Accommodation Zone Research Articles

Does slab-window opening cause uplift of the overriding plate? A case study from the Gulf of California

Complete subduction of an oceanic plate results in slab-window opening. A key uncertainty in this process is whether the higher heat flux and asthenospheric upwelling conventionally associated with slab-window opening generate a detectable topographic signature in the overriding plate. We focus on the Baja California Peninsula, which incorporates the western margin of the Gulf of California rift. The topography and tectonics of the rift flank along the peninsula are strongly bimodal. North of the Puertecitos accommodation zone, the primary drainage divide attains a mean elevation of ca. 1600m above sea level (asl), above an asthenospheric slab-window opened by Pacific-Farallon spreading ridge subduction along this section of the trench at ca. 17–15Ma. To the south, mean topography decreases abruptly to ca. 800m asl (excluding the structurally distinct Los Cabos block at the southern tip of the peninsula), above fragments of the oceanic Farallon slab which stalled following slab tear-off at ca. 15–14Ma. Along the peninsula, a low-relief surface established atop Miocene subduction-related volcaniclastic units has been incised by a west-draining canyon network in response to uplift. These canyons exhibit cut-and-fill relationships with widespread post-subduction lavas. Here, we utilise LANDSAT and digital elevation model (DEM) data, integrated with previously published K-Ar and 40Ar/39Ar lava crystallisation ages, to constrain the onset of rift flank uplift to ca. 9–5Ma later than slab-window formation in the north and ca. 11–10Ma later in the south. These greatly exceed response time estimates of ca. 2Ma or less for uplift triggered by slab-window opening. Instead, uplift timing of the high-elevation northern region is consistent with lower-lithospheric erosion driven by rift-related convective upwelling. To the south, stalled slab fragments likely inhibited convective return flow, preventing lithospheric erosion and limiting uplift to the isostatic response to crustal unloading during rifting.

INVESTIGATION OF LÜDERS DEFORMATION IN THE MILD STEEL

The features of macroscopic strain inhomogeneity in the form of Chernov–Luders bands development on elastic-plastic transition were investigated in the mild steel. The main regularities of nucleation and propagation of the bands are established. Particular attention was paid to the kinetics of moving boundaries (fronts) of bands, characteristic velocities were determined. It is shown that the rate of formation of nucleus of the Chernov–Luders band is more than an order of magnitude higher than the rate of its expansion. Situations are considered when more than one band develops simultaneously in the object and therefore several moving fronts are observed. It is established that in all cases the velocities of fronts of the Chernov–Luders bands are mutually consistent so that at any instant the generalized rate of expansion of the deformed zone is a constant value. The effect of the deformation rate on the kinetics of the Chernov–Luders fronts was analyzed. Both the generalized rate of expansion of the deformed zone and the speed of individual fronts increase with the increase of the loading rate. A nonlinear (power-law) character of this dependence is established. The fronts of the bands have a complex structure. Different parts of the front can move with unequal velocities, so that the front line is locally curved and split. Ahead of the front, in the undeformed part of the sample, the forerunners may appear, the configuration of which resembles the Chernov–Luders bands nuclei. When encountering the fronts of adjacent bands are annihilated. Annihilation of the fronts is a complex process, which is also characterized by the formation of a precursor and secondary diffusion Chernov–Luders bands. These facts demonstrate that a simplified view of the Chernov-Luders band as a deformed region in a loaded sample, and the front of the band as a boundary between deformed and undeformed zones, should be revised. The microscopic theory of Luders deformation is based on the avalanche growth of the density of mobile dislocations due to breaking from an obstacles and subsequent multiplication, which is realized simultaneously at the upper yield point within the crystallite (grain). At the same time, to form a mobile macroscopic deformation front it is necessary that plastic deformation should be transferred to neighboring grains without hardening, that is, grain-boundary accommodation is needed. The results obtained in the paper suggest that such a zone of accommodation is apparently the Chernov–Luders band front, and, therefore, it has a complex structure.

Zygomaticomaxillary Morphology and Maxillary Sinus Form and Function: How Spatial Constraints Influence Pneumatization Patterns among Modern Humans.

Previous research has suggested that the maxillary sinuses may act as "zones of accommodation" for the nasal region, minimizing the impact of climatic-related changes in nasal cavity breadth on surrounding skeletal structures. However, a recent study among modern human crania has identified that, in addition to nasal cavity breadth, sinus morphology also tracks lateral facial form, especially anterior-posterior positioning of the zygomatics. Here, we expand upon this previous study to further investigate these covariation patterns by employing three samples with distinct combinations of nasal and zygomatic morphologies: Northern Asians (n = 28); sub-Saharan Africans (n = 30); and Europeans (n = 29). For each cranium, 30 landmarks were digitized from CT-rendered models and subsequently assigned to either a midfacial or maxillary sinus "block." Two block partial least squares (2B-PLS) analyses indicate that sinus morphology primarily reflects superior-inferior dimensions of the midface, rather than either nasal cavity breadth or zygomatic position. Specifically, individuals with relatively tall midfacial skeletons exhibit more inferiorly and laterally expanded sinuses compared to those with shorter midfaces. Further, separate across-group and within-group 2B-PLS analyses indicate that regional differences between samples primarily build upon a common pattern of midfacial and sinus covariation already present within each regional group. Allometry, while present, only explains a small portion of the midface-sinus covariation pattern. We conclude that previous findings of larger maxillary sinuses among cold-adapted individuals are not predominantly due to possession of relatively narrow nasal cavities, but to greater maxillary and zygomatic heights. Implications for sinus function and midfacial ontogeny are discussed. Anat Rec, 300:209-225, 2017. © 2016 Wiley Periodicals, Inc.

Rift brittle deformation of SE-Brazilian continental margin: Kinematic analysis of onshore structures relative to the transfer and accommodation zones of southern Campos Basin

High-resolution drone-supported mapping and traditional field work were used to refine the hierarchy and kinematics of rift-related faults in the basement rocks and Early Cretaceous mafic dikes onshore of the Campos Basin, SE-Brazil. Two sets of structures were identified. The most significant fault set is NE-SW oriented with predominantly normal displacement. At mesoscale, this fault set is arranged in a rhombic pattern, interpreted here as a breached relay ramp system. The rhombic pattern is a penetrative fabric from the thin-section to regional scale. The second-order set of structures is an E-W/ESE-WNW system of normal faults with sinistral component. These E-W structures are oriented parallel with regional intrabasinal transfer zones associated with the earliest stages of Campos Basin's rift system. The crosscutting relationship between the two fault sets and tholeiitic dikes implies that the NE-SW fault set is the older feature, but remained active until the final stages of rifting in this region as the second-order fault set is older than the tholeiitic dikes. Paleostresses estimated from fault slip inversion method indicated that extension was originally NW-SE, with formation of the E-W transfer, followed by ESE-WNW oblique opening associated with a relay ramp system and related accommodation zones.

Impact of normal faulting and pre‐rift salt tectonics on the structural style of salt‐influenced rifts: the Late Jurassic Norwegian Central Graben, North Sea

AbstractStudies of salt‐influenced rift basins have focused on individual or basin‐scale fault system and/or salt‐related structure. In contrast, the large‐scale rift structure, namely rift segments and rift accommodation zones and the role of pre‐rift tectonics in controlling structural style and syn‐rift basin evolution have received less attention. The Norwegian Central Graben, comprises a complex network of sub‐salt normal faults and pre‐rift salt‐related structures that together influenced the structural style and evolution of the Late Jurassic rift. Beneath the halite‐rich, Permian Zechstein Supergroup, the rift can be divided into two major rift segments, each comprising rift margin and rift axis domains, separated by a rift‐wide accommodation zone – the Steinbit Accommodation Zone. Sub‐salt normal faults in the rift segments are generally larger, in terms of fault throw, length and spacing, than those in the accommodation zone. The pre‐rift structure varies laterally from sheet‐like units, with limited salt tectonics, through domains characterised by isolated salt diapirs, to a network of elongate salt walls with intervening minibasins. Analysis of the interactions between the sub‐salt normal fault network and the pre‐rift salt‐related structures reveals six types of syn‐rift depocentres. Increasing the throw and spacing of sub‐salt normal faults from rift segment to rift accommodation zone generally leads to simpler half‐graben geometries and an increase in the size and thickness of syn‐rift depocentres. In contrast, more complex pre‐rift salt tectonics increases the mechanical heterogeneity of the pre‐rift, leading to increased complexity of structural style. Along the rift margin, syn‐rift depocentres occur as interpods above salt walls and are generally unrelated to the relatively minor sub‐salt normal faults in this structural domain. Along the rift axis, deformation associated with large sub‐salt normal faults created coupled and decoupled supra‐salt faults. Tilting of the hanging wall associated with growth of the large normal faults along the rift axis also promoted a thin‐skinned, gravity‐driven deformation leading to a range of extensional and compressional structures affecting the syn‐rift interval. The Steinbit Accommodation Zone contains rift‐related structural styles that encompass elements seen along both the rift margin and axis. The wide variability in structural style and evolution of syn‐rift depocentres recognised in this study has implications for the geomorphological evolution of rifts, sediment routing systems and stratigraphic evolution in rifts that contain pre‐rift salt units.

The role of gravitational collapse in controlling the evolution of crestal fault systems (Espírito Santo Basin, SE Brazil)

A high-quality 3D seismic volume from offshore Espírito Santo Basin (SE Brazil) is used to assess the importance of gravitational collapse to the formation of crestal faults above salt structures. A crestal fault system is imaged in detail using seismic attributes such as curvature and variance, which are later complemented by analyses of throw vs. distance (T-D) and throw vs. depth (T-Z). In the study area, crestal faults comprise closely spaced arrays and are bounded by large listric faults, herein called border faults. Two episodes of growth are identified in two opposite-dipping fault families separated by a transverse accommodation zone. Statistical analyses for eighty-four (84) faults show that fault spacing is < 250 m, with border faults showing the larger throw values. Fault throw varies between 8 ms and 80 ms two-way time for crestal faults, and 60–80 ms two-way time for border faults. Fault length varies between ∼410 m and 1750 m, with border faults ranging from 1250 m to 1750 m. This work shows that border faults accommodated most of the strain associated with salt growth and collapse. The growth history of crestal faults favours an isolated fault propagation model with fault segment linkage being associated with the lateral propagation of discrete fault segments. Importantly, two episodes of fault growth are identified as synchronous to two phases of seafloor erosion, rendering local unconformities as competent markers of fault reactivation at a local scale. This paper has crucial implications for the understanding of fault growth as a means to assess drilling risk and oil and gas migration on continental margins. As a corollary, this work demonstrates that: 1) a certain degree of spatial organisation occurs in crestal fault systems; 2) transverse accommodation zones can form regions in which fault propagation is enhanced and regional dips of faults change in 4D.

Reconstruction of the Provence Chain evolution, southeastern France

The Provence fold-and-thrust belt forms the eastern limit of the Pyrenean orogenic system in southeastern France. This belt developed during the Late Cretaceous-Eocene Pyrenean-Provence compression and was then deformed by Oligocene-Miocene Ligurian rifting events and Neogene to present-day Alpine compression. In this study, surface structural data, seismic profiles, and crustal-to-lithospheric-scale sequentially balanced cross sections contribute to the understanding of the dynamics of the Provence Chain and its long-term history of deformation. Balanced cross sections show that the thrust system is characterized by various structural styles, including deep-seated basement faults that affect the entire crust, tectonic inversions of Paleozoic-Mesozoic basins, shallower decollements within the sedimentary cover, accommodation zones, and salt tectonics. This study shows the prime control of the structural inheritance over a long period of time on the tectonic evolution of a geological system. This includes mechanical heterogeneities, such as Variscan shear zones, reactivated during Middle Cretaceous Pyrenean rifting between Eurasia and Sardinia. In domains where Mesozoic rifting is well marked, inherited basement normal faults and the thermally weak crust favored the formation of an inner thick-skinned thrust belt during Late Cretaceous-Eocene contraction. Here 155 km (similar to 35%) of shortening was accommodated by inversion of north verging crustal faults, north directed subduction of the Sardinia mantle lithosphere, and ductile thickening of the Provence mantle lithosphere. During the Oligocene, these domains were still predisposed for the localized faulting of the Ligurian basin rifting and the seafloor spreading.

Palaeovalleys in foreland ramp settings: what happens as accommodation decreases down dip?

AbstractRecent advances in our understanding of palaeovalleys are largely guided by examples from passive margins, in which accommodation increases down depositional dip. This study tests these models against a dataset from the Pennsylvanian Breathitt Group of the central Appalachian foreland basin, USA. This fluvio‐deltaic succession contains extensive erosionally based fluvio‐estuarine sand bodies that can be tracked over 80 km down depositional dip from a proximal zone of high accommodation close to the orogenic margin to a distal, lower accommodation zone close to the cratonic margin of the basin. The sand bodies are up to 25 m thick, multi‐storey and characterized in their lower parts by strongly amalgamated storeys containing sandy fluvial to estuarine bar accretion elements, and in their middle to upper parts by more fully preserved storeys up to 10 m thick and laterally extensive over 100s of metres. The upper storeys include abandonment channel‐fills of heterolithic marine or marginal marine deposits or muddy to sandy point‐bar elements. Three major regional‐scale architectures include: (i) Tabular sand bodies that everywhere incise open marine prodelta and mouth bar facies and are interpreted as palaeovalleys formed during falling stage and lowstand systems tracts, when eustatic sea‐level fall outpaced tectonic subsidence across the entire study area. (ii) Sand bodies that incise genetically related floodplain lake and/or bay‐fill minor mouth bar deposits up depositional dip and open marine prodelta and mouth bar facies down dip. These stacked distributary channel deposits map down dip into palaeovalleys and formed when up dip subsidence rate resulted in positive, but reduced rate of accommodation creation, while lower tectonic subsidence rate down‐dip resulted in incision. (iii) Sand bodies that incise genetically related floodplain, lake and/or bay‐fill minor mouth bars up dip and pass down‐dip into genetically related unconfined floodplain, prodelta and mouth bar deposits. These sand bodies represent stacked distributary channel fills and channel amalgamation was the product of high rates of lateral migration, typical of the behaviour of channels above their backwater reach. Case (2) sand bodies demonstrate that in rapidly subsiding foreland basins, cross‐shelf palaeovalleys may form down depositional dip from aggradational, distributive fluival strata. Additionally, the genetic relationship between stacked distributary channels and palaeovalleys supports recent models for palaeovalley formation that emphasize diachronous, cut‐and‐fill during falling stage and lowstands of relative sea level.

Pre-salt to salt stratigraphic architecture in a rift basin: insights from a basin-scale study of the Gulf of Suez (Egypt)

We propose a basin-scale (∼300 × 100 km) study of the pre-salt to salt sedimentary fill from the Suez rift based on outcrop and subsurface data. This study is a new synthesis of existing and newly acquired data using an integrated approach with (1) basin-scale synthesis of the structural framework, (2) stratigraphic architecture characterization of the entire Suez rift using sequence stratigraphy concepts, (3) lithologic maps reconstruction and interpretation, (4) isopach/depocenter maps interpolation to quantify sedimentary volumes, and (5) quantification of the sediment supply, mean carbonate and evaporite accumulation rates, and their integration into the rift dynamic. The Gulf of Suez is ca. 300-km-long and up to 80-km-wide rift structure, resulting from the late Oligocene to early Miocene rifting of the African and Arabian plates. The stratigraphic architecture has recorded five main stages of rift evolution, from rift initiation to finally tectonic quiescence characterized by salt deposits. Rift initiation (ca. 1–4 Myr duration): the Suez rift was initiated at the end of the Oligocene along the NNW-SSE trend of the Red Sea with evidences of active volcanism. Continental to lacustrine deposits only occurred in isolated depocenters. Sediment supply was relatively low. Rift widening (ca. 3 Myr duration): the rift propagated from south to north (Aquitanian), with first marine incursions from the Mediterranean Sea. The rift was subdivided into numerous depocenters controlled by active faults. Sedimentation was characterized by small carbonate platforms and associated sabkha deposits to the south and shallow open marine condition to the north with mixed sedimentation organized into an overall transgressive trend. Rift climax (ca. 5 Myr duration): the rift was then flooded during Burdigalian times recording the connection between the Mediterranean Sea and the Red Sea. The faults were gradually connected and reliefs on the rift shoulders were high as evidenced by a strong increase of the uplift/subsidence rates and sediment supply. Three main depocenters were then individualized across the rift and correspond to the Darag, Central, and Southern basins. Sedimentation was characterized by very large Gilbert-type deltas along the eastern margin and associated submarine fans and turbidite systems along the basin axis. Isolated carbonate platforms and reefs mainly occurred in the Southern basin and along tilted block crests. Late syn-rift to rift narrowing (ca. 4 Myr duration): during the Langhian, the basin recorded several falls of relative sea level and bathymetry in the rift axis was progressively reduced. The former reliefs induced during the rift climax were quickly destroyed as evidenced by the drastic drop in sediment supply. Stratigraphic reconstruction indicates that the Central basin was restricted during lowstand period; meanwhile, open marine conditions prevailed to the north and south of the Suez rift. The Central basin, Zaafarana, and Morgan accommodation zones thus acted as a major divide between the Mediterranean Sea and the Red Sea. During Serravalian times, the Suez rift also recorded several disconnections between the Mediterranean and Red seas as evidenced by massive evaporites in major fault-controlled depocenters. The Suez rift was occasionally characterized by N–S paleogeographic gradient with restricted setting to the north and open marine setting to the south (Red Sea). Tectonic quiescence to latest syn-rift (ca. 7 Myr duration): the Tortonian was then characterized by the deposition of very thick salt series (>1000 m) which recorded a period of maximum restriction for the Suez rift. The basin was still subdivided into several sub-basins bounded by major faults. The basin with a N-S paleogeographic gradient was totally and permanently disconnected from the Mediterranean Sea and connected to open marine condition via the Red Sea. The Messinian was also characterized by a thick salt series, but the evaporite typology and sedimentary systems distribution suggest a more humid climate than during Tortonian times. Pre-salt to salt transition was not sharp and lasted for ca. 4 Myr (Langhian-Serravalian). It was initiated as the result of the combined effect of (1) climatic changes with aridization and low water input from the catchments and (2) rift dynamic induced by plate tectonic reorganization that controlled the interplay between sea level and accommodation zones constituting sills.

Morphological Covariation between the Maxillary Sinus and Midfacial Skeleton among Sub-Saharan and Circumpolar Modern Humans.

Maxillary sinus volume tracks ecogeographic differences in nasal form and may serve as a zone of accommodation for ontogenetic and evolutionary changes in nasal cavity breadth. However, little is known regarding how sinus volume is distributed within the midface. This study investigates morphological covariation between midfacial and sinus shape to better understand structural and functional relationships between the sinus, midface, and nasal cavity. Cranial and sinus models were rendered from CT scans of modern human samples from two disparate climates: sub-Saharan (South Africans [n = 15], West Africans [n = 17]), and circumpolar (Siberian Buriats [n = 18], Alaskan Inuit [n = 20]). Twenty-five 3D coordinate landmarks were placed on the models and subjected to generalized Procrustes analysis. Two-block partial least squares (2B-PLS) analysis was employed to identify patterns of covariation. The 2B-PLS analysis indicates PLS1 (58.6% total covariation) relates to height and breadth relationships between the midface, nasal cavity, and maxillary sinus. Significant regional differences in PLS1 scores are evident: circumpolar samples possess taller/narrower noses with taller/wider sinuses compared to sub-Saharan samples. Importantly, PLS1 indicates that sinus breadth is not exclusively related to nasal cavity breadth; variation in lateral sinus expansion toward the zygoma represents an important contributing factor. PLS2 (16%) relates to supero-inferior positioning of the sinus within the midface. Allometric trends, while statistically significant, explain only a small portion of these covariation patterns. These results suggest that the maxillary sinus serves as a zone of accommodation at the confluence of multiple facial components, potentially minimizing effects of morphological alterations to certain components on adjacent structures. Am J Phys Anthropol 160:483-497, 2016. © 2016 Wiley Periodicals, Inc.

Formal Fire Safety Assessment of Passenger Ships

Fire has been a major cause of ship's accidents throughout maritime history. It is by far the most serious threat to life and the environment as passenger ships get larger and more sophisticated. It is also impossible to protect a passenger vessel against all hazards. Despite the fact that a passenger ship contains potential fire hazards in the engine room space, accommodation zone and electrical systems, etc, the single most important fire hazard onboard a ship may be the man himself, either unintentionally or intentionally. 'Fire safety on passenger vessel' has continued to be the focus of attention on passenger ships. The work described in this thesis is concerned with the application of Formal Fire Safety Assessment to passenger ships. The traditional way of conducting a Formal Safety Assessment (FSA) employs typical fire safety analysis methods that require a certain amount of data. Most fire accident data available for passenger vessels is associated with a high degree of uncertainty and considered to be unreliable. As such, the research carried out in this thesis is directed at the development of novel fire safety analysis methods to address this problem. This thesis proposed several subjective fire safety analysis methods for passenger vessels within the FSA methodology. Also, it concentrates on developing an advanced approach for passenger ships. A few novel safety analysis and synthesis methodologies are presented to integrate fire safety assessment with decision-making techniques so that fire safety can be taken into account from the concept design /operation stages of passenger ships. This is to ensure a more controlled development process permitting decisions regarding design and operation to be made based on fire safety assessment. Finally, this thesis is concluded by summarising the results of this research project and the areas where further effort is required to improve the developed methodologies.

Masters of Empire: Great Lakes Indians and the Making of America by Michael A. McDonnell

Reviewed by: Masters of Empire: Great Lakes Indians and the Making of America by Michael A. McDonnell Christophe Boucher Masters of Empire: Great Lakes Indians and the Making of America. By Michael A. McDonnell. New York, NY: Hill and Wang. 2015. With this monograph, Michael McDonnell joins a growing list of historians who have taken up Daniel Richter’s challenge to reinterpret American history by “facing east from Indian Country.” While the narrative unfolds along a familiar timeline stretching from the early 1600s to Removal, McDonnell gives a new spin to the story by recounting it from an unusual vantage point: that of the Anishinabe Odawa who lived around the straits of Michilimackinac. As a strategic gateway connecting Lakes Huron and Michigan, the region has long been of scholarly interest but no one, until now, had published a detailed ethno-history of its Odawa residents. Although he draws on familiar sources, McDonnell has nonetheless produced a work that is fresh, engaging, and provocative. His book is groundbreaking in that Native Americans are not just actors in a world changing to the beat of colonial drums, as is often the case in Native American History. His Odawa are [End Page 96] actually prime movers in American history and their actions “often changed the course of North American events” (327). McDonnell traces, for instance, the outbreak of the Seven Years War to an Anishinabe raid on a Pickawillany, a Miami village, two years before the Battle of Jumonville Glen in 1754. In his quest to re-assert the centrality of the Odawa in defining historical moments, one could accuse McDonnell of over-reaching by engaging, at times, in rather speculative exercises. But overall, by flipping the frame of reference around and focusing “on one people and one place over the long durée,” the author cleverly destabilizes the traditional master narrative by periodizing known events according to a new historical logic (6). As such, the French and Indian War unfolds in the broader context of the First Anglo-Indian War (1752–1758), a conflict initiated by and for Native Americans with Michili-mackinac––not Quebec City––as ground zero. Far from being peons in a larger imperial contest, the Anishinabeg and their allies only fought alongside the French––and later the English––as long as their interests converged. “The Odawa,” McDonnell remarks, “were able to exploit European imperialism when it came and they did so mostly for their own purposes” (15). Without indigenous support, imperial powers vacillated. In order to keep a foothold in the region, the French, British, and Americans had therefore to adapt their imperial designs to meet the expectations of their hosts, making the Odawa “Masters of Empire.” Interestingly, the Anishinabeg’s influence emanated from the fact that Europeans––and not Native Americans as the story usually goes––were dependent on the locals. Readers familiar with Richard White’s The Middle Ground will discover here a compelling counter-argument to his seminal work. Since its publication in 1991, White’s depiction of European-Indian relations in the Great Lakes region from 1650 to 1815 has had a profound historiographical impact. While scholars initially applied his model indiscriminately to other times and places, more recently, historians have highlighted its shortcomings. McDonnell’s study draws from these more recent works. Echoing Heidi Bohaker’s argument, for instance, the author rejects White’s claim that, after 1650, the Anishinabeg were refugees from Iroquois war parties. Nor did they live in a shattered world or owe their political cohesiveness to French diplomatic endeavors. Instead, they lived in a world “in flux… not in a state of collapse” where the extension of kinship networks, more than French mediation, played a central role to foster political integration among the region’s inhabitants, Indians and Europeans alike. In the colonial Great Lakes region, therefore, “the French could only follow, not lead” (91). This statement stands in sharp contrast to White’s argument that the middle ground, this zone of inter-cultural accommodation, was possible because French and Indians had similar power and none could dictate to the other. As McDonnell points out however, this perspective only reflects distortions in the records. By over-privileging ethno-historic...

Relative role of accommodation zones in controlling stratal architectural variability and facies distribution: Insights from the Fushan Depression, South China Sea

In sedimentary basins, a better understanding of the controlling effect of accommodation/transfer zones on stratal architecture and facies distribution can improve the success rate of locating hydrocarbon reservoirs. The Fushan Depression is a half-graben rift sub-basin located in the southeast of the Beibuwan Basin, South China Sea. In this study, comparative analysis of seismic reflection, palaeogeomorphology, fault activity and depositional facies distribution indicates that three different types of inner-basin slopes (i.e. multi-level step-fault slope in the western area, slope flexure zone in the accommodation zone area and gentle slope in the eastern area) were developed along the southern slope of the Fushan Depression, together with a large-scale accommodation zone located at the intersection of the western and eastern fault systems. Further analysis shows that the accommodation zone played an important role in controlling not only stratal architectural variability in the southern slope but also depositional facies distribution in the accommodation zone area. During the high-stand stage, the deposition of depositional systems was mainly controlled by sediment supply and the NW-trending transfer faults. Major drainage systems entered the Fushan Depression through the accommodation zone along the direction of sediment supply, and sedimentary flow paths were parallel to the accommodation zone axis with sedimentary flows constrained to the adjacent areas of the NW-trending transfer faults. By contrast, during the low-stand stage, the transfer faults had little control over depositional facies distribution, and the sublacustrine fan sediments diverted away from the accommodation zone and flew down along the northeast, oblique to the accommodation zone axis. The flow division in the lowstand stage might be greatly influenced by flow type and topography. In addition, the accommodation zone area demonstrated unique hydrocarbon accumulation models different from the western and eastern areas, suggesting that the future exploration should be conducted at different levels in the Fushan Depression.

Evolution of the Kivu Rift, East Africa: interplay among tectonics, sedimentation and magmatism

AbstractThe seismically and volcanically active Kivu Rift, in the western branch of the East African Rift System, is a type locale for studies of high‐elevation, humid‐climate rift basins, as well as magmatic basin development. Interpretations of offshore multi‐channel seismic (MCS) reflection data, terrestrial radar topography, lake bathymetry and seismicity data recorded on a temporary array provide new insights into the structure, stratigraphy and evolution of the Kivu rift. The Kivu rift is an asymmetric graben controlled on its west side by a ca. 110 km‐long, N‐S striking border fault. The southern basins of the lake and the upper Rusizi river basin are an accommodation zone effectively linking 1470 m‐high Lake Kivu to 770 m‐high Lake Tanganyika. MCS data in the eastern Kivu lake basin reveal a west‐dipping half graben with at least 1.5 km of sedimentary section; most of the ca. 2 km of extension in this sub‐basin is accommodated by the east‐dipping Iwawa normal fault, which bounds an intrabasinal horst. Lake Kivu experienced at least three periods of near desiccation. The two most recent of these approximately correlate to the African Megadrought and Last Glacial Maximum. There was a rapid lake level transgression of at least 400 m in the early Holocene. The line load of the Virunga volcanic chain enhances the fault‐controlled basin subsidence; simple elastic plate models suggest that the line load of the Virunga volcanic chain depresses the basin by more than 1 km, reduces flank uplift locally and broadens the depocentre. Not only do the voluminous magmatism and degassing to the lake pose a hazard to the riparian population, but our studies demonstrate that magmatism has important implications for short‐term processes such as lake levels, inflow and outlets, as well as long term modification of classic half‐graben basin morphology.

Differential Scaling Patterns in Maxillary Sinus Volume and Nasal Cavity Breadth Among Modern Humans.

Among modern humans, nasal cavity size and shape reflect its vital role in air conditioning processes. The ability for the nasal cavity to augment its shape, particularly in inferior breadth, likely relates to the surrounding maxillary sinuses acting as zones of accommodation. However, much is still unknown regarding how nasal and sinus morphology relate to each other and to overall craniofacial form, particularly across diverse populations with varying respiratory demands. As such, this study uses computed tomographic (CT) scans of modern human crania (N = 171) from nine different localities to investigate ecogeographic differences in (1) the interaction between maxillary sinus volume (MSV) and nasal cavity breadth (NCB) and (2) scaling patterns of MSV and NCB in relation to craniofacial size. Reduced major axis (RMA) regression reveals that all samples exhibit an inverse relationship between MSV and NCB, but statistical significance and the strength of that relationship is sample dependent. Individuals from cold-dry climates have larger MSVs with narrower NCBs, while smaller MSVs are associated with wider NCBs in hot-humid climates. MSV and NCB each scale with positive allometry relative to overall craniofacial size. However, sample differences are evident in the both the interaction between MSV and NCB, as well as their correlation with craniofacial size. While these results provide further support that the maxillary sinus and nasal cavity are integrated among populations from opposite ends of the climatic spectrum, additional epigenetic factors are needed to explain variation of these structures among populations from more intermediate climates.

The volcanic, magmatic and tectonic setting of the Taupo Volcanic Zone, New Zealand, reviewed from a geothermal perspective

The Taupo Volcanic Zone (TVZ) in the North Island of New Zealand is one of the world's most spectacular and productive areas of Quaternary silicic volcanism and geothermal manifestations. The TVZ is only the latest manifestation of NNE–SSW-orientated arcs that have migrated in step-wise fashion to the SSE over the past ca. 16Ma. The TVZ began erupting around 2Ma, with early andesitic volcanism being joined and rapidly swamped by voluminous rhyolitic volcanism. The zone shows a pronounced segmentation into northern and southern extremities with andesite composite cones, no calderas and only limited vent-hosted geothermal systems, and a 125km long rhyolite-dominated central segment. About four times as much magma is trapped at depth below the central TVZ than is erupted, feeding heat, volatiles and chemicals into 23 geothermal systems with a total of ca. 4.2GW thermal energy release. The modern (post-61ka) TVZ is an actively rifting arc, widening at 7mm/year at the south end to 15mm/year at the Bay of Plenty coastline, with an associated zone of young to active faulting (Taupo Fault Belt: TFB, or the Taupo Rift), but the axes of the modern TFB and TVZ are offset by 15–20km through much of the central TVZ. Although there is a dominant NNE–SSW tectonic grain within the central TVZ, there are also influences of deeper basement structures that sometimes extend outside the limits of the zone, such as NW–SE, arc-perpendicular accommodation zones linking local domains of extension as well as N–S orientated structures related to the Hauraki Rift that may control fluid flow into the roots of the geothermal systems. Models for the geothermal systems favour either a source in a relatively shallow localised magmatic intrusion (e.g., Kawerau, Ngatamariki) or treat the systems as reflecting large-scale fluid dynamical instabilities from an evenly heated ‘hot plate’ at ∼7km depth. Where controls from dating of host lithologies are available, systems at Kawerau and Ngatamariki are seen to represent renewed activity superimposed on a fossil system fed by past intrusions, and it is unclear what is meant by the lifetime of any single geothermal system. TVZ geothermal systems appear in turn to react too sluggishly to respond to disruptive episodes of volcanism, and recover within geologically short periods of time, as seen at Waimangu and Taupo. In the central TVZ, there are complex inter-relationships between volcanism, magmatism, and tectonism. Magmatism and volcanism are obviously linked, but it is uncertain why intense magmatism at Taupo and Okataina should yield voluminous rhyolite volcanism, whereas more intense magmatism in the Taupo-Reporoa Basin has not yielded significant silicic volcanism but instead feeds multiple large geothermal systems. The central TVZ is unique for an arc segment in the intensity of its magmatic-volcanic-geothermal flux (matching the Yellowstone system), and the cause(s) of this uniqueness are not yet established. Any explanation needs to address the segmented nature of the zone, and why the thermal flux should be so geographically and temporally constrained.

Rift architecture of the Lofoten-Vesterålen margin, offshore Norway

The Lofoten-Vesterålen margin (LVM) located offshore northern Norway, is the least explored and understood margin segment within the Norwegian passive margin. Three previous models have been proposed to explain the evolution and architecture of the margin: 1) segmentation by NW–SE-striking transform faults; 2) temporal and spatial segmentation of different phases of rifting; and 3) segmentation by an E–W-striking accommodation zone. In this study, we utilize industry 2D seismic data, potential field data, and well and outcrop published data information to revise the evolution, structural styles, and segmentation of the margin.The rift evolution of the LVM consists of: a pre-rifting/marginal rifting event during the Triassic and Jurassic, shown by the localized distribution of wedge-shaped sedimentary packages; a main-rift event during the Early Cretaceous with the development of large half-graben depocenters; and a post-rift event during Late Cretaceous to present. Three margin segments were defined (Southern Lofoten, Northern Lofoten and Vesterålen-Andøya) based on variations in the structural styles and basin-fill along the margin. The margin segments are separated by two time-transgressive NW–SE-striking transfer zones. The first transfer zone (North Lofoten transfer zone) separated the Southern Lofoten segment from the Northern Lofoten-Vesterålen-Andøya segments in the Late Jurassic. The second transfer zone (Vesterålen Transfer zone) separated the Northern Lofoten from the Vesterålen-Andøya segments in the Early-Late Cretaceous.The LVM is classified into three distinct rift domains: proximal, necking and oceanic. Each of these domains consistently shows prominent structural similarities across the margin. The necking domain is suggested as the most hydrocarbon prospective area based on similarities with the same domain in mid-Norway where source rock reservoir preservation is the highest.

Epithermal mineralization controlled by synextensional magmatism in the Guazapares Mining District of the Sierra Madre Occidental silicic large igneous province, Mexico

We show here that epithermal mineralization in the Guazapares Mining District is closely related to extensional deformation and magmatism during the mid-Cenozoic ignimbrite flare-up of the Sierra Madre Occidental silicic large igneous province, Mexico. Three Late Oligocene–Early Miocene synextensional formations are identified by detailed volcanic lithofacies mapping in the study area: (1) ca. 27.5 Ma Parajes formation, composed of silicic outflow ignimbrite sheets; (2) ca. 27–24.5 Ma Témoris formation, consisting primarily of locally erupted mafic-intermediate composition lavas and interbedded fluvial and debris flow deposits; (3) ca. 24.5–23 Ma Sierra Guazapares formation, composed of silicic vent to proximal ignimbrites, lavas, subvolcanic intrusions, and volcaniclastic deposits. Epithermal low-to intermediate-sulfidation, gold–silver–lead–zinc vein and breccia mineralization appears to be associated with emplacement of Sierra Guazapares formation rhyolite plugs and is favored where pre-to-synvolcanic extensional structures are in close association with these hypabyssal intrusions.Several resource areas in the Guazapares Mining District are located along the easternmost strands of the Guazapares Fault Zone, a NNW-trending normal fault system that hosts most of the epithermal mineralization in the mining district. This study describes the geology that underlies three of these areas, which are, from north to south: (1) The Monte Cristo resource area, which is underlain primarily by Sierra Guazapares formation rhyolite dome collapse breccia, lapilli-tuffs, and fluvially reworked tuffs that interfinger with lacustrine sedimentary rocks in a synvolcanic half-graben bounded by the Sangre de Cristo Fault. Deposition in the hanging wall of this half-graben was concurrent with the development of a rhyolite lava dome-hypabyssal intrusion complex in the footwall; mineralization is concentrated in the high-silica rhyolite intrusions in the footwall and along the syndepositional fault and adjacent hanging wall graben fill. (2) The San Antonio resource area, underlain by interstratified mafic-intermediate lavas and fluvial sandstone of the Témoris formation, faulted and tilted by two en echelon NW-trending normal faults with opposing dip-directions. Mineralization occurs along subvertical structures in the accommodation zone between these faults. There are no silicic intrusions at the surface within the San Antonio resource area, but they outcrop ∼0.5 km to the east, where they are intruded along the La Palmera Fault, and are located ∼120 m-depth in the subsurface. (3) The La Unión resource area, which is underlain by mineralized andesite lavas and lapilli-tuffs of the Témoris Formation. Adjacent to the La Unión resource area is Cerro Salitrera, one of the largest silicic intrusions in the area. The plug that forms Cerro Salitrera was intruded along the La Palmera Fault, and was not recognized as an intrusion prior to our work.We show here that epithermal mineralization is Late Oligocene to Miocene-age and hosted in extensional structures, younger than Laramide (Cretaceous-Eocene) ages of mineralization inferred from unpublished mining reports for the region. We further infer that mineralization was directly related to the emplacement of silicic intrusions of the Sierra Guazapares formation, when the mid-Cenozoic ignimbrite flare-up of the Sierra Madre Occidental swept westward into the study area about 24.5–23 Ma ago.

Characteristics of low-order accommodation zone and its reservoir formation model in Xin 183 Block, Dongying Sag, Eastern China

This paper investigates the nature of an East-West strike convergent-overlapping accommodation zone in Xin 183 fault block and its influence on the hydrocarbon distribution based on a study of the fault geometry, kinematics and dynamics using 3-D seismic data, well testing data and hydrocarbon distribution data. First, a geometrical analysis indicates that the structural profile of the major fault on the west side and its associated secondary faults has a multi-stage reverse Y appearance; the profile tectonic style on the east side has a cabbage-like appearance. Second, the observed fault activities show that when the activity of the west main fault became weak, the displacement was transferred to east main fault and secondary faults. Third, a dynamical analysis demonstrates that the stress field of the area is left-lateral extensional shear. The reservoir formation model of Xin 183 accommodation zone is constructed, and the preferred hydrocarbon locations are pointed out. [Received: September 23, 2012; Accepted: July 12, 2013]

Strategic Incentives in Teams: Implications of Returns to Scale

This article demonstrates the critical relationship between the characteristics of the production function and the strategic incentives in a team. Equilibrium effort increases in team size when substitutability is low relative to returns to scale. Effort levels are actually strategic complements when returns to scale exceed the substitutability of members' effort. Moreover, even with equal shares the well‐known problem is determined by returns to scale and becomes worse as returns increase. While a target scheme can support the optimal output level as an equilibrium, it does not completely deter free riding. A team member will accommodate shirking by increasing their own effort within a remarkably large “accommodation zone” where the additional effort cost is less than the bonus. This accommodation of shirking by others exists for different returns to scale and even for very low levels of substitutability.