Early Paleozoic and Andean structural evolution in the Rio Jáchal section of the Argentine Precordillera

In the fold-and-thrust belt of the northern Argentine Precordillera, Early Paleozoic basin and slope sediments are affected by a folding event which was combined with a slight greenschist facies metamorphism. The structural geometries are influenced by the former normal faulted boundary towards the eastern carbonate platform. To the east of the slope, Early Paleozoic marine deposits record a ˜ W-vergent folding without a clear metamorphic overprint. This deformation probably took place in the Devonian to pre-Upper Carboniferous interval while in the west an onset during the Late Silurian is reasonable. During Andean (Late Tertiary) compression, the escarpment was again reactivated as an important, east-directed thrust fault, and the folded strata to the east were juxtaposed along distinct, east-directed high-angle reverse faults with some ˜ N-S fold structures interfering with pre-Tertiary folds. Hence, the present architecture of this part of the orogen was largely influenced by different Early Paleozoic depositional realms and structures of one pre-Tertiary compressional event. The latter can be linked with the collision of the Sierras Pampeanas basement complex at the eastern margin of the Precordillera and be related to the collision with the Chilenia Terrane in the west.     

The Rio Sassito sedimentary succession (Ordovician): a pinpoint in the geodynamic evolution of the Argentine Precordillera

In the Precordillera of western Argentina, an isolated outcrop of Llandeilian siliciclastics and Caradocian limestones (Rio Sassito succession) reveals a complex interplay between the tectonic and the sedimentary history of the Precordillera during Middle and Late Ordovician times. The succession is composed of a lower siliciclastic interval and an upper carbonate interval and is bounded below and above by erosional unconformities. Dating of these unconformities, which in many places merged to form one single surface, demonstrates that the most important erosional event took place prior to the deposition of the Rio Sassito succession. This erosional event is correlated to extensional tectonics during continental breakup and the separation of the Precordillera from Laurentia. Block faulting with the formation of horst and graben structures provided the topography for the establishment of a pelagic carbonate platform during the Caradoc. In our view, there are no indications that these phenomena are related to the accretion of the Precordillera to Gondwana or to the formation of an Ordovician supercontinent. The carbonate sediments are typical of temperate-water settings, characterized by the absence of ooids, oncoids, and algae, and by the presence of abundant abraded bioclasts, intraclasts, and peloids. The inference of a temperate-water environment does not, as previously supposed, indicate the accretion of the Precordillera to Gondwana, but is more likely related to global cooling prior to the Ashgillian glaciation.     

Ordovician collision of the Argentine Precordillera with Gondwana, independent of Laurentian Taconic orogeny

The Argentine Precordillera, a rifted fragment of Laurentian crust and sedimentary cover, collided with Gondwana in Middle Ordovician time; the time of collision (Ocloyic orogeny) is similar to that of the Taconic orogeny of eastern Laurentia. Three hypotheses have been proposed to explain Ordovician docking of the Precordillera with western Gondwana: (A) the Precordillera microcontinent was rifted from Laurentia in Cambrian time and, following solitary drift, collided with Gondwana, independent of the Laurentian Taconic orogeny; (B) a continent–continent collision of Laurentia with Gondwana, producing a continuous Taconic–Ocloyic orogenic belt, was followed by rifting that left the Precordillera attached to Gondwana; and (C) the Precordillera at the tip of a distal plateau on greatly stretched Laurentian crust collided with Gondwana and subsequently separated from Laurentia.Contrasts in several aspects of Taconic and Ocloyic orogenic history provide for discrimination between the microcontinent and continent–continent-collision hypotheses. Stratigraphic gradients and lithologic assemblages within the synorogenic clastic wedges are incompatible with a single continuous orogenic belt, which, in palinspastic location, places the thin, fine-grained southern fringe of the Taconic clastic wedge adjacent to the thickest and coarsest part of the Ocloyic clastic wedge. Separate temporal and spatial distribution patterns of volcanic ash (bentonite) beds in Laurentia and the Precordillera indicate originally separate dispersal systems. Late Ordovician Hirnantian Gondwanan glacial deposits in the Precordillera indicate substantial latitudinal separation from Laurentia. Post-collision faults with large vertical separation in the Precordillera have no coeval counterparts on the Laurentian foreland. These contrasts indicate originally separate (not initially continuous, and subsequently dismembered) orogenic belts, favoring the microcontinent hypothesis and eliminating the continent–continent-collision hypothesis.Initial Taconic tectonic loading near the southern corner of the Alabama promontory of Laurentia and the lack of post-Taconic extension there are inconsistent with the tectonic history required by the plateau hypothesis, but are consistent with the tectonic history required by the microcontinent hypothesis. Paleobiogeography, distribution of bentonite beds, and the Hirnantian glacial deposits, all indicate wide separation (Iapetus Ocean) between the Precordillera and southern Laurentia at the time of the Ocloyic and Taconic orogenies, further favoring the microcontinent hypothesis.     

Composition of Ordovician Algoflora of Argentine Precordillera and Its Significance for the Formation of Organic Structures

The Argentine Precordillera, which was part of Gondwana in the Ordovician, is characterized by complex geological evolution, which is reflected in a series of bioevents evident from paleontological data (trilobites, conodonts, brachiopods, graptolites, etc.). An important role also belonged to calcareous algoflora as a major constituent of reefs, which is the main focus of the paper. Some calcareous algae are revised.     

Upper Cambrian carbonate sequences of the Argentine Precordillera and the Steptoean C-Isotope positive excursion (SPICE)

Carbon and Sr-isotope profiles in Upper Cambrian platformal carbonate Formations in the Precordillera, western Argentina (Zonda, La Flecha and La Silla Formations), were constructed for three representative sections: (a) Quebrada de la Flecha, Eastern Precordillera, (b) Cerro La Silla, Central Precordillera and (c) Quebrada de La Angostura, northern part of the Central Precordillera.

At Quebrada de La Angostura, upper part of the La Flecha Formation, δ¹³C_carb varies continuously up-section from − 2.0 to + 5.6‰ (PDB) and records the SPICE anomaly (+ 5‰) reported for the first time in South America. The peak of this excursion is characterized by intercalated 2 m thick beds of black shale with marl and limestone that record the onset of a sea-level change.

The Steptoean Zonda Formation dolomites at the Quebrada de la Flecha exhibit a total δ¹³C range from − 2.7 to + 0.6‰ with discrete positive anomaly about 200 m from the transition to the overlying Sunwaptan La Flecha Formation. Pronounced C-isotope anomaly (− 5.6‰) is observed in the La Flecha Formation at about 300 m below the transition to the La Silla Formation.

At the Cerro La Silla section, the Zonda Formation exhibit δ¹³C values of  − 1‰, increasing slightly at the transition to the La Flecha Formation (− 1 to 0‰). The transition of the La Flecha to the La Silla Formations is characterized by alternation of black shales and dolomitic limestone with a discrete positive C-isotope excursion, probably corresponding to the SPICE.

At the Quebrada de La Flecha, ⁸⁷Sr/⁸⁶Sr for the Zonda Formation varies from 0.70924 to 0.70955 and for the La Flecha Formation from 0.70908 to 0.70942. At Cerro La Silla this ratio varies from 0.70914 to 0.70923 for the La Flecha Formation, and from 0.70898 to 0.70980 for the La Silla Formation. At the Quebrada de La Angostura, ratios for the La Flecha carbonates range from 0.70918 to 0.70993. The overall variation of ⁸⁷Sr/⁸⁶Sr is consistent with globally reported Upper Cambrian seawater values at ca. 500 Ma.

The unambiguous record of SPICE in the La Flecha Formation at the Quebrada de La Angostura supports a Steptoean age for its deposition and allows precise local, regional, and global stratigraphic correlation. The pronounced negative C-isotope excursion recorded in the La Flecha Formation carbonates at the Quebrada de La Flecha is likely equivalent to that registered in Sunwaptan carbonates of North America and Australia, and might be tied to a global event, as a valuable tool in stratigraphic correlation (SNICE, acronym for Sunwaptan negative isotope carbon excursion).     

Zircon and whole-rock Nd-Pb isotopic provenance of Middle and Upper Ordovician siliciclastic rocks, Argentine Precordillera

Graptolite‐bearing Middle and Upper Ordovician siliciclastic facies of the Argentine Precordillera fold‐thrust belt record the disintegration of a long‐lived Cambro‐Mid Ordovician carbonate platform into a series of tectonically partitioned basins. A combination of stratigraphic, petrographic, U‐Pb detrital zircon, and Nd‐Pb whole‐rock isotopic data provide evidence for a variety of clastic sediment sources. Four Upper Ordovician quartzo‐lithic sandstones collected in the eastern and central Precordillera yield complex U‐Pb zircon age spectra dominated by 1·05–1·10 Ga zircons, secondary populations of 1·22, 1·30, and 1·46 Ga, rare 2·2 and 1·8 Ga zircons, and a minor population (<2%) of concordant zircons in the 600–700 Ma range. Archaean‐age grains comprise <1% of all zircons analysed from these rocks. In contrast, a feldspathic arenite from the Middle Ordovician Estancia San Isidro Formation of the central Precordillera has two well‐defined peaks at 1·41 and 1·43 Ga, with no grains in the 600–1200 Ma range and none older than 1·70 Ga. The zircon age spectrum in this unit is similar to that of a Middle Cambrian quartz arenite from the La Laja Formation, suggesting that local basement rocks were a regional source of ca 1·4 Ga detrital zircons in the Precordillera Terrane from the Cambrian onwards. The lack of grains younger than 600 Ma in Upper Ordovician units reinforces petrographic data indicating that Ordovician volcanic arc sources did not supply significant material directly to these sedimentary basins. Nd isotopic data (n = 32) for Middle and Upper Ordovician graptolitic shales from six localities define a poorly mixed signal [ɛ_Nd(450 Ma) = −9·6 to −4·5] that becomes more regionally homogenized in Upper Ordovician rocks (−6·2 ± 1·0; T_DM = 1·51 ± 0·15 Ga; n = 17), a trend reinforced by the U‐Pb detrital zircon data. It is concluded that proximal, recycled orogenic sources dominated the siliciclastic sediment supply for these basins, consistent with rapid unroofing of the Precordillera Terrane platform succession and basement starting in Mid Ordovician time. Common Pb data for Middle and Upper Ordovician shales from the western and eastern Precordillera (n = 15) provide evidence for a minor (<30%) component that was likely derived from a high‐μ (U/Pb) terrane.     

Geology and structural history of the southwest Precordillera margin, northern Mendoza Province, Argentina

Rocks and structures in the southwest Precordillera terrane, located in western Argentina, constrain the Paleozoic distribution of continents and the development of the western margin of Gondwana. Detailed mapping of an area in the southwest Precordillera allowed identification of several pre-Carboniferous rock units formed in distinct tectonic environments and were later tectonically juxtaposed. The pre-Carboniferous rock units comprise carbonate metasiltstone, metasandstone, massive diabase, and quartzo-feldspathic gneiss intruded by ultramafic rocks and layered gabbro. Preliminary structural analysis indicates that the present distribution of units is due to two contractional deformation episodes, an east-directed Devonian ductile event and a west-directed Tertiary brittle event. The metasedimentary rocks, which form the structural base of the area and are part of the western Precordilleran passive margin sequence, were juxtaposed along minor ductile shear zones early in the ductile event. Their contact was then folded during continued ductile deformation; at this time the ultramafic/layered gabbro complex and the massive diabase were emplaced over the metasedimentary units along narrow ductile shear zones. Brittle deformation, associated with the Andean orogeny, involved open folding, thrust faulting, and reactivation of some ductile features.     

Genetic evolution of Permo-Triassic volcaniclastic sequences at Uspallata, Mendoza Precordillera, Argentina

Permo-Triassic volcanic rocks of the Choiyoi Group crop out at Quebrada de Santa Elena and del Telégrafo, Mendoza Precordillera, Argentina. In this area, six volcanic events were generated through three volcanic stages that include collapse of voluminous Plinian columns associated to caldera pressurization and depressurization, emplacement of sub-volcanic bodies and dikes connected to depressurization of the magmatic chamber and establishment of resurgent domes and lava associated to caldera collapse events.

Petrologic and geochemical data indicate increasing acidification toward younger events varying from dacitic to rhyodacitic and rhyolitic rocks. The lithofacial organization allows correlating with the Lower to Upper Permian Horcajo Formation described in the Valle del Río de los Patos, San Juan province. Geochemical data suggests subduction-related arc magmatism in an extensional condition during Lower to Upper Permian.     

The Allochthony of the Argentine Precordillera Ten Years Later (1993–2003): A New Paleobiogeographic Test of the Microcontinental Model

Recent and new faunal data from the Cambrian to Silurian rocks of the Precordillera, Famatina and Northwest Argentina basins are used to discriminate between different paleogeographic models, and especially to establish to what extent they are compatible with a previous conclusion that the Precordillera is a Laurentian-derived microcontinent. There is no paleontological evidence to support a para-autochthonous Gondwanan origin of the Precordillera. The strong differences in the Cambrian trilobite faunas and lithologic successions preclude a common origin of the Precordillera terrane, eastern Antarctica and South Africa. Recent discoveries of brachiopods and organisms of the Phylum Agmata strengthened Laurentian affinities during the Cambrian. The latest Cambrian-early Ordovician faunas that inhabited the autochthonous Northwest Argentina basin, including the western Puna volcaniclastic successions, are mostly peri-Gondwanan. The early Ordovician brachiopods, ostracods and trilobites display mixed Laurentian, Baltic and Avalonian biogeographical links supporting a drifting of the Precordillera across the Iapetus Ocean. Increasing Gondwanan elements during the Llanvirn, along with varied geological evidence, indicate that the first stages of collision may have begun at that time, involving a major change in the plate kinematics. The distribution of facies and faunas, basin development, and timing of deformation are interpreted as resulting from a north to south diachronous closing of the remnant basin during the last phases of convergence and oblique collision of the Precordillera terrane with the Gondwana margin. The high level of endemism of Caradoc faunas may be a consequence of the rearrangement and partial isolation of sedimentary areas during the strike-slip movement of the colliding Precordillera plate with respect to the Gondwana margin. Suggested relationships between facies distribution, geographic barriers and faunal migrations before and during the collision are depicted in a series of schematic reconstructions at five time slices from late Cambrian to Silurian.     

Tectonic evolution of the Tombel graben basement, southwestern Cameroon

Planar structures (foliations and fractures) around the Tombel graben (southwestern end of the Central African Shear zone system) have been investigated and analyzed with the aim of unraveling the tectonic evolution of the basement. The foliations show two major trends, an older N-S-trending gneissose layering of uncertain agereworked by a later Pan-African (600 + 50 Ma) NE-SW ductile trend that is contemporaneous with sinistral shearing and mylonitization. The brittle phase characterized by NW-SE-trending open and partially filled fractures is younger than the mylonitization event and although it has not been dated, it is suggested that the origin of these fractures is linked to the onset of volcanism along the Cameroon volcanic line-31 m.y. ago.The mylonitic foliation is recognized for the first time and supports a tectonic evolution model for the Tombel graben in which ductile non-coaxial deformation was succeeded by brittle failure.     

大别-苏鲁区超高压变质岩的多期构造变质演化

对大别-苏鲁地区超高压(UHP)变质岩的详细构造和岩石学研究揭示了其复杂的构造变质演化历史。除前超高压事件外,至少可识别出5个相继发育的构造变质事件或阶段(D_1-D_5)。D_1和 D_2同超高压事件与三叠纪(250～230Ma)中朝克拉通和扬子克拉通间的大陆深俯冲及碰撞有关,而超高压后的 D_3和 D_4韧性变形及其伴生的减压部分熔融作用和退变质作用事件,则是超高压岩石向中上地壳折返过程中(230～140Ma)发生的。碰撞后形成的 D_4构造,主导了大别-苏鲁超高压和高压变质带区域尺度的构造格架。第5阶段的构造以摩擦或摩擦-粘性过渡性变形机制为主,并伴随有大规模的未变形的花岗质岩体就位,该期构造热事控制了现今大别-苏鲁地区的地貌学特征。新的构造和岩石学资料并结合可利用的地质年代学和地球化学等资料,提出一个涉及中朝与扬子克拉通间三叠纪大陆深俯冲、碰撞及相继超高压变质岩石向地表的多期折返构造变质演化模式。     

黔西南烂泥沟金矿田构造变形及演化分析

烂泥沟金矿是滇黔桂“金三角”内的大型金矿床,具有重要的经济价值。其矿体产出严格受断层控制,构造控矿特征明显。本文通过对烂泥沟金矿及其邻区主要断层之次级构造现象进行详细的野外观察和力学性质鉴定,继而进行几何学、运动学分析,认为研究区构造变形特征分为伸展和挤压两类。其中,晚古生代—中三叠世安尼期早期为伸展阶段,而在安尼期中期及以后为挤压阶段,构造反转时间应为248 Ma或246 Ma稍后。根据作者地震解释剖面,烂泥沟—尾怀断层和隐伏的巧洛断层向下已经延伸到南盘江盆地变质基底,可能为成矿流体提供运移通道,且烂泥沟—尾怀断层南部有较好的地球化学异常和矿化显示,说明烂泥沟金矿深部及外围具有较好的找矿前景。     

四川盆地西南部第三纪盆地原型及其演化

第三纪是四川盆地大范围陆相沉积历史的最后阶段, 同时又是四川盆地重要的构造定形期.探究该时期原型盆地沉积充填规律与构造演化特征, 是揭示四川盆地形成演化过程, 还原其古地理、古气候演变的关键.在综合利用古地磁、地震、野外露头等资料的基础之上, 从盆-山结合的角度出发, 以地质历史时间为主要线索, 对第三纪原型盆地分阶段、分区域地进行了动态化分析研究.重建了各沉积时期原型盆地构造-古地理格局, 分析了盆地沉积充填规律并复原了不同阶段沉积相带的空间展布特征.主要受周缘山系逆冲推覆作用产生的构造负载和盆地基底构造的影响, 第三纪时期四川盆地沉积范围局限于西南部、南部地区, 以河、湖沉积环境为主, 处于持续地挤压、充填过程而具有萎缩消亡的趋势.现今残余第三系地层由老到新, 主要由名山组、芦山组、大邑砾岩组等地层组成(先后经历了: 受造山带挤压推覆作用和温暖干旱气候等因素影响, 发育湖盆边缘冲积扇和沙漠沉积环境, 处于快速沉降阶段的名山组沉积时期; 以及构造活动相对稳定, 湖盆面积不断减小, 以沙泥质沉积互层为特征的芦山组沉积时期; 和沿山前带由南向北迁移, 以发育大型冲积扇为特征的大邑砾岩组沉积时期).纵观整个第三纪构造演化历史, 反映出四川盆地西南部地区在第三纪时期表现为典型的陆内坳陷沉积盆地性质.同期的大地构造活动和气候变化等因素对原型盆地的形成与演化起到了重要的控制作用, 同时也对该时期盆地古地理格局和沉积充填规律产生了重要影响.