Showing papers by "Fred Davey published in 2010"

Postspreading rifting in the Adare Basin, Antarctica: Regional tectonic consequences

Abstract: Extension during the middle Cenozoic (43–26 Ma) in the north end of the West Antarctic rift system (WARS) is well constrained by seafloor magnetic anomalies formed at the extinct Adare spreading axis. Kinematic solutions for this time interval suggest a southward decrease in relative motion between East and West Antarctica. Here we present multichannel seismic reflection and seafloor mapping data acquired within and near the Adare Basin on a recent geophysical cruise. We have traced the ANTOSTRAT seismic stratigraphic framework from the northwest Ross Sea into the Adare Basin, verified and tied to DSDP drill sites 273 and 274. Our results reveal three distinct periods of tectonic activity. An early localized deformational event took place close to the cessation of seafloor spreading in the Adare Basin (~24 Ma). It reactivated a few normal faults and initiated the formation of the Adare Trough. A prominent pulse of rifting in the early Miocene (~17 Ma) resulted in normal faulting that initiated tilted blocks. The overall trend of structures was NE–SW, linking the event with the activity outside the basin. It resulted in major uplift of the Adare Trough and marks the last extensional phase of the Adare Basin. Recent volcanic vents (Pliocene to present day) tend to align with the early Miocene structures and the on-land Hallett volcanic province. This latest phase of tectonic activity also involves near-vertical normal faulting (still active in places) with negligible horizontal consequences. The early Miocene extensional event found within the Adare Basin does not require a change in the relative motion between East and West Antarctica. However, the lack of subsequent rifting within the Adare Basin coupled with the formation of the Terror Rift and an on-land and subice extension within the WARS require a pronounced change in the kinematics of the rift. These observations indicate that extension increased southward, therefore suggesting that a major change in relative plate motion took place in the middle Miocene. The late Miocene pole of rotation might have been located north of the Adare Basin, with opposite opening sign compared to the Eocene-Oligocene pole.

Kinematic and seismic analysis of giant tabular iceberg breakup at Cape Adare, Antarctica

Abstract: [1] Satellite imagery reveals that a series of large icebergs (B15B in April 2001, C19 in June 2003, and B15A in October 2005) broke up or fractured while exiting the Ross Sea in a narrowly defined area off Cape Adare, Antarctica. Examination of recent swath-mapped bathymetric observations revealed that the principle agent of these breakups is a previously unknown 9 km long ridge with minimum depths of 215 m that we call Davey Shoal. Satellite imagery shows that the icebergs are driven into the shoal by coastal currents that converge over the narrow continental shelf. One of the icebergs, the 120 km by 30 km B15A, was instrumented with a seismograph, GPS, and fluxgate compass. This instrumentation provided a unique opportunity to establish the details of the iceberg kinematics that were not revealed by satellite imagery alone and to correlate seismic events observed both on the iceberg and in the far field during breakup. B15A fractured from multiple strikes against Davey Shoal and the adjacent Possession Islands; these strikes were driven by the combination of tidal currents and the coastal mean flow. The periods of iceberg-sourced seismic radiation were correlated with the strikes. The iceberg- and land-based seismic signals showed that the iceberg fracture, its sliding across the shoals, and the ice-on-ice stick-slip contacts among the postbreakup iceberg fragments generated the strong chaotic and harmonic tremor episodes that were observed at distances as far as the South Pole, where these signals propagated as seismically coupled hydroacoustic T phases.

Kinematic and Seismic Analysis of Giant Tabular Iceberg Breakup at Cape Adare

Abstract: 1. University of Washington, School of Oceanography, Box 357940, Seattle, WA 98195-7940, USA (corresponding author: seelye@ocean.washington.edu) 2. Department of Earth and Environmental Science and Geophysical Research Center, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM 87801, USA

Crustal seismic reflection profile across the alpine fault and coastal plain at Whataroa, South Island

Abstract: Crustal seismic reflection data recorded across the Alpine Fault and coastal plain at Whataroa in 1998 are used to derive shallow (4 seconds two-way time or s twt) and deep (14 s twt) seismic reflection images and a simple refraction model. A single 25 km long, 636 channel receiver array recorded energy from 50 kg shots fired at 1 km intervals with intervening shots of 2.5 kg at about 300 m intervals along the profile. Record quality varied from good to poor. Across the Alpine Fault, basement lies at about 300–500 m (0.2–0.5 s twt), and a weak basement reflection may be attributed the Alpine Fault. Basement deepens sharply at about 6 km to the northwest of the fault, and a 3 km section of probably Miocene and younger sediments is imaged to the northwest. The southern boundary of the coastal basin probably corresponds to the South Westland Fault. The deeper section images a distinct band of strong reflectivity at a depth of about 9–10 s twt, the base of which is inferred to be Moho. Depth conversi...