Journal Of The Geological Society Research Articles

Core surprise: What's inside a plate boundary?

Despite the fact that 90% of global seismicity occurs at plate boundary faults, our understanding of their internal structure is lacking. It’s not easy to see inside a plate boundary fault – typically composed of a high-strain fault core surrounded by a fractured damage zone – and when we can, it often requires expensive drilling projects that yield limited information on the internal structure of the whole fault. Understanding the internal structure of large faults is crucial, because their chemical and mechanical properties control how and where earthquakes rupture, nucleate and propagate. This in turn limits the size of the earthquake or the amount of radiated seismic energy, and consequently the severity of surface damage. The 1999 magnitude 7.7 earthquake along the Chelungpu plate boundary fault, for example – the second deadliest earthquake in Taiwan’s recorded history – saw significant variations in slip and ground motion at different locations along the fault which resulted in large local variations in casualties and damage. Subsequent field investigations related these variations to changes in the fault’s structure (i.e., clay width, geometry), which in turn controlled how the fault moved.

Reply to Discussion on ‘Breakup continents at magma poor rifted margins: a seismic v. outcrop perspective’. Journal of the Geological Society, London , 175, 875-882

Dr. Giancarlo Molli constructively replied to our paper ‘Breakup continents at magma poor rifted margins: a seismic v. outcrop perspective’ (Molli 2020) with a twofold argument (i) criticizing the collection of data, and, more importantly, (ii) questioning our discussion on the syn-tectonic behavior of basalts in the study area. 1. Regarding the first point, G. Molli produced a new, high quality interpretative map and a set of retro-deformed sections of the Levanto-Bonassola area to support the idea that Alpine deformation was underestimated by our work. At least three high-quality, detailed maps of the area were published (Decandia and Elter 1969; Barrett 1982; Cortesogno et al. 1987) and, in addition, there are a number of maps produced in the frame of PhD thesis (among which Molli 1995) that are, at present, not yet published. The fact that none of these maps are perfectly identical one to each other does not surprise any field geologist, as each geological map mainly derives from the balance of missing data (cover) and outcrops, especially in highly-anthropized areas, as is the case for the Levanto-Bonassola area. For example, if the maps of Cortesogno et al. (1987) and Decandia and Elter (1969) are compared for the area north of the Bonassola village, it can be seen that different solutions were proposed introducing or not prominent Alpine structures. This example is striking, in particular if one considers the period in which these distinguished geologists worked, because it shows how different maps gave priority to specific primary or late features that were often functional to the proposed framework. For what concerns our work, we wish to stress the concept that the map in Decarlis et al. (2018) was functional to a broader discussion and we therefore highlighted primary structures. The choice of prioritizing some observations (nature of contacts …

Reply to Discussion on ‘The Mesoproterozoic Stac Fada proximal ejecta blanket, NW Scotland: constraints on crater location from field observations, anisotropy of magnetic susceptibility, petrography and geochemistry’, Journal of the Geological Society , 176, 830–846

Simms (2019) states in his discussion: > They dismiss the Lairg Gravity Low as a possible impact crater based on palaeoenvironments, yet the cited paper specifically states ‘the Stoer Group has nothing in common with any of the Moine sequences discussed here' (Friend et al. 2003, p. 254). A direct comparison between the Stoer Group and the Moine Supergroup was never intended; instead the point we made (Amor et al. 2019, p. 844) was that earlier meta-sediments of admittedly uncertain age, as found at the Shin inlier, and over which the much later Moinian sediments were thrust, can be interpreted as marine in origin (Conclusion 1 of Friend et al. 2003), and thus quite different from the sedimentary matrix of the Stac Fada impact debris, which resembles the Stoer Group sediments. These earlier meta-sediments (now metapelites, psammites and marble) lie on a Lewisian-type crustal basement that is located to the east of the present-day outcrop of the Stoer Group, in the vicinity of Lairg, and are presumed to have been thrust an unknown distance from the east during the emplacement of the Moine thrust complex. All geological and plate reconstructions for this time place the deposition of the Stoer Group sediments in a rift valley on the eastern or southeastern margin of Laurentia (Kinnaird et al. 2007; Dalziel 2010). Furthermore, Nicholson (1993), Rainbird et al. (2001), Williams & Foden (2011), Kinnaird et al. (2007), Krabbendam & Rainbird (2012) and Lelpi et al. (2016) agree that this was a discrete and isolated rift, with significant high ground of Lewisian gneiss to the east as evidenced by deeply cut canyons and current direction towards the west (Stewart 2002), although Simms & Ernstson (2019) infer an extension of the Stoer Group deposits much further to the east. While …

Reply to Discussion on ‘A reassessment of the proposed ‘Lairg Impact Structure’ and its potential implications for the deep structure of northern Scotland’. Journal of the Geological Society, London , 176, 817-829

Butler and Alsop provide a clear and thorough explanation as to why our suggestion (Simms & Ernstson 2019), that the Lairg Gravity Low might be a buried impact crater translocated from further east, is incompatible with what is known of the structure of northern Scotland, and I concede that in this particular scenario they are probably right. My original paper (Simms 2015) was based largely on field observations that, amongst other things, indicated that the Stac Fada Member impact ejecta preserved at outcrop had come from the east (despite claims to the contrary by others; Amor et al. 2008, 2019). When subsequently I became aware of the LGL I realized that its …

Discussion on ‘A reassessment of the proposed ‘Lairg impact structure’ and its potential implications for the deep structure of northern Scotland’ Journal of the Geological Society, London, 176, 817-829

The interpretation of the Stac Fada Member of the Stoer Group of NW Scotland as a proximal ejecta blanket of late Mesoproterozoic age (Amor et al. 2008) has led to the search for the impact crater from which it originated. One prominent suggestion has been that this crater lies buried beneath east-central Sutherland, broadly centred on the village of Lairg (Fig. 1; Simms 2015). Simms & Ernstson (2019) develop this suggestion by interpreting the well-known negative anomaly that dominates the Bouguer gravity map of northern Scotland to represent the roots of this crater. However, they qualify their interpretation, noting that the geology at the Stac Fada outcrops implies a more distant crater location, and go on to propose hitherto unreported ‘crustal shortening structures' to explain this discrepancy. In doing so, they challenge current understanding of structural evolution in the Moine Thrust Belt and for crustal structure in NW Scotland. The purpose of this discussion is to demonstrate not only that the Simms & Ernstson (2019) proposals are incompatible with the known structural geology of northern Scotland, but that the geology of the Moine Thrust Belt renders the Lairg site to be one of the very few places that can be regarded as a highly unlikely source for the Stac Fada impact rocks. Internal Earth processes, rather than extra-terrestrial interventions, should be sought as explanations for the ‘Lairg gravity low'. A more general aim is to show, through this discussion, that making small changes to tectonic models commonly carries unintended consequences for geological interpretations that can go unrecognized if only illustrated on unscaled sketches. Fig. 1. Simplified geological map of northern Scotland, with locations discussed in the text, together with the location of the contested ‘Lairg impact crater’. X–Y is the location of the section line for Figure 2a. As recognized …

Salt thickness and composition influence rift structural style, northern North Sea, offshore Norway

Abstract“Salt” giants are typically halite‐dominated, although they invariably contain other evaporite (e.g. anhydrite, bittern salts) and non‐evaporite (e.g. carbonate, clastic) rocks. Rheological differences between these rocks mean they impact or respond to rift‐related, upper crustal deformation in different ways. Our understanding of basin‐scale lithology variations in ancient salt giants, what controls this and how this impacts later rift‐related deformation, is poor, principally due to a lack of subsurface datasets of sufficiently regional extent. Here we use 2D seismic reflection and borehole data from offshore Norway to map compositional variations within the Zechstein Supergroup (ZSG) (Lopingian), relating this to the structural styles developed during Middle Jurassic‐to‐Early Cretaceous rifting. Based on the proportion of halite, we identify and map four intrasalt depositional zones (sensu Clark et al., Journal of the Geological Society, 1998, 155, 663) offshore Norway. We show that, at the basin margins, the ZSG is carbonate‐dominated, whereas towards the basin centre, it becomes increasingly halite‐dominated, a trend observed in the UK sector of the North Sea Basin and in other ancient salt giants. However, we also document abrupt, large magnitude compositional and thickness variations adjacent to large, intra‐basin normal faults; for example, thin, carbonate‐dominated successions occur on fault‐bounded footwall highs, whereas thick, halite‐dominated successions occur only a few kilometres away in adjacent depocentres. It is presently unclear if this variability reflects variations in syn‐depositional relief related to flooding of an underfilled presalt (Early Permian) rift or syn‐depositional (Lopingian) rift‐related faulting. Irrespective of the underlying controls, variations in salt composition and thickness influenced the Middle Jurassic‐to‐Early Cretaceous rift structural style, with diapirism characterising hangingwall basins where autochthonous salt was thick and halite‐rich and salt‐detached normal faulting occurring on the basin margins and on intra‐basin structural highs where the salt was too thin and/or halite‐poor to undergo diapirism. This variability is currently not captured by existing tectono‐stratigraphic models largely based on observations from salt‐free rifts and, we argue, mapping of suprasalt structural styles may provide insights into salt composition and thickness in areas where boreholes are lacking or seismic imaging is poor.

The first occurrence of Phlebopteris dunkeri and P. woodwardii (Matoniaceae) from the middle Jurassic of Iran

The coal horizons of the Middle Jurassic Hojedk Formation of East-Central Iran are fossiliferous, bearing numerous well-preserved fossil plants, comparable to the Shemshak Group/Formation in the Alborz Mountains in Northern Iran. Here we present two recently discovered fossil taxa from the Middle Jurassic Hojedk Formation of the Tabas Block (Central Iran). Based on their distinct morphologies, impression/compression specimens can be assigned to Phlebopteris dunkeri (Schenk) Schenk (Palaeontographica 23: 157–163, 1875) and P. woodwardii Leckenby (Quarterly Journal of the Geological Society 20 (1–2): 74–82, 1864), belonging to Matoniaceae. This is the first record of these two taxa in Iran. Taxa belonging to Equisetaceae, Marattiaceae, Dipteridaceae, Schizaeaceae, Dicksoniaceae, Caytoniales, Bennettitales, Cycadales and Podozamitaceae were observed as accompanying taxa in the studied section. Ecology comparison of the Hojedk Formation plant fossils with the extant relatives of the fossil plant taxa occurring in these deposits indicates accumulation of the host strata under a moist warm (tropical to subtropical) climate during the Middle Jurassic.

Erratum for ‘The provenance of the Devonian Old Red Sandstone of the Dingle Peninsula, SW Ireland; the earliest record of Laurentian and peri-Gondwanan sediment mixing in Ireland,’ Journal of the Geological Society, London, 175, 411–424

Samples in this paper have been assigned formations based on the Geological Survey of Ireland shapefile released prior to the commencement of the study. However, the authors were not aware that, since obtaining the samples, an updated shapefile had been released. This update affects three of the four apatite samples assigned to the Lower Devonian Ballymore Formation. The location of samples Mb-1, Mb-4 and Mb-5 now places them well within the undifferentiated, Upper Devonian Slieve Mish Group. As outlined in our paper, the apatite ages were originally produced concurrently with apatite fission track analysis and were later used in our study to provide additional provenance information in support of the detrital zircon geochronological data. In the second paragraph of the discussion section we say the following: et al. (1999) obtained an age of 411 Ma for the Cooscrawn Tuff Bed in the Ballymore Formation, which is older than 22 of the 70 detrital apatites analysed in this formation. The reassignment of the three samples to the Upper Devonian Slieve Mish Group nullifies the above statement. However, our interpretation that the depositional age of the Ballymore Formation is younger than the 411 Ma age given by Williams et al. (1999) is predominantly based upon the evidence given by the six youngest detrital zircons from the formation which underlies the Ballymore Formation (i.e. the Slea Head Formation). These zircons give a concordia age of 405 ± 4 Ma. This suggests that the Ballymore Formation was more than likely deposited after 409 Ma. We do not believe that the reassignment of the three samples has any major impact on our provenance interpretations. The ∼420 Ma age of the majority of the apatites in samples Mb-1, Mb-4 and Mb-5 actually fits with the range of Palaeozoic detrital zircons in sample AK-17 which was taken from the Slieve Mish Group, thereby supporting minor input of rocks affected by end-Scandian metamorphism.

Geometry, kinematics and regional significance of faulting and related lamprophyric intrusion in the mineralised zone at the Pu Sam Cap complex, Northwest Vietnam

Geometry, kinematics and regional significance of faulting and related lamprophyric intrusion in the mineralised zone at the Pu Sam Cap complex, Northwest Vietnam

Reply to the discussion on ‘Orbital calibration of the late Campanian carbon isotope event in the North Sea’, Journal of the Geological Society, London , 173, 504–517

Smith & Bailey (2017 a ) have criticized the methods applied for time series analysis in our study of a late Campanian chalk core in the North Sea (Perdiou et al. 2016). These critics rely essentially on two points: (1) the resolution spacing that can be applied to our gamma-ray data; (2) the use of the autoregressive order 1 (‘AR1’) noise model, which allows calculation of significance thresholds.

Reply to discussion on ‘Middle Jurassic shear zones at Cap de Creus (eastern Pyrenees, Spain): a record of pre-drift extension of the Piemonte–Ligurian Ocean?’ Journal of the Geological Society, London , 174, 289–300

We welcome the discussion by Elena Druguet, Jordi Carreras and Jochen Mezger (Druguet et al. 2017) of our paper (Vissers et al . 2017 b ) in which we report Jurassic 40Ar/39Ar ages from shear zones at Cap de Creus, NW Spain. The authors argue that our interpretation of these results and the inferred development of the Cap de Creus shear zones during Jurassic stretching and opening of the Piemonte Ligurian ocean contradicts structural and tectonic evidence, and that it is not sufficiently supported by geochronological data. Below we first consider their structural and tectonic arguments from the tectonic–palaeogeographical scale down to the smaller field scale, and then proceed to discuss our Ar/Ar results. Druguet et al. (2017) note that prior to the Oligo-Miocene rifting event the Corso-Sardinian block was fixed east of NE Iberia, hence that the rocks at Cap de Creus were not located at the Jurassic pre-drift continental margin and that the northern Cap de Creus shear zones could not have formed as normal faults related to continental rifting. This palaeogeographical argument builds on a previous hypothesis by Stampfli & Borel (2002), Stampfli et al. (2002) and Stampfli & Hochard (2009) in which Iberia, the Corso-Sardinian block and the Brianconnais domain formed one single microcontinent. This hypothesis, however, must be discarded on the basis of recent palaeomagnetic work on Sardinia by Advokaat et al. (2014) showing that between Late Jurassic and Eocene time Sardinia underwent no vertical-axis rotations relative to Eurasia and was fixed to Europe in a rotated position that results from post-Eocene deformation, whereas all available evidence indicates that Iberia was located much further west and underwent some 35° counterclockwise rotation during the Cretaceous (van Hinsbergen et al. 2017; Vissers et al . 2017 a ). Sardinia, …

Reply to the discussion on ‘A 2.3 Million Year Lacustrine Record of Orbital Forcing from the Devonian of Northern Scotland’ , Journal of the Geological Society, London 173, 474–488

Smith & Bailey (2017 a ) have raised concerns over the methodologies we applied for time series analysis in our study of the cyclic Devonian lacustrine deposits of northern Scotland (Andrews et al. 2016). Similar concerns have been raised on a number of occasions for similar studies (Bailey et al. 2009 on Kemp & Coe 2007; Smith & Bailey 2017 c on Howe et al. 2016; Smith et al. …

Discussion on ‘Middle Jurassic shear zones at Cap de Creus (eastern Pyrenees, Spain): a record of pre-drift extension of the Piemonte–Ligurian Ocean?’ Journal of the Geological Society, London , 174, 289–300

Vissers et al . (2017) performed muscovite 40Ar/39Ar geochronology on shear zones from the Northern Cap de Creus shear belt (Variscan basement of the eastern Pyrenees). They propose a model according to which these shear zones formed during Middle Jurassic extension associated with the opening of the Piemonte–Ligurian Ocean to the east. We argue that this model contradicts structural and tectonic evidence and is not sufficiently supported by geochronological data.

Reply to Discussion on ‘Tectonic and environmental controls on Palaeozoic fluvial environments: reassessing the impacts of early land plants on sedimentation’ Journal of the Geological Society, London , https://doi.org/10.1144/jgs2016-063

Reply to Discussion on ‘Tectonic and environmental controls on Palaeozoic fluvial environments: reassessing the impacts of early land plants on sedimentation’ <i>Journal of the Geological Society, London</i> , https://doi.org/10.1144/jgs2016-063

Erratum for ‘Retro-wedge foreland basin evolution along the ECORS line, eastern Pyrenees, France,’ Journal of the Geological Society , 173, 419–437

The authors would like to apologise for the following error on page 422 in the paragraph beginning ‘Lithologically, the Grey Flysch Group…’: In borehole MZ1 …

Reply to Discussion on ‘Late Cenozoic geological evolution of the northern North Sea: development of a Miocene unconformity reshaped by large-scale Pleistocene sand intrusion’, Journal of the Geological Society , 170, 133–145

Knowledge of small sand injection is far from new (Murchison 1827; Hurst et al . 2011) but that intrusive and extrusive bodies of sand can be more than 100 m thick and extend for kilometres is considered unlikely by Rundberg & Eidvin (2015), who discuss our paper (Loseth et al . 2013). Their discussion disagrees with our interpretation of the origin of these sand bodies and instead favours their previously proposed models (Eidvin & Rundberg 2001; Rundberg & Eidvin 2005). They claim that (1) the early Pleistocene sandstones are turbiditic sands deposited at the toe of the glaciomarine shelf clinoform foresets and not extrusive sands; (2) the Oligocene sandstones, located below mounds at the top Hordaland Group Unconformity, are in situ deposits representing turbiditic gravity sands shed from the Shetland Platform and not intrusive sands; and (3) the top Hordaland Group Unconformity in the northern North Sea was formed by submarine erosion and not during subaerial exposure. We have studied rock properties of Eocene to Pleistocene claystones and sandstones (Oygarden et al . 2015), carried out detailed seismic and well studies in the North Sea (Loseth et al . 2003, 2012, 2013), studied the physical processes of injection, extrusion and depletion of sand in analogue experiments (Rodrigues et al . 2009), and studied outcrops of intrusive sandstones through sand injection research consortiums (e.g. Hurst et al . 2006, 2011; Vigorito et al . 2008; Cartwright 2010; Scott et al . 2012). Our seismic and well observations above the Snorre and Visund fields were in conflict with existing geological models and therefore we proposed new models. This reply builds on knowledge of how the seismic expression of sandstone varies with the type of embedding claystone. Oygarden et al …

Discussion on ‘Late Cenozoic geological evolution of the northern North Sea: development of a Miocene unconformity reshaped by large-scale Pleistocene sand intrusion’, Journal of the Geological Society , 170, 133–145

In their recent paper, Loseth et al . (2013) propose a new model for the late Cenozoic evolution of the northern North Sea. They propose that during the early Pleistocene ( sensu ICS 2013), large volumes of sand were ejected from the Paleocene through the Eocene–Oligocene Hordaland Group, and deposited both as extrusive sand on the Pleistocene seafloor and as intrusive sand within the Oligocene section, c . 180 m below the seafloor. The paper builds on two earlier studies: (1) that by Loseth et al . (2012) in which the Pleistocene extrusive sand is presented in more detail; (2) that by Rodrigues et al . (2009) in which an experimental model related to such sand deposition is presented. The model of Loseth et al . (2013) is illustrated by a seismic section in their figure 12, showing intrusive sands within the Oligocene and extrusive sands within the Pleistocene. Details of this model, such as feeder dykes and blowout fissures that create ditches, are shown in their figure 8 (Visund Field) and figure 11 (Snorre Field). The geological events are summarized in their figure 16. As illustrated on a stratigraphic scheme (their figs 2 and 15), Loseth et al . interpret the Oligocene sands as being injected from a Paleocene parent sand. They propose that the mounds at the top of the Hordaland Group resulted from forced folding over the sand injectites, as exemplified in their figure 11. Furthermore, Loseth et al . interpret escarpments on the Top Hordaland Group Unconformity as cliff sections similar to those of southeastern England today and claim that the northern North Sea was subaerially exposed during a 10 myr Miocene time span. Loseth et al . (2012, 2013) not only challenge our previous turbiditic interpretation of the Pleistocene sand (Eidvin & …

Relationship between authors’ structural position in the collaboration network and research productivity

Purpose – The purpose of this paper is to compute and analyze the topological properties of co-authorship network formed between earth scientists in India. As a case study, the authors evaluate bibliographic data of authors who have contributed research articles in the Journal of the Geological Society of India, a premier earth science journal in India. Design/methodology/approach – Research articles totaling 3,903 records from 1970 to 2011 were harvested from the ISI Web of Science SCI database and analyzed using Social Network Analysis. Findings – The author productivity in terms of number of papers published followed Lotka's law with β=2.1027. A dense giant component was detected that spanned 73 percent of the network with a density of 0.0017 and clustering coefficient of 0.631, suggesting high level of knowledge diffusion and a rapid flow of information and creativity in this network. Local metrics were calculated using degree, betweenness and closeness centralities. A strong correlation was seen between degree and author productivity (number of works) and betweenness centrality and author productivity, suggesting that author's number of connections and controlling “in-between” position in the network may be providing the authors’ with the knowledge and resources to be more productive. Originality/value – The impact of human actions on the earth systems is a hot topic of research in India. This is one of the first works that investigates co-authorship networks of Indian earth science researchers.

Thermochronology, erosion surfaces and missing section in West Greenland

Abstract: In central West Greenland, Palaeogene volcanic sequences deposited during post-rift subsidence are exposed in mountains reaching 2 km above sea level (a.s.l.), with Palaeocene marine deposits within this section at elevations up to 1.2 km a.s.l. This clearly shows that present-day elevated topography of the West Greenland margin is not a remnant of the rifting process but developed later. Integrating such geological constraints with landscape analysis and thermochronological data shows that mountain summits in central West Greenland represent an Oligocene–Miocene peneplain, which is the counterpart of a correlative unconformity offshore separating Eocene from Middle Miocene sedimentary units. Onshore the peneplain has been exhumed, uplifted to its present altitude and progressively dissected since the Late Miocene. Redfield ( Journal of the Geological Society, London , 167 , 261–271, 2010) questioned numerous aspects of this interpretation, suggesting that ‘the AFT model-based hypothesis that [the elevated topography of West Greenland] was constructed in purely Neogene time remains an unproven speculation9. But as we illustrate here, evidence for Neogene uplift is provided by landscape analysis and geological evidence, as well as thermochronology, and integration of these independent lines of investigation provides a consistent synthesis that we regard as highly reliable. The resulting history of episodic burial and exhumation cannot be simply dismissed, and poses a major challenge to accepted tectonic and geomorphological models for the development of rifted continental margins: how do mountains form along passive continental margins millions of years after rifting and breakup?

Umbria–Marche revisited: A refined magnetostratigraphic calibration of dinoflagellate cyst events for the Oligocene of the Western Tethys

In order to contribute to the improvement of biostratigraphic age control for the Oligocene of the Western Tethys, we have carried out a high-resolution study of dinoflagellate cysts from three outcrop sections in the Umbria–Marche Basin of Central Italy. All three sections investigated (Contessa Barbetti Road, Monte Cagnero and Pieve d'Accinelli) have magnetostratigraphic age control, thus allowing us to firmly tie the identified dinoflagellate cyst bioevents to the global geomagnetic polarity timescale. The Oligocene succession of the Umbria–Marche Basin is marked by recurrent acmes of Chiropteridium spp., Lingulodinium machaerophorum and Deflandrea spp. that can be correlated between the sections studied. These acmes originate from long-distance transport from near-shore environments into the pelagic setting represented by the sections studied, probably triggered by eustatic sea-level variations. Four formal dinoflagellate cyst zones of Wilpshaar et al. (1996; Journal of the Geological Society, London 153, 553-561), viz the Hpu, Clo, Dbi, and Ebu zones are revised. Furthermore we propose two new subzones for the Dbi zone. The new taxon Oligokolpoma galeottii is formally described.