Has the Tibetan plateau risen in the Early/Mid–Miocene? Constraints from plate–motion reconstructions and seismicity of the Indian Ocean lithosphere

Summary Magnetisation records and seismic stratigraphy of the Indian Ocean lithosphere indicate that the Early/Mid–Miocene onset of diffuse contractional deformation coincided with slowdowns of the Indian and Capricorn plate motions. At present day such deformation is evidenced by the seismicity of the Indian ocean floor. Deformation onset and past plate–motion slowdowns have been interpreted as consequences of a sudden uplift of the Tibetan plateau by 1 to 2 km, as this – following previous estimates – would generate a tectonically–significant force between 4 · 1012 and 8 · 1012 N/m. However, this view remains at odds with paleo–altimetry estimates from geological and geochemical data, which indicate no increase in plateau altitude throughout the Miocene. Here I use well–established models of viscous/brittle dynamics in inverse mode in order to constrain the amount of force that should be delivered by the Tibetan region to the Indian tectonic setting in order to explain the observations above. Results constrain such a force within the range from 4.3 · 1011 to 3.5 · 1012 N/m. By comparison with previous estimates of the force associated with topography increase, these analyses suggest that the Early/Mid–Miocene onset of contractional deformation and plate–motion slowdowns within the Indian Ocean require minimal uplift of the Tibet plateau of a few hundred meters. The seemingly–contradicting inferences on Early/Mid–Miocene Tibetan uplift that come from geophysical and geological/geochemical observations can be reconciled by noting that the required uplift amount is less than what is resolvable by modern paleo–altimetry techniques.

Evolution and demise of passive margins through grain mixing and damage

How subduction—the sinking of cold lithospheric plates into the mantle—is initiated is one of the key mysteries in understanding why Earth has plate tectonics. One of the favored locations for subduction triggering is at passive margins, where sea floor abuts continental margins. Such passive margin collapse is problematic because the strength of the old, cold ocean lithosphere should prohibit it from bending under its own weight and sinking into the mantle. Some means of mechanical weakening of the passive margin are therefore necessary. Spontaneous and accumulated grain damage can allow for considerable lithospheric weakening and facilitate passive margin collapse. Grain damage is enhanced where mixing between mineral phases in lithospheric rocks occurs. Such mixing is driven both by compositional gradients associated with petrological heterogeneity and by the state of stress in the lithosphere. With lateral compressive stress imposed by ridge push in an opening ocean basin, bands of mixing and weakening can develop, become vertically oriented, and occupy a large portion of lithosphere after about 100 million y. These bands lead to anisotropic viscosity in the lithosphere that is strong to lateral forcing but weak to bending and sinking, thereby greatly facilitating passive margin collapse.

The Xigaze ophiolite: fossil ultraslow-spreading ocean lithosphere in the Tibetan Plateau

The crust and mantle in both ophiolites (fossil ocean lithosphere) and in modern oceans are enormously diverse. Along-axis morphology and lower crustal accretion at ultraslow-spreading ocean ridges are fundamentally different from those at faster-spreading ridges, and are key to understanding how crustal accretion varies with spreading rate and magma supply. Ultraslow-spreading ridges provide analogs for ophiolites, to identify those that may have formed under similar conditions. Parallel studies of modern ocean lithosphere and ophiolites therefore can uniquely inform the origin and genesis of both. Here we report the results of structural and petrological studies on the Xigaze ophiolite in the Tibetan Plateau, and compare it to the morphology and deep drilling results at the ultraslow-spreading Southwest Indian Ridge. The Xigaze ophiolite has a complete but laterally discontinuous crust, with discrete diabase dikes/sills cutting both mantle and lower crust. The gabbro units are thin (∼350 m) and show upward cyclic chemical variations, supporting for an episodic and intermittent magma supply. These features are comparable to the highly focused magmatism and low magma budget at modern ultraslow-spreading ridges. Thus we suggest that the Xigaze ophiolite represents an on-land analog of ultraslow-spreading ocean lithosphere.

Detrital-Zircon Age Spectra of Neoproterozoic-Paleozoic Sedimentary Rocks from the Ereendavaa Terrane in NE Mongolia: Implications for the Early-Stage Evolution of the Ereendavaa Terrane and the Mongol-Okhotsk Ocean

The Mongol-Okhotsk orogenic belt (MOB) is considered to be the youngest division of the huge Central Asian Orogenic Belt, but its origin and evolution are still enigmatic. To better understand the history of the MOB, we conducted U-Pb geochronological analyses of detrital-zircon grains from Neoproterozoic-Paleozoic sedimentary sequences as well as a volcanic suite in the Ereendavaa terrane, the southern framing unit of the MOB, in NE Mongolia. Our results show that the protoliths of the quartzite assemblage of the Ereendavaa terrane basement (or proto-Ereendavaa terrane) was deposited after ca. 1.15 Ga on a passive continental margin. The detrital-zircon age spectra of the Silurian and Devonian sedimentary sequences of the terrane demonstrate that the source areas were dominated by proximal Cambrian-Ordovician arc rocks, likely resulting from the northward subduction of the Kherlen Ocean lithosphere beneath the Ereendavaa terrane. Based on a combination of our new data with those published, we show that the Mongol-Okhotsk Ocean split from an early Paleozoic domain during, or after, the early Silurian by a mantle plume, and developed an Andean-type margin along its northern rim possibly at Middle Devonian times, and a bidirection subduction system in mid-Carboniferous at approximately 325 Ma. This bipolar subduction of the Mongol-Okhotsk Ocean might have lasted until the Triassic.

Four Decades of Lithosphere and Solid Earth Research - ILP's Role as Stimulus

<p>The International Lithosphere Program (ILP) was established in 1980 as the Inter-Union Commission on the Lithosphere (ICL) by the International Council for Science (ICSU), following a request from the International Union of Geodesy and Geophysics (IUGG) and the International Union of Geological Sciences (IUGS). In 2005 ICSU transferred its sponsorship to IUGG and IUGS.</p><p>The ILP focusses on the nature, dynamics, origin, and evolution of the lithosphere, with special attention to the continents and their margins. Targeting these goals through international and interdisciplinary collaboration, ILP established several task forces and coordinating committees to pursue specific research objectives. Topics always follow one of the four ILP themes: global change, contemporary dynamics and deep processes, continental lithosphere, and ocean lithosphere. ILP’s funding is limited to five year periods and just understood as seed money.</p><p>In the last four decades ILP was involved in the composition and set up of a number of worldwide leading light house projects: The GSHAP (Global Seismic Hazard Map), the ICDP (International Continental Drilling Project), the WSM (World Stress Map Project), the TOPO-Europe project and its follow up initiatives TOPO-Asia, TOPO Iberia – just to name a few. Currently ILP supports new initiatives on digitalization.</p><p>With its Flinn-Hart Award (until 2007 Hart Award), honouring outstanding young scientists for contributions in the field of solid Earth sciences, ILP motivated and promoted a generation of early career scientists. The new Evgueni Burov Medal from ILP, established in 2018, pays tribute to an outstanding researcher in solid Earth sciences and recognizes pioneering contributions by mid-career scientists.</p>

Hotspot swells and the lifespan of volcanic ocean islands

<p>The seafloor surrounding ocean hotspots is typically 0.5 - 2 km shallower than expected for its age over areas hundreds to >1000 km wide, but the processes generating these bathymetric swells are uncertain. Two end-member models have been proposed to explain swell uplift. The first model, lithospheric thinning, posits that reheating of the lithosphere causes the seafloor to uplift due to the isostatic effect of replacing colder, denser lithosphere with hotter, less dense upper mantle. The second model, dynamic uplift, proposes that swells are supported by upward flow of ascending mantle plumes and/or hot, buoyant plume material ponded beneath the swell lithosphere. If swells are dominantly produced by lithospheric thinning, the resulting thermal subsidence should approximately mimic the subsidence of young ocean lithosphere. This places an upper bound on the rate of seafloor and island subsidence following swell uplift, since conductive cooling of the lithosphere is a gradual process. On the other hand, if swell topography is dominantly produced by dynamic uplift, then seafloor subsidence depends primarily on how rapidly plate motion carries the seafloor off the swell and the spatial extent of the swell.<br><br>Because these two models predict different patterns of seafloor and island subsidence, swell morphology and the geologic record of island drowning may reveal which of these mechanisms dominates the process of swell uplift. To test this, we isolated regional swell bathymetry at 14 ocean hotspots. Considering the end-member case of lithospheric thinning, we modeled the thermal evolution of the lithosphere at each hotspot following swell uplift, and we compared the resulting thermal subsidence to observed swell subsidence. We also estimated island residence times atop swell bathymetry (swell length/plate velocity), and we compared this residence time to the age at which islands typically drown in each hotspot island chain. We found that observed swell subsidence significantly outpaces thermal subsidence. Moreover, island drowning ages match swell residence times, suggesting that islands and the seafloor subside as tectonic plate motion transports them past mantle sources of swell uplift. This correspondence argues strongly for dynamic uplift of the lithosphere at ocean hotspots. Our results also explain global variations in island lifespan on fast- and slow-moving tectonic plates (e.g. drowned islands in the Galápagos <4 million years (Ma) old versus islands >20 Ma above sea level in the Canary Islands), which profoundly influence island topography, biodiversity, and climate.</p>

VOLCANO-TECTONIC ACTIVITY OF OCEAN LITHOSPHERE IN THE EASTERN SECTOR OF THE PACIFIC OCEAN

Based on the analysis of satellite altimetry and deep-water geological and geophysical surveys, it was established that there is a close connection between underwater volcanoes and tectonics on the crest of the East Pacific Rise, on its western flank and in the deep-water basin. Underwater volcanoes are characterized by different sizes; their basalts are characterized by variable degree of differentiation. The aggregated results denote the existence and relevance of studying the major problem of the volcanic-tectonic activity of the oceanic lithosphere at different stages of its evolution.