Stratigraphic Constraints Research Articles

Sub-seafloor epidosite alteration: Timing, depth and stratigraphic distribution in the Semail ophiolite, Oman

Pervasive epidotization of igneous rocks is a common feature in the ophiolite record of hydrothermally altered oceanic crust. Current genetic models view epidosites as markers of focussed upflow of hydrothermal fluid beneath oceanic spreading ridges. The epidosites are envisaged to form at the base of the sheeted dike complex (SDC) during active plate spreading. Our mapping of the Semail ophiolite in Oman has revealed abundant epidosites in the volcanic sequence, some exceeding 1km2 in extent. They are more frequent and far larger than the mineralogically identical epidosites in the SDC. We have also found epidosites that traverse the entire SDC from bottom to top. Thus, rather than being restricted to the base of the SDC, as implied by current models, epidosites in fact occur throughout the SDC and dominantly within the overlying volcanic pile. We report the occurrence of 19 epidosite bodies and their crosscutting relations with respect to host lava units, dikes, intrusive stocks and also seafloor umbers. The volcanostratigraphic affiliation of the dikes is identified by their whole-rock and clinopyroxene compositions. The relations set constraints on the timing of epidotization with respect to igneous activity in the ophiolite. At least one of the epidosites in the SDC formed during Lasail off-axis volcanism. Another epidosite in the SDC and many in the volcanic units formed later during post-spreading, Alley and Boninitic Alley supra-subduction zone volcanism. Only permissive, not compelling, evidence allows just two of the epidosites to have formed within the main-stage SDC during or shortly after its emplacement. We conclude that epidotization of the oceanic crust is not necessarily coupled to spreading ridges and that it can occur during fore-arc volcanism. This finding is consistent with evidence from the modern seafloor and it requires a different hydrothermal environment to that traditionally associated with alteration beneath spreading axes. The timing and stratigraphic constraints show that some of the epidosites formed within the top 1.4km of the crust, others between 1.2 and 1.7km depth, whereas the deepest formed between 2.6 and 3.8km below the paleo-seafloor. Finally, the timing constraints do not prescribe a strict temporal relationship between epidotization and the formation of the numerous seafloor massive-sulphide (VMS) deposits known in the ophiolite. However, a role for epidosites as source rocks for the metals in the deposits remains permissible.

The geologic history of Margaritifer basin, Mars

AbstractIn this study, we investigate the fluvial, sedimentary, and volcanic history of Margaritifer basin and the Uzboi‐Ladon‐Morava outflow channel system. This network of valleys and basins spans more than 8000 km in length, linking the fluvially dissected southern highlands and Argyre basin with the northern lowlands via Ares Vallis. Compositionally, thermophysically, and morphologically distinct geologic units are identified and are used to place critical relative stratigraphic constraints on the timing of geologic processes in Margaritifer basin. Our analyses show that fluvial activity was separated in time by significant episodes of geologic activity, including the widespread volcanic resurfacing of Margaritifer basin and the formation of chaos terrain. The most recent fluvial activity within Margaritifer basin appears to terminate at a region of chaos terrain, suggesting possible communication between surface and subsurface water reservoirs. We conclude with a discussion of the implications of these observations on our current knowledge of Martian hydrologic evolution in this important region.

XRD-based 40Ar/39Ar age correction for fine-grained illite, with application to folded carbonates in the Monterrey Salient (northern Mexico)

Due to their minute size, 40Ar/39Ar analysis of illite faces significant analytical challenges, including mineral characterization and, especially, effects of grain size and crystallography on 39Ar recoil. Quantifying the effects of 39Ar recoil requires the use of sample vacuum encapsulation during irradiation, which permits the measurement of the fraction of recoiled 39Ar as well as the 39Ar and 40Ar∗ retained within illite crystals that are released during step heating. Total-Gas Ages (TGA) are calculated by using both recoiled and retained argon, which is functionally equivalent to K–Ar ages, while Retention Ages (RA) only involve retained Ar in the crystal. Natural applications have shown that TGA fits stratigraphic constraints of geological processes when the average illite crystallite thickness (ICT) is smaller than 10nm, and that RA matches these constraints for ICTs larger than 50nm. We propose a new age correction method that takes into account the average ICT and corresponding recoiled 39Ar for a sample, with X-ray Corrected Ages (XCA) lying between Total-Gas and Retention Ages depending on ICT. This correction is particularly useful in samples containing authigenic illite formed in the anchizone, with typical ICT values between 10 and 50nm.In three samples containing authigenic illite from Cretaceous carbonates in the Monterrey Salient in northern Mexico, there is a range in TGAs among the different size-fractions of 46–49, 36–43 and 40–52Ma, while RAs range from 54–64, 47–52 and 53–54Ma, respectively. XCA calculations produce tighter age ranges for these samples of 52.5–56, 45.5–48.5 and 49–52.5Ma, respectively. In an apparent age vs ICT or %2M 1illite plot, authigenic illite grains show a slope that is in general slightly positive for TGA, slightly negative for RA, but close to zero for XCA, with thinner crystallites showing more dispersion than thicker ones. In order to test if dispersion is due to a different formation history or the result of retention capability, degassing spectra were modeled for site XCA averages and overall XCA average. Modeling shows that local site age average best match the measured spectra, instead of a global average age, indicating that illite growth reflects local deformation, and is not the result of regional metamorphism. Modeling also shows that Ar-degassing spectra are very sensitive to grain size, such that age interpretation based on Ar-plateaus is meaningless for most fine-grained clays.

Improved U–Th dating of carbonates with high initial 230Th using stratigraphical and coevality constraints

With high precision measurements now achievable by MC-ICPMS, the uncertainty on the initial 230Th content can be the major source of uncertainty for U–Th ages of carbonates. The initial 230Th content is usually derived from the 232Th content of the sample and the initial 230Th/232Th ratio calculated using isochron techniques or using stratigraphical constraints along the speleothem growth axis. These two methods are based on different hypotheses and have not been couple previously. Here, we present a new algorithm called STRUTages that combines stratigraphical and coevality constraints in order to obtain the best estimate on the initial 230Th/232Th ratio of each sample. STRUTages and isochron results are compared on a suite of speleothems and a coral core. Comparison of the U–Th and growth-band counted ages of the coral core demonstrates the validity of the STRUTages approach. An Octave (Matlab-compatible) script allowing the use of stratigraphical and coevality constraints is provided.

Geological uncertainty and geophysical inversion

Existing 3D geological systems are well adapted to high data-density environments, such as at the mine scale where abundant drill core exists, or in basins where 3D seismic provides stratigraphic constraints, but are poorly adapted to regional geological problems. There are three areas where improvements in the 3D workflow need to be made: in the handling of uncertainty; in the model building algorithms themselves; and in the interface with geophysical inversion. All 3D models are under-constrained, and at the regional scale this is especially critical for choosing modelling strategies. The practice of only producing a single model ignores the huge uncertainties that underlie model-building processes, and underpins the difficulty in providing meaningful information to end-users about the inh erent risk involved in applying the model to solve geological problems. Future studies need to recognize this and focus on the characterization of model uncertainty, spatially and in terms of geological features, and produce plausible model suites, rather than single models with unknown validity.

Zircons from the Acraman impact melt rock (South Australia): Shock metamorphism, U–Pb and 40Ar/39Ar systematics, and implications for the isotopic dating of impact events

This study presents the first optical and scanning electron microscopic characterization and U–Pb SHRIMP dating results for zircon grains separated from the most likely autochthonous impact melt rock in the central domain of the large, ∼40–90km eroded Ediacaran Acraman impact structure in South Australia. Microtextural characteristics define five zircon subtypes corresponding to different levels of progressive shock metamorphism, from virtually unshocked monocrystalline zircon grains that exhibit original magmatic zoning in cathodoluminescence images to fully granular zircons that have completely lost their primary zoning pattern and locally contain neocrystallized submicrometer-sized spots of ZrO2 (probably baddeleyite) that pseudomorph pre-impact zircon. The granular zircons correspond to the highest observed level of shock metamorphism and impact-induced recrystallization. ZrO2-bearing granular zircons indicate shock pressures in excess of ∼65–70GPa, which are considerably higher than previous shock pressure estimates for the Acraman impactites. U–Pb systematics of untreated and chemically abraded melt rock zircons indicate that U–Pb ratios of the Acraman zircons were variably reset during impact. Weakly shocked crystalline grains yield ages on concordia at ∼1.59–1.60Ga reflecting the magmatic age of the Gawler Range Volcanics. Only the entirely granular zircon population was apparently impact-reset, but based on an Ediacaran age from stratigraphic constraints on the ejecta layer, experienced significant post-impact Pb loss. The microcrystalline nature of granular zircons could have promoted Pb diffusion and α-recoil in post-impact time, as suggested by grain size-dependent diffusion and recoil modeling. A positive correlation of U concentration and shock level suggests that granularization might have preferentially occurred in initially U-rich, probably metamict, zircons. 40Ar/39Ar dating of a melted Yardea Dacite clast from the Acraman melt rock, as well as K-feldspar separated from shocked Yardea Dacite, resulted in post-impact alteration plateau ages suggestive of hydrothermal events at ∼500Ma and ∼450Ma that selectively affected the impactites exposed in the central domain of the Acraman impact structure. Our study demonstrates that the Acraman impact is particularly difficult to date. In the absence of accurate and precise isotopic ages for Acraman, the Ediacaran ejecta-stratigraphic age of ∼635–541Ma is considered the most reliable age constraint currently available for the timing of the large Acraman impact.

A Bayesian central equivalent dose model for optically stimulated luminescence dating

In this study, we propose and implement a Bayesian model to estimate a central equivalent dose from a set of luminescence measurements. This model is based on assumptions similar to the ones used in the standard statistical pipeline (typically implemented in the Analyst software followed by a subsequent central equivalent dose analysis) but tackles some of its main limitations. More specifically, it consists of a three-stage hierarchical model that has two main advantages over the standard approach: first, it avoids the introduction of auxiliary variables (typically mean and variance), at each step of the inference process, which are likely to fail to characterise the distributions of interest; second, it ensures a homogeneous and consistent inference with respect to the overall model and data. As a Bayesian model, our model requires the specification of prior distributions; we discuss such informative and non-informative distributions and check the relevance of our choices on synthetic data. Then, we use data derived from Single Aliquot and Regenerative (SAR) dose measurements performed on single grains from laboratory-bleached and dosed samples. The results show that our Bayesian approach offers a promising alternative to the standard one. Finally, we conclude by stressing that, relying on a Bayesian hierarchical model, our approach could be modified to incorporate additional information (e.g. stratigraphic constraints) that is difficult to formalise properly with the existing approaches.

Quaternary tephrochronology and deposition in the subsurface Sacramento–San Joaquin Delta, California, U.S.A.

We document characteristics of tephra, including facies and geochemistry, from 27 subsurface sites in the Sacramento–San Joaquin Delta, California, to obtain stratigraphic constraints in a complex setting. Analyzed tephra deposits correlate with: 1) an unnamed tephra from the Carlotta Formation near Ferndale, California, herein informally named the ash of Wildcat Grade (<~1.450 to >~0.780Ma), 2) the Rockland ash bed (~0.575Ma), 3) the Loleta ash bed (~0.390Ma), and 4) middle Pleistocene volcanic ash deposits at Tulelake, California, and Pringle Falls, Bend, and Summer Lake, Oregon, herein informally named the dacitic ash of Hood (<~0.211 to >~0.180Ma). All four tephra are derived from Cascades volcanic sources. The Rockland ash bed erupted from the southern Cascades and occurs in up to >7-m-thick deposits in cores from ~40m subsurface in the Sacramento–San Joaquin Delta. Tephra facies and tephra age constraints suggest rapid tephra deposition within fluvial channel and overbank settings, likely related to flood events shortly following volcanic eruption. Such rapidly deposited tephra are important chronostratigraphic markers that suggest varying sediment accumulation rates in Quaternary deposits below the modern Sacramento–San Joaquin Delta. This study provides the first steps in a subsurface Quaternary stratigraphic framework necessary for future hazard assessment.

Stratigraphic constraints for the upper Oligocene to lower Miocene Puchkirchen Group (North Alpine Foreland Basin, Central Paratethys)

Herein, we present a new stratigraphic model for the upper Oligocene to lower Miocene Puch kirchen Group which comprises deep-marine deposits in the central North Alpine Foreland Basin. Stratigraphic constraints are inferred from the integration of new bio(calcareous nannoplankton) and chemostratigraphic (δCbulk) data from the drill-site Hochburg 1. The first occurrences of Helicosphaera carteri and H. ampliaperta are recorded in the section and indicate a stratigraphic range from nannoplankton zones NP25 to upper NN2/ CNO6 to CNM4. The correlation with the upper Chattian to lower Burdigalian is strongly supported by the δCbulk record that mirrors major trends in global reference records between ~ 24.2 Ma and ~ 20.2–19.8 Ma. Seismic surveys suggest that at least 100 m of sediment have been eroded from the top of the Puchkirchen Group at Hochburg 1, and deposition extended even further into the Burdigalian. Within the Puchkirchen Group, the boundary between the Lower and Upper Puchkirchen formations occurs at ~ 22.2–22 Ma. The new age model for Hochburg 1 significantly improves previous stratigraphic concepts of the Puchkirchen Group and resolves some long-lasting stratigraphic issues in the study area. The implications of the new correlation for analyses of sedimentary budget, subsidence rates, sequence stratigraphy and the regional stage concept challenge our understanding of the development of the North Alpine Foreland Basin as well as the Central Paratethys.

Refining the archaeomagnetic dating curve for the Near East: new intensity data from Bronze Age ceramics at Tell Mozan, Syria

Uncertainty in radiocarbon dates for the Near East, caused by a bimodal distribution of ages due to the natural fluctuations of 14C in the atmosphere, has demonstrated the need for an alternative absolute dating technique to aid in the construction of site chronologies. Here we present a new archaeointensity reference curve model for the first three millennia BCE for the Levant (Syria, Israel, Jordan) for use in archaeomagnetic dating and contribute twelve new intensity results to an increasingly dense geomagnetic field record for the period between 2400 and 1200BCE in the Near East. Archaeomagnetic analysis was conducted on ceramic samples (i.e. pottery sherds) from seven sequential and well-constrained occupational layers at the site of Tell Mozan (Bronze Age Urkesh) in northeastern Syria, resulting in a 90% success rate by specimen (n = 42) for archaeointensity determination and an 86% correspondence between the model and the archaeologically derived dates within one standard deviation (1σ). Age standard deviations as low as ±24 years were obtained after integration with stratigraphic constraints. We also outline the techniques and sampling procedures of archaeomagnetic dating in a manner suitable for the non-paleomagnetist while detailing methodology for archaeomagnetic researchers.

Cenozoic epeirogeny of the Arabian Peninsula from drainage modeling

It is generally accepted that the Arabian Peninsula has been uplifted by subcrustal processes. Positive residual depth anomalies from oceanic crust in the Red Sea and in the Gulf of Aden suggest that a region surrounding this peninsula is dynamically supported. Admittance calculations, surface wave tomography studies, and receiver function analyses all imply that regional topography is generated and maintained by some combination of mantle convective circulation and lithospheric thickness changes. Despite these significant advances, the spatial and temporal uplift rate history of the Arabian Peninsula is not well known. Here we show that a regional uplift rate history can be obtained by jointly inverting 225 longitudinal river profiles that drain this peninsula. Our strategy assumes that shapes of individual river profiles are controlled by uplift rate history and moderated by erosional processes. We used local measurements of incision rate to calibrate the relevant erosional parameters. In our inverse algorithm, uplift rate is permitted to vary smoothly as a function of space and time but upstream drainage area remains invariant. We also assume that knickzone migration is not lithologically controlled. Implications of these important assumptions have been investigated. Our results suggest that the Arabian Peninsula underwent two phases of asymmetric uplift during the last 20–30 Ma at rates of 0.05–0.1 mm a−1. The southwestern flank of the peninsula has been uplifted by 1.5–2.5 km. Regional stratigraphic constraints, the age and composition of volcanism, paleosol formation, incised peneplains, emergent marine terraces, and thermochronometric measurements corroborate our calculated patterns of uplift. Progressive development of three domal swells along the western margin of the peninsula is consistent with localized upwelling of hot asthenospheric mantle.

Facies analysis of a Pliocene river‐dominated deltaic succession (Siena Basin, Italy): Implications for the formation and infilling of terminal distributary channels

AbstractThe recent recognition and description of terminal distributary channels in ancient delta‐front deposits have led to important progress when it comes to understanding: delta construction processes; the plan‐view shapes of both modern and ancient river‐dominated deltas; and sediment dispersal within deltas. Deltaic deposits fed by terminal distributary channels can be observed in the extensive outcrops (approximately up to 300 m wide and 50 to 70 m thick) in the central sector of the Pliocene Siena Basin in Tuscany, Italy. Stratigraphic constraints indicate that the deposition occurred in relatively shallow marine settings, was not affected by tidal effects and was influenced only locally by wave action. As a consequence, these rock exposures provide an opportunity to investigate the facies assemblage of an ancient delta complex, which was built up almost exclusively as the result of river‐related processes. The studied deltaic succession is composed of vertically stacked parasequence‐like units, which are invariably characterized by the lack of coarse‐grained distributary channel deposits at the top and an extensive network of terminal distributary channels that controlled the deltaic deposition. These deposits were interpreted previously as deep‐water turbidites. This study provides new elements for the correct distinction between hyperpycnal‐dominated delta fronts and basinal turbidite deposits. The terminal distributary channel facies documented in this study are more complex than those investigated in previous research. The channels are often passively filled during abandonment phases, and not just due to the aggradation and lateral/upstream migration of mouth bars, as classically proposed. Moreover, the frequent occurrence of mudstone beds just above the basal channel scours may be indicative of an abandonment phase after the channel incision, and before its infilling, with subsequent implications for the formational and infilling mechanisms of terminal distributary channels. Furthermore, the channel fills are often the result of alternating depositional and erosional phases, leading to a complex stratigraphic arrangement.

Unravelling polyphase brittle tectonics through multi-software fault-slip analysis: The case of the Voltri Unit, Western Alps (Italy)

We have analyzed the brittle structures in a key area of the Alps–Apennines transition zone (NW-Italy): here two orogens have interfered with each other since Oligocene times, producing a complex structural evolution and a heterogeneous fault population. Only relative chronologies can be reconstructed as stratigraphic constraints are lacking. We have performed the inversion of fault-slip data with two softwares for paleostress calculations, combined with field observations at selected structural stations and photo-interpretation. The resulting incompatible stress tensors can be grouped in this sequence: i) a strike-slip tensor, with NNW–SSE trending σ1 (Event A); ii) a strike-slip tensor, with NE–SW trending σ1 (Event B) and iii) an extensional/transtensional tensor, with NW–SE or NE–SW trending σ3 (Event C). We correlated our results with structures of known age from adjacent areas. Event A is possibly Rupelian–early Chattian, linked to far-field incipient rifting in the future Ligurian-Provençal basin. Event B fits Oligo–Miocene shortening: the faults may belong to a sinistral strike-slip zone that accommodated the oblique component of deformation during the rotation of the Corsica–Sardinia block. Event C is attributed to a Pliocene/Quaternary? (neotectonic) event. Therefore combining different inversion procedures with a detailed structural analysis has successfully unravelled the polyphase brittle tectonics.

A geochemical perspective on Parisian urban history based on U–Th dating, laminae counting and yttrium and REE concentrations of recent carbonates in underground aqueducts

Two laminated carbonate deposits from the wall of an underground aqueduct, built between the 13th and 15th century, in Paris, France, were studied to provide historical information about nearby human occupation and urban development. To obtain a robust chronology for these small (29 and 45 mm thick) and very young deposits (< 800 a based on the probable date of aqueduct construction), we adopted two methods: laminae counting and U–Th analysis. For laminae, we tested the hypothesis that their deposition is bi-annual. Concerning U–Th dating, the influence of inherited 230Th on calculated ages is discussed and detrital 230Th/232Th values for each speleothem was determined using stratigraphic constraints and found to be significantly different despite their close proximity. In these speleothems, that precipitated over approximately 300 a according to U–Th dating, the number of laminae is similar, and two laminae were deposited each year. The lamina-counting chronology was used to interpret the rare earth elements and yttrium content (REE + Y) variations measured by ICP-QMS as historical variations in water quality in relation with the anthropogenic use of these elements. There is a marked increase of the REE + Y concentrations since the second half of the twentieth century. A marked negative Ce anomaly in the NASC-normalized REE trend is evidenced, and is more pronounced for post 1850s. This anomaly could be due to prior precipitation of Ce4+ before the water reached the aqueduct, related to a high organic matter content of the water in oxidizing conditions. The increase in the Ce anomaly could indicate a second source of water, from a wastewater collection system or a modern water conveyance system. We thus demonstrate the interest of using these annual carbonate deposits in urban areas as a proxy for the history of urbanization or human activities.

Evidence for late Alpine tectonics in the Lake Garda area (northern Italy) and seismogenic implications

Research Article| January 01, 2015 Evidence for late Alpine tectonics in the Lake Garda area (northern Italy) and seismogenic implications Giancarlo Scardia; Giancarlo Scardia 1Istituto di Geologia Ambientale e Geoingegneria, CNR, via Salaria km 29.300, I-00016 Monterotondo Scalo (Roma), Italy †Current address: Instituto Oceanográfico, Universidade de São Paulo, Praça do Oceanográfico 191, 05508-120 São Paulo, SP, Brazil; scardia@usp.br. Search for other works by this author on: GSW Google Scholar Andrea Festa; Andrea Festa 2Dipartimento di Scienze della Terra, Università di Torino, Via Valperga Caluso 35, I-10125 Torino, Italy Search for other works by this author on: GSW Google Scholar Giovanni Monegato; Giovanni Monegato 3Istituto di Geoscienze e Georisorse, CNR, via Valperga Caluso 35, I-10123 Torino, Italy Search for other works by this author on: GSW Google Scholar Roberta Pini; Roberta Pini 4Istituto per la Dinamica dei Processi Ambientali, CNR, Piazza della Scienza 1, I-20126 Milano, Italy Search for other works by this author on: GSW Google Scholar Sergio Rogledi; Sergio Rogledi 5ENI E&P, via Emilia 1, I-20097 San Donato Milanese, Italy Search for other works by this author on: GSW Google Scholar Fabrizio Tremolada; Fabrizio Tremolada 6RPS Energy, Goldsworth House, Denton Way, Goldsworth Park, GU213LG Woking, UK Search for other works by this author on: GSW Google Scholar Fabrizio Galadini Fabrizio Galadini 7Istituto Nazionale di Geofisica e Vulcanologia, via di Vigna Murata 605, I-00143 Roma, Italy Search for other works by this author on: GSW Google Scholar Author and Article Information Giancarlo Scardia †Current address: Instituto Oceanográfico, Universidade de São Paulo, Praça do Oceanográfico 191, 05508-120 São Paulo, SP, Brazil; scardia@usp.br. 1Istituto di Geologia Ambientale e Geoingegneria, CNR, via Salaria km 29.300, I-00016 Monterotondo Scalo (Roma), Italy Andrea Festa 2Dipartimento di Scienze della Terra, Università di Torino, Via Valperga Caluso 35, I-10125 Torino, Italy Giovanni Monegato 3Istituto di Geoscienze e Georisorse, CNR, via Valperga Caluso 35, I-10123 Torino, Italy Roberta Pini 4Istituto per la Dinamica dei Processi Ambientali, CNR, Piazza della Scienza 1, I-20126 Milano, Italy Sergio Rogledi 5ENI E&P, via Emilia 1, I-20097 San Donato Milanese, Italy Fabrizio Tremolada 6RPS Energy, Goldsworth House, Denton Way, Goldsworth Park, GU213LG Woking, UK Fabrizio Galadini 7Istituto Nazionale di Geofisica e Vulcanologia, via di Vigna Murata 605, I-00143 Roma, Italy Publisher: Geological Society of America Received: 04 Sep 2013 Revision Received: 12 May 2014 Accepted: 24 Jun 2014 First Online: 08 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 © 2014 Geological Society of America GSA Bulletin (2015) 127 (1-2): 113–130. https://doi.org/10.1130/B30990.1 Article history Received: 04 Sep 2013 Revision Received: 12 May 2014 Accepted: 24 Jun 2014 First Online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Giancarlo Scardia, Andrea Festa, Giovanni Monegato, Roberta Pini, Sergio Rogledi, Fabrizio Tremolada, Fabrizio Galadini; Evidence for late Alpine tectonics in the Lake Garda area (northern Italy) and seismogenic implications. GSA Bulletin 2015;; 127 (1-2): 113–130. doi: https://doi.org/10.1130/B30990.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract We investigated the recent evolution of the Po Plain–Alps system by integrating subsurface geophysical data from the Po Plain with new stratigraphic and structural observations from the Southern Alps margin. Inversion of structural data and chronology provided by stratigraphic constraints led to the definition of three tectonic events since the Pliocene, namely, the intra-Zanclean, the Gelasian, and the Middle Pleistocene, driven by an axis of maximum compression formerly oriented NE (intra-Zanclean) and then to the NNW (Gelasian and Middle Pleistocene). The associated deformation has been accommodated by two sets of faults consisting of NNE-trending thrust faults, mostly represented in the western sector of Lake Garda, and NW-trending strike-slip faults, observed in the southern and eastern sectors. The interplay between these two sets of faults is interpreted to produce short (<10 km length) thrust ramps activated in left transpression, bounded by longer (30–60 km) transfer faults activated in a right-lateral strike-slip motion. Based on this structural model, we infer moderate seismicity (Mw < 6) associated with the NNE-directed thrusts and stronger earthquakes (also Mw > 6.5) along the NW-trending strike-slip faults. In this framework, the newly defined Nogara fault and the Sant’Ambrogio fault, all pertaining to the NW-trending system, are regarded as potential candidates for the seismogenic source of the January A.D. 1117 event, the most destructive earthquake in the Po Plain. 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Geochronologic and stratigraphic constraints on the Mesoproterozoic and Neoproterozoic Pahrump Group, Death Valley, California: A record of the assembly, stability, and breakup of Rodinia

Research Article| May 01, 2014 Geochronologic and stratigraphic constraints on the Mesoproterozoic and Neoproterozoic Pahrump Group, Death Valley, California: A record of the assembly, stability, and breakup of Rodinia Robert C. Mahon; Robert C. Mahon 1Idaho State University Department of Geosciences, 921 S. 8th Ave., Stop 8072, Pocatello, Idaho 83209-8072, USA †Current address: University of Wyoming Department of Geology and Geophysics, Department 3006, 1000 University Ave., Laramie, Wyoming 82071, USA; e-mail: mahorobe@isu.edu; rmahon1@uwyo.edu. Search for other works by this author on: GSW Google Scholar Carol M. Dehler; Carol M. Dehler 2Utah State University Department of Geology, 4505 Old Main Hill, Logan, Utah 84322-4505, USA Search for other works by this author on: GSW Google Scholar Paul K. Link; Paul K. Link 1Idaho State University Department of Geosciences, 921 S. 8th Ave., Stop 8072, Pocatello, Idaho 83209-8072, USA Search for other works by this author on: GSW Google Scholar Karl E. Karlstrom; Karl E. Karlstrom 3University of New Mexico Department of Earth and Planetary Sciences, MSCO3-2040, Albuquerque, New Mexico 87131, USA Search for other works by this author on: GSW Google Scholar George E. Gehrels George E. Gehrels 4University of Arizona Department of Geosciences, 1040 E. 4th St., Tucson, Arizona 85721, USA Search for other works by this author on: GSW Google Scholar GSA Bulletin (2014) 126 (5-6): 652–664. https://doi.org/10.1130/B30956.1 Article history received: 07 Jul 2013 rev-recd: 03 Dec 2013 accepted: 08 Jan 2014 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Robert C. Mahon, Carol M. Dehler, Paul K. Link, Karl E. Karlstrom, George E. Gehrels; Geochronologic and stratigraphic constraints on the Mesoproterozoic and Neoproterozoic Pahrump Group, Death Valley, California: A record of the assembly, stability, and breakup of Rodinia. GSA Bulletin 2014;; 126 (5-6): 652–664. doi: https://doi.org/10.1130/B30956.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Pahrump Group in the Death Valley region of eastern California records a rich history of Mesoproterozoic to Neoproterozoic tectonic, climatic, and biotic events. These include the formation, stability, and onset of rifting of the Rodinia supercontinent, two potentially low-latitude glaciations correlative with global “snowball Earth” glacial intervals, and the onset of complex microbiota (e.g., testate amoebae). Poor direct age control, however, has significantly hindered the progress of understanding of these important stratigraphic units. New LA-ICPMS (laser ablation–inductively coupled plasma mass spectrometry) detrital zircon data from clastic units directly overlying a major unconformity within the Mesoproterozoic Crystal Spring Formation provide a maximum depositional age of 787 ± 11 Ma for the upper member of the Crystal Spring Formation. This unconformity, representing a duration of ≥300 Ma, is now recognized in sedimentary successions across southwestern Laurentia. These new age data, in addition to the distinct stratigraphic style above and below the unconformity, result in the proposed formal stratigraphic revision to elevate the upper member of the Crystal Spring Formation to the Neoproterozoic Horse Thief Springs Formation and separate it from the remainder of the underlying Mesoproterozoic Crystal Spring Formation (ca. 1100 Ma). New age relations and revised stratigraphic nomenclature significantly clarify stratigraphic and tectonic correlations and imply ca. 1250–1070 Ma assembly, 1070–780 Ma stability, and 780–600 Ma breakup of the supercontinent Rodinia along the southwestern Laurentian margin. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Environmental change across a terrestrial Cretaceous-Paleogene boundary section in eastern Montana, USA, constrained by carbonate clumped isotope paleothermometry

The Cretaceous-Paleogene (K-Pg) mass extinction has been attributed to the impact of a large bolide at the end of the Cretaceous Period, although other potential causes have also been proposed, most notably climate change caused by Deccan Traps (India) flood volcanism. Reconstructing paleoclimate, particularly in terrestrial settings, has been hindered by a lack of reliable proxies. The recent development of carbonate clumped isotope paleothermometry has contributed to temperature reconstructions using geochemical proxies in terrestrial settings. We employ this method, along with new stratigraphic constraints, in the Hell Creek (Cretaceous) and overlying Fort Union (Paleogene) Formations (Montana, USA) to examine changes in temperature leading to and across the K-Pg boundary. We demonstrate that well-preserved ca. 66 Ma aragonitic bivalves serve as suitable paleoclimate archives. Although there are limitations in the stratigraphic availability of fossil bivalves for clumped isotope analysis, we record an apparent 8 °C decrease in summer temperatures over the last 300 k.y. of the Cretaceous that corresponds broadly with patterns observed in other paleotemperature proxies. This observed decrease plausibly could be explained by an absolute temperature decrease or by other environmental stresses on the organisms, but in either case suggests changing living conditions over the interval. Previously documented declines in vertebrate and invertebrate biodiversity occur over the same stratigraphic interval at this study location. These results are consistent with published models of the end-Cretaceous mass extinction in which destabilized ecosystems become more susceptible to an abrupt event like a bolide impact.

Pliocene–Quaternary subsidence and exhumation of the southeastern San Joaquin Basin, California, in response to mantle lithosphere removal

Thermomechanical models of mantle lithosphere removal from beneath the southern Sierra Nevada region, California (USA), predict a complex spatiotemporal pattern of vertical surface displacements. We evaluate these models by using (U-Th)/He thermochronometry, together with other paleothermometry estimates, to investigate such topographic transients. We target Tertiary strata from the Kern arch, a crescent-shaped active uplift located in the southeastern San Joaquin Basin, along the western flank of the southern Sierra Nevada. Kern arch stratigraphy provides a unique record of subsidence and exhumation in a sensitive region immediately adjacent to the delaminating mantle lithosphere at depth. Detrital apatite (U-Th)/He ages from Oligocene–Miocene sandstones collected in Kern arch well cores indicate postdepositional heating to temperatures beyond those corresponding with their present burial depths. When integrated with available geologic and stratigraphic constraints, temperature-time modeling of thermochronometric data suggests partial He loss from apatites at temperatures of 70–90 °C, followed by exhumation to present burial temperatures of 35–60 °C since ca. 6 Ma. By constraining the late Cenozoic geothermal gradient to ∼25 °C/km, our results imply 1.0–1.6 km of rapid (∼0.4 mm/yr) subsidence and sedimentation, and then subsequent uplift and exhumation of southeastern San Joaquin Basin strata in latest Miocene–Quaternary time. Stratigraphic and geomorphic relations further constrain the principal burial episode to ca. 2.5 Ma or later, and exhumation to ca. 1 Ma or later. Subtle differences in the maximum temperatures achieved in various wells may reflect differing degrees of tectonic subsidence and sedimentation as a function of growth faulting and distance from the range front. Our results are consistent with estimates of surface subsidence and uplift from Sierran delamination models, which predict a minimum of ∼0.7 km of tectonic subsidence in regions retaining mantle lithosphere adjacent to the area of delamination, and a minimum of ∼0.8 km of rock uplift in regions where delamination occurred recently. We attribute the marked pulse of tectonic subsidence in the San Joaquin Basin to viscous coupling between the lower crust and a downwelling mass in the delaminating slab. The ensuing episode of exhumation is interpreted to result from the northwestward peeling back of the slab and the associated replacement of dense lithosphere with buoyant asthenosphere that drove rapid rock and surface uplift.

Contribution of stratigraphic constraints of Bulgarian prehistoric multilevel tells and a comparison with archaeomagnetic observations

The general aim of archaeomagnetism is to recover the behavior of the ancient geomagnetic field for a given territory using archaeological baked clay structures that have been well-dated by other independent methods. The Sofia Palaeomagnetic laboratory possesses a large data set for the three characteristic geomagnetic field parameters obtained from one and the same material and covering the last 8000 years. The database accumulated by the authors has been updated several times and the greatest difficulty arose for the prehistoric collections, most of which had no radiocarbon or other chronometric dates. Thus as well as methodological archaeomagnetism problems, the main difficulty concerning the construction of reference curves of the variation of the geomagnetic field is related to the distribution of the experimental results over the absolute time scale. This paper examines the similarity between the archaeomagnetic profiles obtained on multilevel tells from the same prehistoric epochs. It would seem that when 14C dates for multilayer tells are missing, the comparison of such archaeomagnetic stratigraphic profiles with the corresponding chrono-stratigraphic reference profile for the same epoch can be useful in verifying their assumed chronology. However, the described peculiarities connected with the multilevel sites results still show some difficulties for the overall estimation of geomagnetic field variations for a given territory.

Effect of early Pliocene uplift on late Pliocene cooling in the Arctic–Atlantic gateway

Despite the undisputed role of the Arctic Ocean in the modern and Pliocene climate system, the Arctic has only recently attracted public awareness that ongoing, fundamental change in the Arctic cryosphere could be a response to global warming. Clarification of the Arcticʼs role in global climate during the Pliocene is, however, largely hampered by equivocal stratigraphic constraints. From a well-dated Pliocene sequence from the Yermak Plateau, off NW Spitsbergen, we present sedimentological and geochemical data indicating that 4 million years ago terrigenous sediment supply and sources changed abruptly in response to a regional tectonic uplift event. We argue that this event together with contemporary uplift and tilting along the northwestern European continental margin preconditioned the landmasses for glacial ice build-up during intensification of the Northern Hemisphere Glaciation (INHG). Our data further suggest that the final deepening/widening of the Arctic–Atlantic gateway, the Fram Strait, between 6.5 and 5 Ma gradually caused increased deep-water mass exchange which, in turn, likely contributed to the intensification of the North Atlantic thermohaline circulation. Coupled to the North Atlantic warm pool as a regional moisture source, declining atmospheric CO2 levels and other feedback mechanisms during the Pliocene, the regional tectonic activities in the high northern latitudes caused decreased summer ablation and thus allowed the initial build-up of glacial ice both in Scandinavia, and the sub-aerially exposed Svalbard/Barents Sea, culminating in the first large-scale coastline-shelf edge glaciations at ∼2.75Ma ago.