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New Zealand South Island Geophysical Transect (SIGHT)

Cruise Report for R/V Maurice Ewing EW-9601A

Photo of OBS deployment.

Summary

This report documents R/V Maurice Ewing cruise EW-9601, which formed a major part of a multidisciplinary, joint US - New Zealand investigation of continental deformation at convergent plate boundaries (SIGHT - the South Island Geophysical Transect). The Alpine Fault zone of the South Island of New Zealand marks the Pacific/Australian plate boundary, where oblique convergence of two thin continental crustal blocks has built the Southern Alps. The active-source component of SIGHT was designed to exploit the narrowness of the South Island by using onshore/offshore seismic techniques to provide a detailed image of the boundary from both sides.

The Ewing fired over 41,000 shots from its 20-gun tuned airgun source into a wide array of seismic receivers: the Ewing's 4-km-long multichannel streamer, eleven Woods Hole Oceanographic Institution ocean-bottom hydrophones (OBH) and nine U.S. Geological Survey ocean-bottom seismometers (OBS), and 200 Reftek portable seismometers deployed on three major transacts and several cross-lines. All operational goals of the experiment were attained. Forty-four deployments and recoveries of OBH/S were accomplished; the multichannel streamer was deployed and recovered twice; and sixteen sonobuoys were launched. Preliminary velocity models were produced shipboard for one of the onshore explosion transacts and one of the offshore OBH lines. Brute stacks of all MCS lines were produced shipboard, showing reflections from the entire crust and Moho. The resulting data sets will provide fundamental new information on the deep structure of the Pacific/Australian plate boundary and, when combined with the passive seismic, petrophysical, magnetotelluric, and (Geological data being acquired under the auspices of this project, enable new insights into the deformation of continental lithosphere under oblique compression.

SCIENTIFIC OBJECTIVES

This cruise forms a major part of a joint US- New Zealand investigation aimed at improving our understanding of continental deformation at convergent plate boundaries. Mountain belts are the highly visible products of continental collisions and contain unique clues about the Theological properties of continental lithosphere and the processes by which it deforms. Within mountain belts, compression, overthrusting, and erosion often combine to expose large sections of high-grade metamorphic rocks and associated faults. Collisional mountain belts are usually paired with, and mechanically linked to, large asymmetric foreland basins that contain much of the world's hydrocarbons.
On the South Island of New Zealand, along the Pacific/Australian plate boundary, youthful oblique convergence of two thin continental crustal blocks - the submerged Lord Howe Rise to the west and the Chatham Rise/Campbell Plateau to the east - has resulted in a relatively narrow (-80 km wide) zone of active mountain building, the Southern Alps. The plate boundary has recently been forced into a conditemon of transpression through plate boundary reorganization processes. The South Island thus offers a natural laboratory for the study of partitemoning between convergence and translation. In the central Southem Alps of New Zealand, there is also the apparent paradox that the greatest present uplift and deformation is associated with relatively low seismicity. Finally, the narrowness of the South Island provides a unique opportunity to use marine onshore/offshore seismic techniques to provide a detailed image of the boundary from both sides.
The active-source seismic survey was designed to provide fundamental knowledge on deformation in a transpressional continental orogen by identifying potential strain markers in the lithosphere on both sides of the plate boundary. By tracing these strain markers such as the Moho, lower-crustal laminations, and crustal shear zones - we hope to constrain whether, and at what levels of the lithosphere, strain is accommodated by brittle faulting or plastic flow. The specific goals of the survey were to obtain wide-angle onshore/offshore, marine multichannel seismic, and OBH/S wide-angle data along three profiles, two across the orogen and one across undeformed rocks that form the orogen. The data acquired will constrain models of continental collision by providing information on :

Operational ObjectivesThe cruise formed the marine component of an offshore/onshore seismic experiment, comprising seismic measurements alone, two profiles across the central South Island and extending about 200 km offshore from either coast, and one profile across the eastern coast and southeastern part of South Island. Data were recorded by 200 Reftek portable seismographs deployed along the three profiles, by 20 OBH/S on the offshore extensions of the profiles (deployed sequentially on either side), and by a 3- to 4-km-long towed hydrophone streamer. Shots were fired using a 130-liter (8495 cu. in.) airgun array. strike lines were also recorded on either side of the South Island. The survey followed immediately after an onshore seismic refraction/wide angle reflection experiment along the profiles across the South Island.The primary operational goals were to:

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Cruise Participants

Photograph of the research vessel RV Ewing.
RV Ewing

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Cruise Data

Deployment data

Deployment #1 - West Coast (Lines 1W, 12W, 2W, 23W, 3W)

SITE OBS DATE/TIME Deploy LAT. Deploy LON. DEPTH(M) START TIME DATE/TIME Recover LAT. Recover LON.
5 A2 2/09/96 23:56 42 48.702'S 169 53.162'E 520 2/13/96 07:00 2/17/96 16:00 N/A N/A
6 C1 2/10/96 01:28 42 39.077'S 169 41.360'E 859 2/13/96 07:00 2/17/96 17:58 420 39.106'S 169 40.975'E
7 C9 2/10/96 03:27 42 26.972'S 169 26.759'E 1080 2/13/96 07:00 2/17/96 20:26 42 26.952'S 169 26.607'E
8 C3 2/10/96 05:40 42 12.418'S 169 09.356'E 1221 2/13/96 07:00 2/17/96 23:09 42 12.563'S 169 09.008'E
12 C4 2/10/96 18:10 42 29.570'S 168 18.422'E 1572 2/13/96 07:00 2/18/96 18:03 42 29.245'S 168 18.131'E
13 A3 2/10/96 20:47 42 44.142'S 168 38.618'E 1272 2/13/96 07:00 2/18/96 20:59 42 44.189'S 168 38.666'E
14 Al 2/10/96 22:37 42 53.741'S 168 52.026'E 1037 2/12/96 07:00 2/18/96 23:00 42 53.646'S 168 52.110'E
15 A8 2/11/96 00:35 43 05.335'S 169 08.476'E 860 2/13/96 07:00 2/19/96 01:24 43 05.241'S 169 08.609'E
16 A4 2/11/96 04:07 43 14.810'S 169 21.900'E 716 2/13/96 07:00 2/19/96 03:28 N/A N/A

Deployment #2 - East Coast (Lines 4E, 41E, 1E, 12E, 2E, 25E, 5E, 6E)

SITE OBS DATE/TIME Deploy LAT. Deploy LON. DEPTH(M) START TIME DATE/TIME Recover LAT. Recover LON.
24 A2 2/21/96 22:40 44 54.600'S 171º 48.388'E 120 2/24/96 02:00 3/02/96 18:10 44º 54.239'S 171º 48.495'E
25 A3 2/22/96 00:05 45 02.221'S 172 00.293'E 1082 2/24/96 02:00 3/02/96 20:20 45 02.043'S 172 00.294'E
26 C3 2/22/96 01:53 45º 11.268'S 172 14.402'E 1395 2/24/96 02:00 3/02/96 23:30 45º 11.177'S 172 14.496'E
27 Al 2/22/96 04:36 45 21.753'S 172 30.927'E 1452 2/24/96 02:00 3/03/96 02:10 45 21.744'S 172º 31.000'E
32 C1 2/22/96 16:19 45 04.399'S 172 49.309'E 1398 2/24/96 02:00 3/03/96 17:14 45 04.380'S 172 49.316'E
33 C4 2/22/96 18:22 44 53.109'S 172 34.006'E 1194 2/24/96 02:00 3/03/96 18:35 44 52.966'S 172 34.036'E
34 C9 2/22/96 19:52 44 44.499'S 172 22.418'E 324 2/24/96 02:00 3/03/96 21:15 44 44.389'S 172 22.465'E
35 A8 2/22/96 21:29 44 37.085'S 1720 12.524'E 117 2/24/96 02:00 3/03/96 22:35 44 37.071'S 172 12.461'E

Deployment #3 - Southern Cross (Lines SCI, SC2, SC3, SC4)

SITE OBS DATE/TIME Deploy LAT. Deploy LON. DEPTH(M) START TIME DATE/TIME Recover LAT. Recover LON.
43 A8 3/05/96 02:07 45 21.40'S 171º 10.09 E 56 3/05/96 04:00 3/06/96 09:58 45 21.60'S 171 10.15'E
44 A2 3/05/96 03:16 45 16.00'S 171 00.99'E 32 3/05/96 04:00 3/06/96 11:10 N/A N/A

OBS DATA QUALITY

Experiment 1

SITE LAST TRACK START TRACKS RECORDED TRACKS ARCHIVED DATE/TIME PREDEPL(ms) DATE/TIME POSTRECOV(ms) COMMENTS
5-A2 - 44 44 2/09/20:51 6.92 2/17/16:40 N/A Disk crashed. Data logger reset. 2 Channels good - V., Hyd.
6-Cl - 151 151 2/09/19:23 5.64 2/17/16:35 -71.2 2 Channels good - V., Hyd.
7-C9 - 188 188 2/09/20:03 9.2 2/17/20:46 -76.4 2 Channels good - V., Hyd.
8-C3 - 155 155 2/09/17:39 3.84 2/17/23:38 -89.6 2 Channels good - V., Hyd.
12-C4 2/18/18:11 187 187 2/10/08:43 4.68 2/18/18:37 -94.0 2 Channels good - V., Hyd.
13-A3 2/18/18:53 188 188 2/10/09:32 0.45 2/18121:25 -78.0 2 Channels good - V., Hyd.
14-A1 2/18109:42 209 209 2/10/08:10 0.41 2/18/23:43 -96.0 2 Channels good - V., Hyd.
15-A8 2/19101:36 394 394 2/10/07:32 9.60 2/19/01:46 -40.8 Vertical attenuated. Hor. 1, Hor. 2, Hyd. good.
16-A4 2/19/01:36 394 394 2/10/21:37 5.72 2/19/03:52 N/A Data logger reset on recovery. Release damaged. Cannot redeploy. 4 Channels good - V., Hor. 1, Hor. 2, Hyd.

Experiment 2

SITE LAST TRACK START TRACKS RECORDED TRACKS ARCHIVED DATE/TIME PREDEPL(ms) DATE/TIME POSTRECOV(ms) COMMENTS
24-A2 3/02/09:58 500 500 2/21/18:22 9.76 3/02/18:56 -90.4 4 Channels V., Hor. 1, Hor. 2, Hyd.
25-A3 3/01/04:23 209 209 2121/19:16 3.52 3/02/20:48 -92.0 2 Channels good - V., Hyd
26-C3 3/01/04:41 209 209 2/21/23:25 7.16 3/02/23:54 -106.0 2 Channels good - V., Hyd.
27-Al 3/01/04:41 209 209 2/21/19:53 5.56 3/03/02:44 -107.0 2 Channels good - V., Hyd.
32-Cl 3/01/04:41 209 255 2/22/07:26 3.88 3/03/17:34 -103.0 2 Channels good - V., Hyd.
33-C4 3/01/04:41 209 255 2/22/08:42 9.52 3/03/19:58 -101.0 2 Channels good - V., Hyd.
34-C9 3/01/04:41 209 255 2/22/06:43 6.56 3/03/2l:34 -108.0 2 Channels good - V., Hyd.
35-A8 3/02/09:47 500 255 2/22/05:13 1.96 3/03/23:20 -63.28 4 Channels good - V., Hor. 1, Hot. 2, Hyd.

Experiment 3

SITE LAST TRACK START TRACKS RECORDED TRACKS ARCHIVED DATE/TIME PREDEPL(ms) DATE/TIME POSTRECOV(ms) COMMENTS
43-A8 3/06/09:38 84 94 3/04/11:15 6.8 3/06/10:20 2.72 4 Channels good - V., Hot. 1, Hor. 2, Hyd.
44-A2 3/06/10:28 86 86 3/04/09:50 4.04 3/06/11:34 -17.2 4 Channels good - V., Her. 1, Hor. 2, Hyd.

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