Thin-skinned Fold Research Articles

Cenozoic tectonics of the High Andes of west-central Argentina (30–36°S latitude)

The structure of the Central Andes shows three distinctive segments characterized by different geometries. These geometries are superimposed on the present large-scale plate tectonic setting characterized by distinct subduction segments. The northern La Ramada segment is a thick-skinned fold and thrust belt formed by tectonic inversion of a Late Triassic rift. The central Aconcagua segment consists of a thin-skinned fold and thrust belt while the southern Malargüe segment like the first one is a thick-skinned fold and thrust belt developed by tectonic inversion of a Late Triassic-Early Jurassic rift system during late Cenozoic times. The amount of shortening gradually decreases from north to south, as indicated by the crustal roots of the Central Andes. The different geometries along the Principal Cordillera controlled the abrupt changes in the shortening among segments. The structure of Precordillera and Sierras Pampeanas has also been considered in order to account for the total shortening. In the La Ramada segment the main shortening occurred in the Precordillera; in the Aconcagua segment in the Principal Cordillera while in the Malargüe segment the shortening is widely distributed in a broader Principal Cordillera, because south of the flat-slab subduction segment the Precordillera and Sierras Pampeanas are missing.

The Internal-External zone boundary in the eastern Betic Cordillera, SE Spain: Reply

Abstract The Internal-External Zone Boundary (IEZB) in the Betic Cordillera of southern Spain separates the Internal Zone, which was deformed and in part metamorphosed before the early Miocene, from the External Zone, which consists of the cover rocks of the Iberian margin shortened to form a thin-skinned fold and thrust belt in early to middle Miocene times. Over much of its length the IEZB is roughly linear, trending approximately 070°. It has been referred to as a major dextral strike-slip zone, and has even been considered as the northern boundary of a westward moving Alboran microplate. Field and kinematic data from the eastern Betic Cordillera show that the IEZB crops out over a 60 km distance as a gently-dipping thrust with displacement to the southeast or south-southeast, oblique to its regional trend. There is no evidence of dextral strike-slip movement along the boundary. New micropalaeontological studies of calcareous nannoplankton indicate that the Oligo-Miocene basin along the boundary was the site of continuous deposition until the beginning of the middle Miocene. The thrust at the IEZB cuts early Miocene rocks, and is overlapped by the middle Miocene. Thrusting therefore occurred in this time interval. The IEZB is therefore unlikely to have been the dextral boundary of a westward-moving Alboran microplate at this time, but was more likely to have been the locus of NW-directed dextrally oblique convergence.

The building of south China: collision of Yangzi and Cathaysia blocks, problems and tentative answers

This paper aims to give answers to the questions of timing, tectonic style and geodynamic interpretation of south China geologic development. In the middle Jiangnan segment, Yangzi plate and Cathaysia got clearly welded during a Late Proterozoic orogeny of collisional type, marked by HP/LT metamorphism, ophiolite melange obduction, thrusting of greenschist nappes, emplacement of collisional S-type granites. This collision, which built the initial Jiangnan belt, began around 950 ± 40 Ma and was completed at about 770–800 Ma ago. Kinematic study indicates that the Cathaysia plate was underthrust beneath the Yangzi plate. A likely earlier collisional event occurred around 1500 Ma, but is poorly preserved in the studied area. The proposed geodynamic model implies two successive suturings of oceanic domains during the Middle-Late Proterozoic: one about 1500 Ma and one about 950 Ma. A strong remobilization occurred during the Early Paleozoic ‘Caledonian’ orogeny, which induced transpressive ductile deformation. Thin-skinned folding and thrusting took place during the Mesozoic; this intracontinental shortening could be due to collision between the China-Indochina and west Philippines blocks. South China is a composite block, comprising the relics of at least three, maybe four sutures from Jiangnan to the coast.

An inverted metamorphic field gradient in the central Brooks Range, Alaska and implications for exhumation of high-pressure/low-temperature metamorphic rocks

During exhumation of the Brooks Range internal zone, amphibolite-facies rocks were emplaced atop the blueschist/greenschist facies schist belt. The resultant inverted metamorphic field gradient is mappable as a series of isograds encountered as one traverses up structural section. Amphibolite-facies metamorphism occurred at ∼ 110 Ma as determined from 40 Ar 39 Ar analysis of hornblende. This contrasts with 40 Ar 39 Ar phengite cooling ages from the uderlying schist belt, which are clearly older (by 17–22 m.y.). Fabrics in both the amphibolite-facies rocks and schist belt are characterized by repeated cycles of N-vergent crenulation and transposition that was likely associated with out-of-sequence ductile thrusting in the internal zone of the Brooks Range orogen. Contractional deformation occurred in an overall environment of foreland-directed tectonic transport, broadly synchronous with exhumation of the internal zone, and shortening within the thin-skinned fold and thrust belt. These data are inconsistent with a recently postulated mid-Cretaceous episode of lithospheric extension in northern Alaska.

The internal-external zone boundary in the eastern Betic Cordillera, SE Spain

The Internal-External Zone Boundary (IEZB) in the Betic Cordillera of southern Spain separates the Internal Zone, which was deformed and in part metamorphosed before the early Miocene, from the External Zone, which consists of the cover rocks of the Iberian margin shortened to form a thin-skinned fold and thrust belt in early to middle Miocene times. Over much of its length the IEZB is roughly linear, trending approximately 070°. It has been referred to as a major dextral strike-slip zone, and has even been considered as the northern boundary of a westward moving Alboran microplate. Field and kinematic data from the eastern Betic Cordillera show that the IEZB crops out over a 60 km distance as a gently-dipping thrust with displacement to the southeast or south-southeast, oblique to its regional trend. There is no evidence of dextral strike-slip movement along the boundary. New micropalaeontological studies of calcareous nannoplankton indicate that the Oligo-Miocene basin along the boundary was the site of continuous deposition until the beginning of the middle Miocene. The thrust at the IEZB cuts early Miocene rocks, and is overlapped by the middle Miocene. Thrusting therefore occurred in this time interval. The IEZB is therefore unlikely to have been the dextral boundary of a westward-moving Alboran microplate at this time, but was more likely to have been the locus of NW-directed dextrally oblique convergence.

A possible explanation for foreland thrust propagation

A common feature of thin‐skinned fold and thrust belts is the sequential nature of foreland directed thrust systems. As a rule, younger thrusts develop in the footwalls of older thrusts, the whole sequence propagating towards the foreland in the transport direction. As each new younger thrust develops, the entire sequence is thickened; particularly in the frontal region. The compressive toe region can be likened to an advancing wave; as the mountainous thrust belt advanced the down‐surface slope stresses drive thrusts ahead of it much like a surfboard rider. In an attempt to investigate the stresses in the frontal regions of thrustsheets, a numerical method has been devised from the algorithm given by McTigue and Mei [1981]. The algorithm yields a quickly computed approximate solution of the gravity‐ and tectonic‐induced stresses of a two‐dimensional homogeneous elastic half‐space with an arbitrarily shaped free surface of small slope. A comparison of the numerical method with analytical examples shows excellent agreement. The numerical method was devised because it greatly facilitates the stress calculations and frees one from using the restrictive, simple topographic profiles necessary to obtain an analytical solution. The numerical version of the McTigue and Mei algorithm shows that there is a region of increased maximum resolved shear stress, τ, directly beneath the toe of the overthrust sheet. Utilizing the Mohr‐Coulomb failure criterion, predicted fault lines are computed. It is shown that they flatten and become horizontal in some portions of this zone of increased τ. Thrust sheets are known to advance upon weak decollement zones. If there is a coincidence of increased τ, a weak rock layer, and a potential fault line parallel to this weak layer, we have in place all the elements necessary to initiate a new thrusting event. That is, this combination acts as a nucleating center to initiate a new thrusting event. Therefore, thrusts develop in sequence towards the foreland as a consequence of the stress concentrating abilities of the toe of the thrust sheet. The gravity‐ and tectonic‐induced stresses due to the surface topography (usually ignored in previous analyses) of an advancing thrust sheet play a key role in the nature of shallow foreland thrust propagation.

Tectonic Significance of Ross Pass Fault Zone, Central Bridger Range, Montana: ABSTRACT

The Ross Pass fault zone (RPFZ) in the central Bridger Range marks the boundary between Proterozoic Belt Supergroup rocks to the north and Archean metamorphic rocks to the south, and may represent the overlap of two Laramide styles of deformation: thin-skinned fold and thrust deformation to the north and basement-involved foreland deformation to the south. The fault zone consists of three northwest-trending, northeast-dipping, oblique-slip thrust faults with varying amounts of displacement. The middle thrust, the Pass fault, is the most extensive within the zone; to the west, the Proterozoic LaHood Formation is faulted against Archean quartzo-feldspathic gneisses and amphibolites and overlying Middle and Upper Cambrian strata. Displacement decreases eastward as the fault offsets the lower Paleozoic section and dies out within the upper Paleozoic strata. This variation in displacement is caused by folding within the hanging wall strata along the fault plane during thrusting. The folded strata are again offset by another smaller thrust to the northeast, the Peak fault, which may have originated as an out-of-the-syncline thrust. The Dry Fo k fault, southwest of the Pass fault, is largely an intraformational thrust within the Middle Cambrian Meagher Limestone which formed in response to buttressing against the Archean foreland block to the south. South of the RPFZ, both the Archean and the overlying Phanerozoic strata have been deformed into a large asymmetric, steeply dipping, eastward verging fold typical of basement-involved foreland structures. The structural relationships along the RPFZ developed as the southeast margin of the Montana salient of the Cordilleran fold and thrust belt impinged on the northern margin of the Laramide foreland province. The relatively small displacement within the thrusts indicates that the RPFZ is within or near the leading edge of the Montana salient. The RPFZ may be an extension of the inferred ancestral Willow Creek fault zone to the west, which was the southern margin of the Proterozoic Belt basin. Laramide compressional stresses exploited this long active zone of weakness and resulted in the impingement of fold and thrust structures upon a foreland uplift. End_of_Article - Last_Page 1333------------