Woods Hole Science Center
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
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.
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:
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:
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 |
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 |
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 |
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 |
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 |