Workshop report on hard-rock drilling into mid-Cretaceous Pacific oceanic crust on the Hawaiian North Arch

Abstract. The architecture, formation, and modification of oceanic plates are fundamental to our understanding of key geologic processes of the Earth. Geophysical surveys were conducted around a site near the Hawaiian Islands (northeastern Hawaiian North Arch region; Hawaiian North Arch hereafter), which is one of three potential sites for an International Ocean Discovery Program mantle drilling proposal for the Pacific plate that was submitted in 2012. The Hawaiian North Arch site is located in 78–81 Ma Cretaceous crust, which had an estimated full spreading rate of 7–8 cm yr−1. This site fills a major gap in our understanding of oceanic crust. Previously drilling has been skewed to young or older crust (<15 or >110 Ma) and slow-spread crust. P-wave velocity structure in the uppermost mantle of the Hawaiian North Arch shows a strong azimuthal anisotropy, whereas Moho reflections below the basement are variable: strong and continuous, weak, diffuse, or unclear. We assume that the strength of the Moho reflection is related to the aging of the oceanic plate. The Hawaiian volcanic chain (200 km to the southwest of the proposed drill site) and the nearby North Arch magmatism on the proposed Hawaiian North Arch sites might also have affected recognition of the Moho via deformation and/or magma intrusion into the lower crust of the uppermost mantle. This workshop report describes scientific targets for 2 km deep-ocean drilling in the Hawaiian North Arch region in order to provide information about the lower crust from unrecovered age and spreading rate gaps from previous ocean drillings. Other scientific objectives to be achieved by drilling cores before reaching the target depth of the project are also described in this report.

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P-wave Velocity Structure of the Lower Crust and Uppermost Mantle Beneath the Sichuan-Yunnan (China) Region

10.1002/essoar.10509051.1 ◽

2021 ◽

Author(s):

Zhi Wei ◽

Li Zhao

Keyword(s):

Wave Velocity ◽

Lower Crust ◽

Velocity Structure ◽

P Wave ◽

Uppermost Mantle ◽

P Wave Velocity ◽

China Region ◽

P Wave Velocity Structure

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Crustal and Uppermost Mantle Isotropic and Anisotropic P-wave Velocity Variations Beneath Turkey

10.5194/egusphere-egu21-12340 ◽

2021 ◽

Author(s):

Tuna Eken ◽

Haibo Wang ◽

Zhouchuan Huang ◽

Derya Keleş ◽

Tulay Kaya-Eken ◽

...

Keyword(s):

Lower Crust ◽

Azimuthal Anisotropy ◽

P Wave ◽

Uppermost Mantle ◽

Western Turkey ◽

Velocity Models ◽

South Eastern ◽

The North ◽

Eastern Turkey ◽

Major Surface

Turkey has been undergoing compressional and extensional tectonics that greatly influences the major surface features following northward plate convergences since the Miocene. Despite increasing efforts in last few decades aiming to elucidate the current architecture of the crust and mantle beneath Turkey, several issues regarding the depth extent of the deformation zones, crust&#8208;mantle interaction (e.g., coupling and decoupling) in relation to the deformation, and stress transmission in the lithosphere remain elusive. Inversion of 204,531 P wave arrival times from 8,103 local crustal earthquakes yields high&#8208;resolution 3&#8208;D P wave isotropic and azimuthal anisotropic velocity models of the crust and uppermost mantle beneath Turkey. Major outcomes of the present work are low&#8208;velocity anomalies or velocity contrasts down to the uppermost mantle along the North and East Anatolian Fault Zones. We observe the fast velocity directions (FVDs) of azimuthal anisotropy in the lower crust and uppermost mantle parallel to the regional maximum extensional directions in western Turkey, whereas parallel to the surface structures in the crust and uppermost mantle beneath south-eastern Turkey. Our isotropic/anisotropic images strongly imply vertically coherent deformation between the crust and uppermost mantle in western and south-eastern Turkey. However, in central northern Turkey, the FVDs in the uppermost mantle are oblique to both the FVDs in the lower crust and the maximum shear directions derived from GPS measurements, suggesting that the crust and lithospheric mantle are decoupled.

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Reconnaissance seismic refraction-reflection surveys in northwestern New Mexico

Bulletin of the Seismological Society of America ◽

10.1785/bssa0740041263 ◽

1984 ◽

Vol 74 (4) ◽

pp. 1263-1274

Author(s):

Lawrence H. Jaksha ◽

David H. Evans

Keyword(s):

New Mexico ◽

Wave Velocity ◽

Lower Crust ◽

Seismic Waves ◽

Seismic Refraction ◽

Velocity Model ◽

P Wave ◽

Uppermost Mantle ◽

P Wave Velocity ◽

Crustal Thinning

Abstract A velocity model of the crust in northwestern New Mexico has been constructed from an interpretation of direct, refracted, and reflected seismic waves. The model suggests a sedimentary section about 3 km thick with an average P-wave velocity of 3.6 km/sec. The crystalline upper crust is 28 km thick and has a P-wave velocity of 6.1 km/sec. The lower crust below the Conrad discontinuity has an average P-wave velocity of about 7.0 km/sec and a thickness near 17 km. Some evidence suggests that velocity in both the upper and lower crust increases with depth. The P-wave velocity in the uppermost mantle is 7.95 ± 0.15 km/sec. The total crustal thickness near Farmington, New Mexico, is about 48 km (datum = 1.6 km above sea level), and there is evidence for crustal thinning to the southeast.

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Structure of oceanic crust in back-arc basins modulated by mantle source heterogeneity

Geology ◽

10.1130/g48407.1 ◽

2020 ◽

Author(s):

Ingo Grevemeyer ◽

Shuichi Kodaira ◽

Gou Fujie ◽

Narumi Takahashi

Keyword(s):

Oceanic Crust ◽

Mantle Source ◽

Seismic Data ◽

Lower Crust ◽

Subduction Zones ◽

P Wave ◽

Spreading Ridge ◽

Volcanic Arc ◽

Depleted Mantle ◽

Back Arc

Subduction zones may develop submarine spreading centers that occur on the overriding plate behind the volcanic arc. In these back-arc settings, the subducting slab controls the pattern of mantle advection and may entrain hydrous melts from the volcanic arc or slab into the melting region of the spreading ridge. We recorded seismic data across the Western Mariana Ridge (WMR, northwestern Pacific Ocean), a remnant island arc with back-arc basins on either side. Its margins and both basins show distinctly different crustal structure. Crust to the west of the WMR, in the Parece Vela Basin, is 4–5 km thick, and the lower crust indicates seismic P-wave velocities of 6.5–6.8 km/s. To the east of the WMR, in the Mariana Trough Basin, the crust is ~7 km thick, and the lower crust supports seismic velocities of 7.2–7.4 km/s. This structural diversity is corroborated by seismic data from other back-arc basins, arguing that a chemically diverse and heterogeneous mantle, which may differ from a normal mid-ocean-ridge–type mantle source, controls the amount of melting in back-arc basins. Mantle heterogeneity might not be solely controlled by entrainment of hydrous melt, but also by cold or depleted mantle invading the back-arc while a subduction zone reconfigures. Crust formed in back-arc basins may therefore differ in thickness and velocity structure from normal oceanic crust.

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Velocity Structure of the Northeastern End of the Bayan Har Block, China, and the Seismogenic Environment of the Jiuzhaigou and Songpan-Pingwu Earthquakes: Inferences from Double-Difference Tomography

Bulletin of the Seismological Society of America ◽

10.1785/0120200224 ◽

2021 ◽

Author(s):

Wen Yang ◽

Zhifeng Ding ◽

Jie Liu ◽

Jia Cheng ◽

Xuemei Zhang ◽

...

Keyword(s):

Lower Crust ◽

Velocity Structure ◽

Joint Inversion ◽

Earthquake Swarm ◽

P Wave ◽

Double Difference ◽

The Tibetan Plateau ◽

Upper Crust ◽

Low Viscosity ◽

Bayan Har

ABSTRACT The 2017 Mw 6.5 Jiuzhaigou mainshock hit the northeastern end of the Bayan Har block, which has experienced many historical earthquakes, including the 1976 M 7.2 Songpan-Pingwu earthquake swarm. We used the double-difference tomography method to perform a joint inversion of the seismic source and P-wave velocity (VP) structure of the Jiuzhaigou-Songpan-Pingwu region. The results show significant lateral heterogeneity in the VP in the mid-upper crust. The velocity structure in the shallow crust correlates well with the surface geology. The Jiuzhaigou mainshock and Songpan-Pingwu earthquake swarm both occurred at the boundary between high- and low-VP anomalies. The Songpan-Pingwu earthquake swarm may be related to the eastward flow of low-viscosity material in the mid-lower crust of the Tibetan plateau. Low-viscosity material intrudes into the bedrock when it encounters the rigid Motianling massif, resulting in surface uplift and thrust earthquakes. By contrast, the Jiuzhaigou earthquake is associated with strain energy accumulating at the boundary between high- and low-VP anomalies related to the different movement rates of the low-VP material in the mid-lower crust and the high-VP body in the mid-upper crust. In this case, the high-VP body ruptures with a strike-slip sense to the southeast.

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Seismic detection of the low-velocity anomaly at the crust and uppermost mantle beneath the central Tien Shan

10.5194/egusphere-egu2020-3300 ◽

2020 ◽

Author(s):

Ran Cui ◽

Yuanze Zhou

Keyword(s):

Tarim Basin ◽

Velocity Structure ◽

Orogenic Belt ◽

Tien Shan ◽

P Wave ◽

Uppermost Mantle ◽

Low Velocity ◽

Velocity Anomaly ◽

Short Period ◽

Seismic Detection

As one of the most active intracontinental orogenic belts in the world, the Tien Shan orogenic belt&#160;originated&#160;in the&#160;Paleozoic and then experienced tectonic activities such as plate subduction and closure of the Paleo-Asian Ocean.&#160;Previous seismological and geodynamic studies have shown the observed the low-velocity anomaly (LVA)&#160;beneath the central Tien Shan at the uppermost mantle, which has a significant influence on the formation and modification of the crust and mantle lithosphere&#160;( Lei et al, 2007). However, the&#160;distribution, morphology and physical property of the LVA are highly debatable.We conduct 2-D forward waveform modeling based on spectral-element method (SEM) to investigate waveform distortions that were generated by the velocity contrast boundary of the LAV.&#160;The broadband P- and S-&#160;waves from three intermediate-depth&#160;earthquakes at Hindu Kush-Pamir were recorded by the Chinese&#160;Digital Seismograph Network&#160;(Zheng et al., 2010).&#160;We use these records to confirm the location, shape and velocity decrement of the LVA&#160;by fitting the observed records with the synthetics&#160;through SEM based on&#160;the 1D velocity structures (TSTB-B) of the central Tien Shan and northern Tarim basin (Gao et al., 2017). We find the LVA&#160;at 10~100 km beneath the eastern part of the central Tien Shan.&#160;And the northward under-thrusting of the Tarim&#160;Basin may trigger some&#160;mantle upwelling, contributing to the&#160;observed LVA.Lei, J.,&#160;Zhao, D.&#160;(2007). Teleseismic P-wave tomography and the upper mantle structure of the central Tien Shan orogenic belt. Physics of the Earth and Planetary Interiors, 162, 165-185, doi: 10.1016/j.pepi.200704010.Zheng, X., Jiao, W., Zhang, C., et al. (2010). Short-Period Rayleigh-Wave Group Velocity Tomography through Ambient Noise Cross-Correlation in Xinjiang, Northwest China. Bulletin of the Seismological Society of America,&#160;100(3): 1350-1355, doi: 10.1785/0120090225.Gao, Y., Cui, Q., Zhou, Y. (2017). Seismic detection of P-wave velocity structure atop MTZ beneath the Central Tian Shan and Tarim Basin. Chinese Journal of Geophysics ( in Chinese&#160;with English Abstract&#160;), 60 (1) : 98-111, doi: 10.6038 /cjg20170109.

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Crustal and Uppermost Mantle Structure Across Central Myanmar by Joint Analysis of Receiver Functions and Rayleigh-wave Dispersion

10.5194/egusphere-egu2020-7166 ◽

2020 ◽

Author(s):

Yiming Bai ◽

Yumei He ◽

Xiaohui Yuan ◽

Myo Thant ◽

Kyaing Sein ◽

...

Keyword(s):

Phase Velocity ◽

Rayleigh Wave ◽

Velocity Structure ◽

Wave Dispersion ◽

Receiver Functions ◽

P Wave ◽

Central Basin ◽

Automatic Frequency ◽

Uppermost Mantle ◽

Rayleigh Wave Dispersion

The territory of Myanmar, situated at the eastern flank of the India-Asia collision zone, is characterized by complex tectonic structure and high seismicity. From west to east, this region consists of three nearly NS-trending tectonic units: the Indo-Burma Ranges, the Central Basin and the Shan Plateau. Detailed structure of the crust and uppermost mantle beneath Myanmar can provide crucial constraints on regional tectonics, subduction dynamics as well as seismic hazard assessment. Yet seismic velocity structure beneath this region is poorly determined due to sparse regional seismic networks.In this study, we utilize seismic data recorded at 80 broadband stations in Myanmar, among which 70 stations were deployed in 2016 under the project of China-Myanmar Geophysical Survey in the Myanmar Orogen (CMGSMO), 9 stations are operated by IRIS and the remaining one is from GEOFON. We measured the Rayleigh-wave phase velocity dispersion from the ambient noise cross-correlations at periods between 5 s and 40 s by using the automatic frequency-time analysis (AFTAN). A fast marching surface wave tomography (FMST) approach was then adopted to invert the 2-D phase velocity maps in the study region. Our preliminary results show variable crustal structure across central Myanmar, with a strong low-velocity zone north of 22&#176;N in the Indo-Burma Ranges. Since Rayleigh-wave dispersion is more sensitive to absolute velocity speed than to velocity contrasts, the ongoing study jointly inverts the dispersion data with P-wave receiver functions to better determine the velocity discontinuities and thus provides tighter constraints on the shear-velocity structure beneath central Myanmar.

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3-D P-wave velocity structure of oceanic core complexes at 13°N on the Mid-Atlantic Ridge

Geophysical Journal International ◽

10.1093/gji/ggaa093 ◽

2020 ◽

Vol 221 (3) ◽

pp. 1555-1579 ◽

Cited By ~ 3

Author(s):

N M Simão ◽

C Peirce ◽

M J Funnell ◽

A H Robinson ◽

R C Searle ◽

...

Keyword(s):

Wave Velocity ◽

Velocity Structure ◽

P Wave ◽

Ocean Bottom ◽

Uppermost Mantle ◽

Ridge Axis ◽

Magma Supply ◽

P Wave Velocity ◽

Mid Atlantic Ridge ◽

Detachment Faulting

SUMMARY The Mid-Atlantic Ridge at 13°N is regarded as a type locality for oceanic core complexes (OCCs), as it contains, within ∼70 km along the spreading axis, four that are at different stages of their life cycle. The wealth of existing seabed observations and sampling makes this an ideal target to resolve contradictions between the existing models of OCC development. Here we describe the results of P-wave seismic tomographic modelling within a 60 × 60 km footprint, containing several OCCs, the ridge axis and both flanks, which determines OCC crustal structure, detachment geometry and OCC interconnectivity along axis. A grid of wide-angle seismic refraction data was acquired along a series of 17 transects within which a network of 46 ocean-bottom seismographs was deployed. Approximately 130 000 first arrival traveltimes, together with sparse Moho reflections, have been modelled, constraining the crust and uppermost mantle to a depth of ∼10 km below sea level. Depth slices through this 3-D model reveal several independent structures each with a higher P-wave velocity (Vp) than its surrounds. At the seafloor, these features correspond to the OCCs adjacent to the axial valley walls at 13°20′N and 13°30′N, and off axis at 13°25′N. These high-Vp features display dipping trends into the deeper crust, consistent with the surface expression of each OCC's detachment, implying that rocks of the mid-to-lower crust and uppermost mantle within the footwall are juxtaposed against lower Vp material in the hangingwall. The neovolcanic zone of the ridge axis has systematically lower Vp than the surrounding crust at all depths, and is wider between OCCs. On average, throughout the 13°N region, the crust is ∼6 km-thick. However, beneath a deep lava-floored basin between axial OCCs the crust is thinner and is more characteristically oceanic in layering and velocity–depth structure. Thicker crust at the ridge axis suggests a more magmatic phase of current crustal formation, while modelling of the sparse Moho reflections suggests the crust–mantle boundary is a transition zone throughout most of the 13°N segment. Our results support a model in which OCCs are bounded by independent detachment faults whose dip increases with depth and is variable with azimuth around each OCC, suggesting a geometry and mechanism of faulting that is more complicated than previously thought. The steepness of the northern flank of the 13°20′N detachment suggests that it represents a transfer zone between different faulting regimes to the south and north. We propose that individual detachments may not be linked along-axis, and that OCCs act as transfer zones linking areas of normal spreading and detachment faulting. Along ridge variation in magma supply influences the nature of this detachment faulting. Consequently, not only does magma supply control how detachments rotate and migrate off axis before finally becoming inactive, but also how, when and where new OCCs are created.

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Seismic investigation of an active ocean–continent transform margin: the interaction between the Swan Islands Fault Zone and the ultraslow-spreading Mid-Cayman Spreading Centre

Geophysical Journal International ◽

10.1093/gji/ggz283 ◽

2019 ◽

Vol 219 (1) ◽

pp. 159-184 ◽

Cited By ~ 1

Author(s):

C Peirce ◽

A H Robinson ◽

A M Campbell ◽

M J Funnell ◽

I Grevemeyer ◽

...

Keyword(s):

Continental Crust ◽

Oceanic Crust ◽

Fault Zone ◽

Lower Crust ◽

P Wave ◽

Fault System ◽

Transform Fault ◽

Apparent Lack ◽

The North ◽

Transform Margin

SUMMARY The Swan Islands Transform Fault (SITF) marks the southern boundary of the Cayman Trough and the ocean–continent transition of the North American–Caribbean Plate boundary offshore Honduras. The CAYSEIS experiment acquired a 180-km-long seismic refraction and gravity profile across this transform margin, ∼70 km to the west of the Mid-Cayman Spreading Centre (MCSC). This profile shows the crustal structure across a transform fault system that juxtaposes Mesozoic-age continental crust to the south against the ∼10-Myr-old ultraslow spread oceanic crust to the north. Ocean-bottom seismographs were deployed along-profile, and inverse and forward traveltime modelling, supported by gravity analysis, reveals ∼23-km-thick continental crust that has been thinned over a distance of ∼70 km to ∼10 km-thick at the SITF, juxtaposed against ∼4-km-thick oceanic crust. This thinning is primarily accommodated within the lower crust. Since Moho reflections are not widely observed, the 7.0 km s−1 velocity contour is used to define the Moho along-profile. The apparent lack of reflections to the north of the SITF suggests that the Moho is more likely a transition zone between crust and mantle. Where the profile traverses bathymetric highs in the off-axis oceanic crust, higher P-wave velocity is observed at shallow crustal depths. S-wave arrival modelling also reveals elevated velocities at shallow depths, except for crust adjacent to the SITF that would have occupied the inside corner high of the ridge-transform intersection when on axis. We use a Vp/Vs ratio of 1.9 to mark where lithologies of the lower crust and uppermost mantle may be exhumed, and also to locate the upper-to-lower crustal transition, identify relict oceanic core complexes and regions of magmatically formed crust. An elevated Vp/Vs ratio suggests not only that serpentinized peridotite may be exposed at the seafloor in places, but also that seawater has been able to flow deep into the crust and upper mantle over 20–30-km-wide regions which may explain the lack of a distinct Moho. The SITF has higher velocities at shallower depths than observed in the oceanic crust to the north and, at the seabed, it is a relatively wide feature. However, the velocity–depth model subseabed suggests a fault zone no wider than ∼5–10 km, that is mirrored by a narrow seabed depression ∼7500 m deep. Gravity modelling shows that the SITF is also underlain, at >2 km subseabed, by a ∼20-km-wide region of density >3000 kg m−3 that may reflect a broad region of metamorphism. The residual mantle Bouguer anomaly across the survey region, when compared with the bathymetry, suggests that the transform may also have a component of left-lateral trans-tensional displacement that accounts for its apparently broad seabed appearance, and that the focus of magma supply may currently be displaced to the north of the MCSC segment centre. Our results suggest that Swan Islands margin development caused thinning of the adjacent continental crust, and that the adjacent oceanic crust formed in a cool ridge setting, either as a result of reduced mantle upwelling and/or due to fracture enhanced fluid flow.

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Crust and upper mantle velocity structure of the Intermontane belt, southern Canadian Cordillera

Canadian Journal of Earth Sciences ◽

10.1139/e92-121 ◽

1992 ◽

Vol 29 (7) ◽

pp. 1530-1548 ◽

Cited By ~ 23

Author(s):

B. C. Zelt ◽

R. M. Ellis ◽

R. M. Clowes ◽

E. R. Kanasewich ◽

I. Asudeh ◽

...

Keyword(s):

Upper Mantle ◽

Lower Crust ◽

Velocity Structure ◽

P Wave ◽

Seismic Reflection Profile ◽

Wide Angle ◽

Low Velocity ◽

Crust And Upper Mantle ◽

Middle Crust ◽

Refraction Data

As part of the Lithoprobe Southern Cordillera transect, seismic refraction data were recorded along a 330 km long strike profile in the Intermontane belt. An iterative combination of two-dimensional traveltime inversion and amplitude forward modelling was used to interpret crust and upper mantle P-wave velocity structure. This region is characterized by (i) a thin near-surface layer with large variations in velocity between 2.8 and 5.4 km/s, and low-velocity regions that correlate well with surface expressions of Tertiary sedimentary and volcanic rocks; (ii) an upper and middle crust with low average velocity gradient, possibly a weak low-velocity zone, and lateral velocity variations between 6.0 and 6.4 km/s; (iii) a distinctive lower crust characterized by significantly higher average velocities relative to midcrustal values beginning at 23 km depth, approximately 8 km thick with average velocities of 6.5 and 6.7 km/s at top and base; (iv) a depth to Moho, as defined by wide-angle reflections, that averages 33 km with variations up to 2 km; and (v) a Moho transition zone of depth extent 1–3 km, below which lies the upper mantle with velocities decreasing from 7.9 km/s in the south to 7.7 km/s in the north. Where the refraction line obliquely crosses a Lithoprobe deep seismic-reflection profile, good agreement is obtained between the interpreted reflection section and the derived velocity structure model. In particular, depths to wide-angle reflectors in the upper crust agree with depths to prominent reflection events, and Moho depths agree within 1 km. From this comparison, the upper and middle crust probably comprise the upper part of the Quesnellia terrane. The lower crust from the refraction interpretation does not show the division into two components, parautochthonous and cratonic North America, that is inferred from the reflection data, indicating that their physical properties are not significantly different within the resolution of the refraction data. Based on these interpretations, the lower lithosphere of Quesnellia is absent and presumably was recycled in the mantle. At a depth of ~ 16 km below the Moho, an upper mantle reflector may represent the base of the present lithosphere.

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