Earth history events shaped the evolution of uneven biodiversity across tropical moist forests

Far from a uniform band, the biodiversity found across Earth’s tropical moist forests varies widely between the high diversity of the Neotropics and Indomalaya and the relatively lower diversity of the Afrotropics. Explanations for this variation across different regions, the “pantropical diversity disparity” (PDD), remain contentious, due to difficulty teasing apart the effects of contemporary climate and paleoenvironmental history. Here, we assess the ubiquity of the PDD in over 150,000 species of terrestrial plants and vertebrates and investigate the relationship between the present-day climate and patterns of species richness. We then investigate the consequences of paleoenvironmental dynamics on the emergence of biodiversity gradients using a spatially explicit model of diversification coupled with paleoenvironmental and plate tectonic reconstructions. Contemporary climate is insufficient in explaining the PDD; instead, a simple model of diversification and temperature niche evolution coupled with paleoaridity constraints is successful in reproducing the variation in species richness and phylogenetic diversity seen repeatedly among plant and animal taxa, suggesting a prevalent role of paleoenvironmental dynamics in combination with niche conservatism. The model indicates that high biodiversity in Neotropical and Indomalayan moist forests is driven by complex macroevolutionary dynamics associated with mountain uplift. In contrast, lower diversity in Afrotropical forests is associated with lower speciation rates and higher extinction rates driven by sustained aridification over the Cenozoic. Our analyses provide a mechanistic understanding of the emergence of uneven diversity in tropical moist forests across 110 Ma of Earth’s history, highlighting the importance of deep-time paleoenvironmental legacies in determining biodiversity patterns.

Multiple melt source origin of the Line Islands (Pacific Ocean)

The Line Islands volcanic chain in the central Pacific Ocean exhibits many characteristics of a hotspot-generated seamount chain; however, the lack of a predictable age progression has stymied previous models for the origin of this feature. We combined plate-tectonic reconstructions with seamount age dates and available geochemistry to develop a new model that involves multiple melt regions and multiple melt delivery styles to explain the spatial and temporal history of the Line Islands system. Our model identifies a new melt source region (Larson melt region at ~17°S, ~125°W) that contributed to the formation of the Line Islands, as well as the Mid-Pacific Mountains and possibly the Pukapuka Ridge.

Recognising spatially and geochemically anomalous arc magmatism (SGAM) and implications for plate tectonic reconstructions

<p>Subduction zones generate volcanic arcs, but there are many examples where magmatism in convergent plate boundaries occurs in unexpected locations relative to the subducting slab. These magmas are commonly also geochemically anomalous relative to the composition of neighbouring typical subduction-related rocks. The origin of such Spatially and Geochemically Anomalous arc Magmatism (SGAM) may correspond to local variations in subduction parameters, the presence of crustal and lithospheric heterogeneities, or the potential contribution of melts generated by slab tearing and slab edge effects. Using the Holocene volcanoes in South America as a case study, we investigated spatial and geochemical patterns of volcanism along the Andean volcanic belt. Based on a series of geochemical indices, we developed a scoring system for the composition of volcanic rocks, with the lowest and highest scores indicating ‘typical’ and ‘anomalous’ arc melting processes, respectively. The results show that a number of Holocene volcanoes in South America can be unambiguously defined as SGAM. Volcanism in these localities may correspond to disruptions in the geometry of the subducting slab, or to areas affected by mantle flow in the proximity of the slab edge. To test the potential applicability of this method for plate tectonic reconstructions, we calculated geochemical anomaly scores for whole-rock analyses of volcanic rocks from other convergent boundary settings. The results show that high geochemical anomaly scores are obtained in areas where slab tearing has been documented or postulated, such as in Mount Etna (Sicily). The occurrence of anomalous magmatic rocks in older convergent plate boundary settings (e.g., Neogene rocks from the Gibraltar area) corroborates plate tectonic reconstructions that incorporated processes such as subduction segmentation, slab tearing, and the development of asthenospheric windows. Accordingly, we suggest that the recognition of SGAM from other modern and ancient arc settings may inform on similar types of processes, even in cases where the three-dimensional slab structure is no longer detectable.</p>

Implications of the detrital zircon record for global plate tectonic reconstructions in deep time

<p>Full-plate reconstructions describe the history of both past continental motions and how plate boundaries have evolved to accommodate these motions. The fluxes of material into and out of the mantle at plate boundaries is thought to deeply influence the evolution of deep Earth structure, surface environments and biological systems through deep time. Traditionally, plate tectonic reconstructions have relied on geophysical data from the oceans, which provides details of how Pangea broke apart (since ca. 200 Myr) while paleomagnetism is the primary quantitative constraint prior to Pangea formation. However, these data do not directly constrain the extent of subduction zones or other plate boundaries, so reconstructing the past plate configurations of past supercontinents must rely on alternative methods. One source of data that can resolve this problem is to use observations from detrital zircons. Previous studies have proposed classification schemes to determine tectonic settings where samples were deposited, based on the different characteristic shapes of detrital zircon age spectra found in convergent, collisional and extensional settings.</p><p>Here, we investigate the applicability of this method to test and refine global full-plate tectonic reconstructions in deep time, using a published database of zircon ages. We first use reconstructions for relatively recent times (<100 Ma), where reconstructions are reasonable well constrained, to evaluate the effectiveness of the classification method. For older times, where uncertainties in the reconstructions are far larger, we can use the results to discriminate between competing models. We analysed the proximity between reconstructed plate boundaries and zircon sample sites assigned to different tectonic classifications, and found that the classification method does well (~64－79% success depending on distance threshold used) in distinguishing convergent settings. The ability of the classification to define extensional settings such as rift basins is less clear, though samples in this class do lie preferentially further from convergent settings. Based on these insights, we apply the method to evaluate full-plate reconstructions for the Neoproterozoic as well as other competing models for the configuration of Rodinia.</p>

Tomotectonic constraints on the assembly of the western Arctic region and central Alaska: progress, problems and future direction

<p>Alaska is made up of a mosaic of terranes that have enigmatic origins. Several plate restorations for the assembly of Alaska have been proposed, but their validity remains debated, partly due to the removal of vast volumes of oceanic plate material via subduction at the accretionary margins. The position, depth and volume of this subducted lithosphere, recognised as seismically fast anomalies in tomographic images, can be used to track the locations of subduction plate boundaries of the past, thus serving as an important constraint for plate restorations of convergent margins. Existing plate tectonic reconstructions can be assessed and developed further by integrating seismic tomographic models of the mantle with geological and palaeomagnetic bedrock datasets, a procedure which we term “tomotectonic analysis”.</p><p>Previous tomotectonic studies (e.g., Sigloch & Mihalynuk, 2017, GSA Bulletin) have highlighted various discrepancies between the most generally accepted tectonic reconstruction models of the western coast of North America and tomographic observations of slabs in the mantle. For example, the kinematic reconstruction of Laurentia, constrained by the opening of the Atlantic Ocean, places the Cordilleran margin thousands of kilometres east of the tomographically imaged Angayucham and Mezcalera slabs in the mantle during the Early to Late Jurassic. This suggests that there was extensive westward subduction beneath the Insular and Intermontane superterranes that involved multiple plates, rather than a single subduction zone. Though a recent plate reconstruction that employed tomotectonic methods (Clennett et al., 2020, G-Cubed) provided a coherent explanation of bedrock, plate kinematic and mantle observations for the Cordilleran margin, application of this model to Alaska and the Arctic was hindered by low tomographic resolution beneath that region and requires further investigation. In particular, restoration of the Arctic Alaska terrane is complicated further by its possible relationship with the proposed Arctic Alaska-Chukotka microcontinent and its involvement in the accretionary development of the Siberian peninsula and the opening of the Canada Basin, for which several working hypotheses continue to be debated.</p><p>In this study we consider the application of tomotectonic analysis to Mesozoic reconstructions of the western Arctic and central Alaska. We will compare and contrast these tectonic reconstructions with respect to the distribution of slabs in the deep mantle based on observations from the latest seismic tomographic models, such as DETOX-P1, P2 and P3 (Hosseini et al., 2020, GJI). We will also highlight the limitations of current tomographic models and the need for targeted seismic investigations with greater resolution of the underlying mantle. This discussion provides the motivation and rationale for a new seismic tomographic model of the mantle beneath North America currently being produced by the authors using a more complete USArray dataset.</p>

Overview of Cordilleran oceanic terranes and their significance for the tectonic evolution of the northern Cordillera

Ophiolite complexes are an important component of oceanic terranes in the northern Cordillera and constitute a significant amount of juvenile crust added to the Mesozoic Laurentian continental margin during Cordilleran orogenesis. Despite their tectonic importance, few systematic studies of these complexes have been conducted. Detailed studies of the pseudostratigraphy, age, geochemistry, and structural setting of ophiolitic rocks in the northern Cordillera indicate that ophiolites formed in Permian to Middle Triassic suprasubduction zone settings and were obducted onto passive margin sequences. Re-evaluation of ophiolite complexes highlights fundamental gaps in the understanding of the tectonic framework of the northern Cordillera. The previous inclusion of ophiolite complexes into generic 'oceanic' terranes resulted in significant challenges for stratigraphic nomenclature, led to incorrect terrane definitions, and resulted in flawed tectonic reconstructions.

New Tectonic Reconstructions of New Guinea Derived from Biostratigraphy and Geochronology

Biostratigraphic data from exploration wells in Papua, West Papua of Indonesia, Papua New Guinea and Australia were reviewed, revised and updated using modern stratigraphic interpretations. Revised stratigraphic interpretations were combined with zircon U-Pb geochronologic data to produce new tectonic reconstructions of the Indonesian provinces of West Papua and Papua. Zircon U-Pb geochronologic data used in this study include new results from the Papuan Peninsula, combined with existing datasets from West Papua, Papua New Guinea, eastern Australia and New Caledonia. Supplementary geochronologic data were used to provide independent validation of the biostratigraphic data. Findings from a compilation of biostratigraphic and zircon age data provide a framework to produce new tectonic models for the origin of New Guinea’s terranes. Two hypotheses are presented to explain observations from the biostratigraphic and geochronologic data. The ‘Allochthonous Terrane’ Model suggests that many of the terranes are allochthonous in nature and may have been derived from eastern Australia. The ‘Extended Rift’ Model suggests that the New Guinea Terranes may have been separated from north-eastern Australia by an elongate rift system far more extensive than previously described. These new tectonic models are essential for our geological understanding of the regional and can be used to drive successful petroleum exploration in this frontier area.

Pannotia under prosecution

Pannotia is a hypothetical supercontinent that may have existed briefly during the Proterozoic–Cambrian transition. Various lines of evidence used to argue for its existence include global orogenesis in Ediacaran–Cambrian time, the development of Cambrian passive margins and some (but not all) tectonic reconstructions. Indirect measures used to infer Pannotia's veracity include patterns of biological diversity, palaeoclimate, sea level, magmatism and other palaeoenvironmental proxies. It is shown herein that neither the direct records nor the indirect proxies provide compelling support for Pannotia. If that ephemeral contiguous landmass existed at all, its effects on the broader Earth system are inextricably tied to the more fundamental processes of Gondwanaland assembly. This perspective emphasizes the remarkable consolidation of Gondwanaland as a semi-supercontinent within the early stages of the Pangaea cycle. Gondwanaland's size combined with its c. 300 myr longevity might have greater significance for mantle dynamics than the larger, but shorter-lived, Pangaea landmass.

Uncertainties in break-up markers along the Iberia–Newfoundland margins illustrated by new seismic data

Plate tectonic modellers often rely on the identification of “break-up” markers to reconstruct the early stages of continental separation. Along the Iberian-Newfoundland margin, so-called break-up markers include interpretations of old magnetic anomalies from the M series, as well as the “J anomaly”. These have been used as the basis for plate tectonic reconstructions are based on the concept that these anomalies pinpoint the location of first oceanic lithosphere. However, uncertainties in the location and interpretation of break-up markers, as well as the difficulty in dating them precisely, has led to plate models that differ in both the timing and relative palaeo-positions of Iberia and Newfoundland during separation. We use newly available seismic data from the Southern Newfoundland Basin (SNB) to assess the suitability of commonly used break-up markers along the Newfoundland margin for plate kinematic reconstructions. Our data show that basement associated with the younger M-series magnetic anomalies is comprised of exhumed mantle and magmatic additions and most likely represents transitional domains and not true oceanic lithosphere. Because rifting propagated northward, we argue that M-series anomaly identifications further north, although in a region not imaged by our seismic, are also unlikely to be diagnostic of true oceanic crust beneath the SNB. Similarly, our data also allow us to show that the high amplitude of the J Anomaly is associated with a zone of exhumed mantle punctuated by significant volcanic additions and at times characterized by interbedded volcanics and sediments. Magmatic activity in the SNB at a time coinciding with M4 (128 Ma) and the presence of SDR packages onlapping onto a basement fault suggest that, at this time, plate divergence was still being accommodated by tectonic faulting. We illustrate the differences in the relative positions of Iberia and Newfoundland across published plate reconstructions and discuss how these are a direct consequence of the uncertainties introduced into the modelling procedure by the use of extended continental margin data (dubious magnetic anomaly identifications, break-up unconformity interpretations). We conclude that a different approach is needed for constraining plate kinematics of the Iberian plate pre-M0 times.