This same high appears to bound the southern ends of Big Lake and Lake St. Francis sunklands. A third high on the unconformity surface that we have named Joiner ridge, after Joiner, Arkansas, can be traced northwest from the corner of Shelby County, Tennessee Fig. In the Western Lowlands there are large elevation changes on the Pliocene—Pleistocene unconformity surface. A m-deep northeast-trending closed depression is in the central portion of the Western Lowlands that does not appear to be related to Quaternary river erosion.
Bounding this low on the southeast is a high region, and still farther southeast there is a second low that may be related to river erosion. The Precambrian unconformity surface beneath the central Mississippi River valley probably caps eroded 1. Rifting of this landscape formed the northeast-trending Reel-foot rift Mississippi Valley graben. However, it appears that southeast-trending faults also were active during rifting because some of these faults are evident on seismic reflection lines within the rift.
These southeast-trending faults have been mapped in the Ozarks of Arkansas and Missouri as Proterozoic basement faults that predate Reelfoot rifting Sims et al.
Stark proposed that these southeast-trending faults originated within the 1. The Reelfoot rift is subdivided into eight fault-bound blocks Fig.
At the largest scale, the rift consists of two basins divided by a structural high. This intrarift high is bound on the north by the Osceola fault zone and on the south by the Bolivar-Mansfield tectonic zone. Major changes in strike of the Eastern Rift Margin and Western Rift Margin faults occur near their intersection with the Bolivar-Mansfield tectonic zone and the Osceola fault zone, also indicating that these southeast-trending faults influenced the geometry of the Reelfoot rift.
Although we do not have well or seismic data between the Reelfoot rift and the Rough Creek graben, regional depth to magnetic basement maps Hildenbrand, ; Langenheim and Hildenbrand, suggest that the basement rises between these two down-dropped structures Fig.
Thus, we have chosen to close the Reelfoot rift at its northern end with a down-to-the-southwest normal fault Fig. This fault coincides in location with the Reelfoot thrust and reverse fault.
However, the only displacement information we have on the Reelfoot fault shows that there is 70 m of reverse displacement on the top of the Paleozoic section that diminishes up section Van Arsdale, We thus propose that the Reelfoot fault was a normal fault during Reelfoot rifting, but was subsequently inverted and is a reverse fault in the post-Paleozoic section. The southern end of the Reelfoot rift appears to be closed by the up-to-the-southwest White River fault zone.
Our data are also limited in this area and it is possible that the Reelfoot rift continues south beneath the Appalachian-Ouachita thrust belt. Extending down the center of the Reelfoot rift is the Axial fault. Although we have modeled the Axial fault as terminating against the Reel-foot fault, this is speculative. The Axial fault appears to have undergone both dip-slip and strike-slip movement. The northwestern side of the fault is generally downthrown relative to the southeastern side Howe and Thompson, Left-lateral strike-slip offset is also suggested by a series of offset ridges and depressions on the Precambrian surface Fig.
In Figure 9 we project the Reelfoot Rift basement faults to the Pliocene—Pleistocene unconformity surface. In addition, we have included the base of the Pliocene Upland Gravel on Crowley's Ridge in our Pliocene—Pleistocene unconformity surface. The Pliocene—Pleistocene unconformity surface is an erosional surface that has been tectonically modified.
In the Eastern Lowlands, Quaternary uplift has occurred along the Reelfoot fault Grand River tectonic zone to form the Lake County uplift, along the Axial fault to affect regional drainage, and along Joiner ridge. Tectonic subsidence in the Eastern Lowlands has occurred beneath Reelfoot Lake, northwest of the Axial fault, and perhaps immediately east of Joiner ridge.
In the Western Lowlands there are one and perhaps two major structural basins that are separated by an apparently uplifted block bound by the Axial fault and a possible extension of the Eastern Margin faults southwest of the White River fault zone. Perhaps most dramatic in Figure 9 is the elevation of Crowley's Ridge.
Upon viewing Figure 9 stereoscopically see Animation 2 , it becomes apparent that most of the basement faults produce saddles, or appear to divide Crowley's Ridge into sections, as one would expect if the faults were active during the Quaternary.
Hence, based on Figure 9 and neotectonic studies conducted in the Eastern Lowlands, it appears that the entire Reelfoot rift has been tectonically active during the Quaternary Fig. Specifically, we believe that the southeastern half of the Reelfoot rift southeast of the Axial fault has undergone Quaternary uplift. Cox et al. Relative up-to-the-southeast Quaternary displacement along the Axial fault appears to have been responsible for local impoundment of the St.
Francis and Little Rivers to form Lake St. In the Western Lowlands, our proposed Quaternary uplift of the southeastern half of the Reelfoot rift is coincident with the high that is bound by the Axial fault and the southwestern projection of the Eastern Margin faults Fig.
Similarly, it appears that the northwestern half of the Reelfoot rift has undergone Quaternary subsidence. This interpretation is supported by the basin in the Western Lowlands that is bound by the Axial and Western Margin faults and down-to-the-southeast displacement on the Western Rift Margin fault near Jonesboro, Arkansas Van Arsdale et al.
The New Madrid seismic zone has been explained as being caused by right-lateral shear along the Axial fault and the Western Margin fault north of New Madrid, Missouri, with a left-stepping compressional stepover being the Reelfoot fault.
However, the Reelfoot thrust earthquakes and deformation continue southeast beyond the Mississippi River valley beneath a thick loess blanket to near the Eastern Rift Margin faults Figs. Thus, a more accurate assessment would be that there is either one large stepover zone from the Eastern Margin faults to the Western Margin fault or two stepovers separated by the Axial fault.
In assessing the remaining Reelfoot rift area, it appears that there are an additional two aseismic north-trending left stepovers. The most obvious one is the southern half of Crowley's Ridge, which is documented as being bound by young faulting Van Arsdale et al.
We propose that these three stepovers are associated with Reelfoot rift fault intersections; thus, the fault intersections have acted as initiation points for the stepovers Talwani, ; Gangopadhyay and Talwani, Late Proterozoic through Cambrian rifting across southeastern North America created the northeast-trending Reelfoot rift. Proterozoic southeast-trending faults of the Ozarks appear to pass through the Reelfoot rift and were active during rifting Fig.
We speculate that the southeast-trending faults may have propagated upward from Central Plains orogen rocks through the overlying granites and rhyolites of the Eastern Granite Rhyolite Province, as proposed by Stark , Fig. The northeast- and southeast-trending faults that bound the Reelfoot rift and interior subbasins locally have as much as 3 km of vertical displacement Parrish and Van Arsdale, The Rift appears to step up to the north at its northern end across the Grand River tectonic zone Reelfoot normal fault , has a central intrarift uplifted block bound by the Osceola fault zone and Bolivar-Mansfield tectonic zone, and steps up to the south across the White River fault zone.
Thus, the southeast-trending faults appear to be fundamental structures that were involved in the formation of the Reelfoot rift. The Pliocene—Pleistocene unconformity beneath the Mississippi River alluvium is an erosional unconformity that has been tectonically altered at different scales. On a regional scale, local field studies and inspection of Figure 9 suggest that the southeastern half of the Reel-foot rift has undergone Quaternary uplift while the northwestern half has undergone subsidence.
On a more local scale, tectonic uplift has occurred on the Lake County uplift, along the Axial fault, along the southern half of Crowley's Ridge, and on Joiner ridge. Tectonic subsidence has occurred east of the Lake County uplift at Reelfoot Lake and perhaps on the eastern sides of Joiner ridge and the southern half of Crowley's Ridge.
Any tectonic low east of Crowley's Ridge, however, is masked by the Pleistocene river channel. Right-lateral strike slip on the Axial and Western Rift Margin faults near New Madrid, Missouri, has been explained as causing the Reelfoot thrust stepover between these faults. We wish to extend this explanation to propose that the southern half of the Lake County uplift, the southern half of Crowley's Ridge, and Joiner ridge are all right-lateral compressional stepovers Fig.
Essentially, the entire Reelfoot rift has undergone Quaternary right-lateral transpression resulting in relative uplift and subsidence of the eastern and western halves of the Reelfoot rift, respectively, and formation of compressional stepovers.
The stepover that is currently seismically active is at the northern end of the Reelfoot rift. It is reasonable to assume, however, that the different stepovers activate at different times, perhaps as shear displacement propagates along the rift faults. We also thank Landmark Graphics Inc. Figure 1.
Reelfoot rift and New Madrid seismic zone earthquakes. CR—Crowley's Ridge. DEM—digital elevation model. Figure 2. Precambrian basement map of the Reelfoot rift from Dart and Swolfs, Circles—drill holes; thick solid lines—Vibroseis lines. Figure 3. Precambrian geology of the south-central United States from Van Schmus et al. Francois Mountains. Dashed line extending from Texas to Ontario represents inferred eastern limit of pre— Ma continental crust.
Figure 4. A Precambrian unconformity data points red dots and lines from Howe , Wheeler et al. Figure 5. NM—New Madrid. Elevation datum is sea level. Figure 6. Precambrian unconformity structure contour map with the addition of southeast oriented faults.
Animation 1. S1 or the full-text article on www. Figure 7. Distribution of borings used to map the Pliocene—Pleistocene unconformity base of Mississippi River alluvium. Figure 8.
Pliocene—Pleistocene unconformity structure contour map. Francis, J—Joiner ridge. Martins Member, new ; ii highstand-regressive, dysaerobic mudstone - fine-grained sandstone with volcanic ashes Somerset Street Member, new ; and iii upper, regressive, planar and hummocky cross-stratified sandstone Long Island Member, new.
Trilobites are common in the distal Somerset Street Member, and ostracodes and brachiopods dominate the St. Martins and Long Island members. Condensation of the St. Detrital zircon geochronology of Neoproterozoic to Middle Cambrian miogeoclinal and platformal strata: Northwest Sonora, Mexico. Zircons from the Lower? Grains older than 1. It is also possible that the sediments were transported from the south, although source rocks of the appropriate age are not presently exposed south of the study area in northern Mexico.
Three possibilities for the dominant 1. Sampling of additional units in the western U. Lower Cambrian polychaete from China sheds light on early annelid evolution. We herein report a fossilized polychaete annelid, Guanshanchaeta felicia gen. The new taxon has a generalized polychaete morphology, with biramous parapodia most of which preserve the evidence of chaetae , an inferred prostomium bearing a pair of appendages, and a bifid pygidium.
In addition, this new taxon increases our knowledge of early polychaete morphology, which suggests that polychaete annelids considerably diversified in the Cambrian. Compilation and network analyses of cambrian food webs. A rich body of empirically grounded theory has developed about food webs--the networks of feeding relationships among species within habitats. However, detailed food-web data and analyses are lacking for ancient ecosystems, largely because of the low resolution of taxa coupled with uncertain and incomplete information about feeding interactions.
These impediments appear insurmountable for most fossil assemblages; however, a few assemblages with excellent soft-body preservation across trophic levels are candidates for food-web data compilation and topological analysis. Here we present plausible, detailed food webs for the Chengjiang and Burgess Shale assemblages from the Cambrian Period. Analyses of degree distributions and other structural network properties, including sensitivity analyses of the effects of uncertainty associated with Cambrian diet designations, suggest that these early Paleozoic communities share remarkably similar topology with modern food webs.
Observed regularities reflect a systematic dependence of structure on the numbers of taxa and links in a web. Most aspects of Cambrian food-web structure are well-characterized by a simple "niche model," which was developed for modern food webs and takes into account this scale dependence. However, a few aspects of topology differ between the ancient and recent webs: longer path lengths between species and more species in feeding loops in the earlier Chengjiang web, and higher variability in the number of links per species for both Cambrian webs.
Our results are relatively insensitive to the exclusion of low-certainty or random links. The many similarities between Cambrian and recent food webs point toward surprisingly strong and enduring constraints on the organization of complex feeding interactions among metazoan species. The few differences could reflect a transition to more strongly integrated and constrained trophic organization within ecosystems following the rapid diversification of species, body.
Voluminous Paleozoic sandstone sequences were deposited in northern Africa and Arabia following an extended Neoproterozoic orogenic cycle that culminated in the assembly of Gondwana. We measured sensitive high-resolution ion microprobe SHRIMP U-Pb ages of detrital zircons separated from several Cambrian units in the Elat area of southern Israel in order to unravel their provenance.
This sandstone forms the base of the widespread siliciclastic section now exposed on the periphery of the Arabian-Nubian shield in northeastern Africa and Arabia. Most of the detrital zircons we analyzed yielded Neoproterozoic concordant ages with a marked concentration at 0.
The most likely provenance of the Neoproterozoic detritus is the Arabian-Nubian shield; 0. The majority of the pre-Neoproterozoic zircons underwent Pb loss, possibly as a consequence of the Pan-African orogeny resetting their provenance. Kibaran basement rocks are currently exposed more than km south of Elat flanking the Mozambique belt , but the shape of the detrital zircons of that age and the presence of feldspar in the host sandstone are not fully consistent with such a long-distance transport.
Reworking of Neoproteorozoic glacial detritus may explain the presence of Kibaran detrital zircons in the Cambrian of Elat, but the possibility that the Arabian-Nubian shield contains Kibaran rocks hitherto not recognized should also be explored. Both the Baikal rift in Siberia and the Rio Grande rift in New Mexico, Colorado and Texas are major intracontinental extensional structures of Cenozoic age that affect regions about km long and several hundred km wide Figures 1, 2. In the summer of these rifts were visited by study groups of U.
National Academy of Sciences, and U. Geological Survey. Particular emphasis was on the sedimentary record of rift evolution, widespread volcanic activity from inception of rifting to the present, geophysical expression of rift features, and relations between rifting and the larger-scale evolution of the North American Cordillera. In the Baikal region, which presents formidable logistic problems for a workshop, we travelled by bus, truck, helicopter, and ship to examine young seismotectonic features, rift -related basalt, and bounding structures of the Siberian craton that influenced rift development Figure 3.
Stein, C. Rifts are segmented linear depressions, filled with sedimentary and igneous rocks, that form by extension and often evolve into plate boundaries. Flood basalts, a class of Large Igneous Provinces LIPs , are broad regions of extensive volcanism due to sublithospheric processes. Typical rifts are not filled with flood basalts, and typical flood basalts are not associated with significant crustal extension and faulting. Its km length formed as part of the 1.
MCR volcanics are significantly thicker than other flood basalts, due to deposition in a narrow rift rather than a broad region, giving a rift geometry but a LIP's magma volume.
Structural modeling of seismic reflection data shows an initial rift phase where flood basalts filled a fault-controlled extending basin, and a postrift phase where volcanics and sediments were deposited in a thermally subsiding basin without associated faulting.
The crust thinned during rifting and rethickened during the postrift phase and later compression, yielding the present thicker crust. The coincidence of a rift and LIP yielded the world's largest deposit of native copper. This combination arose when a new rift associated with continental breakup interacted with a mantle plume or anomalously hot or fertile upper mantle.
Integration of diverse data types and models will give insight into questions including how the magma source was related to the rifting , how their interaction operated over a long period of rapid plate motion, why the lithospheric mantle below the MCR differs only slightly from its surroundings, how and why extension, volcanism, and compression varied along the rift arms, and how successful seafloor spreading ended the rift phase.
Ages of pre- rift basement and synrift rocks along the conjugate rift and transform margins of the Argintine Precordillera and Laurentia. Chronostratigraphic correlations of synrift and post- rift sedimentary deposits on the Precordillera and on the Texas promontory of Laurentia document initial rifting in the Early Cambrian. Previously published data from synrift plutonic and volcanic rocks in the Wichita and Arbuckle Mountains along the transform-parallel intracratonic Southern Oklahoma fault system inboard from the Ouachita embayment document crystallization ages of — Ma.
In the Moroccan western Anti-Atlas, the combined extensive tectonic events with a long-term sea-level rise is the main factor on building vertical stacking transgressive-regressive sequences.
In the Ait Abdallah-Boussafene axis, the subsidence processes, relayed by a brutal platform tilting generated an elongated NE-SW graben.
This is an evidence of the persistence of the Anti-Atlasic rifting process during the last part of the Lower- Cambrian succession. Early Cambrian origin of modern food webs: evidence from predator arrow worms. Although palaeontological evidence from exceptional biota demonstrates the existence of diverse marine communities in the Early Cambrian approx.
The presence of a diverse zooplankton in Early Cambrian oceans is still an open issue. Here we provide compelling evidence that chaetognaths, an important element of modern zooplankton, were present in the Early Cambrian Chengjiang biota with morphologies almost identical to Recent forms.
New information obtained from the lowermost Cambrian of China added to previous studies provide convincing evidence that protoconodont-bearing animals also belonged to chaetognaths. Chaetognaths were probably widespread and diverse in the earliest Cambrian. The obvious raptorial function of their circumoral apparatuses grasping spines places them among the earliest active predator metazoans.
Morphology, body ratios and distribution suggest that the ancestral chaetognaths were planktonic with possible ecological preferences for hyperbenthic niches close to the sea bottom. Our results point to the early introduction of prey-predator relationships into the pelagic realm, and to the increase of trophic complexity three-level structure during the Precambrian- Cambrian transition, thus laying the foundations of present-day marine food chains.
Our results point to the early introduction of prey—predator relationships into the pelagic realm, and to the increase of trophic complexity three-level structure during the Precambrian— Cambrian transition, thus laying the foundations of present-day marine food chains. Geometry of the neoproterozoic and paleozoic rift margin of western Laurentia: Implications for mineral deposit settings.
Recent field and dating studies across central Idaho and northern Nevada result in identification of two segments of the rift margin. Resulting interpretations of rift geometry in the northern U. Cordillera are compatible with interpretations of northwest- striking asymmetric extensional segments subdivided by northeast-striking transform and transfer segments. The new interpretation permits integration of miogeoclinal segments along the length of the western North American Cordillera.
For the U. Cordillera, miogeoclinal segments include the St. The rift is orthogonal to most older basement domains, but the location of the transform-transfer zones suggests control of them by basement domain boundaries.
The zigzag geometry of reentrants and promontories along the rift is paralleled by salients and recesses in younger thrust belts and by segmentation of younger extensional domains. Likewise, transform transfer zones localized subsequent transcurrent structures and igneous activity.
Sediment-hosted mineral deposits trace the same zigzag geometry along the margin. Sedimentary exhalative sedex Zn-Pb-Ag?? Au and barite mineral deposits formed in continental-slope rocks during the Late Devonian-Mississippian and to a lesser degree, during the Cambrian -Early Ordovician. Such deposits formed during episodes of renewed extension along miogeoclinal segments.
Carbonate-hosted Mississippi Valley- type MVT Zn-Pb deposits formed in structurally reactivated continental shelf rocks during the Late Devonian-Mississippian and Mesozoic due to reactivation of preexisting structures. The distribution and abundance of sedex and MVT deposits are controlled by the. Reconstructing the Avalon continent: Marginal to inner platform transition in the Cambrian of southern New Brunswick.
A west to east, marginal to inner Avalonian platform transition, comparable to that in southeast Newfoundland and southern Britain, is present in the Cambrian of southern New Brunswick. The Saint John - Caton's Island - Hanford Brook area lay on the marginal platform, and its thick, uppermost Precambrian - lower Lower Cambrian is unconformably overlain by trilobite-bearing, upper Lower Cambrian.
An inner platform remnant is preserved in the Cradle Brook outlier 60 km northeast of Saint John. In contrast to the marginal platform sequences, the Cradle Brook outlier has a very thin lower Lower Cambrian and has middle Lower Cambrian strata Bonavista Group not present on the marginal platform.
The Cradle Brook Lower Cambrian closely resembles inner platform successions in eastern Massachusetts and Trinity and Placentia bays, southeast Newfoundland. The Cambrian explosion from million years to million years ago is one of the greatest mysteries in evolutionary biology. It wasn't until this period that complex organisms became common and diverse. The Cambrian side of the " Cambrian explosion" ismore » richly illustrated and contrasts greatly with the Precambrian side.
The study of these extraordinarily preserved fossil biota is extremely difficult. A major challenge is 3-D reconstruction and determining the patter of the cell organization in Weng'an embryos and their buried structures in Maotianshan Shale fossils. This talk will show that two recent technological approaches, propagation phase contrast synchrotron x-ray microtomography and microtomography, provide unique analytical tools that permit the nondestructive computational examination and visualization of the internal and buried characters in virtual sections in any plane, and virtual 3-D depictions of internal structures.
In each province distribute 47 iron- manganese- barite and lead-zinc deposits with tectonic-structural control. The author presents in this paper aspects of position of these deposits in the two provinces with Phanerozoic rifting. The Mediterranean Province belongs to two epochs, Hercynian and Alpine.
The Hercynian Epoch manganese deposits in only Moroccoa- Algeria belong to Paleozoic tectonic zones and Proterozoic volcanics. The Alpine Epoch iron-manganese deposits are of post-orogenic exhalative-sedimentary origin. Manganese deposits in southern Morocco occur in Kabil-Rief quartz-chalcedony veins controlled by faults in andesitic sheets and in bedded pelitic tuffs, strata-form lenses and ore veins, in Precambrian schist and in Triassic and Cretaceous dolomites.
Disseminated manganese with quartz and barite and effusive hydrothermal veins are hosted in Paleocene volcanics. Manganese deposits in Algeria are limited and unrecorded in Tunisia. Strata-form iron deposits in Atlas Heights are widespread in sub- rift zone among Jurassic sediments inter-bedding volcanic rocks. In Algeria, Group Beni-Saf iron deposits are localized along the Mediterranean coast in terrigenous and carbonate rocks of Jurassic, Cretaceous and Eocene age within faults and bedding planes.
In Morocco strata-form hydrothermal lead-zinc deposits occur in contact zone of Tertiary andesite inter-bedding Cambrian shale, Lias dolomites and Eocene andesite. In both Algeria and Tunisia metasomatic Pb-Zn veins occur in Campanian - Maastrichtian carbonates, Triassic breccia, Jurassic limestone, Paleocene sandstones and limestone and Neogene conglomerates and sandstones.
Um-Bogma iron-manganese deposits are closely. The Tarim Basin is the largest, oil-bearing and superimposed basin in the northwest of China.
The development and tectonic property of the initial Tarim basin have been acutely disputed and remain enigmatic.
Urgently need to reveal the origin and formation dynamics of the Tarim Carton and evaluate the potential of the deep energy resources. However, covered by vast desert and huge-thickness sedimentary strata, suffered by multiple tectonic movements, seismic data with low signal- to- noise ratio in the deep are the critical difficulties. We analyse 4 field outcrops, 18 wells, 27 reprocessed seismic reflection profiles with high SNR across the basin and many ancillary ones and aeromagnetic data.
We find about 20 normal fault-controlled rift depressions of the Cryogenian and Ediacaran scattered in the Tarim basin, which developed on the Precambrian metamorphic and crystalline basements and covered by the epeiric sea and basin facies sediments of the Lower Cambrian. The structural styles of the rifts are mainly half grabens, symmetrical troughs and horst-grabens. The regional differences exist obviously in spatial and temporal. Some main faults of the Ediacaran inherited from the Cryogenian and some occurred newly, the more rifting depressions occurred during the Ediacaran.
According to the activities of syn-sedimentary faults, magmatic events and sediments, the tectonic properties of the rifts are different depending on their locations in the Tarim craton. The rifting phases mainly occurred from Ma to Ma. The formation of rifts were associated with the opening of the South Tianshan Ocean and the South Altun. Proterozoic and early Cambrian protists: evidence for accelerating evolutionary tempo.
In rocks of late Paleoproterozoic and Mesoproterozoic age ca. Near the Mesoproterozoic-Neoproterozoic boundary million years ago , red, green, and chromophytic algae diversified; molecular phylogenies suggest that this was part of a broader radiation of "higher" eukaryotic phyla. Observed diversity levels for protistan microfossils increased significantly at this time, as did turnover rates.
Coincident with the Cambrian radiation of marine invertebrates, protistan microfossils again doubled in diversity and rates of turnover increased by an order of magnitude.
Evidently, the Cambrian diversification of animals strongly influenced evolutionary rates, within clades already present in marine communities, implying an important role for ecology in fueling a Cambrian explosion that extends across kingdoms.
Here I propose that despite its step-like function this evolutionary event is the inevitable consequence of Earth and biospheric change.
Two-stage rifting in the Kenya rift : implications for half-graben models. The Kerio sub-basin in the northern Kenya rift is a transitional area between the southern Kenya rift , where crustal thickness is 30 km, and the northern Kenya rift , where crustal thickness is 20 km. The lack of data on the shallow crustal structure, geometry of rift -bounding faults, and rift evolution makes it difficult to determine if the crustal thickness variations are due to pre- rift structure, or along-axis variations in crustal stretching.
We reprocessed reflection seismic data acquired for the National Oil Corporation of Kenya, and integrated results with field and gravity observations to 1 delineate the sub-surface geometry of the Kerio sub-basin, 2 correlate seismic stratigraphic sequences with dated strata exposed along the basin margins, and 3 use new and existing results to propose a two-stage rifting model for the central Kenya rift.
Although a classic half-graben form previously had been inferred from the attitude of uppermost strata, new seismic data show a more complex form in the deeper basin: a narrow full-graben bounded by steep faults.
We suggest that the complex basin form and the northwards increase in crustal thinning are caused by the superposition of two or more rifting events. The first rifting stage may have occurred during Palaeogene time contemporaneous with sedimentation and rifting in northwestern Kenya and southern Sudan. The distribution of seismic sequences suggests that a phase of regional thermal subsidence occurred prior to renewed faulting and subsidence at about 12 Ma after the eruption of flood phonolites throughout the central Kenya rift.
A new border fault developed during the second rifting stage, effectively widening the basin. Gravity and seismic data indicate sedimentary and volcanic strata filling the basin are 6 km thick, with up to 4 km deposited during the first rifting stage. Moresi, L. Thermo-mechanical numerical models and analogue experiments with a layered lithosphere have emphasised the role played by the composition and thermal state of the lithosphere on the style of extension.
The variation in rheological properties and the coupling between lithospheric layers promote depth-dependent extension with the potential for complex rift evolution over space and time. We investigate how erosion of the margins and sedimentation within the basins integrate with the thermo-mechanical processes involved in the structural and stratigraphic evolution of the North West Shelf NWS , one of the most productive and prospective hydrocarbon provinces in Australia.
The complex structural characteristics of the NWS include large-scale extensional detachments, difference between amounts of crustal and lithospheric extension and prolonged episodes of thermal sagging after rifting episodes. It has been proposed that the succession of different extensional styles mechanisms Cambrian detachment faulting, broadly distributed Permo-Carboniferous extension and Late Triassic to Early Cretaceous localised rift development is best described in terms of variation in deformation response of a lithosphere that has strengthened from one extensional episode to the next.
However, previous models invoking large-scale detachments fail to explain changes in extensional styles and overestimate the structural importance of relatively local detachments. We run a series of fully coupled 3D thermo-mechanical numerical experiments that include realistic thermal. The detrital apatites we studied all have extremely rounded cores suggestive of a distant provenance, but some grains also feature distinct euhedral U-rich apatite overgrowth rims.
Authigenic apatite may have grown during the late Devonian thermal event we dated by ZFT, coinciding with existing Rb-Sr ages from authigenic clays in the same deposits and leading to the conclusion that the Devonian event was probably hydrothermal.
Images PMID The East Mediterranean is a land-locked basin, a remnant of Neo-Tethys. It was formed in the Permo-Triassic as a result of the drift of the Tauride block from the Afro-Arabian margin of Gondwana.
Herein we show that rather than being a genuine Afro-Arabia crustal fragment, the Tauride block is underlain by a Late Neoproterozoic Cadomian basement, which differs significantly from the Neoproterozoic "Pan-African" basement of NE Africa from which it was detached. Resembling other Cadomian terranes of Western Europe, the Tauride basement is chiefly a greywacke succession deposited in a mid to late Ediacaran back-arc basin formed on the periphery of Afro-Arabia, above the southward subducting proto-Tethys.
The back-arc region was deformed and metamorphosed to various degrees and intruded by latest Ediacaran- Cambrian granites and volcanics during the Cadomian orogeny. Unlike the protracted ca. Neoproterozoic crustal evolution recorded in Afro-Arabia, the Cadomian basement of the Taurides evolved briefly, over ca.
We show that the entire cycle of sedimentation, metamorphism and magmatism in the Tauribe basement took place in the late Ediacaran- Cambrian and lagged after Neoproterozoic Pan-African orogeny and igneous activity in Afro-Arabia. The Cadomian orogeny had accreted the Taurides, and adjoining peri-Gandwana Cadomian terranes, with an already-consolidated Afro-Arabian continent. Permo-Triassic rifting of the East Mediterranean occurred close to the transition between these two domains.
Rifting has thus been inherited from, and superimposed on late Ediacaran structures formed in front of the current Afro-Arabia margin of Gondwana during Cadomian orogeny. The boundary between the Cadomian edifice and the Pan-African crust of Afro-Arabia appears to lie nowadays on the southern margin of the Mediterranean, extending from Morocco in the west to Arabia in the east.
Hence, the continental margin of the East Mediterranean, including in the Levant basin. Aeromagnetic evidence for a major strike-slip fault zone along the boundary between the Weddell Sea Rift and East Antarctica. Jordan, T. Here we present new aeromagnetic data combined with airborne radar and gravity data collected during the field season over the Institute and Moeller ice stream in West Antarctica.
Digitally enhanced aeromagnetic data and gravity anomalies indicate the extent of Proterozoic basement, Middle Cambrian rift -related volcanic rocks, Jurassic granites, and post Jurassic sedimentary infill. Two new joint magnetic and gravity models were constructed, constrained by 2D and 3D magnetic depth-to-source estimates to assess the extent of Proterozoic basement and the thickness of major Jurassic intrusions and post-Jurassic sedimentary infill.
Background Extant cubozoans are voracious predators characterized by their square shape, four evenly spaced outstretched tentacles and well-developed eyes. Undisputed cubozoan fossils were previously unknown from the early Cambrian ; by that time probably all representatives of the living marine phyla, especially those of basal animals, should have evolved. Methods Microscopic fossils were recovered from a phosphatic limestone in the Lower Cambrian Kuanchuanpu Formation of South China using traditional acetic-acid maceration.
Seven of the pre-hatched pentamerous cubozoan embryos, each of which bears five pairs of subumbrellar tentacle buds, were analyzed in detail through computed microtomography Micro-CT and scanning electron microscopy SEM without coating.
Results The figured microscopic fossils are unequivocal pre-hatching embryos based on their spherical fertilization envelope and the enclosed soft-tissue that has preserved key anatomical features arranged in perfect pentaradial symmetry, allowing detailed comparison with modern cnidarians, especially medusozoans.
A combination of features, such as the claustrum, gonad-lamella, suspensorium and velarium suspended by the frenula, occur exclusively in the gastrovascular system of extant cubozoans, indicating a cubozoan affinity for these fossils.
Additionally, the interior anatomy of these embryonic cubozoan fossils unprecedentedly exhibits the development of many new septum-derived lamellae and well-partitioned gastric pockets unknown in living cubozoans, implying that ancestral cubozoans had already evolved highly specialized structures displaying unexpected complexity at the dawn of the Cambrian.
The well-developed endodermic lamellae and gastric pockets developed in the late embryonic stages of these cubozoan fossils are comparable with extant pelagic. Numerous new specimens reveal a greater presence of chitons in Upper Cambrian rocks than previously suspected. Evidence is presented showing that the chiton esthete sensory system is present in all chiton species in this study at the very beginning of the known polyplacophoran fossil record.
The stratigraphic occurrences and paleobiogeography of Late Cambrian chitons are documented. The 14 previously-named families of Cambrian and Ordovician chitons are reviewed and analyzed. Aulochitonidae n. At the species level, H. The Ordovician species H. In addition, other multivalved Cambrian mollusks are discussed; within this group, Dycheiidae n.
Cladistic analysis indicates a close relationship among the genera here assigned to the Mattheviidae, and between Echinochiton Pojeta. Fossils, molecules and embryos: new perspectives on the Cambrian explosion. The Cambrian explosion is named for the geologically sudden appearance of numerous metazoan body plans many of living phyla between about and million years ago, only 1.
Earlier indications of metazoans are found in the Neoproterozic; minute trails suggesting bilaterian activity date from about million years ago.
Larger and more elaborate fossil burrows appear near million years ago, the beginning of the Cambrian Period. Evidence of metazoan activity in both trace and body fossils then increased during the 13 million years leading to the explosion. All living phyla may have originated by the end of the explosion. Molecular divergences among lineages leading to phyla record speciation events that have been earlier than the origins of the new body plans, which can arise many tens of millions of years after an initial branching.
Various attempts to date those branchings by using molecular clocks have disagreed widely. While the timing of the evolution of the developmental systems of living metazoan body plans is still uncertain, the distribution of Hox and other developmental control genes among metazoans indicates that an extensive patterning system was in place prior to the Cambrian.
However, it is likely that much genomic repatterning occurred during the Early Cambrian , involving both key control genes and regulators within their downstream cascades, as novel body plans evolved. Evolution of the seawater sulfate sulfur composition through the Cambrian Period: Implications from carbonate-associated sulfate.
Cambrian carbonate successions of Australia, W-Gondwana, Kazakhstan, Laurentia, and Siberia were investigated for their sulfur isotopic composition of carbonate-associated sulfate CAS. East African Rift. Places where the earth's crust has formed deep fissures and the plates have begun to move apart develop rift structures in which elongate blocks have subsided relative to the blocks on either side.
The East African Rift is a world-famous example of such rifting. It is characterized by 1 topographic deep valleys in the rift zone, 2 sheer escarpments along the faulted walls of the rift zone, 3 a chain of lakes within the rift , most of the lakes highly saline due to evaporation in the hot temperatures characteristic of climates near the equator, 4 voluminous amounts of volcanic rocks that have flowed from faults along the sides of the rift , and 5 volcanic cones where magma flow was most intense.
This example in Kenya displays most of these features near Lake Begoria. The image was acquired December 18, , covers an area of The Permian-Triassic time interval was a period of high sedimentation rates in the intracontinental Karoo rift basin of northwestern Mozambique, reflecting high exhumation rates in the surrounding high ground Precambrian- Cambrian basement and juxtaposed nappes. Data allow discrimination of U-Pb age populations of ca. The younger U-Pb age population also presents two different groups of zircon grains according to Lu-Hf isotopes.
These Hf isotopes reinforce the presence of unexposed ancient crust in this region. The oldest U-Pb age population resembles the late stages of Grenville Orogeny and the Rodinia Supercontinent geotectonic activity mostly represented by magmatic rocks, which are widely present in the basement of northern Mozambique. The juvenile Hf-isotope signature with an older age component is associated to rocks generated from subduction processes with crust assimilation by continental arcs, which we correlate to rocks of the Nampula Complex, south and east of the Moatize-Minjova Basin.
The U-Pb ages between and Ma were correlated to the calc-alkaline magmatism registered in the Guro Suite, related to the breakup phase of Rodinia, and mark the western limit of the Moatize. Crustal tomographic imaging of a transitional continental rift : the Ethiopian rift. In this study we image crustal structure beneath a magmatic continental rift to understand the interplay between crustal stretching and magmatism during the late stages of continental rifting : the Main Ethiopian Rift MER.
The northern sector of this region marks the transition from continental rifting in the East African Rift to incipient seafloor spreading in the southern Red Sea and western Gulf of Aden. The instruments recorded a total of local earthquakes over a month period. Several synthetic tests show that resolution is good between 12 and 25 km depth below sea level , but some horizontal velocity smearing is evident along the axis of the Main Ethiopian Rift below 16 km.
Our models show high P-wave velocities 6. Early Cambrian sipunculan worms from southwest China. Their sipunculan identity is evidenced by the general morphology of the animals sausage-shaped body with a slender retractable introvert and a wider trunk and by other features, both external e.
The three fossil forms Archaeogolfingia caudata gen. This study suggests that most typical features of extant sipunculans have undergone only limited changes since the Early Cambrian , thus indicating a possible evolutionary stasis over the past Myr.
Recent studies of the Midcontinent Rift MCR near Lake Superior give insights into how some rifts start, evolve, and fail because the rift -filling volcanic and sedimentary rocks are exposed at the surface and well imaged by deep seismic reflection and gravity data. The MCR was traditionally considered to have formed by midplate extension and volcanism 1.
We find that a more plausible scenario is that the MCR formed as part of the rifting of Amazonia from Laurentia and became inactive once seafloor spreading was established. A cusp in Laurentia's apparent polar wander path just before the onset of MCR volcanism likely reflects the rifting. Such cusps have been observed elsewhere when continents separate and a new ocean forms between the two fragments. New analyses also find that the MCR's failure did not result from Grenville compression.
This view is consistent with the observation that many intracontinental rifts form and fail as part of plate boundary reorganizations. Present-day continental extension in the East African Rift and seafloor spreading in the Red Sea and Gulf of Aden form a classic three-arm rift geometry as Africa splits into Nubia, Somalia, and Arabia.
An important feature of the MCR is that it is has aspects both of a continental rift - a segmented linear depression filled with sedimentary and igneous rocks - and a large igneous province LIP. The MCR exhibits many key features of volcanic passive margins: seaward dipping. But the big remaining question is what exactly set off the New Madrid quakes. Some models suggest that spreading from the mid-Atlantic ridge—far to the east of the area—might have generated the necessary forces.
Another possibility is that after the retreat of the heavy ice sheets that once covered North America, the crust may have sprung back in fits and starts, providing a local source of energy to shake the region. Figuring out the answer could help explain the occurrence of devastating earthquakes at other mid-plate zones, such as the Tangshan quake in northern China.
Illustration: Cartoon illustration of the mantle beneath the Reelfoot rift in the New Madrid—Wabash Valley seismic zone. The aligned black lenses represent iron- and water- rich melts that were extracted from the asthenosphere and channeled through the lithosphere.
Image by Chen Chen. Chen C.
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