Small Folds Research Articles

The eastern Jura: Relations between thin-skinned and basement tectonics, local and regional

The thesis area of Hans Cloos in the eastern Jura straddles the boundary between the Paleogene structures of the Tabular Jura, a part of the southeastern end of the Rhine graben, and the Neogene folds and thrusts of the Folded Jura. A recent reflection seismic survey in the area adjacent to the east provides new information on the subsurface structure and the relation between three superposed tectonic events. The main boundary fault zones of a deep and complexly faulted late Paleozoic trough, trending ENE, were reactivated in the early Tertiary. Faulted flexures and small folds, rooted in the basement and Paleozoic, with an amplitude of usually 100 to 300 m were produced, rather than the normal faults typical for the extensional fields of the Rhine-Bresse graben system in the west and the Bavarian Molasse basin in the east. This suggests their being part of a transform zone between these extensional fields. During Jura decollement in the Neogene these inherited, deeply rooted sub-decollement structures remained passive: their small size is kinematically incompatible with the intense subsequent thrusting and folding of the thin Mesozoic-Tertiary skin. However, thrust ramps were apparently nucleated at the respective southern foot of these Paleogene flexures and folds. The best defined and simplest of these composite structures is the Mandach structure within the Tabular Jura, a south-facing flexure of about 300 m relief, developed in the Paleogene by reactivation of the northern border fault zone of the Paleozoic trough. In the Neogene a north-vergent decollement thrust, which is connected with the regional Jura decollement in the middle Triassic evaporites, was initiated at its site.

Structure of the Keel‐Locking Mechanism in Insect‐Pollinated and Self‐Pollinated Alfalfa Species1

Keel petals of the papilionaceous legume Medicago were studied by scanning and transmission electron microscopy to further elucidate the flower‐tripping mechanism. The structures responsible for closure of keel petals in untripped flowers were compared for insect‐pollinated, perennial alfalfa (M. sativa L.), and self‐pollinated, annual M. scutellata (L.) Mill. In both species, the surfaces where the keel petals are locked together are characterized by parallel ridges and grooves. In the closed condition, the ridges on one petal are sunken into the grooves of the opposing petal. The large cell‐wall ridges of M. sativa intermesh in a tongue and groove arrangement and in microscopic sections resemble interlocked jigsaw puzzle pieces. Keel petal ridges of M. scutellata are small parallel folds of the epidermal cuticle, so that strength of attachment between keel petals is weak. The tongue and groove closu?e of the M. sativa keel is consistent with the view that moisture tension facilitates tripping of alfalfa flowers by pollinators. We speculate that intermeshed cell‐wall ridges partially collapse with a loss of turgidity and are more easily disengaged by pollinator activity.

Tectonic evolution of the west Spitsbergen fold belt

Abstract The west Spitsbergen fold belt has a complex tectonic history which is recorded in a thick, nearly complete upper Proterozoic-Phanerozoic layered sequence. Work since 1977 near Bellsund allows recognition of the main deformational events in that segment of the fold belt. The strata are grouped informally into the metamorphosed Hecla Hoek (HH) sequence (Proterozoic) and the Van Keulenfjorden (VK) sequence (Carboniferous-Cretaceous), separated by a pronounced unconformity. The HH is divided into the Antoniabreen succession, the Chamberlindalen succession, and the Kapp Lyell tillite; the first two consist of diverse clastic and carbonate rocks, along with some volcanic rocks. The VK is a platform sequence of shallow marine and terrestrial sedimentary rocks. These layered rocks are intruded by a few Mesozoic dolerite sills and dikes. Subhorizontal Paleogene sedimentary rocks are preserved in a small graben. Both layered sequences have undergone strong deformation. Structures (mainly Caledonian age) in the HH can be grouped as phase 1 (small isoclinal folds, subhorizontal axial planar foliation, ridge-groove lineation in the foliation, and large recumbent folds), phase 2 (tight to isoclinal folds, axial planar foliation), and younger (weak folds and foliations, kink bands, crinkles). Structures (mainly Tertiary age) in the VK include faults (thrust, reverse, down-to-the-east bedding-plane normal, other normal); folds (symmetric, asymmetric, overturned, recumbent, isoclinal); and foliation (in some tightly folded Triassic shales). Six deformational events can be identified here, each described below in terms of age, intensity, and kinematic pattern: • D1 - Vendian or early Paleozoic; very strong; NNW-SSE shortening?, NNW transport? • D2 - Early Paleozoic (pre-Carboniferous); strong; NE-SW shortening, NE vergence. • D3 - Middle (?) Carboniferous; moderate; unclear (limited exposure). • D4 - Early Cretaceous (?); weak; extension, direction unclear. • D5 - Early (?) Tertiary; very strong locally; uplift in W, ENE-WSW shortening, ENE translation. • D6 - Middle (?) Tertiary; moderate; NE-SW extension.

The relationship between foliation and strain: an experimental investigation: reply

Contrary to the criticisms of our ideas by Treagus, we still believe that our experiments show that a secondary preferred orientation of micas or of small kink-like folds, closely resembling similar features in some cleaved rocks, can form at large angles to the λ 1λ 2 plane of strain, where the strain referred to is the strain coeval with development of these features.

Relationship of spaced cleavage to folds and thrusts in the Idaho-Utah-Wyoming thrust belt

The Idaho-Utah-Wyoming thrust belt of the Rocky Mountain Cordillera preserves structures that developed at very shallow levels of the crust ( T < 200°C). The geometry of major structures is well documented and synorogenic conglomerates record the timing of movement on major thrusts. Spaced cleavage and pencil fracturing are developed within micrites of the Jurassic Twin Creek Formation. The cleavage developed dominantly by pressure-solution early in the deformation history of each thrust sheet, aided by elevated temperatures and pressures in areas where increased overburden resulted from emplacement of earlier, overriding sheets. Cleavage within any sheet is well developed in the area covered by the pre-erosional extent of the previous sheet; forelandward from this area cleavage intensity decreases drastically. Thermal modelling of the sheets agrees well with temperature estimates from illite crystallinity studies on material from the cleavage planes. Cleavage is axial planar to small folds and formed concurrently with small-scale buckle folds and contraction faults during the early history of movement on each thrust. However, cleavage strongly fans about large-scale folds formed later during movement, suggesting passive rotation of cleavage with bedding during the later stages of thrust history. The rotated cleavage may also be folded and overprinted by a second weaker cleavage during a second ramping, or during movement on a later, lower thrust. Cross-folding may also develop as a result of lateral ramping. Thus, complex patterns of interfering folds, multiple cleavages, and associated fiber-filled fractures may be produced during motion on a single thrust fault, and so magnify the problem of correlating cleavages and deciphering multiple deformations.

Surface and Subsurface Structural Analysis of a Part of Washita Valley Fault Zone, Southern Oklahoma: ABSTRACT

The Washita Valley fault zone is one of the major northwest-trending structures in southern Oklahoma. This fault system is believed to have originated as a series of normal faults during the formation of the southern Oklahoma aulacogen by late Precambrian or early Cambrian time and to have been reactivated during the Arbuckle orogeny in the Pennsylvanian. Descriptions of movement along the Washita Valley fault zone during Pennsylvanian deformation include numerous interpretations, the most common being left-lateral strike slip with 30-40 mi (50-65 km) of displacement. Structures in the area, however, suggest an alternate model. A detailed field study of small folds, faults, fracture arrays, slickensides, and drainage patterns was conducted along the southeastern half of the Washita Valley fault zone. An attempt has been made to relate these small-scale features to the major structures in the area to determine the orientation of the major compressive stress during deformation and the relative amounts of strike-slip vs. reverse dip-slip movement along the fault zone. Exploration for oil and gas along the Washita Valley fault zone has identified several overturned folds and repeated sections. Field observations in the study area include small drag folds and thrust faults parallel to the trend of the Washita Valley fault zone. The two major anticlines in the area, the Arbuckle and the Tishomingo, are both nearly parallel to the trend of the fault zone. These data suggest a model of deformation involving a large component of reverse dip-slip faulting with major duplication of strata. End_of_Article - Last_Page 514------------

The Macropodid Oesophagus I. Gross Anatomical, Light Microscopic, Scanning and Transmission Electron Microscopic Observations of its Mucosa

The results of examinations on the oesophagus of 248 animals of 36 species of macropodid marsupials are presented in this paper. Four distinct morphological types of oesophageal lining are described and the species examined classified accordingly. The oesophagus of Aepyprymnus: Bettongia, Dendrolagus, Hypsiprymnodon, Onychogalea; Petrogale; Peradorcas, Setonix and Thylogale has a smooth lining with small and simple longitudinal folds (type 1). The large kangaroos of the genus Macropus (M. antilopinus. M. bernardus, M. fuliginosus, M. giganteus. M. robustus, and M. rufus) have smooth oesophageal linings with extensively pleated longitudinal folds (type 11). The so-called brush wallabies of the genus Macropus (M. agilis, M. dorsalis, M. eugenii, M. irma; M. parma and M. parryi) and Wallabia bicolor have papillated oesophageal linings (type 111). In the red-necked wallaby M. rufogriseus banksianus and the Bennett's wallaby M. r. rufogriseus the oesophagus proximal to the diaphragm is lined with longitudinal leaf-like, laminar plates. and distal to the diaphragm the lining consists of finger-like papillae (type IV). The morphology of the oesophagus in the various macropodid species is discussed in relation to dietary preferences, habitat. and phylogenetic associations between species.

Role of subunit interactions in the self-assembly of oligomeric proteins

In oligomeric proteins, the native conformation and its functional properties depend on the interactions which exist between the different chains. The role of these subunit interactions can be studied using either the unfolded state or the native state as a starting point. During the folding process, the properties which appear following a bimolecular reaction are related to the formation of an association area. Similarly, the properties which are lost upon partial dissociation of the native state are related to the association area which is disrupted. Four examples are presented in this article: phosphofructokinase and aspartokinase-homoserine dehydrogenase from E. coli are studied through their folding process, and fatty acid synthetase from B. ammoniagenes and reptilian ovomacroglobulin are studied through their dissociated forms. In all cases, the function of the protein is a sensitive index of the formation of the subunit interactions, and can be more conveniently measured than other size/shape parameters. The extrapolation from the folding of small proteins to the assembly of large and complex structures can be reasonably achieved by admitting that subunit interactions are coupled to the subtle adjustments required by the protein to exert its biological function.

Karstic surface in the Lower Permian sabkha sequence of the Gipshuken Formation, central Spitsbergen, Svalbard

Some unusual karst structures occur in the upper part of the evaporite-dominated sequence of the Gipshuken Formation. This Lower Permian unit is characterized by interbedded anhydrite and dolomites, and is now interpreted in terms of superimposed sabkha cycles. The karst structures are found in the inner part of Skansdalen in Dickson Land, and have not yet been observed elsewhere in corresponding horizons in Svalbard. These structures, often seen as linked hemispheroids. consist of almost pure anhydrite and are here interpreted as representing the remnants of consolidated sabkhas; the original sabkha plain was flooded and partly dissolved, and abandoned channels between the hemispheroidal structures were then filled with sediments of later sabkha cycle. The younger sediments which fill the relief between and above the structures contain small enterolithic folds which indicate primary formed anhydrite. Anhydrite is still the most common subsurface mineral in these sulphatic deposits. an3 there is no evidence of gravitational or tectonic movements within these beds.

Erratum: Kelvin–Helmholtz Instability in a Layered Newtonian Fluid Model of the Geological Phenomenon of Rock Folding

The prototype structural geology problem of the initiation of plane folding in a single more competent rock layer, where both that layer and its embedding medium are originally undergoing a uniform parallel-layer compression at a constant rate of strain, is reexamined. The two-dimensional mathematical model employed represents this situation in terms of a layered quasistatic heterogeneous Newtonian fluid, including the effects of gravity and surface tension, which were ignored during previous investigations of small scale folds. The critical conditions for the onset of such folding are developed by performing the relevant linear stability analysis of an appropriate planar interface solution to this governing system of equations using a modified normal mode approach. These stability results, which depend on both the imposed rate of strain and the layer thickness, are then used to explain the formation of short wavelength minor folds with particular emphasis on the Castile multilayer of Southern New Mexico,...

Tectonic analysis of the Dead Sea Rift Region since the Late‐Cretaceous based on mesostructures

We have measured the orientations of small faults, slickensides, tectonic stylolites, vein with secondary mineralization, small folds and dikes in Israel and Sinai. These structures are indicators of paleo‐stress or strain and provide the pattern of tectonic deformation. Data were collected at 130 stations, most with tens of separate measurements; most stations showed consistent deformation. Stations were located on exposures ranging from Precambrian crystalline rocks to Pleistocene sediments. We have defined two tectonic stress fields, each relatively uniform in both time and space. One stress field, with dominating maximum horizontal compression trending W to WNW, in the Late Cretaceous to Eocene rocks in the folds and plateaus west of the Dead Sea rift. The second field, with dominating horizontal extension trending E to ENE, in all rocks inside the rift and proximal thereto. The first stress field is called the Syrian Arc stress, and the second is called the Dead Sea stress. A change in style of tectonic deformation, which corresponds to the two stress fields, is manifested also in the major structures in Israel. The Late Cretaceous to Neogene deformation is characterized by long wavelength folds and monoclines, whereas the Neogene to Recent deformation is characterized by normal and strike slip faults and volcanic activity.

Structure of the Ordovician succession around Aberdaron, southwest Llŷn, North Wales

AbstractThree deformation phases are recognizable within the Lower Ordovician metasedimentary sequence of the Aberdaron area and they are similar to those described for Lower Palaeozoic sequences in other parts of North Wales. There is no certain evidence however for a major Aberdaron Syncline as described by some previous workers. The first deformation phase produced southcast verging mesoscopic folds with steep to moderate dipping axial surfaces and a sporadic axial plane cleavage. The second deformation was relatively weak and produced only a low‐dipping crenulation cleavage at a few favoured localities. The third phase gave rise to numerous small buckle folds, kinks in some pelitic units where the first cleavage was well developed, an axial plane cleavage, and a suite of quartz veins. The orientation of the third phase minor structures is not uniform and the fold trend and strike of axial plane cleavage varies from east‐northeast to south‐southeast, although retains a constant angular relationship to the local strike of bedding. The distribution of the third cleavage is bimodal and the third deformation phase may have been brought about by conjugate shears during a late brittle fracture stage of NW–SE compression. The structural sequence affecting the Ordovician cannot be correlated with that in the Mona Complex and it seems likely that the Mona Complex was deformed before the Arenig.

Late Jurassic Tectonism on West Side of Colorado Plateau, Utah and Arizona: ABSTRACT

Detailed sedimentologic studies in south-central Utah and north-central Arizona indicate that several major basins and uplifts as well as many smaller folds within them were actively growing during deposition of the Salt Wash Member of the Morrison Formation. The region lies outside that part of the Colorado Plateau underlain by thick Pennsylvanian halite deposits so the structures are not related to salt deformation. Tectonic activity in the region is inferred from several types of sedimentologic features which include thickness variations, facies distribution, cross-bedding parameters, and bedding ratios. Distribution of the various features studied indicates the Emery, Circle Cliffs, Echo Cliffs-Kaibab, Cow Springs, and Monument uplifts, as well as the Henry and Kaipar wits basins, were active during the Late Jurassic. In addition, several anticlines and synclines that were active at the same time can be delineated in or near the Henry and Kaiparowits basins. The apparent absence of local angular unconformities in the Salt Wash near the positive structures suggests that most of the tectonic movements were the result of differential subsidence rather than uplift of the positive structures. It could not be determined if the San Rafael swell near the Emery uplift was active during Late Jurassic time. Also, the relationships are not entirely conclusive but suggest that the Cow Springs uplift may have extended southeast across Black Mesa and that downwarping may not have occurred there in the Late Jurassic. Late Jurassic folds have essentially the same northwest to north-northwest trend as many of the Laramide and possibly younger folds in the region although the older folds tend to be less sinuous. Vertical repetition of lacustrine strata, deposited under conditions especially sensitive to slight tectonic movements, suggest that the movements were episodic. Comparison of the present amount of structural relief with the Late Jurassic structural relief indicates that approximately 3 to 7% of the present-day relief between several of the major basins and uplifts can be attributed to tectonic movements during deposition of the Salt Wash. Thus, it is erroneous to assume that all of the structural deformation in the region occurred during or after the Laramide orogeny. End_of_Article - Last_Page 1351------------

Karstic surface in the Lower Permian sabkha sequence of the Gipshuken Formation, central Spitsbergen, Svalbard

Some unusual karst structures occur in the upper part of the evaporite-dominated sequence of the Gipshuken Formation. This Lower Permian unit is characterized by interbedded anhydrite and dolomites, and is now interpreted in terms of superimposed sabkha cycles. The karst structures are found in the inner part of Skansdalen in Dickson Land, and have not yet been observed elsewhere in corresponding horizons in Svalbard. These structures, often seen as linked hemispheroids. consist of almost pure anhydrite and are here interpreted as representing the remnants of consolidated sabkhas; the original sabkha plain was flooded and partly dissolved, and abandoned channels between the hemispheroidal structures were then filled with sediments of later sabkha cycle. The younger sediments which fill the relief between and above the structures contain small enterolithic folds which indicate primary formed anhydrite. Anhydrite is still the most common subsurface mineral in these sulphatic deposits. an3 there is no evidence of gravitational or tectonic movements within these beds.

Kelvin–Helmholtz Instability in a Layered Newtonian Fluid Model of the Geological Phenomenon of Rock Folding

The prototype structural geology problem of the initiation of plane folding in a single more competent rock layer, where both that layer and its embedding medium are originally undergoing a uniform parallel-layer compression at a constant rate of strain, is reexamined. The two-dimensional mathematical model employed represents this situation in terms of a layered quasistatic heterogeneous Newtonian fluid, including the effects of gravity and surface tension, which were ignored during previous investigations of small scale folds. The critical conditions for the onset of such folding are developed by performing the relevant linear stability analysis of an appropriate planar interface solution to this governing system of equations using a modified normal mode approach. These stability results, which depend on both the imposed rate of strain and the layer thickness, are then used to explain the formation of short wavelength minor folds with particular emphasis on the Castile multilayer of Southern New Mexico, wherein thin planar layers are often found interspersed among thicker folded ones on length scales of 1 mm to 1 cm. The folding-type instability produced is termed Kelvin–Helmholtz because of its similarity in origin to those waves generated by the action of wind over water.

Self-splicing RNA: Autoexcision and autocyclization of the ribosomal RNA intervening sequence of tetrahymena

In the macronuclear rRNA genes of Tetrahymena thermophila, a 413 bp intervening sequence (IVS) interrupts the 26S rRNA-coding region. A restriction fragment of the rDNA containing the IVS and portions of the adjacent rRNA sequences (exons) was inserted downstream from the lac UV5 promoter in a recombinant plasmid. Transcription of this template by purified Escherichia coli RNA polymerase in vitro produced a shortened version of the prerRNA, which was then deproteinized. When incubated with monovalent and divalent cations and a guanosine factor, this RNA underwent splicing. The reactions that were characterized included the precise excision of the IVS, attachment of guanosine to the 5′ end of the IVS, covalent cyclization of the IVS and ligation of the exons. We conclude that splicing activity is intrinsic to the structure of the RNA, and that enzymes, small nuclear RNAs and folding of the pre-rRNA into an RNP are unnecessary for these reactions. We propose that the IVS portion of the RNA has several enzyme-like properties that enable it to break and reform phosphodiester bonds. The finding of autocatalytic rearrangements of RNA molecules has implications for the mechanism and the evolution of other reactions that involve RNA.

Complex evolution of folds and cleavages in Archean rocks, Yellowknife, N.W.T.

Overturned folds and later cleavages in a metasedimentary terrain with marginal volcanics near Yellowknife form a complex succession in which structures repeatedly trend eastward and northward. Early major east–west folds are overturned away from a southern granodioritic complex and terminate westward against a major syncline, with limbs overturned inward, trending north–south along the western granitoid boundary of the terrain. Later smaller folds (limbs ~1 km), which die out away from granitoid plutons, include a progression from northerly to easterly trending structures. Several sets of cleavage foliations, mostly steeply inclined, transgress the folds: an east–west set, succeeded by a regionally dominant set striking northwest to north, and a later north–south set confined to near the western margin of the terrain.The easterly and northerly, near-orthogonal trends followed by successive structures could reflect the influence of fault-bounded crustal blocks. During emplacement of fault-controlled granitoid bodies, the blocks may have tilted successively about easterly or northerly axes with gravitational collapse, folding, and overturning of the supracrustal rocks. Further tilting could in part have led to the compressions that formed the cleavages.

Geometry and origin of large‐scale deformation structures in some ancient wind‐blown sand deposits

ABSTRACTLarge‐scale deformational features that disrupt the cross‐bedded strata of some ancient wind‐blown dune sand deposits (Jurassic Aztec and Navajo Sandstones, U.S.A.) can betraced laterally for at least tens of metres. Information from four exposures leads to an idealized deformation style characterized by (1) a ‘head’ portion marked by collapse features, (2) a middle portion marked by a thrust ramp or a large recumbent fold, and (3) a ‘toe’ portion marked by a planar shear zone with small recumbent folds, drag folds, and possibly small horizontal shear faults. An hypothesis involving earthquake‐induced liquefaction and collapse of the dunes is proposed. Sands liquefied during earthquakes cannot support an unequal surface load, resulting in the collapse of surface dunes above the liquefied substrate. Compensatory lateral squeezing of liquefied sand and buried strata produces the shear zone in the ‘toe’ portion. Engineering studies suggest that buried strata underlying interdunal lows, where over‐burden stress is least, will be most susceptible to liquefaction. Other considerations suggest that it will be the steeper lee sides of advancing surface dunes that collapse, squeezing liquefied sand forward or downwind, i.e. away from the lee slope of surface dunes. If the liquefied condition persists, load structures can be formed when unliquefied layers founder into the liquefied substrate.

The metamorphic and structural evolution of the Phyllite-Quartzite Nappe of western Crete

The Phyllite-Quartzite (PQ) Nappe constitutes an external, allochthonous complex of the Hellenides on the island of Crete and shows a polyphase structural history. A first phase of deformation ( F 1) produced recumbent isoclinal folds, a penetrative schistosity, and boudinage under high-P/low-T metamorphic conditions. Mylonite formation at the top of the PQ Nappe, below the overriding Tripolitza Nappe, further boudinage, and schistosity ( S 2) represent a late tectono-metamorphic episode. Post-metamorphic small folds ( F 3), lineations, and a crenulation cleavage were formed synchronously with transport of the PQ Nappe. A last phase ( F 4) developed small folds, a fracture/crenulation cleavage, and large-scale folds after nappe movement. It is suggested that high-P/low-T metamorphism in the PQ rocks originated during subduction. Nappe transport of the higher, unmetamorphosed units, which were thrust over the PQ Nappe, began under waning metamorphic conditions. Subsequent transport of the PQ Nappe itself also occurred after the completion of metamorphism and after the formation of the mylonite at its top.

Interrelations of mesoscopic structures and strain across a small regional fold, Virginia Appalachians

Small regional folds, such as the Clover Hollow anticline of the Narrows thrust-sheet in southwest Virginia, U.S.A., are considered to be large buckle folds expressing lateral shortening above a subsurface décollement. Cleavage, mesoscopic and regional folds, and contraction faults have developed in these rocks under anchimetamorphic conditions, in a single, protracted deformation during thrust-sheet emplacement. The contraction faults dominate the structure at all scales. Three fault associations (isolated contraction faults, contraction faults in series and complex fault zones with intense folding) determine the pattern and intensity of local structures. Regional displacement transfer of strain along and across faults has produced local variations in structural style. Duplex-like systems of second-order faults terminate laterally into zones of intense folding and third-order faulting. Fold tightness, cleavage intensity, strain magnitude and total longitudinal strain ( ε T) are maximum in these regions. Contraction faults in this thrust-sheet have propagated along zones of high strain rate associated with mesoscopic folding and intense cleavage. Regional hinge migration, and greater structural complexity along the southeast limb of the Clover Hollow anticline, are considered to be due to emplacement of the adjacent thrust-sheet.