Structure of the upper mantle in the north-western and central United States from USArray S-receiver functions

We used more than 40 000 S-receiver functions recorded by the USArray project to study the structure of the upper mantle between the Moho and the 410 km discontinuity from the Phanerozoic western United States to the cratonic central US. In the western United States we observed the lithosphere–asthenosphere boundary (LAB), and in the cratonic United States we observed both the mid-lithospheric discontinuity (MLD) and the LAB of the craton. In the northern and southern United States the western LAB almost reaches the mid-continental rift system. In between these two regions the cratonic MLD is surprisingly plunging towards the west from the Rocky Mountain Front to about 200 km depth near the Sevier thrust belt. We interpret these complex structures of the seismic discontinuities in the mantle lithosphere as an indication of interfingering of the colliding Farallon and Laurentia plates. Unfiltered S-receiver function data reveal that the LAB and MLD are not single discontinuities but consist of many small-scale laminated discontinuities, which only appear as single discontinuities after longer period filtering. We also observe the Lehmann discontinuity below the LAB and a velocity reduction about 30 km above the 410 km discontinuity

The lithosphere-asthenosphere boundary observed with USArray receiver functions

The dense deployment of seismic stations so far in the western half of the United States within the USArray project provides the opportunity to study in greater detail the structure of the lithosphere-asthenosphere system. We use the S receiver function technique for this purpose, which has higher resolution than surface wave tomography, is sensitive to seismic discontinuities, and is free from multiples, unlike P receiver functions. Only two major discontinuities are observed in the entire area down to about 300 km depth. These are the crust-mantle boundary (Moho) and a negative boundary, which we correlate with the lithosphere-asthenosphere boundary (LAB), since a low velocity zone is the classical definition of the seismic observation of the asthenosphere by Gutenberg (1926). Our S receiver function LAB is at a depth of 70–80 km in large parts of westernmost North America. East of the Rocky Mountains, its depth is generally between 90 and 110 km. Regions with LAB depths down to about 140 km occur in a stretch from northern Texas, over the Colorado Plateau to the Columbia basalts. These observations agree well with tomography results in the westernmost USA and on the east coast. However, in the central cratonic part of the USA, the tomography LAB is near 200 km depth. At this depth no discontinuity is seen in the S receiver functions. The negative signal near 100 km depth in the central part of the USA is interpreted by Yuan and Romanowicz (2010) and Lekic and Romanowicz (2011) as a recently discovered mid-lithospheric discontinuity (MLD). A solution for the discrepancy between receiver function imaging and surface wave tomography is not yet obvious and requires more high resolution studies at other cratons before a general solution may be found. Our results agree well with petrophysical models of increased water content in the asthenosphere, which predict a sharp and shallow LAB also in continents (Mierdel et al., 2007)

Seismotectonics of the Pamir

Based on a 2 year seismic record from a local network, we characterize the deformation of the seismogenic crust of the Pamir in the northwestern part of the India-Asia collision zone. We located more than 6000 upper crustal earthquakes in a regional 3-D velocity model. For 132 of these events, we determined source mechanisms, mostly through full waveform moment tensor inversion of locally and regionally recorded seismograms. We also produced a new and comprehensive neotectonic map of the Pamir, which we relate to the seismic deformation. Along Pamir's northern margin, where GPS measurements show significant shortening, we find thrust and dextral strike-slip faulting along west to northwest trending planes, indicating slip partitioning between northward thrusting and westward extrusion. An active, north-northeast trending, sinistral transtensional fault system dissects the Pamir's interior, connecting the lakes Karakul and Sarez, and extends by distributed faulting into the Hindu Kush of Afghanistan. East of this lineament, the Pamir moves northward en bloc, showing little seismicity and internal deformation. The western Pamir exhibits a higher amount of seismic deformation; sinistral strike-slip faulting on northeast trending or conjugate planes and normal faulting indicate east-west extension and north-south shortening. We explain this deformation pattern by the gravitational collapse of the western Pamir Plateau margin and the lateral extrusion of Pamir rocks into the Tajik-Afghan depression, where it causes thin-skinned shortening of basin sediments above an evaporitic décollement. Superposition of Pamir's bulk northward movement and collapse and westward extrusion of its western flank causes the gradual change of surface velocity orientations from north-northwest to due west observed by GPS geodesy. The distributed shear deformation of the western Pamir and the activation of the Sarez-Karakul fault system may ultimately be caused by the northeastward propagation of India's western transform margin into Asia, thereby linking deformation in the Pamir all the way to the Chaman fault in the south in Afghanistan

The crust in the pamir: Insights from receiver functions

The Cenozoic convergence between India and Asia has created Earth's thickest crust in the Pamir‐Tibet Plateau by extreme crustal shortening. Here we study the crustal structure of the Pamir and western Tian Shan, the adjacent margins of the Tajik, Tarim, and Ferghana Basins, and the Hindu Kush, using data collected by temporary seismic experiments. We derive, compare, and combine independent observations from P and S receiver functions. The obtained Moho depth varies from ~40 km below the basins to a double‐normal thickness of 65–75 km underneath the Pamir and Hindu Kush. A Moho doublet—with the deeper interface down to a depth of ~90 km—coincides with the arc of intermediate‐depth seismicity underneath the Pamir, where Asian continental lower crust delaminates and rolls back. The crust beneath most of the Central and South Pamir has a low Vp/Vs ratio (<1.70), suggesting a dominantly felsic composition, probably a result of delamination/foundering of the mafic rocks of the lower crust. Beneath the Cenozoic gneiss domes of the Central and South Pamir, which represent extensional core complexes, the Vp/Vs ratios are moderate to high (~1.75), consistent with the previously observed, midcrustal low‐velocity zones, implying the presence of crustal partial melts. Even higher crustal average Vp/Vs ratios up to 1.90 are found in the sedimentary basins and along the Main Pamir Thrust. The ratios along the latter—the active thrust front of the Pamir—may reflect fluid accumulations within a strongly fractured fault system

Amphibious Seismic Survey Images Plate Interface at 1960 Chile Earthquake

The southern central Chilean margin at the site of the largest historically recorded earthquake in the Valdivia region, in 1960 (Mw = 9.5), is part of the 5000-km-long active subduction system whose geodynamic evolution is controversially debated and poorly understood. Covering the area between 36° and 40°S, the oceanic crust is segmented by prominent fracture zones. The offshore forearc and its onshore continuation show a complex image with segments of varying geophysical character, and several fault systems active during the past 24 m.y. In autumn 2001, the project SPOC was organized to study the Subduction Processes Off Chile, with a focus on the seismogenic coupling zone and the forearc. The acquired seismic data crossing the Chilean subduction system were gathered in a combined offshore-onshore survey and provide new insights into the lithospheric structure and evolution of active margins with insignificant frontal accretion

Сейсмічний експеримент TTZ-South

The wide-angle reflection and refraction (WARR) TTZ-South transect carried out in 2018 crosses the SW region of Ukraine and the SE region of Poland. The TTZ-South profile targeted the structure of the Earth’s crust and upper mantle of the Trans-European Suture Zone, as well as the southwestern segment of the East European Craton (slope of the Ukrainian Shield). The ~550 km long profile (~230 km in Poland and ~320 km in western Ukraine) is an extension of previously realized projects in Poland, TTZ (1993) and CEL03 (2000). The deep seismic sounding study along the TTZ-South profile using TEXAN and DATA-CUBE seismic stations (320 units) made it possible to obtain high-quality seismic records from eleven shot points (six in Ukraine and five in Poland). This paper presents a smooth P-wave velocity model based on first-arrival travel-time inversion using the FAST (First Arrival Seismic Tomography) code.The obtained image represents a preliminary velocity model which, according to the P-wave velocities, consists of a sedimentary layer and the crystalline crust that could comprise  upper, middle and lower crustal layers. The Moho interface, approximated by the 7.5 km/s isoline, is located at 45—47 km depth in the central part of the profile, shallowing to 40 and 37 km depth in the northern (Radom-Łysogóry Unit, Poland) and southern (Volyno-Podolian Monocline, Ukraine) segments of the profile, respectively. A peculiar feature of the velocity cross-section is a number of high-velocity bodies distinguished in the depth range of 10—35 km. Such high-velocity bodies were detected previously in the crust of the Radom-Łysogóry Unit. These bodies, inferred at depths of 10—35 km, could be allochthonous fragments of what was originally a single mafic body or separate mafic bodies intruded into the crust during the break-up of Rodinia in the Neoproterozoic, which was accompanied by considerable rifting. The manifestations of such magmatism are known in the NE part of the Volyno-Podolian Monocline, where the Vendian trap formation occurs at the surface.Сейсмический профиль TTZ-South с использованием преломленных и отраженных в закритической области преломленных волн, отработанный в 2018 г., пересекает юго-западный район Украины и юго-восточный регион Польши. Профиль TTZ-South был направлен на изучение структуры земной коры и верхней мантии Трансъевропейской шовной зоны (ТЕШЗ) и юго-западного сегмента Восточно-Европейского кратона (склона Украинского щита). Профиль длиной ~550 км (~230 км в Польше и ~320 км на западе Украины) является продолжением ранее реализованных проектов в Польше — профиля TTZ (1993 г.) и CEL03 (2000 г.). Глубинное сейсмическое зондирование по профилю TTZ-South, выполненное с использованием 320 сейсмических станций TEXAN и DATA-CUBE, позволило получить сейсмические записи высокого качества из одиннадцати пунктов взрыва (шесть в Украине и пять в Польше). В данной статье представлена упрощенная Р-скоростная модель, основанная на инверсии времен пробега первых вступлений Р-волн, построенная с использованием программы сейсмической томографии первых вступлений FAST. Полученное изображение представляет собой предварительную скоростную модель, которая состоит из осадочного слоя и кристаллической коры, включающей верхний, средний и нижний ее слои. Поверхность Мохо, аппроксимируемая изолинией 7,5 км/с, расположена на глубине 45—47 км в центральной части профиля, воздымаясь до 40 и 37 км в северной (Радом-Лысогорский блок в Польше) и южной (Волыно-Подольская моноклиналь в Украине) частях профиля соответственно. Особенностью скоростного разреза является ряд высокоскоростных тел, выявленных в диапазоне глубин 10—35 км. Аналогичные высокоскоростные тела ранее были обнаружены в коре Радом-Лысогорского блока. Тела, обнаруженные на глубине 10—35 км, могут быть аллохтонными фрагментами изначально единого массива основных пород или отдельными телами основного состава, внедрившимися в кору в неопротерозое во время раскола суперконтинета Родиния, который сопровождался мощным рифтогенезом. Проявления рифтогенного магматизма известны в северо-восточной части Волыно-Подольской моноклинали, где на поверхность выходят вендские трапы.Сейсмічний профіль TTZ-South з використанням заломлених і відбитих у за критичній зоні заломлених хвиль, відпрацьований у 2018 р., перетинає південно-західний район України і південно-східний регіон Польщі. Профіль TTZ-South був спрямований на вивчення структури земної кори і верхньої мантії Транс'єв ропейської шовної зони (ТЄШЗ) і південно-західного сегмента Східно-Європейського кратона (схила Українського щита). Профіль довжиною ~550 км (~230 км в Польщі і ~320 км на заході України) є продовженням раніше реалізованих проєктів у Польщі — профілю TTZ (1993 р.) і CEL03 (2000 р). Глибинне сейсмічне зондування за профілем TTZ-South, виконане з використанням 320 сейсмічних станцій TEXAN і DATA-CUBE, дало змогу отримати сейсмічні записи високої якості з одинадцяти пунктів вибуху (шість в Україні і п'ять у Польщі). У даній статті представлена спрощена Р-швидкісна модель, що базується на інверсії часів пробігу перших вступів Р-хвиль, побудована з використанням програми сейсмічної томографії перших вступів FAST. Отримане зображення являє собою попередню швидкісну модель, яка складається з осадового шару і кристалічної кори, що включає верхній, середній і нижній її шари. Поверхня Мохо, що апроксимується ізолінією 7,5 км/с, розташована на глибині 45—47 км у центральній частині профілю, здіймається до 40 і 37 км у північній (Радом-Лисогорський блок у Польщі) і південній (Волино-Подільська монокліналь в Україні) частинах профілю відповідно. Особливістю швидкісного розрізу є ряд високошвидкісних тіл, виявлених у діапазоні глибин 10—35 км. Подібні високошвидкісні тіла раніше були виявлені в корі Радом-Лисогірського блоку. Тіла, виявлені на глибині 10—35 км, можуть бути алохтонними фрагментами спочатку єдиного масиву основних порід або окремими тілами основного складу, що впровадилися в кору в неопротерозої під час розколу суперконтінета Родінія, який супроводжувався потужним рифтогенезом. Прояви рифтогенного магматизму відомі в північно-східній частині Волино-Подільської моноклінали, де на поверхню виходять вендські трапи

Images of crustal and mantle structure across the northern margin of the Tibet-Pamir plateau

Abstract HKT-ISTP 2013 A

Evidence for subduction of Asian continental lithosphere under the Pamir from lithospheric imaging

Abstract HKT-ISTP 2013 A

Upper mantle temperature and the onset of extension and break-up in Afar, Africa

It is debated to what extent mantle plumes play a role in continental rifting and eventual break-up. Afar lies at the northern end of the largest and most active present-day continental rift, where the East African Rift forms a triple junction with the Red Sea and Gulf of Aden rifts. It has a history of plume activity yet recent studies have reached conflicting conclusions on whether a plume still contributes to current Afar tectonics. A geochemical study concluded that Afar is a mature hot rift with 80 km thick lithosphere, while seismic data have been interpreted to reflect the structure of a young, oceanic rift basin above mantle of normal temperature. We develop a self-consistent forward model of mantle flow that incorporates melt generation and retention to test whether predictions of melt chemistry, melt volume and lithosphere–asthenosphere seismic structure can be reconciled with observations. The rare- earth element composition of mafic samples at the Erta Ale, Dabbahu and Asal magmatic segments can be used as both a thermometer and chronometer of the rifting process. Low seismic velocities require a lithosphere thinned to 50 km or less. A strong positive impedance contrast at 50 to 70 km below the rift seems linked to the melt zone, but is not reproduced by isotropic seismic velocity alone. Combined, the simplest interpretation is that mantle temperature below Afar is still elevated at 1450◦C, rifting started around 22–23 Ma, and the lithosphere has thinned from 100 to 50 km to allow significant decompressional melting