Intrafolial Folds Research Articles

Alpine structures in northwestern Calabria, Italy

The Tyrrhenian Coastal Chain of northwestern Calabria is underlain by five tectono-stratigraphic units that represent large thrust sheets originating in the Tyrrhenian area and emplaced in Calabria before late Miocene time. Rocks of these nappe structures were originally Mesozoic-Tertiary platform-type carbonates, Jurassic-Cretaceous oceanic crust, and segments of a Hercynian Paleozoic microcontinent. Calcareous-dolomitic, metavolcanic, and metasedimentary rocks of the three geometrically lowest units exhibit similar structural successions and comparable meta-morphic patterns. The first deformational event recognized in such terranes generated recumbent, isoclinal, intrafolial folds with an axial-surface schistosity or slaty cleavage virtually parallel to bedding or volcanic layering. This was accompanied by metamorphism in the blueschist and lower greenschist facies and was followed by a second main deformational phase, which formed tight, inclined to recumbent folds with various types of axial-plane crenulation cleavages. Deformation and metamorphism are interpreted to have occurred before nappe emplacement, whereas during overthrusting, previous linear features such as L2 lineations underwent reorientation toward parallelism, with the stretching direction generating a structural pattern that was radial in relation to the Calabrian arc. The uppermost tectono-stratigraphic units made of Hercynian gneissic terranes display early structures largely obliterated by Alpine cataclasis and retrograde metamorphism. Two minor deformational phases occurred throughout the nappe pile and predated the late Miocene postorogenic transgression. The Pliocene-Quaternary arching of this foldbelt was accompanied by widespread normal faulting, which determined the present outcrop pattern.

The structure and stratigraphy of the anorthosite complex in the area north of Bjørnesund, near Fiskenæsset

Over the area extending about 15 km north of the inner part of Bjørnesund, near Fiskenæsset (fig. 16), numerous conformable layers of anorthositic rocks occur within gneisses. The rocks have been metamorphosed in the amphibolite and granulite facies, and affected by five episodes of folding. Only minor intrafolial and isoclinal folds have been recognised from the early phases (F1 and F2) but major isoclinal folds which were initially recumbent with NE trending axes were produced by F3 folds. These were subsequently refolded by F4 and F5 fold phases which produced major and minor tight to open upright folds with axial planes striking BSE and NE respectively. It is possibie that the anorthosites have been affected by all five episodes of folding, but only F3 to F5 have been recognised in these rocks with certainty.

Structure and metamorphism of the Lewisian of east Tiree, Inner Hebrides

Synopsis A study of the Lewisian of east Tiree has shown the presence of previously unmapped psammitic, calcareous and semipelitic paragneisses which occur, often with basic gneisses, in the ‘grey gneiss’ of the area. Four main phases of folding have been determined: F 1 —isoclinal intrafolial folds; F 2 —tight similar folds; F 3 —close and open folds with NW.–SE. axial planes; and F 4 —open and gentle folds with NE.–SW. axial planes. Fold phases (F 1 –F 3 ) have been related to successive metamorphic episodes of granulite facies, upper amphibolite facies and lower amphibolite facies which have been assigned to the Scourian, Early and Late Laxfordian respectively on the basis of their structural relationship with intrusive basic bodies which are now amphibolites. The relations of the ‘grey gneiss’ to the paragneisses and basic gneisses suggest the existence of a pre-Scourian basement. The region is cut by late shears associated with the formation of pseudotachylite. The sequence in Tiree is related to the structural and metamorphic sequences given by other authors for the Outer Hebrides.

Kink Bands along the Denali Fault, Alaska

The possible application of kink bands in dynamic structural analysis is shown by a recent study along a short segment of the Denali fault, Alaska. In laboratory experiments, kink-band planes form at measured angles of 45° to 60° with the maximum principal stress (σ 1 ) in properly oriented foliated rocks; this relation can be applied in the study of fault-stress histories. Two stress patterns are postulated during the history of the Denali fault; one is compatible with reverse or thrust faulting and is based primarily on the geometry of intrafolial folds, slip cleavage, and crinkles. A second (and presumably younger) stress pattern is compatible with strike-slip movement and is based solely on kink-band geometry.

Structure of the Lewisian rocks between Strath Dionard and Rhiconich, Sutherland, Scotland

Synopsis The Lewisian rocks of the Strath Dionard–Rhiconich district consist essentially of acid gneiss, amphibolite and calc-silicate rock (Rhiconich group) with pegmatitic, granitic and basic minor intrusions (post-Rhiconich group rocks). The earliest structural element in the Rhiconich group is a lithological layering, possibly an original sedimentary feature, now largely obscured due to the subsequent deformation and metamorphism. Penetrative foliation associated with small intrafolial folds and mineral lineation related to the deformation of the Gualin phase are affected by generally asymmetrical folds including the large Strath Dionard antiform and the associated structures related to the Dionard phase of deformation. The later open folds, pseudotachylitic crush-rock and joints are related to the Leacach phase of deformation. The three phases of deformation were variously accompanied by recrystallisation and migmatitic activity and developed in a series of rocks comparable with those of the southern belt of Lewisian at a crustal level above that of the pyroxene-granulites of the central belt.

Polyphase Deformation in the Archean Basement Complex, Beartooth Mountains, Montana and Wyoming

Minor tectonic structures and related metamorphic mineral growths in the Archean crystalline rocks (circa 2.7 b.y.) of the Beartooth Mountains indicate the effects of at least four successive phases of deformation. The most intense metamorphic recrystallization was associated with the formation of intrafolial passive flow folds in the earliest recognized phase. Flexural-flow folds, which are probably parasitic on the major structures of the region, and an associated secondary foliation, were developed subsequently. At least two successive sets of flexural folds, with related axial-planar structures, were formed in later deformational phases. The common granitic rocks of the region, the quartzo-feldspathic gneisses, are of early development. Cross-cutting pegmatitic veins were emplaced after the formation of the structures of the second phase while xenolith-bearing granite is of late formation. The tectonic, metamorphic, and igneous events recognized are considered to have been developed during a major erogenic episode which is named the Beartooth orogeny.

Superposed deformations of the Ketilidian gneisses in the Sârdloq area, South Greenland

A large part of the Sårdloq area comprises intercalated horizons of homogeneous gneiss, veined gneiss and banded gneiss of Precambrian age. The gneisses were deformed during the Ketilidian orogeny by three phases of folding, F1-F3. F1 are in the form of mesoscopic intrafolial folds in varying states of preservation within the regional foliation, S1. F2 are mesoscopic and macroscopic folds that have a shallow plunge to the NE and SW, where unaffected by the third folding. Asymmetrical, similar, disharmonic folds with curviplanar axial planes but constantly oriented axes are the characteristic F. structures formed under strong lithological control mostly in the veined gneiss horizons under syn-migmatisation and amphibolite facies conditions. The crystallisation of constituent minerals taking place under syn- to post-kinematic conditions outlasted the deformation. S. axial plane structures are rarely developed. F3 folds have a constant orientation with vertical axial planes and plunge variably to the NW or SE. The F3 folds are in the form of a macroscopic buckling of the S1 foliation formed under post-migmatisation and epidote-amphibolite facies conditions. A tectonic mélange was locally formed in response to the third phase movements. A structural correlation is made with neighbouring areas. It is suggested that the metamorphism and migmatisation are diachronous with respect to the fold phases, arriving during the third folding at a higher level in the Nanortalik area to the SE.