The Hudson Bay Lithospheric Experiment (HuBLE) : Insights into Precambrian Plate Tectonics and the Development of Mantle Keels

The UK component of HuBLE was supported by Natural Environment Research Council (NERC) grant NE/F007337/1, with financial and logistical support from the Geological Survey of Canada, Canada–Nunavut Geoscience Office, SEIS-UK (the seismic node of NERC), and First Nations communities of Nunavut. J. Beauchesne and J. Kendall provided invaluable assistance in the field. Discussions with M. St-Onge, T. Skulski, D. Corrigan and M. Sanborne-Barrie were helpful for interpretation of the data. D. Eaton and F. A. Darbyshire acknowledge the Natural Sciences and Engineering Research Council. Four stations on the Belcher Islands and northern Quebec were installed by the University of Western Ontario and funded through a grant to D. Eaton (UWO Academic Development Fund). I. Bastow is funded by the Leverhulme Trust. This is Natural Resources Canada Contribution 20130084 to its Geomapping for Energy and Minerals Program. This work has received funding from the European Research Council under the European Unions Seventh Framework Programme (FP7/2007-2013)/ERC Grant agreement no. 240473 ‘CoMITAC’.Peer reviewedPublisher PD

Lateral variations in the crustal structure of the Indo-Eurasian collision zone

We thank Michael Ritzwoller and two anonymous reviewers for constructive comments that have helped improve the manuscript. The majority of the seismic data used in this study were downloaded from IRIS DMC. Data for the NGRI stations in India were provided by S. S. Rai, and Zahid Rafi provided the PMD Pakistan data. Kajal Borah provided the ambient noise cross-correlations for the Uttaranchal network. Figures were prepared using Generic Mapping Tools (GMT) software (Wessel and Smith, 1998). We thank Robert Herrmann for making the Computer Programs in Seismology freely available.Peer reviewedPublisher PD

Thermo-compositional structure of the North and South American cratonic lithosphere

It remains unanswered how cratons - the ancient cores of the continents - formed, remained stable, and occasionally lose their deep roots. In this thesis, I analyse geophysical data from North and South America to constrain the thermo-compositional structure of their cratonic lithosphere. For northeastern North America, I model Rayleigh-wave phase velocities, and for eastern South America, I jointly invert Rayleigh-wave group velocities, topography, and geoid height. I use a grid search to find structures that match the data, searching a large set of plausible shield geotherms and a comprehensive set of compositional structures with the option of metasomatic minerals and eclogite as seismic slow and fast compositions. The data require larger variations in cratonic thermal and compositional structures than often considered; structures that can be correlated to tectonic evolution stages. Craton assembly appears to often involve subduction, where we find evidence of a diachronous secular evolution during the Proterozoic from (shallow) subduction where eclogitised crust was preserved in the lithosphere, to (steeper) subduction leaving only a volatile-altered shallow lithosphere. Thick roots (350-150 km) remain under much of the North American Craton and part of the South American Platform. These regions include preserved Archean/Paleoproterozoic cores and neighbouring regions with roots metasomatized throughout much of their depth by plume activity/rifting. Evidence of root loss/erosion is observed mainly under South America, which we attribute to major plume and subduction interaction during the Phanerozoic. Our results, confirmed by preliminary waveform analysis, indicate that metasomatism is a widespread feature of cratonic lithosphere, particularly at shallow depths. The main implications of this work are that the cratonic lithosphere preserves more signatures of its tectonic evolution than previously realised and likely holds larger quantities of volatiles than often assumed, at a level that affects its density, viscosity, and possibly solid-earth volatile cycling.Open Acces

3-D multiobservable probabilistic inversion for the compositional and thermal structure of the lithosphere and upper mantle: III. Thermochemical tomography in the Western-Central U.S.

Acknowledgments We are indebted to F. Darbyshire and J. von Hunen for useful comments on earlier versions of this work. This manuscript benefited from thorough and constructive reviews by W. Levandowski and an anonymous reviewer. We also thank J. Connolly, M. Sambridge, B. Kennett, S. Lebedev, B. Shan, U. Faul, and M. Qashqai for insightful discussions about, and contributions to, some of the concepts presented in this paper. The work of J.C.A. has been supported by two Australian Research Council Discovery grants (DP120102372 and DP110104145). Seismic data are from the IRIS DMS. D.L.S. acknowledges support from NSF grant EAR-135866. This is contribution 848 from the ARC Centre of Excellence for Core to Crust Fluid Systems (http://www.ccfs.mq.edu.au) and 1106 in the GEMOC Key Centre (http://www.gemoc.mq.edu.au).Peer reviewedPublisher PD

A review of crust and upper mantle structure beneath the Indian subcontinent

This review presents an account of the variations in crustal and upper mantle structure beneath the Indian subcontinent and its environs, with emphasis on passive seismic results supplemented by results using controlled seismic sources. Receiver function results from more than 600 seismic stations, and over 10,000 km of deep seismic profiles have been exploited to produce maps of average crustal velocities and thickness across the region. The crustal thickness varies from 29 km at the southern tip of India to 88 km under the Himalayan collision zone, and the patterns of variation show significant deviations from the predictions of global models. The average crustal shear velocity (Vs) is low in the Himalaya–Tibet collision zone compared to Indian shield. Major crustal features are as follows: (a) the Eastern Dharwar Craton has a thinner and simpler crustal structure crust than the Western Dharwar Craton, (b) Himalayan crustal thickness picks clearly follow a trend with elevation, (c) the rift zones of the Godavari graben and Narmada–Son Lineament show deeper depths of crust than their surroundings, and (d) most of the Indian cratonic fragments, Bundelkhand, Bhandara and Singhbhum, show thick crust in comparison to the Eastern Dharwar Craton. Heat flow and crustal thickness estimates do not show any positive correlations for India. Estimates of the thickness of the lithosphere show large inconsistencies among various techniques not only in terms of thickness but also in the nature of the transition to the asthenosphere (gradual or sharp). The lithosphere beneath India shows signs of attrition and preservation in different regions, with a highly heterogeneous nature, and does not appear to have been thinned on broader scale during India's rapid motion north towards Asia. The mantle transition zone beneath India is predominantly normal with some clear variations in the Himalayan region (early arrivals) and Southwest Deccan Volcanic Province and Southern Granulite Terrain (delayed arrivals). No clear patterns on influence on the mantle transition zone discontinuities can be associated with lithospheric thickness. Over 1000 anisotropic splitting parameters from SKS/SKKS phases and 139 using direct S waves are available from various studies. The shear-wave splitting results clearly show the dominance of absolute-plate-motion related strain of a highly anisotropic Indian lithospheric mantle with delay times between the split S phases close to 1 s. There are still many parts of India where there is, at best, limited information on the character of the crust and the mantle beneath. It is to be hoped that further installations of permanent and temporary stations will fill these gaps and improve understanding of the geodynamic environment of the Indian subcontinent.This study has been supported by a grant from the Ministry of Earth Sciences (MoES), IITKGP/CKH

Seismic properties of the crust and uppermost mantle of North America

Seismic refraction profiles for the North American continent were compiled. The crustal models compiled data on the upper mantle seismic velocity (P sub n), the crustal thickness (H sub c) and the average seismic velocity of the crystalline crust (V sub p). Compressional wave parameters were compared with shear wave data derived from surface wave dispersion models and indicate an average value for Poisson's ratio of 0.252 for the crust and of 0.273 for the uppermost mantle. Contour maps illustrate lateral variations in crustal thickness, upper mantle velocity and average seismic velocity of the crystalline crust. The distribution of seismic parameters are compared with a smoothed free air anomaly map of North America and indicate that a complidated mechanism of isostatic compensation exists for the North American continent. Several features on the seismic contour maps also correlate with regional magnetic anomalies

Seismological structure of the 1.8 Ga Trans-Hudson Orogen of North America

Precambrian tectonic processes are debated: what was the nature and scale of orogenic events on the younger, hotter, and more ductile Earth? Northern Hudson Bay records the Paleoproterozoic collision between the Western Churchill and Superior plates—the ∼1.8 Ga Trans-Hudson Orogeny (THO)—and is an ideal locality to study Precambrian tectonic structure. Integrated field, geochronological, and thermobarometric studies suggest that the THO was comparable to the present-day Himalayan-Karakoram-Tibet Orogen (HKTO). However, detailed understanding of the deep crustal architecture of the THO, and how it compares to that of the evolving HKTO, is lacking. The joint inversion of receiver functions and surface wave data provides new Moho depth estimates and shear velocity models for the crust and uppermost mantle of the THO. Most of the Archean crust is relatively thin (∼39 km) and structurally simple, with a sharp Moho; upper-crustal wave speed variations are attributed to postformation events. However, the Quebec-Baffin segment of the THO has a deeper Moho (∼45 km) and a more complex crustal structure. Observations show some similarity to recent models, computed using the same methods, of the HKTO crust. Based on Moho character, present-day crustal thickness, and metamorphic grade, we support the view that southern Baffin Island experienced thickening during the THO of a similar magnitude and width to present-day Tibet. Fast seismic velocities at >10 km below southern Baffin Island may be the result of partial eclogitization of the lower crust during the THO, as is currently thought to be happening in Tibet

Integrated geophysical-petrological modeling of lithosphere-asthenosphere boundary in central Tibet using electromagnetic and seismic data

We undertake a petrologically driven approach to jointly model magnetotelluric (MT) and seismic surface wave dispersion (SW) data from central Tibet, constrained by topographic height. The approach derives realistic temperature and pressure distributions within the upper mantle and characterizes mineral assemblages of given bulk chemical compositions as well as water content. This allows us to define a bulk geophysical model of the upper mantle based on laboratory and xenolith data for the most relevant mantle mineral assemblages and to derive corresponding predicted geophysical observables. One-dimensional deep resistivity models were derived for two groups of MT stations. One group, located in the Lhasa Terrane, shows the existence of an electrically conductive upper mantle layer and shallower conductive upper mantle layer for the other group, located in the Qiangtang Terrane. The subsequent one-dimensional integrated petrological-geophysical modeling suggests a lithosphere-asthenosphere boundary (LAB) at a depth of 80¿120 km with a dry lithosphere for the Qiangtang Terrane. In contrast, for the Lhasa Terrane the LAB is located at about 180 km but the presence of a small amount of water in the lithospheric mantle (<0.02 wt%) is required to fit the longest period MT responses. Our results suggest two different lithospheric configurations beneath the southern and central Tibetan Plateau. The model for the Lhasa Terrane implies underthrusting of a moderately wet Indian plate. The model for the Qiangtang Terrane shows relatively thick and conductive crust and implies thin and dry Tibetan lithosphere.Peer Reviewe

Constraints on the Structure and Evolution of the Malawi Rift from Active- and Passive-Source Seismic Imaging

Located at the southernmost sector of the Western Branch of the East African Rift System, the Malawi Rift exemplifies an active, magma-poor, weakly extended continental rift. This work focuses on the northern portion of the Malawi Rift, which is flanked by long (>100 km) basin-bounding border faults and crosses several significant remnant structures. This combination of characteristics makes the Malawi Rift the ideal location to investigate the controlling processes governing present-day extension throughout the lithosphere. To investigate these processes I image shallow basin- to uppermost-mantle structure beneath the region using a combination of passive- and active-source seismic datasets. I conduct passive-source imaging of the crust and upper mantle using ambient-noise and teleseismic Rayleigh-wave phase velocities between 9 and 100 s period. This study includes six lake-bottom seismometers located in Lake Malawi (Nyasa), the first time seismometers have been deployed in any of the African rift lakes. I utilize the resulting phase-velocity maps to invert for a shear velocity model of the Malawi Rift discussed below. I utilize active-source tomographic imaging to obtain new constraints on rift basin structure in the Malawi Rift from a 3-D compressional velocity (Vp) model. The velocity model uses observations from the first wide-angle refraction study conducted using lake-bottom seismometers in one of the great lakes of East Africa. The 3-D velocity model reveals up to ~5 km of synrift sediments, which smoothly transition from eastward thickening against the Livingstone Border Fault in the North Basin to westward thickening against the Usisya Border Fault in the Central Basin. I use new constraints on synrift sediment thickness to construct displacement profiles for both faults. Both faults accommodate large throws (> 7 km) but the Livingstone Fault is ~30 km longer. The dimensions of these faults suggest they are nearing their maximum size. The presence of >4 km of sediment within the accommodation zone suggests fault length was established early pointing the "constant length" model of fault growth. The presence of an intermediate velocity unit with velocities of 3.75-4.5 km/s is interpreted to represent prior rifting (Permo-Triassic and/or Cretaceous) sedimentary deposits beneath Lake Malawi. These thick (up to 4.6 km) packages of preexisting sedimentary strata improve the understanding of the Tanganyika-Rukwa-Malawi rift system and the role of earlier stretching phases on synrift basin development. I use the previously obtained local-scale measurements of Rayleigh wave phase velocities between 9 and 100 s combined with constraints on basin structure and crustal thickness to robustly invert for shear velocity from the surface to 135 km for the Malawi Rift. We compare our resulting 3-D model to a 3-D model of shear velocity obtained for the mature Main Ethiopian Rift and Afar Depression using commensurate datasets and identical methodologies. Comparing the Vs models for the two regions reveals markedly different seismic velocities particularly pronounced in the upper mantle (average velocities in the Malawi Rift are ~9% faster than the Main Ethiopian Rift). Our 3-D Vs model of the Malawi Rift reveals a strong, localized low velocity anomaly associated with the Rungwe Volcanic Province within the crust and upper mantle that can be explained without requiring the presence of partial melt. Away from the Rungwe Volcanic Province, velocities within the plateau regions are fast (> 4.6 km/s) and representative of depleted lithospheric mantle to depths of 100 and >135 km to the west and east of the rift, respectively. Thinned lithosphere, represented by the absence of similarly high velocities, is centered directly beneath the rift axis and footwall escarpments of the rift basins. The correlation between the localization of lithospheric thinning, the boundaries between abutting Proterozoic mobile belts, and the positions of the basin-bounding border faults may point to the controlling role of preexisting large-scale structures in localizing strain and allowing extension to occur here