The physiographic and geological differences between central Argentina and Patagonia led pioneer authors to consider Patagonia as being geologically different from South America, mainly during Gondwana time (Keidel, 1925; Windhausen, 1931, Dalmayrac et al., 1980).
The transcontinental boundary between these two crustal terranes was inferred from several photolineaments, including the Huincul Fault zone, the Río Limay Lineament and the Salinas Trapalcó–Laguna Curicó Lineament (Turner and Baldis, 1978). The zone where this interpreted boundary is located is covered by two Mesozoic basins and Tertiary–Quaternary sediments. There are no outcrops of Neopaleozoic rocks and thus the geological configuration at that time is unknown.
Permo-Triassic igneous rocks in the North Patagonia Massif and the Carboniferous marine Tepuel Basin occur to the south of the proposed boundary. To the north of the boundary are several Carboniferous marine and continental basins occur (Andacollo, El Imperial, Carapacha, and Claromecó), Contemporaneous igneous complexes are located in the Chadí Leuvú and San Rafael Blocks, which are also located to the north of the boundary.
Geología regional del norte de la patagonia |
Reconstrucción del sector norte de Patagonia |
Estructura
transcontinental límite Patagonia
Gondwana |
Ubicación
límite Patagonia
Gondwana y regiones cercanas |
In order to explain the differences between central Argentina and Patagonia, Ramos (1984) proposed that Patagonia represent an allochthonous terrane separated from Gondwana–South America by a marine basin before Carboniferous times. This terrane approached Gondwana–South America, initiating north–south directed subduction with a final stage of collision between both continental blocks in the Permian.
The subduction stage produced Permo-Triassic arc-related magmatism in the North Patagonia Massif, while the terrane accretion produced intense Permian folding and thrusting in the Sierras Australes. There are several doubts about this hypothesis. For example, no outcrops of basic-ultramafic rocks with oceanic crustal affinities have been found. The pattern of the gravimetric and magnetic anomalies (Kostadinoff and Labudía, 1991, Kostadinoff et al. 2005) north, south and over the proposed boundary are incompatible with the existence of a belt of high-density rocks below the Quaternary cover, as would be expected in the Ramos (1984) hypothesis.
Paleomagnetic studies by Rapalini (1998) in the Famatinian rocks of the North Patagonian Massif (Silurian– Devonian Sierra Grande Formation) indicate that Patagonia has not undergone latitudinal displacements relative to South America since Devonian times. The continuity of the Pampean belt southwards of the supposed boundary (González et al., 2002; Rapela and Pankhurst, 2002; Kostadinoff et al., 2005) is also at odds with the tectonic model of Ramos (1984).
Additionally, several authors (e.g., Rapalini and Vizán, 1993, Rosello et al., 1997) have interpreted the Gondwana deformation in the Sierras Australes and North Patagonian Massif as resulting from intraplate compression. This deformational event is also preserved in the Cape Fold Belt (South Africa), the Malvinas (Falkland) Islands, and in Antarctica and constitutes the ‘Samfrau Orogenic Zone’ (Söhnge, 1983, Davidson et al., 1987, Dalziel et al., 1987).
Interacción Terreno
Panthalassan-Gondwana |
Hipótesis
interacción Patagonia Gondwana |
Arcos del N de
Patagonia relacionados con la interacción Patagonia Gondwana |
Interacción final
entre Patagonia Gondwan |
In La Pampa Province the Gondwana magmatic rocks crop out as a NW–SE trending belt that extends from the San Rafael and Chadí Leuvú Blocks to López Lecube (Linares et al., 1980, Llambías and Leveratto, 1975, Llambías et al., 1996, Gregori et al., 2003).
Sedimentary successions deposited in a continental setting form the small Carapacha Basin in the La Pampa Province. In the Sierras Australes and the Claromecó Basin occurs glacio-marine to fluvial sedimentary sequences of the Permo-Carboniferous Pillahuincó Group. These basins display lithological and structural features similar to the Karoo Basin in South Africa (Keidel, 1916).
In the North Patagonian Massif, magmatic rocks are widely represented by three separate belts that are oriented SW–NE, W–E and NW–SE. The NW–SE trending belt is present in the studied area and extends from Yaminué to the Atlantic coast.
Foliated granites (Yaminué, Tapera, María Teresa, Laguna Medina, Peñas Blancas and La Laguna) and non-foliated plutonic complexes (Donosa, Calvo, Navarrete, Flores, Pailemán, Tembrao, La Verde) outcrop along the belt (Llambías and Rapela, 1984, Rapela and Caminos, 1987, Grecco et al., 1994, Giacosa, 1997). Several small Middle to Upper Triassic continental rifts are located in the Los Menucos and the Comallo areas. These rifts mainly contain pyroclastic deposits.
Kostadinoff et al. (2005) described several gravimetric and magnetic anomalies, as well as the continuation of the Huincul Fault zone and the Choele Choel High. Unpublished internal reports on seismic surveys by Yacimientos Petrolíferos Fiscales (YPF) provide information about the subsurface architecture in this region.
Belts of steep gravity gradients
In order to validate the continuity of the Huincul Fault zone and other structural lineaments proposed by Turner and Baldis (1978), we focused on areas with steep gravity gradients in the gravity anomaly map. Such steep gradients are usually interpreted as representing fault structures (Nettleton, 1976), boundaries of rocks with contrasting petrophysical characteristics or sutures (Gibb and Thomas, 1976, Wellman 1978, Coward and Fairhead 1980, Gibb et al. 1983).
Differences in density or thickness between blocks are often responsible for steep gradients (Gibb and Thomas, 1976, Thomas, 1983, Kunar et al. 2004, Williams et al., 2006). Values between 0.5 and 2 mGal/km, were obtained using the gradient and second derivate operator. A 3D image generated using Surfer 8™, show several lineaments defined by belts with steep gravity gradients.
The table indicates the name, location, gradient, any surface evidence and possible age of each lineament.
Lineament |
Location |
Gradient (mGal/km) |
Direction |
Surface evidence |
Related to |
Possible age |
A |
Salitral Bajo de Menucos to Chimpay |
1–1.3 |
SW–NE |
Not surface evidence, due to the Tertiary–Quaternary coveR |
The continuity of the Sierra Blanca de la Totora, Patú Co and El Loro fault system (200 km long) |
The Sierra Blanca de la Totora, Patú Co and El Loro fault system were active during emplacement of the Upper Paleozoic granitic bodies located between these lineaments (Saini-Eidukat et al., in press) |
B |
Cerro Mesa to Laguna La Larga |
1–1.2 in the NW segment. 0.5 in theW– E segment. |
NW–SE |
A system of topographic lows (Salitral Ojo de Agua, Laguna Larga) |
Parallel to the Pangaré Mylonite (Gregori et a., 2000) and Salinas Trapalcó–Laguna Curicó lineament (Ramos and Cortés, 1984 |
The Pangaré Mylonite was active during Upper Paleozoic (Saini-Eidukat et al., in press) |
C |
Laguna La Larga to Bajo La Salamanca |
0.5 |
SW–NE |
Not surficial expression, extensive Tertiary–Quaternary cover. |
The continuity of the Nahuel Niyeu lineament system, which is a major structure 25 km wide, 65 km long between the Salinas Trapalcó–Laguna Cuiricó Lineament (Ramos and Cortés, 1984) and the Somoncura Plateau. Include the Arroyo Seco, Barda de Lucho, Arroyo Salado, Laguna Negra, Quiroga and Ramos lineaments, the Tardugno, Musters and Huanteleo faults and the Nahuel Niyeu, Railer and Rana thrust sheets |
According to von Gosen (2003) the Nahuel Niyeu, Railer and Rana thrust sheets indicate N–S transport during the Neopaleozoic. |
D |
Puesto Medina to Estación Mancha Blanca |
1 |
NW–SE |
Parallel to the Salinas Trapalcó– Laguna Curicó lineament (Ramos and Cortés, 1984. |
Possibly related to the El Jaguelito Fault zone (Ramos, 1975 |
El Jaguelito Fault zone and associated minor mylonitic belts were active during the Upper Paleozoic (Giacosa, 2001) |
E |
Bajo La Salamanca to La Salina |
0.5 |
NW–SE and E–W |
The Rio Negro follows approximately the lineament direction |
Parallel to the Salinas Trapalcó–Laguna Curicó lineament (Ramos and Cortés, 1984) |
Since this lineament shows a strike and development similar to lineament D it is believed that both were generated during the same episode |
F |
San Antonio Oeste to Zanjón de Oyuela |
2 |
W–E and ENE direction |
Not surficial expression was recognized ? |
? |
? |
G |
La Salina to Salinas del Gualicho |
2 |
W–E |
Not topographic features represent this lineament. |
Located inside of the Viedma High |
Possibly a continuation of the Huincul Fault zone ? |
Three main lineament orientations directions can be recognized: lineaments A and C are NE–SW trending; lineaments B, D and E are NW–SE trending; and lineaments F, G and the Huincul Fault zone are E–W trending. The E–W lineaments exhibit more complex geometries than the other lineaments. Most lineaments form the boundary of the Choele Choel High (e.g. A, B, C, E and the Huincul Fault zone). |
Huincul Fault zone
Because the Huincul Fault zone (Kostadinoff et al., 2005)
crops out in the Neuquén Basin (Windhausen, 1914 and Keidel, 1925), its configuration and kinematics are relatively well constrained. The Huincul Fault Zone displays dextral strike-slip movement (Orchuela and Ploszkiewicz, 1984) and a zig-zag along strike pattern. This pattern suggests that both transtensional and transpressional conditions occurred during fault movement, resulting in the contemporaneous development of positive and negative flower structures. This fault system was active from Upper Paleozoic–Triassic times, as recorded by continental depocenters associated with the transtensional subsidence (Vergani et al. 1995).
Its eastwards continuity was suggested by Turner and Baldis (1978), and established by Orchuela and Ploszkiewicz (1984) based on interpretation of seismic reflection data. The high gravimetric gradients between Neuquén and Cubanea support the continuity of the Huincul Fault Zone until 63° 40´W, with maximum gradients (2 m Gal/km) between the Neuquén Basin and 64° 45´W–39° 20´S.
The section between Neuquén and Chimpay shows a smooth releasing bend, that changes to a restraining bend in the Chimpay-63° 40´W section. The gravity anomalies are discontinuous along the fault zone. In the area between Chimpay and Cubanea, the differences in magnitude of the anomaly between the Choele Choel High and the Carapacha Basin and the Río Colorado High mark the trace of this fault zone. In the section between Chimpay and Neuquén, such differences in the anomalies are less evident, but high gravimetric gradients (2 mGal/km) are interpreted to reflect the location of the fault zone.
Magnetic anomalies are diffusely coincident with the interpreted fault zone between 5 and 10 km north of Chichinales and Choele Choel. These anomalies are low-amplitude (175 to 75 nT) and are interpreted to be related to the Albian– Coniacian red beds of the Neuquén Group, which contain abundant Fe-oxides (hematite). The sediments have a magnetic susceptibility of 0.006500 SI (Telford et al., 1990), which is sufficient to explain the magnitude of the magnetic anomalies.
These low-magnitude anomalies do not compared with magnetic anomalies typically observed in sutures zones because suture zones are generally composed by serpentinite zones associated with sheared peridotites, which have a magnetic response N500 nT (Ferraccioli et al., 2002).
The Huincul Fault zone is therefore unlikely to be a suture. In the Neuquén–Chichinales section the Neuquén Group is 1000 m thick, but in the cliffs located south of the Rio Negro, only the upper section of this unit crop outs.
In contrast, the area north of the fault zone, Upper Cretaceous–Lower Tertiary marine Malargue Group crop out, suggesting more than 500 m of vertical displacement across a fault zone. Due to the outline of the Huincul Fault Zone, the section between Plaza Huincul and Picún Leufú and between Neuquén and Chimpay represent domains of dextral transpressive shear (Orchuela and Ploszkiewicz, 1984), where basement rocks are exposed in the fault zone. In the section between Chimpay and 64° 15´W, deformation appears to be transtensional based on the existence of small depocenters associated to the fault zone.
Mylonitic belts and lineaments with steep gravimetric and high magnetic gradients in northern Patagonia, southern La Pampa and Buenos Aires provinces
Northern Patagonia is characterized by the presence of several mylonitic belts and regional structural lineaments, whose geometries, deformational mechanisms and age mostly remains unclear.
The better-known of these lineaments and mylonites include:
a) Pangaré and La Seña mylonites (Gregori et al., 2000), are formed by two belts, each 1 km wide, 15 km long, and comprising granitic augen-mylonites, protomylonites, mylonites and ultramylonites, with strikes between 310° and 330°. They exhibit sinistral movement associated with an east–west compression.
b) Peynecura mylonitic belt (Llambías et al., 2002) is more than 500mwide, 15 kmlong, and has a NNW–SSE strike. The deformation is interpreted to have occurred during the Early Triassic because is related to foliated granites of such age. No information about strain orientation is available.
c) The El Jaguelito Fault zone is a 50 km long NW–SE dextral lineament that appears in the eastern area of the North Patagonian Massif. According to Ramos and Cortés (1984), deformation occurred during Upper Paleozoic times.
d) Peñas Blancas and La Laguna mylonites are described by Giacosa (1996). They are 10 km long, and are temporal related to the foliated Permian-aged Peñas Blancas and La Laguna Granites. Four more NW–SE mylonitic belts were recognized during regional mapping in this area, but information on strain orientation is unavailable. The timing of deformation is interpreted to be Gondwanan because the foliated granites were intruded at the same time as mylonitization occur.
 |
e) The Nahuel Niyeu lineament system is a major structure that is 25 km wide, and extends for 65 km long between the Salinas Trapalcó–Laguna Curicó Lineament and the Somoncura Plateau
Nahuel Niyeu lineament includes the Arroyo Seco, Barda de Lucho, Arroyo Salado, Laguna Negra, Quiroga and Ramos lineaments, the Tardugno, Musters and Huanteleo faults and the Nahuel Niyeu, Railer and Rana thrust sheets
In the La Pampa province, a mylonitic belt was described in Cerro de los Viejos by Tickyj et al. (1997).
A further four belts of steep gravity gradients which were interpreted as major structures were described by Kostadinoff et al. (2001). These four belts include:
a) The Quehué lineaments is a 350 km long, 15 km wide. It is defined in the gravity data by a 2 mGal/km gradient which defines a structure interpreted to represent the boundaries of the Triassic Quehué basin.
b) The Valle Daza-Cuchillo Có lineament is a 310–320° trending belt that is 100 kmlong and characterised by a steep gradient in gravimetry data. This lineament is interpreted to represent the western boundary of the Rio Colorado High.
|
c) The Cerro los Viejos lineament extends in a NW-direction for more than 100 km from Cerro Los Viejos and is possibly related to the Cerro Los Viejos mylonite. According to Kostadinoff and Llambías (2002), the Cerro los Viejos lineament represents the contact between the Permian Carapacha Basin and the Río Colorado High. This lineament and its associated steep gravity gradient disappears in the central part of the La Pampa province, but is identified near Algarrobo del Aguila as a 50 km long, 15 km wide lineament with steep gravity gradient. This NW-trending structure is considered to be the boundary of a Triassic basin (Kostadinoff and Llambías, 2002).
 |
d) The Macachín Rift includes Paleozoic and Mesozoic sedimentary sequences deposited in a NNW–SSE extensional basin.
Outcrops of igneous and sedimentary Gondwanan rocks in the central part of the La Pampa province follow WNW or NW trends, which suggest structural control.
In the Buenos Aires Province strike-slip faults located north and south have been used to explain the structure of the Sierras Australes. For example, Newton and Cingolani (1990) showed a series of WNW–ESE trending sinistral strike-slip faults near Pigue and Cerros Colorados, which are evident in gravity data (Alvarez, 2004). In the Bahía Blanca area, Bonorino et al. (1987) showed the occurrence of several lineaments with steep gravity gradients south of Sierras Australes. Lineaments of importance include the Salitral de la Vidriera and Nueva Roma belts, which are strike-slip fault zones longer than 100 km in strike extent. |
Gondwana deformation in the Neuquén Basin, northern Patagonia and southern La Pampa and Buenos Aires provinces
Windhausen (1914), Keidel (1925) and Groeber (1929) described NE–SW structures transverse to the N–S Andean Orogeny in the Neuquén Basin. According to these authors, as well as Herrero Ducloux (1946) and De Ferraris (1947) the stratigraphic and structural evidences suggest folding during the Cenomanian or the Middle Jurassic.
They observed that the structures mimic the northwestern and northern border of the North Patagonian Massif and that they possibly formed during NW- and N-directed lateral translation of the massif. These structures were grouped in the “Dorsal de Huincul” (Huincul Fault zone) by De Ferraris (1947).
In the North Patagonian Massif, the Gondwana deformation is recorded by the Pangaré and La Seña mylonites. Clearly defined δ and σ type porphyroclasts of K-feldspar, and shaped lenses of Qz–K-feldspar, support sinistral movement. These belts and the associated foliated granite (Gregori et al., 2000) indicate maximum compression (σ1) in a WNW–ESE direction during Middle to Upper Triassic (Saini-Eidukat et al., in press). In the El Jaguelito Fault zone, the fault pattern suggests a dextral transcurrence along N 310° during Upper Paleozoic times (Ramos and Cortés, 1984).
SE, S and SW vergence occurred along the Nahuel Niyeu lineament system ), each of which was related to three phases of deformation, with principle shortening directions in NE–SW, NW–SE and E–W directions during Permo-Triassic times (von Gosen, 2003).
In the Sierra Grande area, the Pampean El Jaguelito Formation contains evidence which supports a D1 event that was generated byNW–SE or E–Wshortening (Giacosa and Paredes, 2001, von Gosen, 2002). In the same area, the Famatinian Sierra Grande Formation is folded around open synclines and anticlines that have N–S trending axial traces.
The Permian Laguna Medina granite intrudes the Sierra Grande Formation and records a ductile E–W shortening deformation event, which was coincident with deformation of the Sierra Grande Formation (Rossello et al., 1997, von Gosen, 2002).
Transport directions from NE to SW during the Gondwana deformation was accommodated along mylonites within the Peñas Blancas and La Laguna Granites (Giacosa, 2001, von Gosen, 2002).
In the North Patagonian Massif, the Sierra Grande Formation, the Laguna Medina Granite and foliated granites and mylonites in the Estancias La Seña and Pangaré area all indicate shortening in aWNW–ESE or E–Wdirection.
The mylonites in the Peñas Blancas and La Laguna Granites and thrust sheets and faults in the Nahuel Niyeu area are related to a N–S or SW–NE shortening (Giacosa, 2001, von Gosen, 2002).
In the La Pampa Province, the Carapacha Basin exhibits a NW–SE elongation, suggesting basin development occurred during NE–SWdirection extension. Folding of the sedimentary successions and inversion in the Carapacha Basin (Melchor, 1999) and mylonitization of the Cerro de los Viejos granite (Tickyj et al., 1997) dated as Late Permian (254±2 Ma), suggests that both areas were affected by the same deformational phase related to a NE–SW shortening.
In the Sierras Australes of Buenos Aires Province, the Pillahuincó Group (Carboniferous-Permian) was deformed between the late Early Permian and the early Late Permian (Tomezzoli and Vilas, 1996).
Deformation generated a sigmoidally-shaped mountain chain. Folding and overthrusting occurred during NE vergence (von Gosen et al., 1990). This deformation was related to sinistral transpressive deformation (Sellés Martinez, 1989) along the WNW–ESE Pigué transcurrent fault system (Alvarez, 2004) and the Salitral de la Vidriera and Nueva Roma transcurrent fault systems (Bonorino et al., 1987).
The deformation was interpreted to be related to transpressive movement of the “Patagonian block” relative to the South American craton during Permo-Triassic time (Martinez, 1980), although little supporting evidence was provided. Sellés Martínez (1989) considered the deformation in Sierras Australes to be due to a transpressive regime between the Patagonian block and the South American craton during the Late Permian oblique subduction.
Detailed structural studies in the Sierras Australes define a sigmoidal belt, with fold trends rotating N60° clockwise, from WNW to NNW, a configuration attributable to dextral transpression (Cobbold et al., 1986, 1991, 1992).
Inferred directions of principle shortening are oblique to the orogenic belt, and to the convergent southern margin of Gondwana and are interpreted to result from oblique (dextral) subduction beneath an Andean-type margin (Cobbold et al., 1992, Craddock et al., 1998). Consequently, the sedimentary basins, the emplacement of the Gondwana intrusive and volcanic rocks and the regional structure north of the Colorado and Negro rivers are related to a first SW–NE extension, which later switched to a shortening regime in a NE–SW or N–S direction.
In their structural analysis of northern Patagonia, Turner and Baldis (1978) identified several NW–SE lineaments in the area between Bajo del Gualicho and Golfo San Matías.
They suggested variable compressive stresses in the North Patagonian Massif including: west-shortening directed (Río Limay area); northeast-directed shortening (Salinas del Gualicho); eastdirected shortening (Sierra Grande); and southwest-directed shortening (El Cuy). The variation in stress directions is attributed to counterclockwise rotation of the North Patagonian Massif.
Deformation in the southern area of the Gondwana supercontinent
The first detailed reconstruction of the relative position of Africa and South America during the Upper Paleozoic was given by du Toit (1927). The continuity of an orogenic system was extended from South America to Australia, passing just south of the Permo-Triassic fold belts, introducing the name of Samfrau Orogenic Zone (du Toit, 1937). This belt is nearly equivalent to what we understand today as the Gondwanide Orogenic Belt (du Toit, 1937).
In the South African segment, the arrangement of the Gondwanide Orogenic Belt and the associated basins is strongly conditioned by the configuration of older basements blocks (Kaapvaal, ≥2.5 Ga, Namaqua-Natal, ≈1 Ga and Gariep, Saldania and Mozambique, 1–0.5 Ga). The Ordovician–Devonian Cape Basin follows a main W–E trend along the Saldania province with branches to the NW and NE. The Cape Fold Belt includes southern and western tectonic domains (Söhnge, 1983), separated by the Cape Syntaxis, across which the structural strike, and the intensity of deformation changes markedly (de Beer, 1995).
The southern domain extends from the eastern margin of the Cape Syntaxis to Port Elizabeth and is characterized by east–west-trending, north-verging structures including bedding- parallel thrusts and north-verging folds. No evidence for dextral shearing was found in this domain.
In the western domain, strata is mildly deformed (de Beer, 1992). Folds and faults trend northwest to north (de Beer, 1990). North-trending slickensides developed on thrust planes (Ransome and de Wit, 1992) and stretching lineations (Cobbold et al., 1992) indicate a component of strike-parallel displacement. The pattern of folds, faults and lineations are consistent with overall dextral strike-slip deformation (Cobbold et al., 1992).
The Cape Syntaxis is a northeast-trending structural domain that separates the southern and western domains (de Beer, 1995). Based on a reconnaissance of the structure of the syntaxis and adjacent domains, Ransome and de Wit (1992) proposed a model in which the continental crust to the west of Cape Syntaxis was rotated clockwise during syntaxis formation, while crust immediately east is rotated counter-clockwise.
In the Malvinas (Falkland) Islands, Late Permian fold and thrust belt structures form the eastward continuation of the Cape Fold Belt when restored into a pre-Gondwana break-up configuration. Detailed structural studies by Curtis and Hyam (1998) and Hyam (1998) suggest that these structures developed in a complex strain system of combined NW–SE shortening and dextral shear.
In Antarctica, the Gondwanide Orogen extends through the Ellsworth–Whitmore Mountain block and the Pensacola Mountains (Curtis and Storey, 1996). During Gondwanide orogenesis the Ellsworth–Whitmore Mountain block is assumed to have occupied a site intermediate between the more easterly Falkland Islands and the more westerly Pensacola Mountains (Curtis and Storey, 1996).
Structural analyses (fold orientation, shortening directions and strain partitioning) indicate contemporaneous development of gently plunging foreland-verging folds and asymmetric, steeply plunging folds symptomatic of deformation within a highly oblique dextral transpressive belt (Curtis, 1997, 1998, Craddock et al., 1998). Dextral transpressional structures are evident throughout much of the Gondwanide Belt, suggesting a component of dextral transpressional during the evolution of this orogenic belt.
Johnston (2000) indicated that the east–west-trending domain of the Cape Fold Belt ends to the west and east against the northwest-trending South American and Antarctic portions of the Gondwanide Orogen respectively.
Tectonic model
The diverse shortening directions and lateral crustal translations during Gondwana time north and south of the interpreted boundary between northern Patagonia and Gondwana South America cannot be explained as the result of homogeneous stress regime (i.e. NE–SW compression due to the subduction and collision of Patagonia with the southern margin of Gondwana South America) as speculated by Ramos (1984), and von Gosen (2002, 2003).
All lineaments plotted (those both related and unrelated to mylonites, and the belts with steep gravity and magnetic gradients, presumably active during Gondwana times) suggest a lineament network that defines a block system.
This collage of continental crustal blocks and its movement was the result of a tectonic event that produced crustal fragmentation. The geometry of individual blocks controlled the block movement and therefore the development of mylonite belts and other lineaments.
In order to explain the variable stress regime in which zone of compressional and extensional tectonism were active at the same time over such a large area, we require movement of crustal blocks along arcuate strike-slip faults. This style of deformation is common during collisional orogeny, where frontal zones are deformed during indentor tectonics, where the lateral terminations of the indentor involve extensive escape tectonics (Ratschbacher et al., 1991).
The size and shape of the continental fragments involved, the orientation of the indentor, and the collision vector, influence the extent and geometry of indentation and later escape. Modern examples of large-scale indentation-escape orogens are found in the Himalaya-Alpine belt. These models include the collision of the Arabian and the European plates (west-directed escape) and the east- and southeast-directed escape of southeast Asia following the collision of India and Asia (Tapponnier and Molnar, 1976).
The transcurrent shear zones separating elongate crustal slices are the most important large-scale structural features in these Himalaya–Alpine models. Indentation-escape tectonics is widely observed and accepted in younger orogens, but has not been recognized to the same extent in older mountain belts, probably because the structures associated with such a tectonic regime are less obvious.
In addition, subsequent orogenic events may obscure the presence of an ancient indentation-escape regime (Ratschbacher et al., 1991). Several examples of Precambrian indentation-escape tectonics have been described. In southern Africa, the proto-Kalahari craton acted as a southwest-directed indenter into juvenile Mesoproterozoic island-arc terranes at ca. 1100 Ma (Jacobs et al., 1993). The associated escape tectonics regime in the orogen is shown by extensive craton-parallel ductile shear zones and the development of pull-apart basins in the preserved African foreland (Koras– Sinclair basins).
Other example of indenter-escape tectonic has been reported in Australia, where the Yilgara craton indented the Pilbara–Gawler craton during the late Paleoproterozoic Capricorn orogeny (Krapez, 1999). Another example is the East African–Antarctic orogen, which resulted from the collision of various parts of proto-East and West Gondwana during late Neoproterozoic–early Paleozoic time (between 650 and 500Ma).
The southern part of this Himalayan-type orogen can be interpreted in terms of a lateral-escape tectonic model. Small microplates (the Falkland, Ellsworth–Haag, and Filchner blocks) probably represent shear-zone-bounded blocks produced by tectonic translation during lateral escape, similar to those currently evolving in Southeast Asia.
Assuming amodel of indentation-escape tectonics for northern Patagonia, the belts with steep gravity gradients described in this study represent transcurrent shear zones separating shear-zonebounded blocks. The block situated between lineaments C, D and E has possibly acted as a northwest-directed indenter associated with west-directed movements of the area located south of the Huincul Fault zone.
Several small blocks were displaced west and southwest, possibly the western sector of the Choele Choel High and the areas located west of the line A and southwest of line B, as recorded in the La Seña–Pangaré mylonites and Nahuel Niyeu system.
The eastern sector of the Choele Choel High, between the Huincul Fault zone and lineament E was displaced to the east and southeast during lateral escape. In areas of transpression, such as along the Huincul Fault Zone between Estancia El Caldén and Chimpay, rocks were faulted upwards to form positive flower structures.
 |
Such a configuration can explain the uplift of the Choele Choel High against the Carapacha Basin and accounts for the change of detritus composition as detected in the Carapacha and Claromecó Basins (Rossello et al, 1997, Melchor, 1999). It seems probable that some blocks in the North Patagonian Massif, located south of the Huincul Fault Zone, rotated counterclockwise during Permo-Triassic time. It is doubtful that the deformation can be explained in terms of the collision of two major crustal blocks, principally because there is no conclusive evidence for such a collision.
The development of the Gondwanide deformation in northern Patagonia–Sierras Australes–La Pampa province in a location far from the active convergent margin of Gondwana is attributable to two factors:
a) dextral margin-parallel translation of a crustal block or blocks outboard of the Gondwanide Orogen; and
b) the oblique orientation of this area relative to the east–west trend (present coordinates) of the orogen in the Cape Fold Belt, as shown in Fig. 12B. |
Coincidences in timing of deformation and the orientation of principal stress directions, point to an evolutionary history that is similar to other portions of the Gondwanide Orogen (de Wit and Ransome, 1992; Cobbold et al., 1991, Vaughan and Pankhurst, 2008).
In the southern Canadian Rocky Mountain fold and thrust belt, Late Cretaceous and Tertiary deformation developed ~1500 km inboard of the active convergent margin (McMechan and Thompson, 1989).
Oblique (dextral) subduction of oceanic plates beneath western North America resulted in the development of major dextral strike-slip faults within the overriding North American crust.
The development of this fold and thrust belt 1500 km inboard of the cordilleran margin in response to oblique subduction-driven, margin-parallel dextral translation was considered as an analogue for development of the Cape Fold Belt (Johnston, 2000).
According to the evidence presented here, this model can also be applied to northern Patagonia. However, the relatively extensive width of this active margin is at present not fully understood.
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