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Silicic Volcanism at an Off-Axis Geothermal Field in the Mariana Trough Back-Arc Basin

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Silicic Volcanism at an Off-Axis Geothermal Field in the Mariana Trough Back-Arc Basin Silicic volcanism at an off-axis geothermal field in the Mariana Trough back-arc basin PETER LONSDALE 1 . _ . } Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California 92093 JAMto HAWKINS I ABSTRACT values, their lateral variability, and some nonlin- a few high values (1.5-2.5 W/m2) in the imme- ear temperature gradients led them to infer ac- diate vicinity (within 100 m) of small rocky hil- An abyssal-hill site with localized high heat tive hydrothermal discharge in this area. This locks (Hobart and others, 1983). flow on the 3-m.y.-old west flank of the active was surprising because observations of sedi- ridge in the Mariana Trough has sets of ment-covered crust of similar age on the flanks METHODS OF STUDY oblique fault scarps, mapped by multi-beam of slow-spreading mid-ocean ridges (for exam- Surface-Ship Transits transits, and cluster of 50- to 1,000-m-diam- ple, Scott and others, 1976) suggest that local- eter domes examined with a Deep Tow ized high-temperature discharge probably ceases About 90% of the 250-km2 area of Figure 2 survey. Some of the domes, including small, after the crust leaves the axial rift valley, al- is covered by 3 almost overlapping swaths of 20- to 30-m-high rocky knobs encrusted with though some hydrothermal circulation may per- closely spaced soundings from a SONARF.AY manganese oxide, crop out through the sedi- sist within the igneous crust. multi-narrow-beam survey conducted by the ment cover, but, despite heat-flow evidence To learn more about the hydrothermal U.S. Naval Oceanographic Office (who kindly that they are sites of hydrothermal discharge, phenomena and the heat source at this off-axis furnished the soundings). The principles and these bedrock features are fundamentally dif- site, we made a near-bottom survey as part of an bathymetric capabilities of this system are ferent from the hydrothermal mounds found extensive Deep Tow program in the Mariana described in Glenn (1970) and Phillips and on some mid-ocean ridges. Dredging one Trough that focused on the spreading axis. We Fleming (1978). dome recovered a rhyodacite pumice that we used the same equipment that had proved effec- The site lies within the 18°N corridor selected believe erupted at a depth of 2,600-3,600 m tive for mapping the Galapagos Mounds hy- as a "study transect" of the international within the basin. Ilie siliceous rock may be drothermal field (Lonsdale, 1977), and we SEATAR program, so it is crossed by several an extreme differentiate of a tboleiitic magma, intended to explore the similarities with that in- seismic profiles (for example, Fig. 3), including but a more likely origin is partial melting of tensively studied area (for example, Williams the multichannel line described by Hegarty and the basin's sedimentary fill. and others, 1979; Green and others, 1981), others (1980). The profiles tie into Deep Sea which is also off-axis but on the flank of a mid- Drilling Project Site 454 (Hussong, Uyeda, and INTRODUCTION ocean ridge. A comparative study seemed prom- others, 1981) located 20 km east of the geo- ising because echograms from the back-arc basin thermal field. The Mariana Trough is an archetype of an had led Anderson and others (1981) to infer the Deep Tow Survey actively opening back-arc (or "inter-arc") basin presence of hydrothermal mounds, perhaps (Karig, 1971). Fomiation and extension of analogous to the piles of manganese oxide and The Scripps Deep Tow instrument package oceanic crust, with fissure eruption of tholeiitic ferruginous clay that were precipitated at the (Spiess and Mudie, 1970) was towed 10—1C0 m basalt, is concentrated at a well-defined spread- Galapagos Rift site (Corliss and others, 1978). above the sea floor at 3 km/hr along 80 kin of ing center between the Philippine plate and a This paper concentrates on results of the track (Fig. 2) navigated by an array of 3 moored Mariana Arc microplate that overlies the west- Deep Tow survey, with supplementary data transponders. The most useful sensors on the ward-dipping subduction zone (Bibee and oth- from surface-ship transits of the area, and on de- towed vehicle were its narrow-beam 125-kHz ers, 1980). Deep-sea drilling results (Scott and scription and interpretation of dredged rock bathymetric profiler, 4-kHz subbottom profiler, others, 1981) suggest that the 250-km-wide samples. The poorly navigated heat-flow sta- paired side-scan sonars (with a 500-750-m swath basin (Fig. 1) was created in the past 5 m.y., and tions made in 1979 (Anderson and others, width), paired stereo-cameras (which collected magnetic anomalies indicate that the youngest 1981) cannot be located precisely on our revised 400 sets of photographs in 3 runs), and Neil parts of the basin floor, the rift mountains flank- chart. One month after the Deep Tow effort, Brown CTD. ing the axial rift valley, accreted symmetrically navigating with respect to the same transponder at half-rates of 15-17 mm/yr (Bibee and others, net that we had used, a new suite of heat-flow Geologic Sampling and Rock Analysis 1980; Hussong and Fryer, 1980). stations and cores was collected around targets In 1979, Andersoa and others (1981) discov- identified by the near-bottom surveys. The heat- Appreciation of the technique we used for ered very high (>2 W/m2) heat flow at a hill 45 flow results are reported elsewhere: they show a rock dredging is crucial to our discussion of the km west of the spreading axis, and the high generally low flux (0.05-0.2 W/m2) except for significance of the recovered sample. Rock out- Geological Society of America Bulletin, v. 96, p. 940-951, 9 figs., 3 tables, July 1985. 940 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/96/7/940/3434811/i0016-7606-96-7-940.pdf by guest on 30 September 2021 Figure 1. Regional bathymetry of Mariana Trough, showing location of study area pretation include Fryer and Hussong (1981), Bibee and others (1980), Mrozowski relative to modern plate boundary. Contours in kilometres. P = sites where Fryer and and Hayes (1980), and unpublished SONARRAY transits and Deep Tow surveys. Hussong (1981) dredged silicic pumice. Sources for bathymetry and tectonic inter- Profile is a composite interpretation of several airgun lines along 18°N. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/96/7/940/3434811/i0016-7606-96-7-940.pdf by guest on 30 September 2021 942 LONSDALE AND HAWKINS 144° 14 E 144°15'E 144° 16' E 144° 17 E 144° 18'E 144° 19' E 144° 20'E I44°2 I E 144°22 E 144°23 E Figure 2. Bathymetry around Deep Tow survey area, from SONARRAY data (dashed lines show interpolations between swaths), and Deep Tow bathymetry ;ilong tracks shown. Contours in metres corrected according to Matthews (his Table 42). The multi-beam data was collected by the U.S. Naval Oceanographic Office, which kindly furnished the soundings. crops were first mapped by Deep Tow's sonars as the dredge and its contents were immediately BATHYMETRY AND patches of high reflxtivity that protrude through raised to the surface. SHALLOW STRUCTURE the prevailing sediment blanket, and some were The pumiceous rock recovered by dredging then photographed. The largest of these outcrops was weighed to establish bulk density and poros- Multi-beam and Seismic Profiler Results was sampled by a transponder-positioned ity, and then it was analyzed with light- dredge. Soon aftei the transponder navigation microscope and X-ray diffraction techniques. The central region of the Mariana Trough and the character of the pinger trace indicated Chemistry was determined by X-ray fluores- resembles the rift mountains and fractured that the dredge had arrived at the mapped out- cence and by an electron probe micro-analyzer, plateaus of slow-spreading mid-ocean ridges like crop, there was a sharp 5,000-lb increase in wire using well-rinsed samples of the glass web of the the Mid-Atlantic Ridge. The northern half of the tension followed by a sudden tension drop, and pumice and of fused glass chips of the bulk rock. multi-beam survey area (Fig. 2) is occupied by a Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/96/7/940/3434811/i0016-7606-96-7-940.pdf by guest on 30 September 2021 SILICIC VOLCANISM, OFF-AXIS GEOTHERMAL FIELD, MARIANA TROUGH 943 Figure 3. Single-channel seismic profiles across the study area; located in Figure 2. 20-km-wide tilted plateau, the eastern rim of which is almost as high as the rift mountains beside the axial rift valley (Fig. 3a). The pla- teau's 1-km-high boundary escarpment strikes -350°, and its surface has a 100-400-m relief of subparallel abyssal hills, spaced 1-3 km apart (Fig. 4). South of 18°N, the plateau surface is less tilted (Fig. 3b), the boundary escarpment is offset 10 km westward, and this scarp and the abyssal hills strike almost due north. The lineated structural fabric of this central part of the trough floor is inherited from the period of crustal accretion at the plate boundary BROAD INTERMONTANE zone. The abyssal hills, like those on the flanks BASINS IN OLDER CRUST of mid-ocean ridges (Macdonald and Luyendyk, 1977; Searle and Laughton, 1977), are probably fault blocks created in the axial rift valley. Seis- mic profiles (Fig. 3) and 3.5-kHz records show that in the study area, these fault blocks have been buried and smoothed by deposition of 30-50 m of unconsolidated sediment.
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