Structural evolution of the Llanos foothills, Eastern Cordillera, Colombia

The Llanos foothills are located in the frontal thrust zone of the Eastern Cordillera in Colombia in a complex environment that BP has been exploring actively since 1988. This exploration has resulted in the discovery of several fields with a variety of hydrocarbon fluids (gas condensate and volatile oil) in very tight quartz-arenites. The structural style and complexity of this fold-and-thrust belt changes along the trend from single frontal structures to an imbricate of up to five thrust sheets in a triangle zone. In highly complex environments, the seismic image quality is poor, and interpretation becomes very challenging. The structural models of the area have evolved as more data have been acquired. The initial structural model required inversion of the basin at the end of the Andean orogeny. The structural style changed to an in-sequence imbricate thrust stack with very long, trailing back limbs that return to regional elevation and finalize in a tighter structures with short back limbs. The concept of early deformation and multiple phases has been introduced. Three main phases have been distinguished: (1) an early event during the deposition of the Lower Carbonera (39–29 Ma), with incipient structures formed to create syntectonic deposition; (2) a phase of steady subsidence that increased notably at the end of the period (29–7 Ma); and (3) the latest phase (7–0 Ma), when most deformation and uplifting occurred. The migration of hydrocarbons happened simultaneously with the deformation, and its final distribution, amount, and variation in composition is related to the structural evolution of the area.     

Structure and tectonics of the central segment of the Eastern Cordillera of Colombia

In the Eastern Cordillera of Colombia, a new structural model constrained by field data, paleontologic determinations, and interpretations of seismic reflection profiles is proposed. The model implies 70 km of shortening, including reactivation of basement structures as inverse faults in both flanks of the chain. These faults propagated within the lower Cretaceous strata, inducing passively rooted and transported thrust sheets as the successive basement faults were reactivated. Two structural styles are identified in the western flank: (1) positive flower structures in a transpressive regime, which affected rocks older than upper Paleocene and were unconformably covered by post–late Paleocene sediments, and (2) compressive structures during the late Miocene–Recent Andean phase. Presently, WNW-ESE compression reactivates Late Paleocene structures, which locally affect Andean trends. In the western margin of the Eastern Cordillera, the Cambao thrust takes up most displacement, whereas the Bituima fault takes only a minor part. To the south, this relationship reverses, suggesting complementary behavior by the Bituima and Cambao faults, as well as a transfer zone. This suggestion explains the southward termination of the Guaduas syncline as a structure related to the Cambao fault, whereas the Bituima fault increases its displacement southward, generating the Girardot foldbelt that takes over the structural position of the Guaduas syncline.     

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.     

Mesozoic transtensional basin history of the Eastern Cordillera, Colombian Andes: Inferences from tectonic models

Backstripping analysis and forward modeling of 162 stratigraphic columns and wells of the Eastern Cordillera (EC), Llanos, and Magdalena Valley shows the Mesozoic Colombian Basin is marked by five lithosphere stretching pulses. Three stretching events are suggested during the Triassic–Jurassic, but additional biostratigraphical data are needed to identify them precisely. The spatial distribution of lithosphere stretching values suggests that small, narrow (<150 km), asymmetric graben basins were located on opposite sides of the paleo-Magdalena–La Salina fault system, which probably was active as a master transtensional or strike-slip fault system. Paleomagnetic data suggesting a significant (at least 10°) northward translation of terranes west of the Bucaramanga fault during the Early Jurassic, and the similarity between the early Mesozoic stratigraphy and tectonic setting of the Payandé terrane with the Late Permian transtensional rift of the Eastern Cordillera of Peru and Bolivia indicate that the areas were adjacent in early Mesozoic times. New geochronological, petrological, stratigraphic, and structural research is necessary to test this hypothesis, including additional paleomagnetic investigations to determine the paleolatitudinal position of the Central Cordillera and adjacent tectonic terranes during the Triassic–Jurassic. Two stretching events are suggested for the Cretaceous: Berriasian–Hauterivian (144–127 Ma) and Aptian–Albian (121–102 Ma). During the Early Cretaceous, marine facies accumulated on an extensional basin system. Shallow-marine sedimentation ended at the end of the Cretaceous due to the accretion of oceanic terranes of the Western Cordillera. In Berriasian–Hauterivian subsidence curves, isopach maps and paleomagnetic data imply a (>180 km) wide, asymmetrical, transtensional half-rift basin existed, divided by the Santander Floresta horst or high. The location of small mafic intrusions coincides with areas of thin crust (crustal stretching factors >1.4) and maximum stretching of the subcrustal lithosphere. During the Aptian–early Albian, the basin extended toward the south in the Upper Magdalena Valley. Differences between crustal and subcrustal stretching values suggest some lowermost crustal decoupling between the crust and subcrustal lithosphere or that increased thermal thinning affected the mantle lithosphere. Late Cretaceous subsidence was mainly driven by lithospheric cooling, water loading, and horizontal compressional stresses generated by collision of oceanic terranes in western Colombia. Triassic transtensional basins were narrow and increased in width during the Triassic and Jurassic. Cretaceous transtensional basins were wider than Triassic–Jurassic basins. During the Mesozoic, the strike-slip component gradually decreased at the expense of the increase of the extensional component, as suggested by paleomagnetic data and lithosphere stretching values. During the Berriasian–Hauterivian, the eastern side of the extensional basin may have developed by reactivation of an older Paleozoic rift system associated with the Guaicáramo fault system. The western side probably developed through reactivation of an earlier normal fault system developed during Triassic–Jurassic transtension. Alternatively, the eastern and western margins of the graben may have developed along older strike-slip faults, which were the boundaries of the accretion of terranes west of the Guaicáramo fault during the Late Triassic and Jurassic. The increasing width of the graben system likely was the result of progressive tensional reactivation of preexisting upper crustal weakness zones. Lateral changes in Mesozoic sediment thickness suggest the reverse or thrust faults that now define the eastern and western borders of the EC were originally normal faults with a strike-slip component that inverted during the Cenozoic Andean orogeny. Thus, the Guaicáramo, La Salina, Bitúima, Magdalena, and Boyacá originally were transtensional faults. Their oblique orientation relative to the Mesozoic magmatic arc of the Central Cordillera may be the result of oblique slip extension during the Cretaceous or inherited from the pre-Mesozoic structural grains. However, not all Mesozoic transtensional faults were inverted.     

Present-day stress field and deformation in eastern Austria

 We analysed fault plane solutions and borehole breakouts in the eastern part of the Eastern Alps and found a heterogeneous stress field which we interpret as a transition zone of three different stress provinces: the western European stress province with NW to NNW S_H orientation and mainly strike-slip faulting regime; the Adriatic stress province with a radial stress pattern and thrust faulting to strike-slip faulting regime; and the Dinaric-Pannonian stress province with NE S_H orientation and strike-slip faulting regime. The western Pannonian basin seems to be a part of the transition zone with WNW to NW S_H orientation. A stress regime stimulating strike-slip faulting prevails in the Eastern Alps. The south Bohemian basement spur as a major tectonic structure with a high rheological contrast to surrounding units has a strong influence on the stress field and exhibits the highest seismicity at its tip due to stress concentration. From a constructed vertical stress orientation profile we found stress decoupling of the Northern Calcareous Alps from the underlying European foreland. Both the Molasse and the Flysch-Helvetic zone are considered as candidates for decoupling horizons due to stress orientation observations and due to their rheological behaviour. From seismological and rheological data, we suggest a horizontal stress decoupling across the Eastern Alps caused by a weakened central Alpine lithosphere. Received: 3 July 1998 / Accepted: 4 April 1999     

Provenance and Evolution of the Guarguariiz Complex, Cordillera Frontal, Argentina

The Guarguardz Complex, basement of the Cordillera Frontal, included in the proposed Chilenia Terrane, consists of metasedimentary rocks deposited in clastic and carbonatic platforms. Turbiditic sequences point out to slope or external platform environments. According to geochemical data, the sedimentary protoliths derived through erosion of a mature cratonic continental basement. Volcanic and subvolcanic rocks with N and E-MORB signature were interbeded in the metasedimentary rocks during basin development. A compressional stage, starting with progressive deformation and metamorphism, followed this extensional stage. Continuing deformation led to the emplacement of slices of oceanic crust, conforming an accretionary prism during Late Devonian. The Guarguardz Complex and equivalent units in western Precordillera and also in the Chilean Coastal Cordillera share common evolutional stages, widely represented along the western Gondwana margin. These evidences imply that Chilenia is not an allochthonous terrane to Gondwana, but a portion of its Early Paleozoic margin. Regional configuration indicates that the Guarguardz Complex and equivalent units represent the accretionary prism of the Famatinian arc (Middle Ordovician-Late Devonian).     

On the lag time between internal strain and basement involved thrust induced exhumation: The case of the Colombian Eastern Cordillera

Thrust sheets accumulate internal strain before they start moving along discrete fault planes. However, there are no previous studies evaluating the time difference between initiation of strain and fault displacement. In this paper we use observations from the Eastern Cordillera of Colombia to evaluate this interval. We utilize multiple thermochronometers and paleothermometers to refine the timing of deformation. Based on these new data we build time-temperature path estimates that together with geometric outcrop-based structural analysis and fluid inclusions allow us to assign relative timing to features associated with strain, such as cleavage, veins and certain types of fractures, and compare that with the timing of thrusting. We find that cleavage was only formed close to maximum paleotemperatures, almost coeval with the onset of thrust-induced denudation by the Late Oligocene. The corresponding structural level of fold-related veins suggest that they were formed later but still when the country rocks were at temperatures higher than 160 °C, mostly during the Early Miocene and still coexisted with the latest stages of cleavage formation. Our data show that the main period of strain hardening was short (probably a few million years) and occurred before first-order basement thrusting was dominant, but was associated with second-order folding.     

Ongoing tectonic reactivation of the Outer Carpathians and its impact on the foreland: Results of borehole breakout measurements in Poland

The direction of present-day maximum horizontal compression (= S_Hmax = tectonic stress) was interpreted for 62 wells in Poland, using the method of borehole breakout analysis of 4-arm and 6-arm dipmeter logs. The study area covers complex tectonic junction of the Carpathian orogen and its foreland, which comprises the East European Craton (EEC) divided by the Teisseyre–Tornquist zone (TTZ) from the Palaeozoic platform of western Poland. For this area, frequent deviation of the S_Hmax direction from NW–SE characteristic for the Atlantic ridge push has been interpreted in terms of the ALCAPA tectonic push. In the Upper Silesian segment of the Polish Outer Carpathians (POC), NNE–SSW-oriented S_Hmax in the accretionary wedge differs significantly from NNW–SSE S_Hmax in the autochthonous basement. The above discrepancy points to uncoupled type of the suture in this segment of the POC. In this scenario the ALCAPA push involves the nappes and is compensated in the top of the basement, which is expressed by systematic S_Hmax rotations. In the accretionary wedge of the eastern Małopolska segment, S_Hmax follows the trend perpendicular to the strike of nappes. It is in general agreement with NNE–SSW-oriented S_Hmax in the autochthonous basement that also parallels the ALCAPA push direction. Similarity in stress orientation between these structural levels implies coupled suture zone in this segment of the POC. Further to the north, ALCAPA push is transmitted into the foreland plate where it causes N–S orientation of S_Hmax, as determined for sedimentary cover of the EEC. Within the Baltic portion of the EEC, further S_Hmax rotation towards the intermediate NNW–SSE position suggests balance between the ALCAPA push and the ridge push components. Within the TTZ, common S_Hmax rotations from N–S to NW–SE indicate structurally controlled accommodation of the ALCAPA push. In the Palaeozoic platform of western Poland, Mesozoic complex of the Fore-Sudetic Monocline reveals NNE–SSW-directed S_Hmax that differs from NW–SE-oriented S_Hmax in the Variscan accretionary wedge. Here, mechanical decoupling along Zechstein evaporates is likely.

Presented set of breakout data from Poland shows that recent compressive reactivation of the Carpathians exerts strong impact on the stress field of the foreland plate at a distance of 700 km from the suture. Due to this effect, the Fore-Carpathian stress domain within the North European stress province can be discriminated.     

The magmatic history of the Vetas-California mining district,Santander Massif,Eastern Cordillera,Colombia

The Vetas-California Mining District (VCMD), located in the central part of the Santander Massif (Colombian Eastern Cordillera), based on U–Pb dating of zircons, records the following principal tectono-magmatic events: (1) the Grenville Orogenic event and high grade metamorphism and migmatitization between ∼1240 and 957 Ma; (2) early Ordovician calc–alkalic magmatism, which was synchronous with the Caparonensis–Famatinian Orogeny (∼477 Ma); (3) middle to late Ordovician post-collisional calc–alkalic magmatism (∼466–436 Ma); (4) late Triassic to early Jurassic magmatism between ∼204 and 196 Ma, characterized by both S- and I-type calc–alkalic intrusions and; (5) a late Miocene shallowly emplaced intermediate calc–alkaline intrusions (10.9 ± 0.2 and 8.4 ± 0.2 Ma). The presence of even younger igneous rocks is possible, given the widespread magmatic–hydrothermal alteration affecting all rock units in the area.The igneous rocks from the late Triassic–early Jurassic magmatic episodes are the volumetrically most important igneous rocks in the study area and in the Colombian Eastern Cordillera. They can be divided into three groups based on their field relationships, whole rock geochemistry and geochronology. These are early leucogranites herein termed Alaskites-I (204–199 Ma), Intermediate rocks (199–198 Ma), and late leucogranites, herein referred to as Alaskites-II (198–196 Ma). This Mesozoic magmatism is reflecting subtle changes in the crustal stress in a setting above an oblique subduction of the Panthalassa plate beneath Pangea.The lower Cretaceous siliciclastic Tambor Formation has detrital zircons of the same age populations as the metamorphic and igneous rocks present in the study area, suggesting that the provenance is related to the erosion of these local rocks during the late Jurassic or early Cretaceous, implying a local supply of sediments to the local depositional basins.     

Late quaternary glacial stades in the Cordillera Central,Colombia, based on glacial geomorphology,tephra–soil stratigraphy,palynology, and radiocarbon dating

Using data from glacial geomorphology, tephra–soil stratigraphy and mineralogy, palynology, and radiocarbon dating, a sequence of glacial and bioclimatic stades and interstades has been identified for the last ca. 50000 yr in the Ruiz-Tolima massif, Cordillera Central, Colombia. Six Pleistocene cold stades separated by warmer interstades occurred: before 48000, between 48000 and 33000, between 28000 and 21000, from ≥16000 to ca. 14000, ca. 13000–12400, and ca. 11000–10000 yr BP. Although these radiocarbon ages are minimum-limiting ages obtained from tephra layers on top of tills, the tills are not significantly older because most are bracketed by dated tephra sets in measured stratigraphic sections. Two minor moraine stages likely reflect glacier standstill during cold intervals ca. 7400 yr BP and slightly earlier. Finally, glaciers readvanced between the seventeenth and nineteenth centuries. In contrast to the ice-clad volcanoes of the massif, ca. 34 km² in area above an altitude of ca. 4800 m, the ice cover expanded to 1200 km² during the Last Glacial Maximum (LGM) and was still 800 km² during Late-glacial time (LGT). Glacier reconstructions based on the moraines suggest depression of the equilibrium line altitude (ELA) by ca. 1100 m during the LGM and 500–600 m during LGT relative to the modern ELA, which lies at ca. 5100 m in the Cordillera Central. Glaciers in this region apparently reached their greatest extent when the climate was cold and wet, e.g. during stades corresponding to Oxygen Isotope Stage 3; glaciers were still expanding during the LGM ca. 28000–21000 yr BP, but they shrank considerably after 21000 yr BP because of greatly reduced precipitation. © 1997 John Wiley & Sons, Ltd.     

Late Jurassic terrane collision in the northwestern margin of Gondwana (Cajamarca Complex,eastern flank of the Central Cordillera,Colombia)

Medium-grade metabasites and metapelites from the Cajamarca Complex (Central Cordillera of Colombia) are in fault contact with the Jurassic Ibague batholith and show a penetrative foliation, locally mylonitic, suggesting intense dynamic–thermal metamorphism. The amphibolites are composed of calcic amphibole + epidote + plagioclase + quartz plus rutile + titanite + apatite + carbonate as accessory phases. Chlorite and albite appear as retrograde replacements. The metapelites are mainly composed of phengite + quartz + garnet + chlorite, plus epidote + albite + apatite + titanite + haematite as accessory phases. Bulk geochemistry of the amphibolites indicates basaltic protoliths with a mid-ocean ridge basalt (MORB) signature, although enrichment in the mobile large-ion lithophile elements compared to MORB suggests pre- and/or syn-metamorphic alteration by fluids. Peak pressure–temperature determinations for both types of rocks are similar, ranging 550–580°C and 8 kbar (approximately 26 km depth and an apparent geothermal gradient of 22°C/km). ⁴⁰Ar-³⁹Ar dating of amphibole from two amphibolite samples and one phengitic mica from a pelitic schist yielded plateau ages of 146.5 ± 1.1 Ma and 157.8 ± 0.6 Ma, and 157.5 ± 0.4 Ma, respectively. These Late Jurassic ages contrast with previously published (Permian)Triassic ages of metamorphism in the Cajamarca Complex. Taken together, our data indicate tectonic-driven burial of oceanic supracrustal sequences down to mid-crustal depths during Late Jurassic times and are best explained as the result of terrane collision-related metamorphism and deformation in a fore-arc/volcanic-arc environment of the active western margin of Gondwana rather than as a result of Jurassic thermal–metamorphic resetting of a (Permian)Triassic metamorphic sequence during intrusion of the Jurassic Ibague batholith. Our results represent the first report of Jurassic terrane collision tectonics involving supracrustal oceanic rocks in the northwestern margin of Gondwana in Colombia.     

Transverse zones controlling the structural evolution of the Zipaquira Anticline (Eastern Cordillera,Colombia): Regional implications

We report paleomagnetic, magnetic fabric and structural results from 21 sites collected in Cretaceous marine mudstones and Paleogene continental sandstones from the limbs, hinge and transverse zones of the Zipaquira Anticline (ZA). The ZA is an asymmetrical fold with one limb completely overturned by processes like gravity and salt tectonics, and marked by several axis curvatures. The ZA is controlled by at least two (2) transverse zones known as the Neusa and Zipaquira Transverse Zones (NTZ and ZTZ, respectively). Magnetic mineralogy methods were applied at different sites and the main carriers of the magnetic properties are paramagnetic components with some sites being controlled by hematite and magnetite. Magnetic fabric analysis shows rigid-body rotation for the back-limb in the ZA, while the forelimb is subjected to internal deformation. Structural and paleomagnetic data shows the influence of the NTZ and ZTZ in the evolution of the different structures like the ZA and the Zipaquira, Carupa, Rio Guandoque, Las Margaritas and Neusa faults, controlling several factors as vergence, extension, fold axis curvature and stratigraphic detatchment. Clockwise rotations unraveled a block segmentation following a discontinuos model caused by transverse zones and one site reported a counter clockwise rotation associated with a left-lateral strike slip component for transverse faults (e.g. the Neusa Fault). We propose that diverse transverse zones have been active since Paleogene times, playing an important role in the tectonic evolution of the Cundinamarca sub-basin and controlling the structural evolution of folds and faults with block segmentation and rotations.     

Pressure-temperature Evolution of the Metapelites in the Motuo Area,the Eastern Himalayan Syntaxis

The Motuo area is located in the east of the Eastern Himalayan Syntaxis. There outcrops a sequence of high-grade metamorphic rocks, such as metapelites. Petrology and mineralogy data suggest that these rocks have experienced three stages of metamorphism. The prograde metamorphic mineral assemblages(M1) are mineral inclusions(biotite + plagioclase + quartz ± sillimanite ± Fe-Ti oxides) preserved in garnet porphyroblasts, and the peak metamorphic assemblages(M2) are represented by garnet with the lowest XSps values and the lowest XFe# ratios and the matrix minerals(plagioclase + quartz ± Kfeldspar + biotite + muscovite + kyanite ± sillimanite), whereas the retrograde assemblages(M3) are composed of biotite + plagioclase + quartz symplectites rimming the garnet porphyroblasts. Thermobarometric computation shows that the metamorphic conditions are 562–714°C at 7.3–7.4 kbar for the M1 stage, 661–800°C at 9.4–11.6 kbar for the M2 stage, and 579–713°C at 5.5–6.6 kbar for the M3 stage. These rocks are deciphered to have undergone metamorphism characterized by clockwise P-T paths involving nearly isothermal decompression(ITD) segments, which is inferred to be related to the collision of the India and Eurasia plates.     

Evolution of intraplate stress fields under multiple remote compressions: The case of the Iberian Chain (NE Spain)

The stress evolution of the central-eastern Iberian Chain during the Tertiary compression has been a matter of discussion during the last decades. In particular, there is not a complete agreement on whether the tectonic evolution is controlled by different external stress fields or it is essentially related to a single stress field with multiple stress perturbations. A systematic procedure to discriminate between these two hypotheses is proposed. The procedure involves statistical computing of local compression directions, identifying and ‘filtering’ stress deviations on outcrop to map scale, and timing of paleostresses. The latter has been interpreted from both analysis of cross-cut relationships of structures and consideration of the palaeostress record through the sequence of syntectonic sedimentary units. The results suggest that a single stress field with multiple perturbations cannot explain the ensemble of compression directions inferred in the region. The final proposed model includes three different, partially superposed Intraplate Stress Fields ISF (NE–SW, ESE–WNW to SSE–NNW, and NNE–SSW ISFs), driven by genetically independent far-field tectonic forces related with the active Iberia plate margins, and showing both local and regional deflection of stress trajectories.     

Geochemistry of black shales from the Lower Cretaceous Paja Formation,Eastern Cordillera,Colombia: Source weathering,provenance, and tectonic setting

The major and trace element characteristics of black shales from the Lower Cretaceous Paja Formation of Colombia are broadly comparable with those of the average upper continental crust. Among the exceptions are marked enrichments in V, Cr, and Ni. These enrichments are associated with high organic carbon contents. CaO and Na₂O are strongly depleted, leading to high values for both the Chemical Index of Alteration (77–96) and the Plagioclase Index of Alteration (86–99), which indicates derivation from a stable, intensely weathered felsic source terrane. The REE abundances and patterns vary considerably but can be divided into three main groups according to their characteristics and stratigraphic position. Four samples from the lower part of the Paja Formation (Group 1) are characterized by LREE-enriched chondrite-normalized patterns (average La_N/Yb_N = 8.41) and significant negative Eu anomalies (average Eu/Eu¹ = 0.63). A second group of five samples (Group 2), also from the lower part, have relatively flat REE patterns (average La_N/Yb_N = 1.84) and only slightly smaller Eu anomalies (average Eu/Eu¹ = 0.69). Six samples from the middle and upper parts (Group 3) have highly fractionated patterns (average La_N/Yb_N = 15.35), resembling those of Group 1, and an identical average Eu/Eu¹ of 0.63. The fractionated REE patterns and significant negative Eu anomalies in Groups 1 and 3 are consistent with derivation from an evolved felsic source. The flatter patterns of Group 2 shale and strongly concave MREE-depleted patterns in two additional shales likely were produced during diagenesis, rather than reflecting more mafic detrital inputs. An analysis of a single sandstone suggests diagenetic modification of the REE, because its REE pattern is identical to that of the upper continental crust except for the presence of a significant positive Eu anomaly (Eu/Eu¹ = 1.15). Felsic provenance for all samples is suggested by the clustering on the Th/Sc–Zr/Sc and Gd_N/Yb_N–Eu/Eu¹ diagrams. Averages of unmodified Groups 1 and 3 REE patterns compare well with cratonic sediments from the Roraima Formation in the Guyana Shield, suggesting derivation from a continental source of similar composition. In comparison with modern sediments, the geochemical parameters (K₂O/Na₂O, La_N/Yb_N, La_N/Sm_N, Eu/Eu¹, La/Sc, La/Y, Ce/Sc) suggest the Paja Formation was deposited at a passive margin. The Paja shales thus represent highly mature sediments recycled from deeply weathered, older, sedimentary/metasedimentary rocks, possibly in the Guyana Shield, though Na-rich volcanic/granitic rocks may have contributed to some extent.     

北黄海盆地东部坳陷中生界沉积特征及演化

北黄海盆地是我国近海勘探及研究程度均较低的中新生代叠合断陷盆地, 而东部坳陷是其内最具油气勘探前景的二级构造单元。通过岩芯观察与测井相分析, 结合三维地震资料, 认为东部坳陷中生界主要发育扇三角洲、辫状河三角洲、三角洲与湖泊沉积。受构造演化阶段的制约, 不同时期具有不同的沉积演化特征。其中中侏罗世处于湖盆的初期断陷期, 半深湖-深湖亚相发育, 仅在坳陷边缘见少量小规模的扇三角洲沉积。晚侏罗世处于断陷扩展期, 整体为一个完整的湖进-湖退旋回, 早期北部缓坡发育三角洲沉积, 南部及东部陡坡见扇三角洲沉积, 中后期则由于湖平面上升, 发育暗色泥岩为主的湖泊沉积, 晚期北部缓坡见小规模三角洲前缘沉积。早白垩世处于断陷萎缩期, 垂向发育2个正旋回沉积序列, 其中旋回下部以扇三角洲、辫状河三角洲相为主, 向上渐变为滨浅湖沉积。     

Pollen‐ and diatom based environmental history since the Last Glacial Maximum from the Andean core Fúquene‐7, Colombia

The late Pleistocene–Holocene ecological and limnological history of Lake Fúquene (2580 m a.s.l.), in the Colombian Andes, is reconstructed on the basis of diatom, pollen and sediment analyses of the upper 7 m of the core Fúquene‐7. Time control is provided by 11 accelerator mass spectrometry (AMS) ¹⁴C dates ranging from 19 670 ± 240 to 6040 ± 60 yr BP. In this paper we present the evolution of the lake and its surroundings. Glacial times were cold and dry, lake‐levels were low and the area was surrounded by paramo and subparamo vegetation. Late‐glacial conditions were warm and humid. The El Abra Stadial, a Younger Dryas equivalent, is reflected by a gap in the sedimentary record, a consequence of the cessation of deposition owing to a drop in lake‐level. The early Holocene was warm and humid; at this time the lake reached its maximum extension and was surrounded by Andean forest. The onset of the drier climate prevailing today took place in the middle Holocene, a process that is reflected earlier in the diatom and sediment records than in the pollen records. In the late Holocene human activity reduced the forest and transformed the landscape. Climate patterns from the Late‐glacial and throughout the Holocene, as represented in our record, are similar to other records from Colombia and northern South America (the Caribbean, Venezuela and Panama) and suggest that the changes in lake‐level were the result of precipitation variations driven by latitudinal shifts of the Intertropical Convergence Zone. Copyright © 2003 John Wiley & Sons, Ltd.     

黑龙江东部鸡西盆地构造层序划分与盆地动力学演化

依据钻孔及露头资料,对黑龙江东部鸡西盆地进行了构造层序划分及研究,共划分了1个Ⅰ级、2个Ⅱ级、5个Ⅲ级构造层序,在此基础上讨论了鸡西盆地动力学演化机制,证实鸡西盆地主要由敦密断裂在白垩纪左旋走滑拉分而成。纵向上表现为两个构造演化阶段：早白垩世早中期表现为断陷盆地特点,控制了鸡西群含煤层序的生长和发育;早白垩世晚期-晚白垩世早期表现为坳陷盆地特征,控制了桦山群红层的生长、发育。该盆地的层序划分及动力学分析对黑龙江东部白垩纪地层对比、构造作用及成矿作用的分析提供了重要依据。     

Travels of the Cache Creek Terrane: a paleomagnetic, geobarometric and Ar/Ar study of the Jurassic Fourth of July Batholith, Canadian Cordillera

The Middle Jurassic Fourth of July Batholith and cross-cutting mafic dikes have been studied geochronologically, geobarometrically and paleomagnetically to estimate subsequent tectonic motion of the Cache Creek Terrane (CCT) in the northern Canadian Cordillera. ⁴⁰Ar/³⁹Ar hornblende ages from a granodiorite phase are similar to U–Pb zircon ages and indicate rapid cooling of the batholith upon intrusion, suggesting that the magnetization age is coincident with the 173-Ma crystallization age. Argon ages of biotite from the granodiorite and two mafic dikes have similar ages of 165 Ma, which dates cooling through 280 °C.Aluminum-in-hornblende geobarometry indicates differential uplift of the batholith across a north–south fault zone along Atlin Lake with >6 km more uplift on its eastern side. Also, the eastern side has been tilted downward to the south–southwest by 9°.Combined paleomagnetic data from 20 granitoid and 11 mafic dike sites yield an in situ paleopole at 55°W, 63°N (d_p=5°, d_m=5°) and a tilt-corrected paleopole at 81°W, 55°N (d_p=5°, d_m=6°). Compared to the 173-Ma reference pole for the North American craton, the tilt-corrected pole suggests a significant southward translation of 16.1±3.7° and a significant clockwise rotation of 107±7°. The translation estimate is similar to the Jurassic Teslin Crossing pluton in the Stikine Terrane, however, the rotation estimate is very different. This could indicate that the Cache Creek Terrane was at a similar latitude of the Stikine Terrane, but the two were not yet amalgamated.