Paul F. Hoffman

Did the Breakout of Laurentia Turn Gondwanaland Inside-Out?

Comparative geology suggests that the continents adjacent to northern, western, southern, and eastern Laurentia in the Late Proterozoic were Siberia, Australia-Antarctica, southern Africa, and Amazonia-Baltica, respectively. Late Proterozoic fragmentation of the supercontinent centered on Laurentia would then have been followed by rapid fan-like collapse of the (present) southern continents and eventual consolidation of East and West Gondwanaland. In this scenario, a pole of rotation near the Weddell Sea would explain the observed dominance of wrench tectonics in (present) east-west trending Pan-African mobile belts and subduction-accretion tectonics in north-south trending belts. In the process of fragmentation, rifts originating in the interior of the Late Proterozoic supercontinent became the external margins of Paleozoic Gondwanaland; exterior margins of the Late Proterozoic supercontinent became landlocked within the interior of Gondwanaland.

Toward a Neoproterozoic composite carbon-isotope record

Glacial deposits of Sturtian and Marinoan age occur in the well-studied Neoproterozoic successions of northern Namibia, South Australia, and northwestern Canada. In all three regions, the Marinoan glaciation is presaged by a large negative δ 13 C anomaly, and the cap carbonates to both glacial units share a suite of unique sedimentological, stratigraphic, and geochemical features. These global chronostratigraphic markers are the bases of a new correlation scheme for the Neoproterozoic that corroborates radiometric data that indicate that there were three glacial epochs between ca. 750 and 580 Ma. Intraregional correlation of Neoproterozoic successions in the present-day North Atlantic region suggests that glacial diamictite pairs in the Polarisbreen Group in northeastern Svalbard and the Tillite Group in eastern Greenland were deposited during the Marinoan glaciation, whereas the younger of a pair of glacials (Mortensnes Formation) in the Vestertana Group of northern Norway was deposited during the third (Gaskiers) Neoproterozoic glaciation. Gaskiers-aged glacial deposits are neither globally distributed nor overlain by a widespread cap carbonate but are associated with an extremely negative δ 13 C anomaly. framework for a new, high-resolution model carbon-isotope record for the Neoproterozoic comprising new δ 13 C (carbonate) data from Svalbard (Akademikerbreen Group) and Namibia (Otavi Group) and data in the literature from Svalbard, Namibia, and Oman. A new U-Pb zircon age of 760 ± 1 Ma from an ash bed in the Ombombo Subgroup in Namibia provides the oldest direct time-calibration point in the compilation, but the time scale of this preliminary δ 13 C record remains

Speculations on Laurentia's first gigayear (2.0 to 1.0 Ga)

It is postulated that a mantle superswell (convective upwelling thousands of kilometres in diameter) developed beneath a stationary supercontinent aggregated in the late Early Proterozoic, giving rise to Middle Proterozoic anorogenic magmatism across Laurentia. Magmatism is attributed to invasion and ponding of mantle melts in the crust, causing uplift and anhydrous partial melting of the lower crust. Products include vast sheets of rhyolite, syenogranite, anorthosite, and gabbro, extensive mafic dike swarms, and rifts containing up to 20 km of basalt. Rifting occurred as a consequence rather than the cause of mantle upwelling. Thermal dissipation of lithospheric mantle may have provided rheological conditions favorable for whole-crustal imbrication observed in the parautochthonous Grenville orogen. The Middle Proterozoic supercontinent engendered more pronounced anorogenic magmatism than subsequent supercontinents because of secular cooling of the mantle. Before the Middle Proterozoic, crustal mass may have been insufficient to produce supercontinents large enough to effectively insulate the mantle and promote superswells. The theory of supercontinental episodicity makes testable predictions concerning paleolatitudes, relative sea levels, and globally synchronous episodes of Proterozoic orogenic and anorogenic activity.

The break-up of Rodinia, birth of Gondwana, true polar wander and the snowball Earth

Abstract A major global plate reorganisation occurred between ∼750 and ∼550 Ma. Gondwana was assembled following the dispersal of Rodinia, a supercontinent centred on Laurentia in existence since ∼1050 Ma. The reorganisation began when tectonic elements, later composing East Gondwana, rotated piecemeal away from the Pacific margin of Laurentia. These elements swept across the ancestral Pacific (Mozambique) Ocean that lay between Laurentia and the combined African cratons of Congo and Kalahari, which were loosely joined after ∼820 Ma. Simultaneously, the Adamastor (Brasilide) Ocean closed by subduction bordering the West Gondwana cratons, drawing virtually all of Gondwana together by ∼550 Ma. The final assembly of Gondwana occurred contemporaneously with the separation of Laurentia from West Gondwana. It has been postulated that the imprint of Rodinias long-lived existence on lower mantleconvection produced a prolate ellipsoidal geoid figure. This could give rise to episodic inertial interchange true polar wander (IITPW), meaning that the entire silicate shell of the Earth (above the core-mantle boundary) rolled through 90° with respect to the diurnal spin axis in ∼15 Ma (equivalent to an apparent polar wander velocity of ∼66 cm a −1 . Although empirical arguments for IITPW of Cambrian age appear to be flawed, evidence for an ultra-fast ( > 40 cm a −1 ) meridional component of apparent polar wander for Laurentia between 564 and 550 Ma suggests that IITPW might have occurred at that time. The break-up of Rodinia increased the continental margin area and preferential organic C burial globally, which is reflected by high δ 13 C values in seawater proxies. The consequent drawdown of CO 2 is implicated in a succession of runaway ice-albedo catastrophes between ∼750 and ∼570 Ma, during each of which the oceans completely froze over. Each “snowball” Earth event must have lasted for millions of years because their terminations depended on extreme CO 2 levels, built up by subaerial volcanic outgassing in the absence of sinks for C. A succession of ice-albedo catastrophes, each terminated under ultra-greenhouse conditions, must have imposed an intense environmental filter on the evolution of life. They may have triggered the radiation of Ediacaran fauna in the aftermath of the final snowball event. It is increasingly recognised that the Late Neoproterozoic was one of the most remarkable periods in Earth history, and it appears to exemplify the interplay of tectonics, the environment and biology in deep time.

Anatomy of North America: thematic geologic portrayals of the continent

Abstract Six thematic tectonic maps are used to analyse the makeup of the North American continent. Themes are: 1. (1) major tectonic elements of the continent 2. (2) time of last major deformation 3. (3) time of first major deformation 4. (4) miogeoclines and terranes by kindred 5. (5) suture zones and terrane boundaries by age, and 6. (6) time of accretion. Features illustrated include distribution of orogenic belts and their extensions beneath cover sequences to the continental edge, contrast between juvenile and reworked crust in orogenic belts, geometry of ancient continental margins, distribution and classification of accreted terranes, geometry of suture zones and courses of ancient oceans, and how the continent evolved from an assemblage of Archean minicontinents to its present configuration. It is suggested that essentially similar plate tectonic processes controlled continental breakup and assembly from the Archean onwards, albeit with gradual increase in size of continental lithospheric plates and quantitative change in other parameters such as heatflow and character of the mantle.

Extreme winds and waves in the aftermath of a Neoproterozoic glaciation.

The most severe excursions in the Earths climatic history are thought to be associated with Proterozoic glaciations. According to the ‘Snowball Earth’ hypothesis, the Marinoan glaciation, which ended about 635 million years ago, involved global or nearly global ice cover. At the termination of this glacial period, rapid melting of continental ice sheets must have caused a large rise in sea level. Here we show that sediments deposited during this sea level rise contain remarkable structures that we interpret as giant wave ripples. These structures occur at homologous stratigraphic levels in Australia, Brazil, Canada, Namibia and Svalbard. Our hydrodynamic analysis of these structures suggests maximum wave periods of 21 to 30 seconds, significantly longer than those typical for todays oceans. The reconstructed wave conditions could only have been generated under sustained high wind velocities exceeding 20 metres per second in fetch-unlimited ocean basins. We propose that these extraordinary wind and wave conditions were characteristic of the climatic transit, and provide observational targets for atmospheric circulation models.

Short-lived 1.9 Ga continental margin and its destruction, Wopmay orogen, northwest Canada

A 1.9 Ga continental margin in the northwest of the Canadian Shield evolved in less than 15 m.y. from a zone of crustal stretching, through a phase of passive-margin subsidence, to attempted subduction beneath an exotic(?) microcontinent. Reversal of subduction polarity and generation of a major episutural magmatic arc occurred within 15 m.y. following microcontinental accretion. Terminal collision occurred 90 m.y., at most, after initial rifting. Certain differences in detail between this collided margin and those of Phanerozoic age may be related to subduction of newly rifted lithosphere. Rapid evolution of the orogen, the contracted passive-margin phase in particular, is consistent with (but does not prove) more rapid recycling of oceanic plates at that time.

Continental transform tectonics: Great Slave Lake shear zone (ca. 1.9 Ga), northwest Canada

Great Slave Lake shear zone (GSLsz) is a northeast-trending dextral mylonite zone across which the coeval north-trending Thelon and Taltson magmatic zones on the west margin of the Churchill province are offset 300–700 km. It is proposed that the GSLsz is a continental transform structure related to oblique collisional indentation of the active margin of the Churchill province by the microcontinental Slave province. This accounts for geologic and geophysical differences between the Thelon and Taltson zones and explains the localization of a Tibetan-type (Queen Maud) uplift north of the GSLsz and the abrupt loss of slip on the GSLsz east of the uplift. It also provides a new model for the “Athapuscow aulacogen,” analogous to the modern Bay of Bengal–Indo-Burman Ranges–Sagaing transform–Andaman Sea situation.

Tectono-magmatic evolution of the 1.9-Ga great bear magmatic zone, Wopmay orogen, northwestern Canada☆

Hildebrand, R.S., Hoffman, P.F. and Bowring, S.A., 1987. Tectono-magmatic evolution of the 1.9-Ga Great Bear magmatic zone, Wopmay orogen, northwestern Canada. In: S.D. Weaver and R.W. Johnson (Editors), Tectonic Controls on Magma Chemistry. J. Volcanol. Geotherm. Res., 32: 99-118. The 1875-1840-Ma Great Bear magmatic zone is a 100-km wide by at least 900-km-long belt of predominantly subgreenschist facies volcanic and plutonic rocks that unconformably overlie and intrude an older sialic basement complex. The basement complex comprises older arc and back-arc rocks metamorphosed and deformed during the Calderian orogeny, 5-15 Ma before the onset of Great Bear magmatism. The Great Bear magmatic zone contains the products of two magmatic episodes, separated temporally by an oblique folding event caused by dextral transpression of the zone: (1) a 1875-1860-Ma pre-folding suite of mainly calc-alkaline rocks ranging continuously in composition from basalt to rhyolite, cut by allied biotite-hornblende-bearing epizonal plutons; and (2) a 1.85-1.84-Ga post-folding suite of discordant, epizonal, biotite syenogranitic plutons, associated dikes, and hornblende-diorites, quartz diorites, and monzodiorites. The pre-folding suite of volcanic and plutonic rocks is interpreted as a continental magmatic arc generated by eastward subduction of oceanic lithosphere. Cessation of arc magmatism and subsequent dextral transpression may have resulted from ridge subduction and resultant change in relative plate motion. Increased heat flux due to ridge subduction coupled with crustal thickening during transpression may have caused crustal melting as evidenced by the late syenogranite suite. Final closure of the western ocean by collision with a substantial continental fragment, now forming the neoautochthonous basement of the northern Canadian Cordillera, is manifested by a major swarm of transcurrent faults found throughout the Great Bear zone and the Wopmay orogen. Although there is probably no single evolutionary template for magmatism at convergent plate margins, the main Andean phase of magmatism, exemplified by the pre-folding Great Bear magmatic suite, evolves as larger quantities of subduction-related mafic magma rise into and heat the crust. This results in magmas that are more homogeneous, siliceous, and explosive with time, ultimately leading to overturn and fractionation of the continental crust.

On the coevolution of Ediacaran oceans and animals

Fe speciation and S-isotope of pyrite data from the terminal Proterozoic Sheepbed Formation in Canada and Doushantuo Formation in China reveal that ocean deep waters were anoxic after the global glaciations (snowball Earth) ending 635 million years ago, but that marine sulfate concentrations and inferentially atmospheric oxygen levels were higher than before the glaciations. This supports a long-postulated link between oxygen levels and the emergence of eumetazoa. Subsequent ventilation of the deep ocean, inferred from shifts in Fe speciation in Newfoundland (previously published data) and western Canada (this report), paved the way for Ediacaran macrobiota to colonize the deep seafloors.