) has developed subparallel to the axial planes ofF 2 folds, particularly in the psammopelitic rocks. The plunge and trend ofF 2 folds vary widely over the area. Deformation ofF 2 folds into hook-shaped geometry and development of another set of axial planar crenulation cleavage are the main imprints of the third generation folds (F 3) in the region. In addition to these, there are at least two other sets of cleavage planes with corresponding folds in small scales. More common among these is a set of recumbent and reclined folds (F 4), developed on steeply dipping early-formed planes. Kink bands and associated sharp-hinged folds represent the other set (F 5). Two major refolded folds are recognizable in the map pattern of the Zawar mineralised belt. The larger of the two, the Main Zawar Fold (MZF), shows a broad hook-shaped geometry. The other large-scale structure is the Zawarmala fold, lying south-west of the MZF. Both the major structures show truncation of lithological units along their respective east ‘limbs’, and extreme variation in the width of formations. The MZF is primarily the result of superimposition ofF 3 onF 2.F 1 folds are relatively smaller in scale and are recognizable in the quartzite unit which responded to deformation mainly by buckle shortening. Large-scale pinching-and-swelling that appears in the outcrop pattern seems to be a pre-F2 feature. The structural evolutionary model worked out to explain the chronology of the deformational features and the large-scale out-crop pattern envisages extreme east-west shortening following formation ofF 1 structures, resulting in the formation of tight and isoclinal antiforms (F 2) with pinched-in synforms in between. These latter zones evolved into a number of ductile shear zones (DSZs). The east-west refolding of the large-scaleF 2 isoclinal antiforms seems to be the consequence of a continuous deformation and resultant migration of folds along the DSZs. The main shear zone which wraps the Zawar folds followed a curved path. Because of the penetrative nature of theF 2 movement, the early lineations which were at high angles to the later ones (as is evident in the west of Zawarmala), became subparallel to the trend ofF 2 folding over a large part of the area. Further, the virtually coaxial nature ofF 2 andF 3 folds and the refolding ofF 3 folds by a new set of N-S folds is an indication of continuous progressive deformation.  相似文献   

9.
Strongly deformed volcaniclastic metasediments and ophiolitic slices hosting the Sukari gold mineralization display evidence of a complex structural evolution involving three main ductile deformational events (D1–D3). D1 produced ENE-trending folds associated with NNW-propagating thrust slices and intrusion of the Sukari granite (689 ± 3 Ma). D2 formed a moderately to steeply dipping, NNW-trending S2 foliation curved to NE and developed arcuate structure constituting the Kurdeman shear zone (≤ 595 Ma) and East Sukari imbricate thrust belt. Major NE-trending F2 folds, NW-dipping high-angle thrusts, shallow and steeply plunging mineral lineation and shear indicators recorded both subhorizontal and subvertical transport direction during D2. D3 (560–540 Ma) formed NNE-trending S3 crenulation cleavage, tight F3 folds, Sukari Thrust and West Sukari imbricate thrust. The system of NW-trending sinistral Kurdeman shear zone (lateral ramps and tear faults) and imbricate thrusts (frontal ramps) forming the actuate structure developed during SE-directed thrusting, whereas the prevailing pattern of NNE-trending dextral Sukari shear zone and imbricate thrusts forming Sukari thrust duplex developed during NE-directed tectonic shearing. Sukari granite intruded in different pluses between 689 and 540 Ma and associated with at least four phases of quartz veins with different geometry and orientation. Structural analysis of the shear fabrics indicates that the geometry of the mineralized quartz veins and alteration patterns are controlled by the regional NNW- and NE-trending conjugate zones of transpression. Gold-bearing quartz veins are located within NNW-oriented sinistral shear zones in Kurdeman gold mine area, within steeply dipping NW- and SE dipping thrusts and NE- and NS-oriented dextral and sinistral shear zones around Sukari mine area, and along E-dipping backthrusts and NW-SE and N-S fractures in Sukari granite. The high grade of gold mineralization in Sukari is mainly controlled by SE-dipping back-thrusts branched from the major NW-dipping Sukari Thrust. The gold mineralization in Sukari gold mine and neighboring areas in the Central Eastern Desert of Egypt is mainly controlled by the conjugate shear zones of the Najd Fault System and related to E-W directed shortening associated with oblique convergence between East and West Gondwana.  相似文献   

10.
In southern Taiwan the initial collision of the Luzon volcanic arc with the passive continental margin of China results in the emergence of an accretionary prism of, predominantly, turbidites in composition, thus providing an appropriate place to study the temporal and spatial variation of deformation during the transition of subduction to arc-continent collision. Field surveys have recently been carried out in slightly metamorphosed rocks along the well-exposed Jinlun-Taimali coast in southeastern Taiwan. Three folding phases are identified in the area. The first phase is characterized by gently dipping but widely distributed phyllitic cleavage (S1). The second phase is represented by sparsely distributed crenulation cleavage (S2) that folded the phyllitic cleavage. The third phase is characterized by E–W trending antiforms (F3) that involved both types of pre-existing cleavages. Restoration of such an antiform in the north using a method proposed in this paper reveals that phyllitic cleavage in the overturned beds dips gently towards the southeast or east-southeast before the antiform, in relation to the first-phase thrusting or folding under regional ESE-WNW compression. From the first to third phase, the maximum horizontal compression underwent an about 90° anticlockwise rotation from ESE-WNW to E–W or NE–SW to N–S, and the deformation depth seems to decrease drastically, in terms of the decreasing proportion of pervasive deformation. All these variations are attributed to the oblique arc-continent collision that exhumed the whole accretionary prism and induced a local stress perturbation in southeastern Taiwan.  相似文献   

11.
In the Rhoscolyn area of Anglesey, the late Precambrian interbedded psammites and pelites of the Monian Supergroup are folded into a kilometre‐scale antiform, plunging about 25°NE and with an axial surface dipping about 40°NW. Numerous folds of up to a few tens of metres in wavelength are present on both limbs of this antiform. These smaller‐scale folds also plunge about 25°NE but clearly belong to two separate episodes of folding, and it has become a matter of longstanding controversy as to whether the larger antiform belongs to the first or second of these episodes. Close examination of the cleavage/bedding asymmetries from all the lithologies, however, shows that the large antiform is a second‐generation structure, and that on the gently dipping northwest limb, the sense of cleavage/bedding asymmetry of the earlier cleavage in the psammitic units has been almost uniformly and homogeneously reversed (so that it appears to be axial planar to the antiform), while in the pelitic units the sense of cleavage/bedding asymmetry of the earlier cleavage has been preserved. Many of the small‐scale complexities of the observed cleavage/bedding relationships may be explained by appealing to differences in the timing of the formation of buckling instabilities relative to this reorientation of the early cleavage in the psammites during the second deformation. A first‐order analysis of the finite strains from around the large‐scale antiform shows that the orientation of the first cleavage prior to the second deformation was steeply dipping to the southeast. The second deformation correlates with the southeast‐verging Caledonian deformation affecting the Monian and Ordovician units elsewhere in northwest Anglesey, while the northwest‐verging first deformation event, which is not present in the Ordovician rocks, must have occurred before they were deposited. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

12.
The ENE-plunging macroscopic folds, traced by calc gneiss interbanded with marble and sillimanite schist within the Peninsular Gneiss around Suganapuram in the ‘Palghat gap’ in southern India, represent structures of the second generation (D2). They have folded the axial planes of a set of D1 isoclinal folds on stratification coaxially, so that the mesoscopic D1 folds range from reclined in the hinge zones, through inclined to upright in the limb zones of the D2 folds. Orthogonal relation between stratification and axial planar cleavage, and ‘M’ shaped folds on layering locate the hinge zones of the D1 folds, whereas folds on axial planar cleavage with ‘M’ shaped folds are the sites of the D2 fold hinges. Extreme variation in the shapes of the isoclinal D1 folds from class 1B through class 1C to nearly class 2 of Ramsay is a consequence of buckling followed by flattening on layers of widely varying viscosity contrast. The large ENE-trending structures in this supracrustal belt within the Peninsular Gneiss in the ‘Palghat gap’ could not have evolved by reorientation of NS-trending structures of the Dharwar tectonic province to the north by movement along the Moyar-Bhavani shear zone which marks the boundary between the two provinces. This is because the Moyar and Bhavani faults are steep dipping reverse faults with dominant dip-slip component. Deceased  相似文献   

13.
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14.
Within the Cambrian Jodoigne Formation in the easternmost part of the Anglo-Brabant Deformation Belt, sub-horizontal to gently plunging folds occur within the limbs of steeply plunging folds. The latter folds are cogenetic with cleavage and are attributed to the Brabantian deformation event. In contrast, although cleavage is also (1) virtually axial planar to the sub-horizontal to gently plunging higher-order folds, shows (2) a well-developed divergent fanning across these folds, (3) an opposing sense of cleavage refraction on opposite fold limbs, and (4) only very small cleavage transection angles, an analysis of the cleavage/bedding intersection lineation suggests that these higher-order folds have a pre-cleavage origin. On the basis of a comparison of structural and sedimentological features these higher-order folds are interpreted as slump folds. The seemingly ‘normal’ cleavage/fold relationship across the slump folds within the limbs of the large steeply plunging folds is due to the very small angle between cleavage and bedding.As such, a ‘normal’ cleavage/fold relationship is no guarantee for a syn-cleavage fold origin. It is not unlikely that also within undeformed, recumbent slump folds, a well-developed compaction fabric, formed parallel to the axial surface of the slump folds, may show fanning and contrasting senses of cleavage refraction on opposite fold limbs.  相似文献   

15.
The Late Ordovician Abercrombie Beds, south of Reids Flat, New South Wales, and adjacent to the Wyangala Batholith, show evidence of three successive fold episodes. First generation folds are tight to isoclinal, with fold axes ranging from vertical to horizontal and north‐trending, and steep axial‐plane slaty cleavage. Second generation folds are steeply plunging, tight to open with north‐striking axial planes. In pelitic rocks the axial plane structure is a crenulation cleavage which overprints the slaty cleavage. The first two fold episodes were accompanied by greenschist‐facies metamorphism. Granite emplacement occurred prior to the second fold episode. A third deformation was of relatively mild intensity and produced open, north‐trending folds with axial planes dipping moderately to the east, and crenulation cleavage as the axial plane structure in pelitic rocks. These latest folds are correlated with the latest folds in the Abercrombie Beds north of the Abercrombie River. The mapped area has no apparent macroscopic structure and may be considered as a single domain.  相似文献   

16.
The Beit Bridge Complex of the Central Zone (CZ) of the Limpopo Belt hosts the 519 ± 6 Ma Venetia kimberlite diatremes. Deformed shelf- or platform-type supracrustal sequences include the Mount Dowe, Malala Drift and Gumbu Groups, comprising quartzofeldspathic units, biotite-bearing gneiss, quartzite, metapelite, metacalcsilicate and ortho- and para-amphibolite. Previous studies define tectonometamorphic events at 3.3–3.1 Ga, 2.7–2.5 Ga and 2.04 Ga. Detailed structural mapping over 10 years highlights four deformation events at Venetia. Rules-based implicit 3D modelling in Leapfrog Geo™ provides an unprecedented insight into CZ ductile deformation and sheath folding. D1 juxtaposed gneisses against metasediments. D2 produced a pervasive axial planar foliation (S2) to isoclinal F2 folds. Sheared lithological contacts and S2 were refolded into regional, open, predominantly southward-verging, E–W trending F3 folds. Intrusion of a hornblendite protolith occurred at high angles to incipient S2. Constrictional-prolate D4 shows moderately NE-plunging azimuths defined by elongated hornblendite lenses, andalusite crystals in metapelite, crenulations in fuchsitic quartzite and sheath folding. D4 overlaps with a: 1) 2.03–2.01 Ga regional M3 metamorphic overprint; b) transpressional deformation at 2.2–1.9 Ga and c) 2.03 Ga transpressional, dextral shearing and thrusting around the CZ and d) formation of the Avoca, Bellavue and Baklykraal sheath folds and parallel lineations.  相似文献   

17.
The moderately metamorphosed and deformed rocks exposed in the Hampden Synform, Eastern Fold Belt, in the Mt Isa terrane, underwent complex multiple deformations during the early Mesoproterozoic Isan Orogeny (ca 1590–1500 Ma). The earliest deformation elements preserved in the Hampden Synform are first‐generation tight to isoclinal folds and an associated axial‐planar slaty cleavage. Preservation of recumbent first‐generation folds in the hinge zones of second‐generation folds, and the approximately northeast‐southwest orientation of restored L1 0 intersection lineation suggest recumbent folding occurred during east‐west to northwest‐southeast shortening. First‐generation folds are refolded by north‐south‐oriented upright non‐cylindrical tight to isoclinal second‐generation folds. A differentiated axial‐planar cleavage to the second‐generation fold is the dominant fabric in the study area. This fabric crenulates an earlier fabric in the hinge zones of second‐generation folds, but forms a composite cleavage on the fold limbs. Two weakly developed steeply dipping crenulation cleavages overprint the dominant composite cleavage at a relatively high angle (>45°). These deformations appear to have had little regional effect. The composite cleavage is also overprinted by a subhorizontal crenulation cleavage inferred to have developed during vertical shortening associated with late‐orogenic pluton emplacement. We interpret the sequence of deformation events in the Hampden Synform to reflect the progression from thin‐skinned crustal shortening during the development of first‐generation structures to thick‐skinned crustal shortening during subsequent events. The Hampden Synform is interpreted to occur within a progressively deformed thrust slice located in the hangingwall of the Overhang Shear.  相似文献   

18.
The wedge‐shaped Moornambool Metamorphic Complex is bounded by the Coongee Fault to the east and the Moyston Fault to the west. This complex was juxtaposed between stable Delamerian crust to the west and the eastward migrating deformation that occurred in the western Lachlan Fold Belt during the Ordovician and Silurian. The complex comprises Cambrian turbidites and mafic volcanics and is subdivided into a lower greenschist eastern zone and a higher grade amphibolite facies western zone, with sub‐greenschist rocks occurring on either side of the complex. The boundary between the two zones is defined by steeply dipping L‐S tectonites of the Mt Ararat ductile high‐strain zone. Deformation reflects marked structural thickening that produced garnet‐bearing amphibolites followed by exhumation via ductile shearing and brittle faulting. Pressure‐temperature estimates on garnet‐bearing amphibolites in the western zone suggest metamorphic pressures of ～0.7–0.8 GPa and temperatures of ～540–590°C. Metamorphic grade variations suggest that between 15 and 20 km of vertical offset occurs across the east‐dipping Moyston Fault. Bounding fault structures show evidence for early ductile deformation followed by later brittle deformation/reactivation. Ductile deformation within the complex is initially marked by early bedding‐parallel cleavages. Later deformation produced tight to isoclinal D2 folds and steeply dipping ductile high‐strain zones. The S2 foliation is the dominant fabric in the complex and is shallowly west‐dipping to flat‐lying in the western zone and steeply west‐dipping in the eastern zone. Peak metamorphism is pre‐ to syn‐D2. Later ductile deformation reoriented the S2 foliation, produced S3 crenulation cleavages across both zones and localised S4 fabrics. The transition to brittle deformation is defined by the development of east‐ and west‐dipping reverse faults that produce a neutral vergence and not the predominant east‐vergent transport observed throughout the rest of the western Lachlan Fold Belt. Later north‐dipping thrusts overprint these fault structures. The majority of fault transport along ductile and brittle structures occurred prior to the intrusion of the Early Devonian Ararat Granodiorite. Late west‐ and east‐dipping faults represent the final stages of major brittle deformation: these are post plutonism.  相似文献   

19.
The Paleoproterozoic Liaohe assemblage and associated Liaoji granitoids represent the youngest basement in the Eastern Block of the North China Craton. Various structural elements and metamorphic reaction relations indicate that the Liaohe assemblage has experienced three distinct deformational events (D1 to D3) and four episodes of metamorphism (M1 to M4). The earliest greenschist facies event (M1) is recognized in undeformed or weakly deformed domains wrapped by the S1 schistosity, suggesting that M1 occurred before D1. The D1 deformation produced small, mostly meter-scale, isoclinal and recumbent folds (F1), an associated penetrative axial planar schistosity (S1), a mineral stretching lineation (L1) and regional-scale ductile shear zones. Concurrent with D1 was M2 metamorphism, which occurred before D2 and produced low- to medium-pressure amphibolite facies assemblages. Regionally divergent motion senses reflected by the asymmetric F1 folds and other sense-of-shear indicators, together with the radial distribution of the L1 lineation surrounding the Liaoji granitoids, imply that D1 represents an extensional event. The D2 deformation produced open to tight F2 folds of varying scales, S2 axial crenulation cleavages and ENE-NE-striking thrust faults, involving broadly NW–SE compression. Following D2 was M3 metamorphism that led to the formation of sillimanite and cordierite in low-pressure type rocks and kyanite in medium-pressure rocks. The last deformational event (D3) formed NW-WNW-trending folds (F3), axial planar kink bands, spaced cleavages (S3), and strike–slip and thrust faults, which deflect the earlier D1 and D2 structures. D3 occurred at a shallow crustal level and was associated with, or followed by, a greenschist facies retrograde metamorphic event (M4).The Liaohe assemblage and associated Liaoji granitoids are considered to have formed in a Paleoproterozoic rift, the late spreading of which led to the occurrence of the early extensional deformation (D1) and the M1 and M2 metamorphism, and the final closing of which was associated with the D2 and D3 phases of deformation and M3 and M4 metamorphism.  相似文献   

20.
Two well-developed mesoscopic folds, D_2 and D_3, which postdate the middle amphibolite metamorphism, were recognized in the western hinterland zone of Pakistan. NW–SE trending D_2 folds developed during NE–SW horizontal bulk shortening followed by NE–SW trending D_3 folds, which developed during SE–NW shortening. Micro- to mesoscopically the NW–SE trending S2 crenulation cleavage, boudins and mineral stretching lineations are overprinted by D_3. The newly established NW–SE trending micro- to mesoscopic structures in Munda termed D_2, which postdated F_1/F_2, is synchronously developed with F3 structures in the western hinterland zone of Pakistan. We interpret that D_2 and D_3 folds are counterclockwise rotated in the tectonic event that has evolved the Hazara Kashmir Syntaxis after the main phase Indian plate and Kohistan Island Arc collision. Chlorite replacement by biotite in the main matrix crenulation cleavages indicates prograde metamorphism related with D_2. The inclusion of muscovite and biotite in garnet porphyroblasts and the presence of staurolite in these rocks indicate that the Barrovian metamorphic conditions predate D_2 and D_3. We interpret that garnet, staurolite and calcite porphyroblasts grew before D_2 because the well developed S2 crenulation cleavage wraps around these porphyroblasts.  相似文献   

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