Christoph Heubeck

Early Cambrian ocean anoxia in South China

Arising from: M. Wille, T. F. Nägler, B. Lehmann, S. Schröder & J. D. Kramers 453, 767–769 (2008)10.1038/nature07072; Wille et al. replyThe cause of the most marked changes in the evolution of life, which define the first-order stratigraphic boundary between the Precambrian and the Phanerozoic eon, remains enigmatic and a highly topical subject of debate. A global ocean anoxic event, triggered by large-scale hydrogen sulphide (H2S) release to surface waters, has been suggested by Wille et al., on the basis of two data sets from South China and Oman, to explain the fundamental biological changes across the Precambrian/Cambrian (PC/C) boundary. Here we report a new precise SHRIMP U–Pb zircon age of 532.3 ± 0.7 million years (Myr) ago (Fig. 1) for a volcanic ash bed in the critical unit that reflects the ocean anoxic event, the lowermost black shale sequence of the Niutitang Formation in the Guizhou Province, South China. This age is significantly younger than the precise PC/C boundary age of 542.0 ± 0.3 Myr ago, approximately 10 Myr younger than the extinction of the Ediacaran fauna, and thus challenging the view of a major ocean anoxic event having been responsible for the major changes in the direction of evolution at the PC/C boundary.

Depositional and tectonic setting of the Archean Moodies Group, Barberton Greenstone Belt, South Africa

The 3.22-3.10 Ga old Moodies Group, uppermost unit of the Swaziland Supergroup in the Barberton Greenstone Belt (BGB), is the oldest exposed, well-preserved quartz-rich sedimentary sequence on earth. It is preserved in structurally separate blocks in a heavily deformed fold-and-thrust belt. North of the Inyoka Fault, Moodies strata reach up to 3700 m in thickness. Detailed mapping, correlation of measured sections, and systematic analysis of paleocurrents show that the lower Moodies Group north of the Inyoka Fault forms a deepening- and fining-upward sequence from a basal alluvial conglomerate through braided fluvial, tidal, and deltaic sandstones to offshore sandy shelf deposits. The basal conglomerate and overlying fluvial facies were derived from the north and include abundant detritus eroded from underlying Fig Tree Group dacitic volcanic rocks. Shoreline-parallel transport and extensive reworking dominate overlying deltaic, tidal, and marine facies. The lithologies and arrangement of Moodies Group facies, sandstone petrology, the unconformable relationship between Moodies strata and older deformed rocks, presence of at least one syndepositional normal fault, and presence of basaltic flow rocks and airfall fall tuffs interbedded with the terrestrial strata collectively suggest that the lower Moodies Group was deposited in one or more intramontane basins in an extensional setting. Thinner Moodies sections south of the Inyoka Fault, generally less than 1000 m thick, may be correlative with the basal Moodies Group north of the Inyoka Fault and were probably deposited in separate basins. A northerly derived, southward-thinning fan-delta conglomerate in the upper part of the Moodies Group in the central BGB overlies lower strata with an angular unconformity. This and associated upper Moodies conglomerates mark the beginning of basin shortening by south- to southeast-directed thrust faulting along the northern margin of the BGB and suggest that the upper Moodies Group was deposited in a foreland basin. Timing, orientation, and style of shortening suggest that this deformation eventually incorporated most of the BGB into a major fold-and-thrust belt.

Latitudinal variation in sedimentary processes in the Peru-Chile Trench off Central Chile

Four cruises of the German research vessel RV SONNE (cruises SO101, SO103, SO104 and SO161) surveyed the Chilean continental margin and oceanic plate using seismic measurements across the Peru-Chile Trench, swath-mapping bathymetry, sediment echosounding, dredges and gravity-core sampling. In this paper, we present data from cruise SO161 derived from the sediment-filled sector of the trench between 35 and 44° S. South of 33°10′ S, sediment fill in the trench ranges from 2200 to 3500 m thickness. The sediment volume decreases northwards, as the trench width narrows from 80 km at 41° S to 25 km at 33° S. Turbidity currents enter the trench mainly via nine canyon systems that are deeply incised into the continental slope. Reflection patterns from the trench fill exhibit a cyclicity that can be linked to Milankovic cycles. Turbiditic deposits at elevated positions within the trench indicate Pleistocene mass-wasting events that were able to overcome a height difference of some hundred meters. Within the trench, a fraction of the turbidity currents is channelled by a northward-dipping, axial channel. This axial channel has eroded up to 200 m into the trench fill and from 42° S, it extends northwards some 1000 km, terminating at the foot of the Juan Fernandez Ridge. The channel has no continuous precursor and might have evolved its present-day form during the last glaciation.

Tectonic fabric map of the ocean basins from satellite altimetry data

Abstract Satellite altimetry data provide a new source of information on the bathymetry of the ocean floor. The tectonic fabric of the oceans (i.e., the arrangement of fracture zones, ridges, volcanic plateaus and trenches) is revealed by changes in the horizontal gravity gradient as recorded by satellite altimetry measurements. SEASAT and GEOSAT altimetry data have been analyzed and a global map of the horizontal gravity gradient has been produced that can be used to identify a variety of marine tectonic features. The uniformity of the satellite coverage provides greater resolution and continuity than maps based solely on ship-track data. This map is also the first global map to incorporate the results of the GEOSAT mission, and as a result, new tectonic features are revealed at high southerly latitudes. This map permits the extension of many tectonic features well beyond what was previously known. For instance, various fracture zones, such as the Ascension, Tasman, and Udintsev fracture zones, can be extended much closer to adjacent coninental margins. The tectonic fabric map also reveals many features that have not been previously mapped. These features include extinct ridges, minor fracture zone lineations and seamounts. In several areas, especially across aseismic plateaus or along the margins of the continents, the map displays broad gravity anomalies whose origin may be related to basement structures.

Late syndepositional deformation and detachment tectonics in the Barberton Greenstone Belt, South Africa

The thick-bedded, competent quartzose sandstones, conglomerates, and siltstones of the Moodies Group (<3,225 Ma) influence structural style in the ∼3.1–3.5 Ga Barberton Greenstone Belt. Moodies Group rocks are folded into a train of tight, subparallel, doubly plunging, overturned synclines separated by narrow, strike-parallel fault zones. Structural analysis along a transect across the central part of the greenstone belt suggests that much of the volcanic-sedimentary greenstone sequence was buckle-folded above a detachment within altered ultramafic rocks located at a depth of 3–6 km. Early development of brittle structures concentrated shortening and displacement near the fold hinges and in the anticlinal fault zones. Penetrative strain during folding was low. The sense of displacement along faults, the original fold vergence, and the provenance of Moodies Group sandstone and conglomerate suggest that tectonic transport was directed toward the interior of the belt from its northwestern and southeastern margins. Orogenic contraction due to folding alone was at least −56% and subsequent ductile shortening strain accounted for an additional contraction of approximately −10%. Buoyant rise of the Kaap Valley Tonalite to the north at approximately 3.22 Ga and lateral intrusions of sheetlike granitic plutons along both sides of the preserved greenstone belt at 3.1 and 3.0 Ga, respectively, imposed significant ductile strain on the greenstone belt margins and obscured or displaced the orogenic hinterlands of the facing fold-and-thrust belts. The syn-and post-Moodies tectonic history of the central Barberton Greenstone Belt involves the formation and deformation of several shortlived alluvial to shallow marine sedimentary basins on unstable “protocontinental” crust. The probable extensional origin of at least some of these basins and their subsequent shortening, buckle folding, and displacement above a subhorizontal detachment emphasizes the dominance of horizontal tectonics in the late-stage deformation of this Archean greenstone belt.

Geologic evaluation of plate kinematic models for the North American-Caribbean plate boundary zone

Heubeck, Ch. and Mann, P., 1991. Geologic evaluation of plate kinematic models for the North American-Caribbean plate boundary zone. Tectonophysics, 191: l-26. Four published plate kinematic models for the present-day North American-Caribbean plate boundary predict significantly different fault behavior along this dominantly strike-slip plate boundary zone (PBZ). Using a computer graphics system, we compare the deformational pattern predicted by the four kinematic models with geologic and seismic observations along the entire length of the PBZ from Central America to the northern Lesser Antilles. Our results indicate that none of the kinematic models predict all the geologic and seismic observations across the entire PBZ. However, all four models predict mapped fault character and observed earthquake focal mechanisms along certain segments of the PBZ. We conclude that the present-day deformation within the North American-Caribbean plate boundary zone cannot be described by rotation about a single pole. Instead, the Caribbean “plate” appears to be segmented into at least three major “sub-plates” or blocks, each of which requires an individual pole of rotation to describe its motion relative to the North American plate. Sub-plate boundaries correspond to two poorly studied zones of diffuse deformation within the Caribbean plate (Honduras Depression of Central America and Beata Ridge of the Caribbean Sea).

An early ecosystem of Archean tidal microbial mats (Moodies Group, South Africa, ca. 3.2 Ga)

Abundant microbial mats from the Mesoarchean Moodies Group (Barberton Greenstone Belt, South Africa, ca. 3.2 Ga) are densely interbedded with coarse-grained and gravelly sandstones in a nearly mud-free setting. They apparently grew in marginal marine and possibly subaerial coastal and tidal environments. Characteristic sedimentary structures include anastomosing bedding, gas or fluid escape structures, sand volcanoes, biomat doming, patchy silicified microstromatolites, and microbial sand-chip conglomerates. They indicate rapid growth of mechanically tough microbial mats, possibly aided by early seafloor silicification, in a high-energy, high-sedimentation-rate environment. The observations expand the knowledge of the habitat of Archean microbial mats and life on early Earth. If tidal environments were more widespread in the Archean than today, biomats in extensive intertidal settings may have contributed to immobilize large volumes of sand.

U–Pb and Hf isotope data of detrital zircons from the Barberton Greenstone Belt: constraints on provenance and Archaean crustal evolution

Combined U–Pb and Hf isotope analyses of detrital zircons from the Fig Tree and Moodies Groups of the Barberton Greenstone Belt, South Africa, yield similar Hf isotope compositions and age populations, thus pointing to a similar provenance. Zircon populations of Fig Tree Group greywacke and Moodies Group quartzarenite are both dominated by age clusters at 3.53, 3.47, and 3.28 Ga, and a minor cluster at 3.36 Ga. The Moodies quartzarenite sample additionally contains a younger age population at 3.23–3.19 Ga. Hafnium isotope data indicate that the source area of both sediments was affected by new crust formation from depleted mantle sources at 3.53, 3.47, and perhaps at 3.36 Ga (ϵHft between −1.7 and +4.5), accompanied by partial reworking of an Eoarchaean crustal component as old as 3.75–3.95 Ga. In contrast, crustal reworking was the predominant process between 3.28 and 3.22 Ga (ϵHft between −6.0 and +0.9), probably related to subduction and collision of terranes along the Inyoka Fault system. The zircon U–Pb and Hf isotope datasets favour a southern provenance for the Fig Tree and Moodies sediments, comprising granitoids in the vicinity of the southern Barberton Greenstone Belt and in Swaziland. This finding is in contrast to the sedimentary record of the Moodies Group, which mostly suggests a northern and along-strike provenance. This discrepancy may be due to reworking of sediments during extensive syn- and postorogenic strike-slip faulting and high uplift or subsidence between 3.26 and 3.19 Ga. Supplementary material Results of in situ U–Pb and Lu–Hf isotope zircon analyses are available at www.geolsoc.org.uk/SUP18561.

Exhumation and Basin Development Related to Formation of the Central Andean Plateau, 21° S

Thermochronological (apatite fission track, AFT) and sedimentological data from the central Andean high plateau and its eastern foreland reflect the Tertiary tectonic evolution of this plateau. AFT data define several stages of exhumational cooling in the plateau and its foreland: A first stage (AFT dates of 40 and 36 Ma), restricted to the central Eastern Cordillera, can be attributed to initial thrusting increments following the Incaic Phase. Around 33–30 Ma, cooling occurred along basement highs over the entire plateau; kinematics were thrusting (eastern Cordillera) as well as normal faulting (Altiplano). From 17 Ma onwards, exhumational cooling took place in the eastern foreland (Interandean-Subandean), reflecting the eastward migration of the thrust front into the Chaco foreland basin.

Diachronous uplift and recycling of sedimentary basins during Cenozoic tectonic transpression, northeastern Caribbean plate margin

Abstract Four marine sedimentary sequences of Late Cretaceous to Pleistocene age crop out in the 320 km long, 1 to 30 km wide Peralta-Rio Ocoa belt of Hispaniola (Haiti and Dominican Republic). The four sedimentary sequences are dominated by marine turbiditic rocks ranging up to 11.5 km in apparent thickness and exhibit mainly northwest to southeast, belt-parallel paleocurrents. The three younger sequences are generally in fault or unconformable contact with the underlying sequence. From northwest to southeast, the four belts become progressively wider, younger, less metamorphosed, less folded and faulted, and lower in topographic elevation. We interpret the three younger sequences as a syn-tectonic stratigraphic record of diachronous, northwest to southeast transpressional closure of a Coniacian-Danian back-arc basin represented by the oldest exposed sedimentary sequence at the northwest end of the belt and inferred at depth beneath the three younger sequences. Using the stratigraphic record, we infer the following three stages in the closure of the back-arc basin and the overlying younger basins. Stage One: latest Cretaceous to early Late Eocene closure . A minimum of 11 km of Paleocene turbidites and limestone in the Padre las Casas area and a minimum of 11.5 km of Early Eocene to early Late Eocene pelagic limestone, mudstone, sandstone, and siltstone in the Peralta and Sierra El Numero areas was deposited in an elongate basin derived from the first stage of latest Cretaceous-early Late Eocene closure and erosion of the Coniacian-Danian back-arc basin to the northwest. Paleocene-Eocene turbidites in both areas contain large amounts of reworked Campanian-Paleocene microfauna and exhibit northwest to southeast belt-parallel paleocurrents. Syn-deformational features in Eocene sedimentary rocks in the Peralta and Sierra El Numero area indicate that convergent deformation accompanied sedimentation (Witschard and Dolan, 1990). Stage Two: Middle Eocene to Early Miocene closure . Up to 8.6 km of Middle Eocene to Early Miocene turbidites, debris flows, and olistostromes of the Rio Ocoa Group of the Dominican Republic were deposited in an elongate basin derived from Middle Eocene to Early Miocene closure, uplift, and erosion of the Peralta Group to the west and northwest. Turbidites contain reworked Eocene faunas and lithologies similar to those in the underlying Peralta Group and exhibit northwest to southeast, belt-parallel paleocurrents. Erosion of Late Cretaceous island arc and overlying Paleogene carbonate rocks of the Cordillera Central provided additional belt-perpendicular sources of coarser-grained conglomerate and olistostromes. Following deposition of the Rio Ocoa Group, both the Peralta and the Rio Ocoa Group were deformed in a southwest-verging fold-and-thrust belt during the Early Miocene. Stage Three: Middle Miocene to Recent closure . Up to 1.5 km of Middle Miocene to Pleistocene sandstone, conglomerate, and reefal limestone of the Ingenio Caei Group of the Dominican Republic were deposited in an elongate basin above a pronounced angular unconformity developed on the older, deformed rocks of the Rio Ocoa Group. Sediments of the Ingenio Caei Group were derived from Middle Miocene to Recent closure, uplift, and erosion of the Rio Ocoa and Peralta Groups to the northwest. Onshore exposures of the Ingenio Caei Group can be correlated with submerged strata seen on seismic reflection profiles across the offshore San Pedro basin. Recent submarine clastic sedimentation in the San Pedro basin is dominated by northwest to southeast influx of clastic turbidites derived from active erosion of onshore exposures of the Rio Ocoa and Ingenio Caei Groups. Middle Miocene (?) to Recent sediments of the San Pedro basin are being actively shortened by northward to northeastward-directed underthrusting along the Muertos trench. Underthrusting at the Muertos trench is suggested to represent ongoing closure of the back-arc basin thought to be deeply buried beneath the San Pedro basin. Tectonic transpression along the closing back-arc basin marks one edge of an actively deforming microplate within the North America-Caribbean strike-slip plate boundary zone.