scholarly journals No ring fracture in Mono Basin, California

2021 ◽  
Author(s):  
Wes Hildreth ◽  
Judy Fierstein ◽  
Juliet Ryan-Davis
Keyword(s):  
Shear Zone ◽  
Circular Ring ◽  
Middle Pleistocene ◽  
Low Velocity ◽  
Ring Fracture ◽  
Magma Body ◽  
Rhyolitic Magma ◽  
Velocity Anomaly ◽  
Shear Structures ◽  
Northern Half

In Mono Basin, California, USA, a near-circular ring fracture 12 km in diameter was proposed by R.W. Kistler in 1966 to have originated as the protoclastic margin of the Cretaceous Aeolian Buttes pluton, to have been reactivated in the middle Pleistocene, and to have influenced the arcuate trend of the chain of 30 young (62−0.7 ka) rhyolite domes called the Mono Craters. In view of the frequency and recency of explosive eruptions along the Mono chain, and because many geophysicists accepted the ring fracture model, we assembled evidence to test its plausibility. The shear zone interpreted as the margin of the Aeolian Buttes pluton by Kistler is 50−400 m wide but is exposed only along a 7-km-long set of four southwesterly outcrops that subtend only a 70° sector of the proposed ring. The southeast end of the exposed shear zone is largely within the older June Lake pluton, and at its northwest end, the contact of the Aeolian Buttes pluton with a much older one crosses the shear zone obliquely. Conflicting attitudes of shear structures are hard to reconcile with intrusive protoclasis. Also inconsistent with the margin of the ovoid intrusion proposed by Kistler, unsheared salients of the pluton extend ∼1 km north of its postulated circular outline at Williams Butte, where there is no fault or other structure to define the northern half of the hypothetical ring. The shear zone may represent regional Cretaceous transpression rather than the margin of a single intrusion. There is no evidence for the Aeolian Buttes pluton along the aqueduct tunnel beneath the Mono chain, nor is there evidence for a fault that could have influenced its vent pattern. The apparently arcuate chain actually consists of three linear segments that reflect Quaternary tectonic influence and not Cretaceous inheritance. A rhyolitic magma reservoir under the central segment of the Mono chain has erupted many times in the late Holocene and as recently as 700 years ago. The ring fracture idea, however, prompted several geophysical investigations that sought a much broader magma body, but none identified a low-density or low-velocity anomaly beneath the purported 12-km-wide ring, which we conclude does not exist.

The case for a shallow-crustal anomalous zone (magma body?) near the south end of Hilton Creek fault, California, including new evidence from an interpretation of pre-S arrivals

1989 ◽  
Vol 79 (3) ◽  
pp. 805-812
Author(s):  
William A. Peppin ◽  
William Honjas ◽  
Thomas W. Delaplain ◽  
Ute R. Vetter
Keyword(s):  
Seismic Gap ◽  
The South ◽  
S Wave ◽  
Low Velocity ◽  
Magma Body ◽  
Velocity Anomaly ◽  
Anomalous Zone ◽  
Double Couple ◽  
A Site ◽  
New Evidence

Abstract Seven independent lines of evidence can be cited for the existence of a shallow-crustal anomalous zone at a site near the south end of Hilton Creek fault, near Mammoth Lakes, California. They are: (1) the presence of a persistent seismic gap since detailed observations began in 1979; (2) S-wave shadowing for travel paths crossing the site; (3) a low-velocity anomaly associated with the south end of Hilton Creek fault discovered by seismic tomography; (4) the observation of two non-double-couple mechanisms for large earthquakes a few km east and west of the site; (5) a concentration of pre-S-arrivals indicative of a possible reflection in the vicinity of this site; (6) a geometric symmetry of source-receiver paths showing pre-S arrivals about this site; and (7) [new evidence presented herein] travel-time fits of strong pre-S arrivals recorded at epicenter of the 1978 Wheeler Crest event (ML 5.7).

Magma accumulation beneath Santorini volcano, Greece, from P-wave tomography

Geology ◽  
2019 ◽  
Vol 48 (3) ◽  
pp. 231-235 ◽  
Author(s):  
B.G. McVey ◽  
E.E.E. Hooft ◽  
B.A. Heath ◽  
D.R. Toomey ◽  
M. Paulatto ◽  
...  
Keyword(s):  
Three Dimensional ◽  
Seismic Attenuation ◽  
P Wave ◽  
Structural Constraints ◽  
Low Velocity ◽  
Magma Body ◽  
Velocity Anomaly ◽  
Velocity Models ◽  
Santorini Volcano ◽  
Ray Bending

Abstract Despite multidisciplinary evidence for crustal magma accumulation below Santorini volcano, Greece, the structure and melt content of the shallow magmatic system remain poorly constrained. We use three-dimensional (3-D) velocity models from tomographic inversions of active-source seismic P-wave travel times to identify a pronounced low-velocity anomaly (–21%) from 2.8 km to 5 km depth localized below the northern caldera basin. This anomaly is consistent with depth estimates of pre-eruptive storage and a recent inflation episode, supporting the interpretation of a shallow magma body that causes seismic attenuation and ray bending. A suite of synthetic tests shows that the geometry is well recovered while a range of melt contents (4%–13% to fully molten) are allowable. A thin mush region (2%–7% to 3%–10% melt) extends from the main magma body toward the northeast, observed as low velocities confined by tectono-magmatic lineaments. This anomaly terminates northwest of Kolumbo; little to no melt underlies the seamount from 3 to 5 km depth. These structural constraints suggest that crustal extension and edifice loads control the geometry of magma accumulation and emphasize that the shallow crust remains conducive to melt storage shortly after a caldera-forming eruption.

Evolution of the East Africa-Arabia plume head

2021 ◽  
Author(s):  
Chiara Civiero ◽  
Sergei Lebedev ◽  
Nicolas L. Celli
Keyword(s):  
East Africa ◽  
Basic Mechanism ◽  
Continental Lithosphere ◽  
Gulf Of Aden ◽  
Low Velocity ◽  
Velocity Anomaly ◽  
Seismic Waveform ◽  
Igneous Provinces ◽  
Lithospheric Plates ◽  
Plate Reconstructions

<p>Hot plumes rising from Earth’s deep mantle are thought to form broad plume heads beneath lithospheric plates. In continents, mantle plumes cause uplift, rifting and volcanism, often dispersed over surprisingly broad areas. Using seismic waveform tomography, we image <span>a star-shaped, low-velocity anomaly centered at Afar and composed of three narrow branches: beneath East Africa, beneath the Gulf of Aden, and beneath the Red Sea and West Arabia, extending north to Levant. We interpret this anomaly as the seismic expression of </span>interconnected corridors of hot, partially molten rock beneath the East Africa-Arabia region. The corridors underlie areas of uplift, rifting and volcanism and accommodate an integral, active plume head. Eruption ages and plate reconstructions indicate that it developed south-to-north, and tomography shows it being fed by three deep upwellings beneath Kenya, Afar and Levant. <span>These results demonstrate the complex feedbacks between the continental-lithosphere heterogeneity and plume-head evolution. </span>Star-shaped plume heads sprawling within thin-lithosphere valleys can account for the enigmatic dispersed volcanism in large igneous provinces and are likely to be a basic mechanism of plume-continent interaction.</p>

Three-dimensional seismic structure of the lithosphere under Montana Lasa

1976 ◽  
Vol 66 (2) ◽  
pp. 501-524
Author(s):  
Keiiti Aki ◽  
Anders Christoffersson ◽  
Eystein S. Husebye
Keyword(s):  
Generalized Inverse ◽  
Random Medium ◽  
Three Dimensional ◽  
P Wave ◽  
Seismic Structure ◽  
Low Velocity ◽  
Medium Theory ◽  
Velocity Anomaly ◽  
Inversion Techniques ◽  
Teleseismic Events

abstract Using P-wave residuals for teleseismic events observed at the Montana Large Aperture Seismic Array (LASA), we have determined the three-dimensional seismic structure of the lithosphere under the array to a depth of 140 km. The root-mean-square velocity fluctuation was found to be at least 3.2 per cent which may be compared to estimate of ca. 2 per cent based on the Chernov random medium theory. The solutions are given by both the generalized inverse and stochastic inverse methods in order to demonstrate the relative merit of different inversion techniques. The most conspicuous feature of the lithosphere under LASA is a low-velocity anomaly in the central and northeast part of the array siting area with the N60°E trend and persisting from the upper crust to depths greater than 100 km. We interpret this low-velocity anomaly as a zone of weakness caused by faulting and shearing associated with the building of the Rocky Mountains.

scholarly journals Crustal velocity structure of the Apennines (Italy) from P-wave travel time tomography

1996 ◽  
Vol 39 (6) ◽  
Author(s):  
C. Chiarabba ◽  
A. Amato
Keyword(s):  
Velocity Structure ◽  
P Wave ◽  
Depth Interval ◽  
Low Velocity ◽  
Data Set ◽  
Arrival Times ◽  
Velocity Anomaly ◽  
Minimum Number ◽  
Velocity Images ◽  
Lower Crustal

In this paper we provide P-wave velocity images of the crust underneath the Apennines (Italy), focusing on the lower crustal structure and the Moho topography. We inverted P-wave arrival times of earthquakes which occurred from 1986 to 1993 within the Apenninic area. To overcome inversion instabilities due to noisy data (we used bulletin data) we decided to resolve a minimum number of velocity parameters, inverting for only two layers in the crust and one in the uppermost mantle underneath the Moho. A partial inversion of only 55% of the overall dataset yields velocity images similar to those obtained with the whole data set, indicating that the depicted tomograms are stable and fairly insensitive to the number of data used. We find a low-velocity anomaly in the lower crust extending underneath the whole Apenninic belt. This feature is segmented by a relative high-velocity zone in correspondence with the Ortona-Roccamonfina line, that separates the northern from the southern Apenninic arcs. The Moho has a variable depth in the study area, and is deeper (more than 37 km) in the Adriatic side of the Northern Apennines with respect to the Tyrrhenian side, where it is found in the depth interval 22-34 km.

scholarly journals Transition from continental rifting to oceanic spreading in the northern Red Sea area

2021 ◽  
Author(s):  
Sami El Khrepy ◽  
Ivan Koulakov ◽  
Nassir Al-Arifi ◽  
Mamdouh S. Alajmi ◽  
Ayman N. Qadrouh
Keyword(s):  
Red Sea ◽  
Plate Tectonics ◽  
Continental Rifting ◽  
Low Velocity ◽  
Lower Velocity ◽  
Velocity Anomaly ◽  
Oceanic Spreading ◽  
Lithospheric Extension ◽  
Sea Area ◽  
Northern Red Sea

<p><strong>Lithosphere extension, which plays an essential role in plate tectonics, occurs both in continents (as rift systems) and oceans (spreading along mid-oceanic ridges). The northern Red Sea area is a unique natural geodynamic laboratory, where the ongoing transition from continental rifting to oceanic spreading can be observed. Here, we analyze travel time data from a merged catalogue provided by the Egyptian and Saudi Arabian seismic networks to build a three-dimensional model of seismic velocities in the crust and uppermost mantle beneath the northern Red Sea and surroundings. The derived structures clearly reveal a high-velocity anomaly coinciding with the Red Sea basin and a narrow low-velocity anomaly centered along the rift axis. We interpret these structures as a transition of lithospheric extension from continental rifting to oceanic spreading. The transitional lithosphere is manifested by a dominantly positive seismic anomaly indicating the presence of a 50–70-km-thick and 200–300-km-wide cold lithosphere. Along the forming oceanic ridge axis, an elongated low-velocity anomaly marks a narrow localized nascent spreading zone that disrupts the transitional lithosphere. Along the eastern margins of the Red Sea, the lithosphere is disturbed by the lower-velocity anomalies coinciding with areas of basaltic magmatism.</strong></p>

The upper mantle beneath the South Atlantic Ocean, South America and Africa from waveform tomography with massive data sets

2020 ◽  
Vol 221 (1) ◽  
pp. 178-204 ◽  
Author(s):  
N L Celli ◽  
S Lebedev ◽  
A J Schaeffer ◽  
M Ravenna ◽  
C Gaina
Keyword(s):  
South America ◽  
Upper Mantle ◽  
High Velocity ◽  
South Atlantic ◽  
The South ◽  
Low Velocity ◽  
The West ◽  
Velocity Anomaly ◽  
Archean Crust ◽  
Velocity Anomalies

SUMMARY We present a tomographic model of the crust, upper mantle and transition zone beneath the South Atlantic, South America and Africa. Taking advantage of the recent growth in broadband data sampling, we compute the model using waveform fits of over 1.2 million vertical-component seismograms, obtained with the automated multimode inversion of surface, S and multiple S waves. Each waveform provides a set of linear equations constraining perturbations with respect to a 3-D reference model within an approximate sensitivity volume. We then combine all equations into a large linear system and solve it for a 3-D model of S- and P-wave speeds and azimuthal anisotropy within the crust, upper mantle and uppermost lower mantle. In South America and Africa, our new model SA2019 reveals detailed structure of the lithosphere, with structure of the cratons within the continents much more complex than seen previously. In South America, lower seismic velocities underneath the transbrasilian lineament (TBL) separate the high-velocity anomalies beneath the Amazon Craton from those beneath the São Francisco and Paraná Cratons. We image the buried portions of the Amazon Craton, the thick cratonic lithosphere of the Paraná and Parnaíba Basins and an apparently cratonic block wedged between western Guyana and the slab to the west of it, unexposed at the surface. Thick cratonic lithosphere is absent under the Archean crust of the São Luis, Luis Álves and Rio de La Plata Cratons, next to the continental margin. The Guyana Highlands are underlain by low velocities, indicating hot asthenosphere. In the transition zone, we map the subduction of the Nazca Plate and the Chile Rise under Patagonia. Cratonic lithosphere beneath Africa is more fragmented than seen previously, with separate cratonic units observed within the West African and Congo Cratons, and with cratonic lithosphere absent beneath large portions of Archean crust. We image the lateral extent of the Niassa Craton, hypothesized previously and identify a new unit, the Cubango Craton, near the southeast boundary of the grater Congo Craton, with both of these smaller cratons unexposed at the surface. In the South Atlantic, the model reveals the patterns of interaction between the Mid-Atlantic Ridge (MAR) and the nearby hotspots. Low-velocity anomalies beneath major hotspots extend substantially deeper than those beneath the MAR. The Vema Hotspot, in particular, displays a pronounced low-velocity anomaly under the thick, high-velocity lithosphere of the Cape Basin. A strong low velocity anomaly also underlies the Cameroon Volcanic Line and its offshore extension, between Africa and the MAR. Subtracting the global, age-dependent VS averages from those in the South Atlantic Basins, we observe areas where the cooling lithosphere is locally hotter than average, corresponding to the location of the Tristan da Cunha, Vema and Trindade hotspots. Beneath the anomalously deep Argentine Basin, we image unusually thick, high-velocity lithosphere, which suggests that its anomalously great depth can be explained, at least to a large extent, by isostatic, negative lithospheric buoyancy.

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