J. Brendan Murphy

St. Francis Xavier University · Department of Earth Sciences

The late Neoproterozoic-Cambrian interval is characterized by global-scale orogenesis, rapid continental growth, and profound changes in Earth systems. Orogenic activity involved continental collisions spanning more than 100 million years, culminating in Gondwana amalgamation. Avalonia is an example of arc magmatism and accretionary tectonics as su...

Plain Language Summary Paleogeographic reconstructions, often presented as black‐box models of the paleolocations of continents, are typically constructed by minimizing paleomagnetic bias while satisfying geologic constraints. Such paleogeographic models are created to fit some informed selection of paleomagnetic and geologic data. However, underfi...

Appinite bodies are a suite of plutonic rocks, ranging from ultramafic to felsic in composition, that are characterized by idiomorphic hornblende as the dominant mafic mineral in all lithologies and by spectacularly diverse textures, including planar and linear magmatic fabrics, mafic pegmatites and widespread evidence of mingling between coeval ma...

The tectonic evolution of the Neoproterozoic Arabian-Nubian Shield (ANS) is widely held as an example of crustal growth processes. The ANS consists of continental crust formed prior to and during the collision between East and West Gondwana. In this contribution, we compile, review, and synthesize published whole-rock Nd-Sr isotopic and zircon U-Pb...

Evaluation of the potential geodynamic connections between the evolution of Palaeozoic oceans in NW Gondwana and NE Gondwana is challenging. Until recently, most syntheses emphasized only two Palaeozoic oceans (Proto-Tethys and Palaeo-Tethys) in the east Tethys realm. However, the discovery of early Palaeozoic ophiolites along the Palaeo-Tethys sut...

The origin of the Cambrian−Ordovician tectono-magmatic events affecting NE Gondwana and the adjacent peri-Gondwanan terranes (e.g., Himalaya, Lhasa, Southern Qiangtang, Baoshan, Tengchong, Sibumasu, Helmand, and Karakorum) is controversial. Although its volume is poorly constrained, we propose that an extensive belt of granitic rocks that formed in...

The Neoproterozoic Arabian-Nubian Shield (ANS) consists of continental crust formed prior to and during the collision between East and West Gondwana. The Eastern Desert of Egypt (i.e., northern Nubian Shield) constitutes the north-western part of the ANS. Neoproterozoic magmatism in the Eastern Desert region occurred between ∼ 800 Ma and ∼ 550 Ma....

Marine ironstone is a Phanerozoic biochemical sedimentary rock that contains abundant primary iron. Although rare, ironstone is conspicuous in the Paleozoic sedimentary record. Its iron source remains contentious, with traditional models invoking a continentally derived source. Increasing sedimentologic evidence suggests that many Paleozoic ironsto...

Following a decade during which its presence was widely accepted, the existence of the putative Ediacaran supercontinent Pannotia has come into question since the turn of the millenium, largely due to the geochro-nology of Ediacaran-Cambrian orogens, which suggests that the supposed landmass had begun to break up well before it was fully assembled....

This Special Paper covering “New Developments in the Appalachian-Caledonian-Variscan Orogen” follows our 2019 GSA Annual Meeting session with the same title (convened by Kuiper, Nance, Murphy, and Strachan; 32 contributions) and our 2019 GSA Northeastern (NEGSA) Section meeting session “Peri- Gondwanan Terranes and Their Origins: What Do We Really...

New analytical and field techniques, as well as increased international communication and collaboration, have resulted in significant new geological discoveries within the Appalachian-Caledonian-Variscan orogen. Cross-Atlantic correlations are more tightly constrained and the database that helps us understand the origins of Gondwanan terranes conti...

Granitoid batholiths dominated by felsic-intermediate compositions are commonly associated with mafic plutons and enclaves; however, the genetic relationship between the apparently coeval but compositionally dissimilar magmas is unclear. Here we review the age, lithogeochemical and Nd-Sr isotopic compositions of some classic plutonic rocks emplaced...

The Paleozoic Era begins with the final assembly of Gondwana and ends with the amalgamation of the supercontinent Pangea. Although this tectonic progression is generally well documented, one fundamental but under-studied phenomenon during this era is the transition from two-way to one-way (northward) migration of peripheral terranes between Gondwan...

Middle Ordovician phosphatic ironstone of the Welsh Basin provides new insight into the paleoenvironmental significance of ironstone and Ordovician ocean chemistry. Deposition occurred in a back-arc basin along the southern margin of Avalonia as the Rheic Ocean opened to the south. Ironstone is interpreted to have accumulated as part of an aggradat...

Northward drift of the Cimmerian microcontinents and opening of the Neo-Tethys Ocean in Early Permian are prominent events in northern Gondwana's protracted breakup history. However, the geodynamic setting responsible for these events is controversial. The dispersal of the eastern Cimmerian microcontinents was accompanied by the emplacement of Qian...

After its Ediacaran-Early Cambrian assembly, Gondwana was flanked by a system of peripheral orogens, Terra Australis, Avalonian-Cadomian and newly defined North Indo-Australie, which display broad temporal correlations of their lithotectonic records. Prior to assembly, their initial histories were primarily controlled by the early Neoproterozoic br...

The South Mountain Batholith (SMB; Nova Scotia, Canada) is the largest composite batholith exposed in the Appalachians and lies entirely within the most outboard Meguma terrane. In-situ and CA-TIMS U-Pb dating and in-situ isotopes (Lu-Hf, O) and geochemistry for zircon from all phases of the SMB constrain its source as well as its evolution. CA-ID-...

The growth of continental crust through melt extraction from the mantle is a critical component of the chemical evolution of the Earth and the development of plate tectonics. However, the mechanisms involved remain debated. Here, we conduct petrological and geochemical analyses on a large (up to 5000 km 2) granitoid body in the Arabian-Nubian shiel...

Island-arc accretion during the assembly of Gondwana has been widely regarded as the main mechanism for Neoproterozoic crustal growth in the Arabian-Nubian Shield (ANS). However, processes involved to transform the newly accreted juvenile terranes into a typical continental crust remain unclear. Here, we present geochemical, isotopic, and U-Pb geoc...

A widespread provenance shift recorded by passive margin strata of western Laurentia, from predominant Stenian (1.2 ‐ 1.0 Ga) detrital zircon age‐components to their absence, occurred during the Neoproterozoic‐Cambrian Sauk transgression and is commonly used as a ca. 540 Ma chronostratigraphic marker throughout the west/southwestern United States....

Supercontinents signify self-organization in plate tectonics. Over the past ~2 63 billion years, 3 major supercontinents have been identified, with increasing age: Pangaea, 64 Rodinia, and Columbia. In a prototypal form, a cyclic pattern of continental assembly and 65 breakup likely extends back to ~3 billion years ago, albeit on the smaller scale...

Magmatic and tectonic processes can transport large volumes of magma generated in the deep crust as discrete pulses to shallower crustal depths, resulting in the incremental construction of large, composite batholiths over thousands to tens of millions of years. The Silurian to Early Devonian Donegal composite batholith in Ireland is a classic exam...

Supercontinent Pangea was preceded by the formation of Gondwana, a “megacontinent” about half the size of Pangea. There is much debate, however, over what role the assembly of the precursor megacontinent played in the Pangean supercontinent cycle. Here we demonstrate that the past three cycles of supercontinent amalgamation were each preceded by ~2...

The Australia-Laurentia connection in the Paleoproterozoic to Mesoproterozoic supercontinent Nuna is thought to have initiated by ca. 1.6 Ga when both continents were locked in a proto-SWEAT (southwestern U.S.–East Antarctic) configuration. However, the longevity of that configuration is poorly constrained. Here, we present a new high-quality paleo...

Middle to Upper Ordovician volcanic rocks in the Arisaig area of Nova Scotia, Canada, constitute the only known record of volcanism in West Avalonia during that interval. Hence, they have been extensively studied to test paleocontinental reconstructions that consistently show Avalonia as a drifting microcontinent during that period. Identification...

A supercontinent is generally considered to reflect the assembly of all, or most, of the Earth's continental lithosphere. Previous studies have used geological, atmospheric and biogenic ‘geomarkers’ to supplement supercontinent identification. However, there is no formal definition of how much continental material is required to be assembled, or in...

The Earth’s mantle is currently divided into the African and Pacific domains, separated by the circum-Pacific subduction girdle, and each domain features a large low shear-wave velocity province (LLSVP) in the lower mantle. However, it remains controversial as to whether the LLSVPs have been stationary through time or dynamic, changing in response...

Progressive mantle melting during the Earth's earliest evolution led to the formation of a depleted mantle and a continental crust enriched in highly incompatible elements. Re-enrichment of Earth's mantle can occur when continental crustal materials begin to founder into the mantle by either subduction or, to a lesser degree, by delamination proces...

The supercontinent Pangea formed by the subduction of the Iapetus and Rheic oceans between Gondwana, Laurentia, and Baltica during mid-to-late Paleozoic times. However, there remains much debate regarding how this amalgamation was achieved. Most paleogeographic models based on paleomagnetic data argue that the juxtaposition of Gondwana and Laurussi...

Continental arcs are the sites of production of continental crust, but the origin of these magmatic systems is not well understood. Although a number of processes have been suggested to be important, the role of water migrating from slab to surface during subduction has been underappreciated. Directly below the Moho, hot (approximately 1,100 °C), h...

Middle to Upper Ordovician ironstone and associated sedimentary rocks of the West Asturian‐Leonese and Cantabrian tectonostratigraphic zones, Spain, provide new information regarding the Palaeozoic Fe cycle and the palaeoceanography of the Rheic Ocean. Examination of drill cores and outcrops indicates the southeastern margin of this narrow seaway w...

The formation and subsequent breakup of the supercontinent Pangaea has dominated Earth’s evolution for the last 320 million years. Although its configuration at the time of breakup is widely accepted, there remains uncertainty about its configuration at the time of its amalgamation. The classic Pangaea-A model, widely known as “Wegenerian” configur...

Paleogeography can be reconstructed using various crust- or mantle-based reference frames that make fundamentally different assumptions. The various reconstruction models differ significantly in continental paleolongitude, but it has been difficult to assess which models are more valid. We suggest here a “LLSVP test”, where an assumed correlation b...

Synorogenic basins formed in all lithospheric units involved in the Variscan orogeny, i.e. the Gondwanan and Laurussian continental margins and also in the oceanic realm. However, examples of the latter are only preserved in SW Iberia. On the other hand, synorogenic deposits of the Gondwanan foreland are only preserved in NW Iberia, whereas remains...

Hafnium isotopes of zircon represent a well-dated proxy for the evolution of magmatic systems through Earth history. Time series analysis on the hafnium isotopes of zircon reveals a hierarchy of statistically significant periodic signals spanning multiple orders of magnitude (10⁶–10⁹ year cycles). We attribute the hierarchy of cyclicity to organizi...

In northwest Donegal, Ireland, a large number of coeval appinitic (hornblende-plagioclase-rich) plutons and lampro-phyre dykes occur around the Ardara pluton, a granitic satellite body and one of the oldest phases of the ca. 428-400 Ma composite Donegal Batholith. The appinite units form a bimodal (mafic-felsic) suite in which hornblende is the dom...

The evolution of the mantle source beneath the Teplá-Barrandian (TBB) and the adjacent southern part of the Saxo-Thuringian (S-STB) and northern part of the Moldanubian (N-MB) blocks of the Bohemian Massif (Czech Republic) is tracked from the Late Neoproterozoic to the Cenozoic in order to examine the coupling between the crust and underlying conti...

There is an emerging consensus that Earth's landmasses amalgamate quasi-periodically into supercontinents, interpreted to be rigid super-plates essentially lacking tectonically active inner boundaries and showing little internal lithosphere–mantle interactions. The formation and disruption of supercontinents have been linked to changes in sea-level...

In the original Wilson cycle, the northern Appalachian–Caledonide orogen resulted from the collision of two continental masses separated by a single ocean. One of these corresponds to the modern concept of Laurentia, but the colliding continent to the east has been variously subdivided into many smaller terranes and domains, including Ganderia, Ava...

The geological evolution of Avalonia was fundamental to the first application of plate tectonic principles to the pre-Mesozoic world. Four tectonic phases have now been identified. The oldest phase (760–660 Ma) produced a series of oceanic arcs, some possibly underlain by thin slivers of Baltica crust, which accreted to the northern margin of Gondw...

Disagreement about the existence of the late Neoproterozoic supercontinent Pannotia highlights the limitation of defining supercontinents simply on the basis of size, which, for pre-Pangaean supercontinents, is difficult to determine. In the context of the supercontinent cycle, however, supercontinent assembly and break-up, respectively, mark the e...

Magmatic activity is an integral component of orogenic processes, from arc magmatism during convergence to post-collisional crustal melting. Southern Iberia exposes a Late Paleozoic suture zone within Pangea and where a crustal fragment of Laurussia (South Portuguese Zone) is juxtaposed with parautochthonous Gondwana (Ossa Morena Zone). Fault-bound...

Paleozoic continental reconstructions indicate that subduction of Rheic oceanic lithosphere led to collision between Laurussia and Gondwana which was a major event in the formation of the Ouachita-Appalachian-Variscan orogenic belt and the amalgamation of Pangea. However, arc systems which record that subduction are poorly preserved. The preservati...

Periodic assembly and dispersal of continental fragments has been a characteristic of the solid Earth for much of its history. Geodynamic drivers of this cyclic activity are inferred to be either top-down processes related to near surface lithospheric stresses at plate boundaries or bottom-up processes related to mantle convection and, in particula...

In northwest Donegal, Ireland, a large number of appinitic (hornblende/intermediate plagioclase-rich) intrusions occur around the ca. 405 Ma Ardara granitic pluton. They form a group of coeval, plutonic and hypabyssal rocks, ranging in composition from ultramafic to felsic, in which hornblende is the dominant mafic mineral and typically occurs both...

In the development of the Appalachian-Caledonide Orogen, the earliest stages of deformation of peri-Laurentian and peri-Gondwanan rocks took place concurrently from latest Cambrian into Early Ordovician. Deformation close to the Laurentian margin, preserved in the foreland belt of the orogen, is characterized as Taconian (in N. America) or Grampian...

The northern margin of Gondwana was dominated by arc activity throughout much of the Neoproterozoic and Early Cambrian. However, the composition and duration of arc activity varied considerably along and across strike. Sm-Nd isotopic systematics suggest arc magmas were derived from both juvenile (outboard) and ancient (inboard) sources, analogous w...

The spatial and temporal association of post-collisional granites and lamprophyre dykes is a common but enigmatic relationship in many orogenic belts, including the Variscan orogenic belt of SW England. The geology of SW England has long been interpreted to reflect orogenic processes associated with the closure of the Rheic Ocean and the formation...

The Late Paleozoic Variscan orogen in Europe is widely acknowledged to be the result of convergence and collision between Laurussia and Gondwana during closure of the Rheic Ocean. The orogen is classically divided into a number of tectonostratigraphic zones that have a distinctive curvature (IberoArmorican Arc, IAA) and record different aspects of...