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Seismic Investigation of the Greenland Margin (SIGMA)

Cruise Report for R/V Maurice Ewing EW-9607

Map of cruise operations.

Based on WHOI cruise report by
W. Steven Holbrook - Dept. of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole,MA 02543 USA
Trine Dahl-Jensen - Danish Lithosphere Centre, stervoldgade 10, Copenhagen, Denmark

Summary

R/V Maurice Ewing cruise EW-9607 formed the major data acquisition phase of SIGMA (Seismic Investigation of the Greenland MArgin), a joint U.S.-Danish investigation of the deep crustal structure of the Southeast Greenland continental margin. Previous seismic data indicate that Southeast Greenland is a volcanic rifted margin that contains significant thicknesses of mafic igneous material emplaced during early Tertiary continental breakup. The purpose of the SIGMA survey was to provide critical missing information on (1) the thickness, velocity structure, and composition of the Tertiary igneous crust; (2) variations in these properties both across the margin and along the margin at increasing distance from the Iceland plume track; and (3) the detailed structure of the continent-ocean boundary. This was achieved by acquiring seismic data along four margin-crossing transacts, one along the Greenland-Iceland Ridge and three at successively increasing distances of 200-1100 km from the presumed plume track. The Ewing fired over 33,000 shots from its 20-gun tuned airgun source into an array of offshore and onshore seismic receivers: the Ewing`s 4-km-long multichannel streamer, eleven Woods Hole Oceanographic Institution ocean-bottom hydrophones (OBH), eight U.S. Geological Survey ocean-bottom seismometers (OBS), and 35 Reftek portable seismometers deployed on two transacts in Greenland and one in Iceland. Despite several gales, two storms, heavy fog, cold water, and numerous icebergs, the operational goals of the experiment were largely attained. Sixty-nine deployments and 68 recoveries of OBH/S were accomplished (one OBH was lost during recovery); over 1300 km of MCS data were acquired in four deployments of the multichannel streamer; and four sonobuoys were launched. Preliminary velocity models were produced shipboard for two of the offshore lines. Migrated brute stacks of all MCS lines were produced in near-real-time shipboard, and further shipboard MCS processing, including velocity picldng and multiple suppression, provided improved crustal images. The data collected during this experiment will provide key information on the degree of structural and magmatic symmetry on conjugate VRM segments and on the influence of the Iceland hotspot on margm magmatism during and immediately after continental breakup. This information will provide an important framework for interpreting geological and drilling results on the margin and for constraining then-nomechanical models of VRM evolution. This study is a joint venture between US and Danish co-investigators, including data acquisition, processing, interpretation, and project financing.

Scientific Objectives

The SIGMA (Seismic Investigation of the Greenland Margin) project is designed to make accurate measurements of crustal thickness, velocity structure, and seismic reflectivity along the hotspot-influenced volcanic rifted margin (VRM) off Southeast Greenland.

VRM`s are characterized by a prism of igneous rocks, several times thicker than normal oceanic crust, that occupies the continent-ocean transition zone in an 80- to 150-km-wide belt and extends in some regions more than 1500 km along strike. This thick igneous crust has two characteristics in seismic data: a seaward-dipping reflector sequence interpreted as subaerially erupted basalt flows and intercalated volcaniclastics, and a high-velocity lower crust with P-wave velocities (7.2-7.6 km/s) suggestive of mafic/ultrainafic intrusive rocks. Several models for the thermal and mechanical processes involved in the formation of VRM have been proposed, including decompression melting during passive upwelling near a mantle plume; actively upwelling plume heads impinging on the base of the lithosphere; enhanced upper mantle convection driven by steep, cold lithospheric edges adjacent to the rift; and hot upper mantle due to non-plume "hot cells" or insulation by supercontinents.

SIGMA consists of four transacts that systematically sample the structure of the SE Greenland margin and the continent-ocean transition at increasing distance from the Iceland hotspot track. The resulting data will provide answers to several questions regarding the SE Greenland VRM, including:

Operational Objectives

The SIGMA cruise comprised a seismic survey of the Southeast Greenland rifted margin along four transects, one along the Greenland-Iceland Ridge and three at successively greater distances from the presumed track of the Iceland hotspot. Wide-angle seismic data were recorded by 37 onshore Reftek portable seismogaphs deployed along three profiles, by 11 Woods Hole Oceanographic Institution OBH and 8 U.S. Geological Survey OBS offshore, and vertical-incidence data were recorded by the Ewing`s 4-km-long towed hydrophone streamer. Shots were fired using the Ewing`s 130-liter (8495 cu. in.) airgun array and by chemical explosions detonated by the onshore team on Transect 11.

The primary operational goals were to:

Cruise Plan

Task Duration Elapsed Hours Elasped Days Date Notes Time
Steam St. John`s - Angmagssalik 126 126 5.25 30 Steam: 0.00
Wireline release tests 6 132 5.50 30 - 6.00
Transfer oil, Angmagssalik 6 138 5.75 30 - 12.00
Steam to Line II 8 146 6.08 30 6.08 20.00
Deploy 19 OBH/S, Line II 30 176 7.33 1 - 2.00
Deploy MCS streamer and 39 215 8.96 2 - 17.00
Shoot Line II 48 263 10.96 4 - 17.00
Reshoot MCS, shelf 14 277 11.52 5 - 7.00
Recover streamer and steam to EOL 12 289 12.02 5 - 19.00
Reshoot for OBH 377 326 13.56 7 - 8.00
Recover 19 OBH/S, Line II 41 367 15.27 9 Profile II: 1.00
Contingencyy 0 367 15.27 9 9.19 1.00
Steam to Line II 13 380 15.81 9 - 14.00
Deploy 15 OBH/S, Line II 30 410 17.06 10 - 20.00
Deploy streamer + guns 12 422 17.56 11 - 8.00
Shoot Line II 37 459 19.10 12 - 21.00
Recover guns + streamer 8 467 19.44 13 - 5.00
Recover 13 OBH/S, Line II 33 499 20.79 14 - 13.00
Steam to last OBH + deploy guns 1 500 20.83 14 - 14.00
Shoot to Iceland + recover guns 15 515 21.46 15 - 5.00
Steam back to OBH + recover 2 OBH 12 527 21.96 15 Profile I: 17.00
Contingency 0 527 21.96 15 6.69 17.00
Steam-to Line III 36 563 23.46 17 - 5.00
Weather + deploy 2 OBS 50 613 25.54 19 - 7.00
Deploy 17 OBH/S, Line III 25 638 26.58 20 - 8.00
Deploy streamer + guns 10 648 27.00 20 - 18.00
Shoot Line III 30 678 28.28 22 - 0.00
Recover streamer + wait for daylight 9 687 28.60 22 - 9.00
Reshoot line III 27 714 29.73 23 - 12.00
Recover 18 OBH/S, Line III 49 762 31.75 25 Profile III: 12.00
Contingency 0 762 31.75 25 9.79 12.00
Steam to line IV 27 789 32.88 26 - 15.00
Wait for weather 25 814 33.92 27 - 16.00
Deploy 16 OBH/S, Line IV 28 842 35.06 28 - 20.00
Deploy streamer + guns 7 849 35.35 29 - 2.00
Shoot line IV 53 902 37.56 1 - 8.00
Recover guns+streamer 4 906 37.73 1 - 11.00
Recover 3 OBH + wait for weather 51 956 39.83 3 - 14.00
Recover remaining 13 OBH/S 24 980 40.83 4 Profile IV: 14.00
Contingency - Unsed 40 - - - 9.08 -
Steam to St. Jothn`s 73 1053 43.88 7 - 15.00

Cruise Participants

Photograph of the research vessel RV Ewing.
RV Ewing

Top

Cruise Data

Deployment #1 Transect II

SITE OBH/S TTID # DEPLOYMENT DATE/TIME LAT. LON. DEPTH(M) START TIME RECOVERY DATE/TIME LAT. LON.
2.1 Al 132 8/30/96 22:48 66 09.00'N 35 11.44'W 243 9/l/0400 9/08/96 20:20 66 08.86'N 35 11.59'W
2.2 A3 133 8/30/96 23:43 66 05.320'N 35 01.681'W 245 9/l/0400 9/08/96 21:28 66 05.39'N 35 01.69'W
2.9 C3 134 8/31/96 08:24 65 30.628'N 33 33.13l'W 275 9/l/0400 9/08/96 10:24 65 30.64'N 33 33.17'W
2.10 C9 131 8/31/96 09:41 65 24.977'N 33 19.300'W 329 9/l/0400 9/08/96 08:47 65 24.92'N 33 19.50'W
2.11 C4 491 8/31/96 11:25 65 18.087'N 33 02.796'W 940 9/l/0400 9/08/96 07:21 65 17.490'N 33 04.229'W
2.12 C1 239 8/31/96 12:49 65 11.213'N 32 46.363'W 1483 9/l/0400 9/08/96 03:49 65 11.118'N 32 46.620'W
2.13 A8 381 8/31/96 14:13 65 04.332'N 32 30.080'W 1769 9/l/0400 9/08/96 01:25 65 04.37'N 32 30.01'W
2.14 A4 7299 8/31/96 15:35 64 57.439'N 32 13.937'W 2025 9/l/0400 9/07/96 23:37 64 57.36'N 32 14.32'W

Deployment #2 Transect I

SITE OBH/S TTID # DEPLOYMENT DATE/TIME LAT. LON. DEPTH(M) START TIME RECOVERY DATE/TIME LAT. LON.
1.1 A3 133 9/09/96 15:50 67 34.320'N 32 47.154'W 375 9/11/1000 9/13/96 09:30 67 02.25'N 32 46.97'W
1.2 Al 132 9/09/96 18:32 66 59.7647'N 32 26.1255'W 374 9/11/1000 9/13/96 07:16 66 59.610'N 32 26.089'W
1.9 A8 381 9/10/96 08:03 66 34.266'N 29 25.803'W 319 9/11/0200 9/14/96 06:32 66 34.284'N 29 26.795'W
1.10 C4 491 9/10/96 10:25 66 30.367'N 29 00.344'W 334 9/11/0200 9/14/96 07:58 66 30.423'N 29 00.243'W
1.11 C9 131 9/10/96 13:14 66 26.39'N 28 35.300'W 327 9/11/0200 9/14/96 09:35 66 26.442'N 28 35.213'W
1.12 C3 134 9/10/96 15:47 66 22.324'N 28 10.255'W 351 9/11/0200 9/14/96 11:47 66 22.42'N 28 09.97'W
1.13 A4 7299 9/10/96 17:53 66 18.201'N 27 45.469'W 392 9/11/0200 9/14/96 14:27 66 18.150'N 27 45.643'W

Deployment #3 Transect III

SITE OBH/S TTID # DEPLOYMENT DATE/TIME LAT. LON. DEPTH(M) START TIME RECOVERY DATE/TIME LAT. LON.
3.1 Al 132 9/19/96 09:48 63 38.44'N 40 19.233'W 179 9/18/2000 9/25/96 13:22 63 38.104'N 40 19.014'W
3.2 A3 133 9/18/96 14:47 63 35.841'N 40 10.608'W 263 9/18/2000 9/25/96 11:31 63 35.335'N 40 10.420'W
3.3 C1 239 9/18/96 13:23 63 32.473'N 40 01.362'W 545 9/18/2000 9/25/96 10:19 63 31.947'N 40 01.178'W
3.10 C4 491 9/19/96 20:32 63 02.26'N 38 28.42'W 1919 9/20/1400 9/24/96 22:48 63 02.07'N 38 28.93'W
3.11 C3 380 9/19/96 21:51 63 56.40'N 38 11.30'W 2050 9/20/1400 9/24/96 20:34 62 55.51'N 38 11.39'W
3.12 C9 131 9/19/96 23:17 63 50.57'N 37 54.39'W 2173 9/20/1400 9/24/96 17:49 62 50.556'N 37 54.554'W
3.13 A4 7299 9/20/96 00:33 63 44.64'N 37 37.44'W 2130 9/20/1400 9/24/96 15:29 62 44.802'N 37 36.424'W
3.14 A8 381 9/20/96 01:50 63 38.71'N 37 20.77'W 2115 9/20/1400 9/24/96 12:58 62 3 8.122'N 37 20.723'W

Deployment #4 Transect IV

SITE OBH/S TTID # DEPLOYMENT DATE/TIME LAT. LON. DEPTH(M) START TIME RECOVERY DATE/TIME LAT. LON.
4.1 Al 132 9/28/96 21:18 59 52.89'N 45 21.80'W 131 9/29/0500 10/4/96 15:04 59 53.047'N 45 22.373'W
4.2 C4 491 9/28/96 19:49 59 46.77'N 44 57.80'W 135 9/29/0500 10/4/96 14:19 59 47.004'N 44 58.037'W
4.3 A3 133 9/28/96 18:32 59 42.010'N 44 38.626'W 147 9/29/0500 10/4/96 12:41 59 41.978'N 44 38.727'W
4.9 C9 131 9/28/96 09:30 59 11.789'N 42 46.091'W 1805 9/28/1300 10/4/96 03:00 59 11.411'N 42 47.584'W
4.10 C3 380 9/28/96 07:52 59 06.617'N 42 27.530'W 2129 9/28/1300 10/4/96 00:45 59 05.87'N 42 28.78'W
4.11 A8 381 9/28/96 05:10 59 00.077'N 42 04.648'W 2527 9/28/1300 10/3/96 21:56 58 59.91'N 42 05.30'W
4.12 A4 7299 9/28/96 02:49 58 53.459'N 41 41.870'W 2743 9/28/1300 10/3/96 19:28 58 53.756'N 41 40.881'W
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