Kinematic variations across Eastern Cordillera at 24°S (Central Andes): Tectonic and magmatic implications

The Eastern Cordillera (Central Andes,  24°S) consists of a basement-involved thrust system, resulting from Miocene–Quaternary eastward migrating compression, separating the Puna plateau from the Santa Barbara System foreland. The inferred Tertiary strains arising from shortening in the Eastern Cordillera and Santa Barbara System are similar, higher than in the Puna. Slip data collected on the major  N–S trending faults of Eastern Cordillera show a westward progression from dip-slip (contraction) to dextral and sinistral motions. This, consistently with established tectonic models, may result from partitioning due to the oblique Mio-Quaternary underthrusting of the Brazilian Shield north of 24°S. This strain partitioning has three main implications. (1) As the dextral and sinistral shear in the Eastern Cordillera are  62% and 29% of the compressive strain respectively, the Eastern Cordillera results more strained than Santa Barbara System foreland, contrary to previous estimates. (2) The partitioning in the Eastern Cordillera may find its counterpart in that to the west of the Central Andes, giving a possible structural symmetry to the Central Andes. (3) The easternmost N–S strike-slip structures in the Eastern Cordillera coincide with the easternmost Mio-Pliocene magmatic centres in the Central Andes, at  24°S. Provided that, further to the east, the crust is partially molten, the absence of magmatic centres may be explained by the presence of pure compressive structures in this portion of the Eastern Cordillera.     

Geochronology of Quaternary glaciations from the tropical Cordillera Huayhuash, Peru

The Cordillera Huayhuash in the central Peruvian Andes (10.3°S, 76.9°W) is an ideal mountain range in which to study regional climate through variations in paleoglacier extents. The range trends nearly north-south with modern glaciers confined to peaks >4800 m a.s.l. Geomorphology and geochronology in the nearby Cordillera Blanca and Junin Plain reveal that the Peruvian Andes preserve a detailed record of tropical glaciation. Here, we use ASTER imagery, aerial photographs, and GPS to map and date glacial features in both the western and eastern drainages of the Cordillera Huayhuash. We have used in situ produced cosmogenic ¹⁰Be concentrations in quartz bearing erratics on moraine crests and ice-polished bedrock surfaces to develop an exposure age chronology for Pleistocene glaciation within the range. We have also collected sediment cores from moraine-dammed lakes and bogs to provide limiting ¹⁴C ages for glacial deposits. In contrast to the ranges to the north and south, most glacial features within the Cordillera Huayhuash are Lateglacial in age, however we have identified features with ages that span 0.2 to 38 ka with moraine sets marking the onset of glacier retreat at 0.3 ka, 9–10 ka, 13–14 ka, 20–22 ka, and >26 ka. The range displays a pronounced east-west variation in maximum down-valley distance from the headwall of moraine crests with considerably longer paleoglaciers in the eastern drainages. Importantly, Lateglacial paleoglaciers reached a terminal elevation of 4000 m a.s.l. on both sides of the Cordillera Huayhuash; suggesting that temperature may have been a dominant factor in controlling the maximum glacier extent. We suggest that valley morphology, specifically valley slope, strongly influences down-valley distance to the maximum glacier extent and potential for moraine preservation. While regionally there is an extensive record of older (>50 ka) advances to the north (Cordillera Blanca) and to the south (Junin region), the apparent lack of old moraines in this locality may be explained by the confined morphology of the Cordillera Huayhuash valleys that has inhibited the preservation of older glacial geomorphic features.     

智利中部埃尔特尼恩特斑岩型铜钼矿床地质、成矿特征及成因

智利中部埃尔特尼恩特矿床是世界级超大型铜钼矿床之一,其铜矿石(含铜0.62%)储量达12.4×10~8t,钼矿石(含钼0.018%)储量达7.8×10~8t。矿床位于智利中部安第斯山脉晚中新世—早上新世铜-钼成矿省。该成矿省赋存于晚中新世火山活动带。埃尔特尼恩斑岩型铜-钼矿床赋存于中—晚中新世代伐尔隆斯建造,后者伏于科亚-玛查理建造之下,两者之间呈构造不整合或局部不整合接触关系。矿床产于伐尔隆斯的组成部分晚中新世火山深成杂岩中,该杂岩由厚层的玄武质至流纹质喷出岩及侵入岩组成。矿床围岩为安山岩、长英质-中性侵入岩和布莱登岩筒角砾岩。该矿床流体包裹体组合特征表明,岩浆热液演化及成矿作用经历了4个阶段。流体包裹体成分研究表明,该矿床的形成是富含Cu和可能还富含S的深源流体向不断脱挥发分的巨型次火山岩浆房发生贯入作用的结果。     

Coastal neotectonics in Southern Central Andes: uplift and deformation of marine terraces in Northern Chile (27°S)

Neotectonic observations allow a new interpretation of the recent tectonic behaviour of the outer fore arc in the Caldera area, northern Chile (27°S). Two periods of deformation are distinguished, based on large-scale Neogene to Quaternary features of the westernmost part of the Coastal Cordillera: Late Miocene to Early Pliocene deformations, characterized by a weak NE–SW to E–W extension is followed by uppermost Pliocene NW–SE to E–W compression. The Middle Pleistocene to Recent time is characterized by vertical uplift and NW–SE extension. These deformations provide clear indications of the occurrence of moderate to large earthquakes. Microseismic observations, however, indicate a lack of shallow crustal seismicity in coastal zone. We propose that both long-term brittle deformation and uplift are linked to the subduction seismic cycle.     

Late Paleozoic–Early Triassic magmatism on the western margin of Gondwana: Collahuasi area, Northern Chile

The basement in the ‘Altiplano’ high plateau of the Andes of northern Chile mostly consists of late Paleozoic to Early Triassic felsic igneous rocks (Collahuasi Group) that were emplaced and extruded along the western margin of the Gondwana supercontinent. This igneous suite crops out in the Collahuasi area and forms the backbone of most of the high Andes from latitude 20° to 22°S. Rocks of the Collahuasi Group and correlative formations form an extensive belt of volcanic and subvolcanic rocks throughout the main Andes of Chile, the Frontal Cordillera of Argentina (Choiyoi Group or Choiyoi Granite-Rhyolite Province), and the Eastern Cordillera of Peru.Thirteen new SHRIMP U–Pb zircon ages from the Collahuasi area document a bimodal timing for magmatism, with a dominant peak at about 300 Ma and a less significant one at 244 Ma. Copper–Mo porphyry mineralization is related to the younger igneous event.Initial Hf isotopic ratios for the ~ 300 Ma zircons range from about − 2 to + 6 indicating that the magmas incorporated components with a significant crustal residence time. The 244 Ma magmas were derived from a less enriched source, with the initial Hf values ranging from + 2 to + 6, suggestive of a mixture with a more depleted component. Limited whole rock ¹⁴⁴Nd/¹⁴³Nd and ⁸⁷Sr/⁸⁶Sr isotopic ratios further support the likelihood that the Collahuasi Group magmatism incorporated significant older crustal components, or at least a mixture of crustal sources with more and less evolved isotopic signatures.     

Curved structures and interference fold patterns associated with lateral ramps in the Eastern Cordillera, Central Andes of Argentina

The conspicuous curved structures located at the eastern front of the Eastern Cordillera between 25° and 26° south latitude is coincident with the salient recognized as the El Crestón arc. Major oblique strike-slip faults associated with these strongly curved structures were interpreted as lateral ramps of an eastward displaced thrust sheet. The displacement along these oblique lateral ramps generated the local N–S stress components responsible for the complex hanging wall deformation. Accompanying each lateral ramp, there are two belts of strong oblique fault and folding: the upper Juramento River valley area and El Brete area.On both margins of the Juramento River upper valley, there is extensive map-scale evidence of complex deformation above an oblique ramp. The N–S striking folds originated during Pliocene Andean orogeny were subsequently or simultaneously folded by E–W oriented folds. The lateral ramps delimiting the thrust sheet coincident with the El Crestón arc salient are strike-slip faults emplaced in the abrupt transitions between thick strata forming the salient and thin strata outside of it. El Crestón arc is a salient related to the pre-deformational Cretaceous rift geometry, which developed over a portion of this basin (Metán depocenter) that was initially thicker. The displacement along the northern lateral ramp is sinistral, whereas it is dextral in the southern ramp. The southern end of the Eastern Cordillera of Argentina shows a particular structure reflecting a pronounced along strike variations related to the pre-deformational sedimentary thickness of the Cretaceous basin.     

智利斑岩型铜矿地质特征及成矿规律

智利斑岩型铜矿闻名全球,世界上最大的丘基卡马塔和特尼恩特斑岩型铜矿就产于此地,主要分布在圣地亚哥-依基克一带,是地震-火山作用和侵入作用特别强烈的地区,成矿时代为拉拉米期（早白垩世-古近纪）.通过对智利斑岩型铜矿区域地质特征和3个典型的斑岩型铜矿床成矿地质条件及矿床地质特征分析,得出智利斑岩型铜矿成矿与多期次岩浆活动密切相关,呈南北纵向成带展布、成群出现,成矿时代新、埋藏浅、储量大.Lowell-Guilbert的斑岩铜矿成矿模式和Hollister的“闪长岩模式”对智利斑岩型铜矿的勘查起了重要的指导作用.     

Late Holocene sea-surface temperature and precipitation variability in northern Patagonia, Chile (Jacaf Fjord, 44°S)

A high-resolution multi-proxy study including the elemental and isotopic composition of bulk organic matter, land plant-derived biomarkers, and alkenone-based sea-surface temperature (SST) from a marine sedimentary record obtained from the Jacaf Fjord in northern Chilean Patagonia (44°20′S) provided a detailed reconstruction of continental runoff, precipitation, and summer SST spanning the last 1750 yr. We observed two different regimes of climate variability in our record: a relatively dry/warm period before 900 cal yr BP (lower runoff and average SST 1°C warmer than present day) and a wet/cold period after 750 cal yr BP (higher runoff and average SST 1°C colder than present day). Relatively colder SSTs were found during 750–600 and 450–250 cal yr BP, where the latter period roughly corresponds to the interval defined for the Little Ice Age (LIA). Similar climatic swings have been observed previously in continental and marine archives of the last two millennia from central and southern Chile, suggesting a strong latitudinal sensitivity to changes in the Southern Westerly Winds, the main source of precipitation in southern Chile, and validating the regional nature of the LIA. Our results reveal the importance of the Chilean fjord system for recording climate changes of regional and global significance.     

Differential Late Paleozoic active margin evolution in South-Central Chile (37°S–40°S) – the Lanalhue Fault Zone

The N–S oriented Coastal Cordillera of South Central Chile shows marked lithological contrasts along strike at 38°S. Here, the sinistral NW–SE-striking Lanalhue Fault Zone (nomen novum) juxtaposes Permo-Carboniferous magmatic arc granitoids and associated, frontally accreted metasediments (Eastern Series) in the northeast with a Late Carboniferous to Triassic basal-accretionary forearc wedge complex (Western Series) in the southwest. The fault is interpreted as an initially ductile deformation zone with divergent character, located in the eastern flank of the basally growing, upwarping, and exhuming Western Series. It was later transformed and reactivated as a semiductile to brittle sinistral transform fault. Rb–Sr data and fluid inclusion studies of late-stage fault-related mineralizations revealed Early Permian ages between 280 and 270 Ma for fault activity, with subsequent minor erosion. Regionally, crystallization of arc intrusives and related metamorphism occurred between 306 and 286 Ma, preceded by early increments of convergence-related deformation. Basal Western Series accretion started at >290 Ma and lasted to 250 Ma. North of the Lanalhue fault, Late Paleozoic magmatic arc granitoids are nearly 100 km closer to the present day Andean trench than further south. We hypothesize that this marked difference in paleo-forearc width is due to an Early Permian period of subduction erosion north of 38°S, contrasting with ongoing accretion further south, which kinematically triggered the evolution of the Lanalhue Fault Zone. Permo-Triassic margin segmentation was due to differential forearc accretion and denudation characteristics, and is now expressed in contrasting lithologies and metamorphic signatures in todays Andean forearc region north and south of the Lanalhue Fault Zone.     

SHRIMP U–Pb dating of the Antucoya porphyry copper deposit: new evidence for an Early Cretaceous porphyry-related metallogenic epoch in the Coastal Cordillera of northern Chile

The Antucoya porphyry copper deposit (300 Mt at 0.45% total Cu) is one of the largest deposits of a poorly known Early Cretaceous porphyry belt in the Coastal Cordillera of northern Chile. It is related to a succession of granodioritic and tonalitic porphyritic stocks and dikes that were emplaced within Jurassic andesitic rocks of the La Negra Formation immediately west of the N–S trending sinistral strike-slip Atacama Fault Zone. New zircon SHRIMP U–Pb data indicate that the porphyries of Antucoya crystallized within the time span from 142.7 ± 1.6 to 140.6 ± 1.5 Ma (±2 σ), and late, unmineralized, NW–SE trending dacite dikes with potassic alteration and internal deformation crystallized at 141.9 ± 1.4 Ma. The Antucoya porphyry copper system appears to be formed after a change of stress conditions along the magmatic arc from extensional in the Late Jurassic to transpressive during the Early Cretaceous and provides support for an Early Cretaceous metallogenic episode of porphyry-type mineralization along the Coastal Cordillera of northern Chile.     

The influence of tectonically derived relief and climate on the extent of the last Glaciation east of the Patagonian ice fields (Argentina, Chile)

Two large ice fields between 46°30′ and 51°30′S cover the Patagonian Andes. The North and South Patagonian Ice Fields are separated by the transandine depth line at 47°45′ to 48°15′S. Canal and Río Baker run through this depression. The two ice fields are generally considered relics of a continuous ice cap, which covered the entire Patagonian Andes from 39° to 52°S and extended far into the eastern foreland of the Andes. This assumption is not correct for the 200-km-long section of the Andes between Lago Pueyrredón (Lago Cochrane in Chile) (47°15′S) and Lago San Martín (Lago O'Higgins in Chile) (48°45′S). The lack of a continuous ice cap extending far into the east is caused by the transandine depth line, playing a crucial role in the fluvial erosion and the glacial scouring of this tectonic zone. This depression formed a river system (e.g. Río Baker, Río Bravo and Río Mayer) that drains towards the west. Reconstruction of the maximum glacial advance of the last ice age shows that the eastern outlet glaciers of the two ice fields between Lago San Martín and Lago Pueyrredón did not drain towards the east, but rather followed the general gradient of the transandine depth line. In this area the eastern flank of the Andes between Monte San Lorenzo (3770 m) and Sa. de Sangra (2155 m) supported valley glaciers, which were independent of the expanding ice fields. Only a few valley glaciers advanced towards the Patagonian Meseta. The terminal moraines of these glaciers were erroneously interpreted as the eastern edge of a continuous ice cap. North of 47°30′S the outlet glaciers of the NPI advanced 200 km during the LGM and the late glacial advances nearly reached to 71°W. In contrast, south of 49°S glacier expansion was comparatively less: The LGM is situated only 85–115 km east of the present margins of the large outlet glaciers (O'Higgins, Viedma, and Upsala), and no late glacial advance reached 72°W. These considerable differences of glacier expansion were influenced by the northward migration of the westerly precipitation belt during glacial cycles. There is tentative evidence that the glaciers advanced three times in the period from 14 000 to 9 500 ¹⁴C years BP.     

Origin and evolution of the Steamboat Springs siliceous sinter deposit, Nevada, U.S.A.

Siliceous hot spring deposits from Steamboat Springs, Nevada, U.S.A., record a complex interplay of multiple, changing, primary environmental conditions, fluid overprinting and diagenesis. Consequently these deposits reflect dynamic geologic and geothermal processes. Two surface sinters were examined—the high terrace, and the distal apron-slope, as well as 13.11 m (43 ft) of core material from drill hole SNLG 87-29. The high terrace sinter consists of vitreous and massive-mottled silica horizons, while the distal deposit and core comprise dominantly porous, indurated fragmental sinters. Collectively, the three sinter deposits archive a complete sequence of silica phase diagenetic minerals from opal-A to quartz. X-ray powder diffraction analyses and infrared spectroscopy of the sinters indicate that the distal apron-slope consists of opal-A and opal-A/CT mineralogy; the core yielded opal-A/CT and opal-CT with minor opal-A; and the high terrace constitutes opal-C, moganite, and quartz. Mineralogical maturation of the deposit produced alternating nano–micro–nano-sized silica particle changes. Based on filament diameters of microbial fossils preserved within the sinter, discharging thermal outflows fluctuated between low-temperatures (< 35 °C, coarse filaments) and mid-temperatures ( 35–60 °C, fine filaments). Despite transformation to quartz, primary coarse and fine filaments were preserved in the high terrace sinter. AMS ¹⁴C dating of pollen from three horizons within core SNLG 87-29, from depths of 8.13 to 8.21 m (26′8″ to 26′11″), 10.13 to 10.21 m (33′3″ to 33′6″), and 14.81 to 14.88 m (48′7″ to 48′10″), yielded dates of 8684 ± 64 years, 11,493 ± 70 years and 6283 ±60 years, respectively. In the upper section of the core, the stratigraphically out-of-sequence age likely reflects physical mixing of younger sinter with quartzose sinter fragments derived from the high terrace. Within single horizons, mineralogical and morphological components of the sinter matrix were spatially patchy. Overall, the deposit was modified by sub-surface flow of alkali-chloride thermal fluids depositing a second generation of silica, and periodically, by acidic steam condensate formed during periods when the water table was low. Local faulting produced considerable fracturing of the sinter. Hence, the Steamboat Springs sinter experienced a complex history of primary and secondary hydrothermal, geologic and diagenetic events, and their inter-relationships and effects are locked within the physical, chemical and biological signatures of the deposit.     

Tectonic and climate driven fluctuations in the stratigraphic base level of a Cenozoic continental coal basin, northwestern Andes

Changes in the sedimentologic and stratigraphic characteristics of the coal-bearing middle Oligocene–late Miocene siliciclastic Amagá Formation, northwestern Colombia, reflect major fluctuations in the stratigraphic base level within the Amagá Basin, which paralleled three major stages of evolution of the middle Cenozoic Andean Orogeny. These stages, which are also traceable by the changes in the compositional modes of sandstones, controlled the occurrence of important coal deposits. The initial stage of evolution of the Amagá Basin was related to the initial uplift of the Central Cordillera of Colombia around 25 Ma, which promoted moderate subsidence rates and high rates of sediment supply into the basin. This allowed the development of aggradational braided rivers and widespread channel amalgamation resulting in poor preservation of both, low energy facies and geomorphic elements. The presence of poorly preserved Alfisols within the scarce flood plains and the absence of swamp deposits suggest arid climate during this stage. The compositional modes of sandstones suggest sediment supply from uplifted basement-cored blocks. The second stage of evolution was related to the late Oligocene eastward migration of the Pre-Andean tholeitic magmatic arc from the Western Cordillera towards the Cauca depression. This generated extensional movements along the Amagá Basin, enhancing the subsidence and increasing the accommodation space along the basin. As a result of the enhanced subsidence rates, meandering rivers developed, allowing the formation of extensive swamps deposits (currently coal beds). The excellent preservation of Entisols and Alfisols within the flood plain deposits suggests rapid channels migration and a humid climate during deposition. Moderate to highly mature channel sandstones support this contention, and point out the Central Cordillera of Colombia as the main source of sediment. Enhanced subsidence during this stage also prevented channels amalgamation and promoted both, high preservation of geomorphic elements and high diversity of sedimentary facies. This resulted in the most symmetric stratigraphic cycles of the entire Amagá Formation. The final stage of evolution of the Amagá Basin was related to the early stage of development of the late Miocene northwestern Andes tholeitic volcanism (from 10 to 8 Ma). The extensive thrusting and folding associated to this volcanism reduced the subsidence rates along the basin and thus the accommodation space. This permitted the development of highly aggradational braided rivers and promoted channels amalgamation. Little preservation of low energy facies, poor preservation of the geomorphic elements and a complete obliteration of important swamp deposits (coal beds) within the basin are reflected by the most asymmetric stratigraphic cycles of the whole formation. The presence of greenish/reddish flood plain deposits and Alfisols suggests a dry climate during this depositional stage. The presence of channel sandstones with high contents of volcanic rock fragments supports a dry climate, and suggests an incipient phase of the Combia tholeiitic magmatism present during deposition of the Amagá Formation. The subsequent eastward migration of the NW Andes magmatic arc (after 8 Ma) may have produced basin inversion and suppressed deposition along the Amagá Basin.     

Tephrostratigraphy of the Bedded Tuff Member (Kapthurin Formation, Kenya) and the nature of archaeological change in the later middle Pleistocene

Correlation of volcaniclastic deposits of the Bedded Tuff Member (K4) of the Kapthurin Formation (Kenya) provides the means to assess the nature of archaeological change during the later middle Pleistocene, a time period critical to human evolution but poorly represented at other African localities. Field stratigraphic evidence, and petrographic and electron microprobe geochemical analyses of volcanic glass and phenocrysts, define eight subdivisions of K4 tephra. These include a succession of deposits from a local volcanic source that erupted intermittently, as well as other tuffs likely from different sources outside the Baringo basin. Upper portions of the Bedded Tuff Member date to 235,000 yr. The Bedded Tuff Member is underlain by sediments that include the Grey Tuff, dated to 509,000 ± 9000 yr. The tephrostratigraphic framework defined here is used to place Acheulian and Middle Stone Age (MSA) archaeological sites in chronological order. Results show the persistence of Acheulian large cutting tool manufacture after the advent of points, considered an MSA artifact type. Two assemblages from the site of Koimilot record the appearance at 200,000–250,000 yr of a variety of Levallois flake production methods, an integral if incompletely understood feature of the MSA, here likely derived from local technological antecedents. Combined evidence from the tools and flake production methods suggest an incremental and mosaic pattern of change in hominin adaptive strategies during the Acheulian–MSA transition.     

Late Triassic fluvial and marine shelf succession, Quebrada Doña Inés Chica, Atacama region, northern Chile

A 400-meter-thick volcanic and fine-grained clastic sedimentary succession in Quebrada Doña Ines Chica (26°07′S latitude; 69°20'W longitude) provides a record of Late Triassic deposition in the Atacama region of northern Chile. The strata are conformably overlain by fossiliferous marine limestones and sandstones of Late Triassic to Early Jurassic (Sinemurian) age which contain the oldest ichthyosaur remains known from Central and South America. The clastic succession is interpreted as coastal fluvial deposits, with the overlying limestones representing shelf deposits.     

Chronologic implications of new Miocene mammals from the Cura-Mallín and Trapa Trapa formations, Laguna del Laja area, south central Chile

Recent work in the central Andean Main Range of Chile near Laguna del Laja (37.5°S, 71°W) has produced the first mammal fossils for the region. Fossils, locally abundant and well preserved, occur patchily across a wide area southeast of the lake. Mammalian remains are derived from generally strongly folded (kilometer-scale) exposures of the locally 1.8 km thick, early to middle Miocene Cura-Mallín Formation; two identifiable specimens have been recovered from the overlying Trapa Trapa Formation as well. Both formations consist primarily of well-stratified (1–5 m thick layers) volcaniclastic and volcanic strata, deposited predominantly in fluviatile systems. The Cura-Mallín Formation is possibly the southern continuation of (or lateral equivalent to) the richly fossiliferous Abanico Formation mapped between 32°S and 36°S. Intensive sampling in a series of localities east and south of Laguna del Laja has yielded diverse faunas, in addition to radioisotopically dateable horizons. The new fossil mammal faunas represent as many as six South American Land Mammal “Ages” (SALMAs). Fossils, together with preliminary ⁴⁰Ar/³⁹Ar radioisotopic dates, ranging from 9 to 20 Ma across the exposed thickness of the Cura-Mallín Formation and into the overlying Trapa Trapa Formation, provide a robust geochronological framework for middle Cenozoic strata in the Laguna del Laja region. The sequence of directly superposed mammalian assemblages at Laguna del Laja is one of the longest in all of South America, rivaled only by the classic Gran Barranca section of Patagonian Argentina. These data illuminate the geological history of the area and its record of mammalian evolution. The potential to isotopically date these diverse faunas with high precision (error ± 0.5 Ma) presents a rare opportunity to calibrate related portions of the SALMA sequence.     

The time-space distribution of Eocene to Miocene magmatism in the central Peruvian polymetallic province and its metallogenetic implications

Eocene to late Miocene magmatism in the central Peruvian high-plain (approx. between Cerro de Pasco and Huancayo; Lats. 10.2–12°S) and east of the Cordillera Occidental is represented by scattered shallow-level intrusions as well as subaerial domes and volcanic deposits. These igneous rocks are calc-alkalic and range from basalt to rhyolite in composition, and many of them are spatially, temporally and, by inference, genetically associated with varied styles of major polymetallic mineralization. Forty-four new ⁴⁰Ar–³⁹Ar and three U/Pb zircon dates are presented, many for previously undated intrusions. Our new time constraints together with data from the literature now cover most of the Cenozoic igneous rocks of this Andean segment and provide foundation for geodynamic and metallogenetic research.The oldest Cenozoic bodies are of Eocene age and include dacitic domes to the west of Cerro de Pasco with ages ranging from 38.5 to 33.5 Ma. South of the Domo de Yauli structural dome, Eocene igneous rocks occur some 15 km east of the Cordillera Occidental and include a 39.34 ± 0.28 Ma granodioritic intrusion and a 40.14 ± 0.61 Ma rhyolite sill, whereas several diorite stocks were emplaced between 36 and 33 Ma. Eocene mineralization is restricted to the Quicay high-sulfidation epithermal deposit some 10 km to the west of Cerro de Pasco.Igneous activity in the earliest Oligocene was concentrated up to 70 km east of the Cordillera Occidental and is represented by a number of granodioritic intrusions in the Milpo–Atacocha area. Relatively voluminous early Oligocene dacitic to andesitic volcanism gave rise to the Astabamba Formation to the southeast of Domo de Yauli. Some stocks at Milpo and Atacocha generated important Zn–Pb (–Ag) skarn mineralization. After about 29.3 Ma, magmatism ceased throughout the study region. Late Oligocene igneous activity was restricted to andesitic and dacitic volcanic deposits and intrusions around Uchucchacua (approx. 25 Ma) and felsic rocks west of Tarma (21–20 Ma). A relationship between the Oligocene intrusions and polymetallic mineralization at Uchucchacua is possible, but evidence remains inconclusive.Widespread magmatism resumed in the middle Miocene and includes large igneous complexes in the Cordillera Occidental to the south of Domo de Yauli, and smaller scattered intrusive centers to the north thereof. Ore deposits of modest size are widely associated with middle Miocene intrusions along the Cordillera Occidental, north of Domo de Yauli. However, small volcanic centers were also active up to 50 km east of the continental divide and include dacitic dikes and domes, spatially associated with major base and precious metal mineralization at Cerro de Pasco and Colquijirca. Basaltic volcanism (14.54 ± 0.49 Ma) is locally observed in the back-arc domain south of Domo de Yauli approximately 30 km east of the Cordillera Occidental.After about 10 Ma intrusive activity decreased throughout Central Perú and ceased between 6 and 5 Ma. Late Miocene magmatism was locally related to important mineralization including San Cristobal (Domo de Yauli), Huarón and Yauricocha.Overall, there is no evidence for a systematic eastward migration of the magmatic arc through time. The arc broadened in the late Eocene to early Oligocene, and thereafter ceased over wide areas until the early Miocene, when magmatism resumed in a narrow arc. A renewed widening and subsequent cessation of the arc occurred in the late middle and late Miocene. The pattern of magmatism probably reflects two cycles of flattening of the subduction in the Oligocene and late Miocene. Contrasting crustal architecture between areas south and north of Domo de Yauli probably account for the differences in the temporal and aerial distribution of magmatism in these areas.Ore deposits are most abundant between Domo de Yauli and Cerro de Pasco and were generally emplaced in the middle and late Miocene during the transition to flat subduction and prior to cessation of the arc. Eocene to early Oligocene mineralization also occurred, but was restricted to a broad east–west corridor from Uchucchacua to Milpo–Atacocha, indicating a major upper-plate metallogenetic control.     

Cretaceous and Cenozoic cooling history across the ultrahigh pressure Tongbai–Dabie belt, central China, from apatite fission-track thermochronology

The crystalline terrane of the Tongbai–Dabie region, central China, comprising the Earth's largest ultrahigh-pressure (UHP) exposure was formed during Triassic collision between the Sino–Korean and Yangtze cratons. New apatite fission-track (AFT) data presented here from the UHP terrane, extends over a significantly greater area than reported in previous studies, and includes the (eastern) Dabie, the Hong'an (northwestern Dabie) and Tongbai regions. The new data yield ages ranging from 44 ± 3 to 142 ± 36 Ma and mean track lengths between 10 and 14.4 μm. Thermal history models based on the AFT data taken together with published ⁴⁰Ar/³⁹Ar, K–Ar, apatite and zircon (U–Th)/He and U–Pb data, exhibit a three-stage cooling pattern that is similar across the study region, commencing with an Early Cretaceous rapid cooling event, followed by a period of relative thermal stability during which rocks remained at temperatures within the AFT partial annealing zone (60–110 °C) and ending with a possible renewed phase of accelerated cooling during Pliocene to Recent time. The first cooling phase followed large-scale transtensional deformation between 140 and 110 Ma and is related to Early Cretaceous eastward tectonic escape and Pacific back arc extension. Between this phase and the subsequent slow cooling phase, a transition period from 120 to 80 Ma (to 70 to 45 Ma along the Tan–Lu fault) was characterised by a relatively low cooling rate (3–5 °C/Ma). This transition is likely related to a tectonic response associated with the mid-Cretaceous subduction of the Izanagi–Pacific plate as well as lithospheric extension and thinning in eastern Asia. The present regional AFT age pattern is therefore basically controlled by the Early Cretaceous rapid cooling event, but finally shaped through active Cenozoic faulting. Following the transition phase the subsequent slow cooling phase pattern implies a net reduction in horizontal compressional stress corresponding to increased extension rates along the continental margin due to the decrease in plate convergence. Modelling of the AFT data suggests a possible Pliocene–Recent cooling episode, which may be supported by increased rates of sedimentation observed in adjacent basins. This cooling phase may be interpreted as a response to the far-field effects of the frontal India–Eurasia collision to the west. Approximate estimates suggest that the total amount of post 120 Ma denudation across the UHP orogen ranged from 2.4 to 13.2 km for different tectonic blocks and ranged from 0.8 to 9.7 km during the Cretaceous to between 1.7 and 3.8 km during the Cenozoic.     

Paleomagnetism of the earliest Cretaceous to early late Cretaceous sandstones, Khorat Group, Northeast Thailand: Implications for tectonic plate movement of the Indochina block

A total of 400 samples (33 sites) were collected from the earliest Cretaceous to early Late Cretaceous sandstones of the Khorat Group in the Indochina block for paleomagnetic study to unravel the tectonic evolution of the region. The sites were adopted from 3 traverses located in the northern edge of the Khorat Plateau, northeastern Thailand. Results indicate that almost all the sandstones exhibit similar magnetic values with an average declination (D) = 31.7°, inclination (I) = 30.3°, λ = 59.7°,  = 190.9°, K = 54.4, and A₉₅ = 3.7 at reference point 17°30′N and 103°30′E. The calculated paleolatitude points are inferred to deviate from the present latitude point by 1.2 ± 2.3°. Only the lowermost part of the Cretaceous sandstones can pass a positive fold test at 95% confidence level. The relationship between the virtual geomagnetic poles (VGPs) of Cretaceous rocks of the Indochina plate in Thailand and those of the South China plate advocate that there is a major displacement of Indochina along the northwest-trending Red River and associated faults by about 950 ± 150 km with a 16.0–17.0° clockwise rotation relative to the South China plate during earliest Cretaceous times. Paleomagnetic results of the early Late Cretaceous Indochina plate point to a 20–25° clockwise rotation relative to the present occurring since very Late Cretaceous (65 Myrs)–Early Neogene times which may be due to the collision between India and Asia.     

Thermochronology of allochthonous terranes in Ecuador: Unravelling the accretionary and post-accretionary history of the Northern Andes

The western cordilleras of the Northern Andes (north of 5°S) are constructed from allochthonous terranes floored by oceanic crust. We present ⁴⁰Ar/³⁹Ar and fission-track data from the Cordillera Occidental and Amotape Complex of Ecuador that probably constrain the time of terrane collision and post-accretionary tectonism in the western Andes. The data record cooling rates of 80–2 °C/my from temperatures of 540 °C, during 85 to 60 Ma, in a highly tectonised mélange (Pujilí unit) at the continent–ocean suture and in the northern Amotape Complex. The rates were highest during 85–80 Ma and decelerated towards 60 Ma. Cooling was a consequence of exhumation of the continental margin, which probably occurred in response to the accretion of the presently juxtaposing Pallatanga Terrane. The northern Amotape Complex and the Pujilí unit may have formed part of a single, regional scale, tectonic mélange that started to develop at ~85 Ma, part of which currently comprises the basement of the Interandean Depression. Cooling and rotation in the allochthonous, continental, Amotape Complex and along parts of the continent–ocean suture during 43–29 Ma, record the second accretionary phase, during which the Macuchi Island Arc system collided with the Pallatanga Terrane. Distinct periods of regional scale cooling in the Cordillera Occidental at 13 and 9 Ma were synchronous with exhumation in the Cordillera Real and were probably driven by the collision of the Carnegie Ridge with the Ecuador Trench. Finally, late Miocene–Pliocene reactivation of the Chimbo–Toachi Shear Zone was coincident with the formation of the oldest basins in the Interandean Depression and probably formed part of a transcurrent or thrust system that was responsible for the inception and subsequent growth of the valley since 6 Ma.