Crustal Structure of Wanganui Basin: Implications for Back-Arc Basin Formation

Onshore-offshore and onshore wide-angle reflection and refraction seismic data are analysed in order to constrain the crustal P-wave velocity structure within Wanganui Basin and the southern Hikurangi margin, western North Island, New Zealand. Two, two-dimensional, P-wave velocity models are constructed using reflection and refraction phase travel-times based on forty-four onshore-offshore receiver gathers, twelve onshore shot gathers and two marine multi-channel-seismic reflection profiles collected along two transects within Wanganui Basin. The profiles are orientated both sub-parallel and perpendicular to the strike of the Hikurangi margin. Within Wanganui Basin, a north-south increase in Moho depth is resolved sub-parallel to the strike of the Hikurangi margin from 25 km depth north of Mt Ruapehu to 36.5 km within offshore Wanganui Basin at the latitude of Foxton. This increase is punctuated by an abrupt step in Moho depth of 7±3 km over a lateral distance of 42 km by up to 5 km based on wide-angle data. The Australian/Pacific plate interface is imaged as a relatively smooth surface at depths of 28.15 - 35.6 km between the latitudes of Paekakariki and Foxton with apparent dip to the north-north-east increasing from 2.3 to 9.5°. Perpendicular to the strike of the Hikurangi margin, this interface increases in depth from 16 - 33 km with an increase in dip of 5.4° beneath Featherston to 19° beneath central Wanganui Basin. P-wave velocities range between 5.8 - 6.5 kms-¹ in the upper ~18 km of the crust within Wanganui Basin. These are interpreted to represent mid and lower-crustal rocks that have been emplaced in the upper crust by exhumation during the Miocene. In the lower crust P-wave velocities are inferred as 6.5 - 6.85 kms-1 sub-parallel and 6.5 - 6.65 kms-¹ perpendicular to Hikurangi margin, suggesting a lower crustal P-wave anisotropy of up to ~ 3% This may reflect the preferential alignment of mineral fabrics in the direction of relative transpressive plate motion during the Neogene and/or structural anisotropy parallel to the strike of the Taranaki Fault Zone and Kapiti-Manawatu Fault System. The structure of the Pacific plate crust is resolved as two layers. The top layer is inferred as a low-velocity sediment sheet with a P-wave velocity of 5.00 - 5.20 kms-¹ and thickness of 1.5 - 3.5 km. The second layer is interpreted as 9.5 - 10 km thick oceanic crust with P-wave velocities between 6.7 and 7.2 kms-¹. This confirms that anomalously thick (up to 13.5 km) Hikurangi Plateau crust has subducted at least as far east as the Tararua Ranges. Subducted sediment appears to be pooling beneath the western side of North Island between Featherston and Kapiti Island, reaching a maximum thickness of 3.5 km. Rock uplift driven by the relative buoyancy of this material is capable of explaining the average elevation of the Tararua Ranges and coupled with previous studies from the Raukumara Basin in the north, suggest accretion of subducted sediment to the overlying plate may be characteristic of the entire Hikurangi margin

Experimental Design for Bathymetry Editing

We describe an application of machine learning to a real-world computer assisted labeling task. Our experimental results expose significant deviations from the IID assumption commonly used in machine learning. These results suggest that the common random split of all data into training and testing can often lead to poor performance.Comment: Published as a workshop paper at ICML 2020 Workshop on Real World Experiment Design and Active Learnin

Crustal structure, gravity anomalies and subsidence history of the Parnaíba cratonic basin, Northeast Brazil

Cratonic basins cover more than 10&amp;percnt; of Earth's continental surface area, yet their origin remains enigmatic. In this thesis a suite of new and legacy geophysical and geological data are integrated to constrain the origin of the Parnaíba basin, a cratonic basin in Northeast Brazil. These data include a 1400 km long, deep (20 s two-way travel time) seismic reflection profile, five +/- 110 km offset wide-angle split-spread receiver gathers, gravity anomaly, and well data. In the centre of the basin, the depth to pre-Paleozoic basement is &amp;Tilde; 3.3 km, a zone of midcrustal reflectivity (MCR) can be traced laterally for &amp;Tilde; 250 km at depths between 17-25 km and Moho depth is &amp;Tilde; 42 +/- 2 km. Gravity and P-wave modelling suggests that the MCR represents the upper surface of a high density (2985 kg m3) and Vp (6.7–7.0 km s-1) lower crustal body, likely of magmatic origin. Backstripping of well data shows a concave up decreasing tectonic subsidence, similar in form to that commonly observed in rift-type basins. It is shown, however, that the seismic and gravity data are inconsistent with an extensional origin. It is shown that an intrusive body in the lower crust that has loaded and flexed the surface of the crust, combined with sediment loading, provides a satisfactory fit to the observed gravity anomaly, sediment thickness and basin shape. A buried load model is also consistent with seismic data, which suggest that the Moho is as deep or deeper beneath the basin centre than its flanks and accounts for at least part of the tectonic subsidence through a viscoelastic stress relaxation that occurs in the lithosphere following load emplacement. Comparative analysis of the Michigan and Congo basins shows gravity data from these basins is also consistent with a lower crustal mass excess, while subsidence analysis shows viscoelastic stress relaxation may also contribute to their early subsidence histories. However, unlike Parnaíba, both of these basins appear to have been subjected to secondary tectonic processes that obscure the primary 'cratonic basin' subsidence signals. Parnaíba basin, therefore, offers an excellent record for the investigation of cratonic basin formation.</p

Crustal structure, gravity anomalies and subsidence history of the Parnaíba cratonic basin, Northeast Brazil

Cratonic basins cover more than 10&percnt; of Earth's continental surface area, yet their origin remains enigmatic. In this thesis a suite of new and legacy geophysical and geological data are integrated to constrain the origin of the Parna&iacute;ba basin, a cratonic basin in Northeast Brazil. These data include a 1400 km long, deep (20 s two-way travel time) seismic reflection profile, five +/- 110 km offset wide-angle split-spread receiver gathers, gravity anomaly, and well data. In the centre of the basin, the depth to pre-Paleozoic basement is &Tilde; 3.3 km, a zone of midcrustal reflectivity (MCR) can be traced laterally for &Tilde; 250 km at depths between 17-25 km and Moho depth is &Tilde; 42 +/- 2 km. Gravity and P-wave modelling suggests that the MCR represents the upper surface of a high density (2985 kg m3) and Vp (6.7â7.0 km s-1) lower crustal body, likely of magmatic origin. Backstripping of well data shows a concave up decreasing tectonic subsidence, similar in form to that commonly observed in rift-type basins. It is shown, however, that the seismic and gravity data are inconsistent with an extensional origin. It is shown that an intrusive body in the lower crust that has loaded and flexed the surface of the crust, combined with sediment loading, provides a satisfactory fit to the observed gravity anomaly, sediment thickness and basin shape. A buried load model is also consistent with seismic data, which suggest that the Moho is as deep or deeper beneath the basin centre than its flanks and accounts for at least part of the tectonic subsidence through a viscoelastic stress relaxation that occurs in the lithosphere following load emplacement. Comparative analysis of the Michigan and Congo basins shows gravity data from these basins is also consistent with a lower crustal mass excess, while subsidence analysis shows viscoelastic stress relaxation may also contribute to their early subsidence histories. However, unlike Parnaíba, both of these basins appear to have been subjected to secondary tectonic processes that obscure the primary 'cratonic basin' subsidence signals. Parnaíba basin, therefore, offers an excellent record for the investigation of cratonic basin formation.</p

Crustal Structure of Wanganui Basin: Implications for Back-Arc Basin Formation

Onshore-offshore and onshore wide-angle reflection and refraction seismic data are analysed in order to constrain the crustal P-wave velocity structure within Wanganui Basin and the southern Hikurangi margin, western North Island, New Zealand. Two, two-dimensional, P-wave velocity models are constructed using reflection and refraction phase travel-times based on forty-four onshore-offshore receiver gathers, twelve onshore shot gathers and two marine multi-channel-seismic reflection profiles collected along two transects within Wanganui Basin. The profiles are orientated both sub-parallel and perpendicular to the strike of the Hikurangi margin

Experimental Design for Bathymetry Editing

We describe an application of machine learning to a real-world computer assisted labeling task. Our experimental results expose significant deviations from the IID assumption commonly used in machine learning. These results suggest that the common random split of all data into training and testing can often lead to poor performance

Modeling Uncertainties of Bathymetry Predicted With Satellite Altimetry Data and Application to Tsunami Hazard Assessments

10.1029/2020JB019735Journal of Geophysical Research: Solid Earth125

Supporting Information for "Modeling Uncertainties of Satellite Altimetry-predicted Bathymetry"

JOURNAL OF GEOPHYSICAL RESEARC

Modeling Uncertainties of Bathymetry Predicted With Satellite Altimetry Data and Application to Tsunami Hazard Assessments

10.1029/2020JB019735Journal of Geophysical Research: Solid Earth125