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Volcanic Petrology and Geologic History of Northeast Bank, Southern California Borderknd

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Volcanic Petrology and Geologic History of Northeast Bank, Southern California Borderknd JAMES W. HAWKINS Geological Research Division, Scripps Institution of Oceanography, La Jolla, California 92037 EDWIN C. ALLISON Geology Department, San Diego State College, and Research Associate, Scripps Institution of Oceanography, La Jolla, California 92037 DOUG MACDOUGALL Department ofSIO, University of California, San Diego, La Jolla, California 92037 Volcanic Petrology and Geologic History of Northeast Bank, Southern California Borderknd ABSTRACT (see Fig. 1), is a flat-topped, steep-sided sub- marine hill rising about 900 m above the sur- Basaltic rocks, hyaloclastites and fossil rounding sea floor (Fig. 2). The bank top is fragments incorporated in volcanic material 17 km X 6 km within the 500 m depth contour were dredged from the flanks of Northeast which delineates the flattened summit. Maxi- Bank (lat 32°20' N., long. 119°40' W.) on the mum relief on the summit is less than 150 m Southern California Borderland. Unaltered, and local relief probably is only a few tens of fine-grained alkali basalt has 2.6 percent Ne and meters; the shallowest part of the bank is near 10.8 percent Ol in the norm. Trace element its south end where it rises to a depth of 357 m. abundances are typical of eastern Pacific basin The bank is 20 km east of the crest of the alkali basalts but Ba is enriched (880 ppm). 2700 m high Patton Escarpment which marks The fossil fragments, incorporated in breccia, the seaward edge of the Southern California hyaloclastite and agglomerate include a fauna Borderland (Emery, 1960). West of the Patton which lived in less than 50 m of water, as well as Escarpment are several seamounts ranging in forms from the intertidal zone. Bathymetry of height from 2000 to 3600 m. San Juan seamount the bank and the depth range of the dredged (3100 m high), the closest of these to the zone indicate that there has been at least 300 or escarpment, is encrusted with basaltic rocks as much as 500 m of subsidence since the (Emery and Shepard, 1945); it is believed to volcanism which incorporated the fauna in be a submarine volcano. East of the escarpment volcanic material. Isostatic adjustments due to the borderland comprises a series of north- crustal load of the bank can account for this westerly trending basins and ridges; in part, subsidence. these trends and the bathymetric features may A fission track date of unaltered basaltic glass be fault controlled. The basins are floored with gives an age of 4.5 ± 0.5 m.y.; a K/Ar date on Pliocene and younger silts and sands (Emery, unaltered holocrystalline basalt gave an age of 1960) while on the banks the rocks range in age 4.36 + 0.8 m.y.; the faunal assemblage is Plio- from probable Jurassic (Franciscan-type lithol- Pleistocene and manganese crust thicknesses ogies) to Pleistocene or Recent. Some of the are 0.2 to 1 cm. This is the youngest reported ridges rise high enough to form islands; these volcanism for the borderland. The magnetic islands are geologically similar to the nearby anomaly due to the bank lies on the projected coastal land areas. Some of the ridges or banks trend of anomaly 5c of the sea floor. It seems are covered with volcanic rocks and several, clear that this is fortuitous; there is no genetic such as Northeast Bank, have the morphologic relationship between these anomalies and these characteristics of submarine volcanoes. Krause data do not support the suggestions that (1965) reported widespread occurrences of Mason-Raff lineations can be traced through volcanic rock, some of it unaltered, young- the Borderland. looking basalt, in the borderland south of the region described here. North of the 32nd INTRODUCTION parallel young volcanic material is much less Northeast Bank, located on the continental abundant than in the area to the south discussed margin 230 km west of San Diego, California by Krause (1965). Northeast Bank is one of the Geological Society of America Bulletin, v. 82, p. 219-228, 4 figs., January 1971 219 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/1/219/3428483/i0016-7606-82-1-219.pdf by guest on 27 September 2021 SOUNDINGS IN FATHOMS CONTOUR INTERVAL-ZOO FATHOMS MERCATOK PROJECTION PREPARED BY U.S. BUREAU OF COMMERCIAL FISHERIES seUE AND UNIVERSITY OF CALIFORNIA INSTITUTE OF MARINE RESOURCES /! VI Figure 1. Generalized bathymetry of Southern California Borderland and location of Northeast Bank. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/1/219/3428483/i0016-7606-82-1-219.pdf by guest on 27 September 2021 PETROLOGY OF THE SAMPLES 221 32°30'N 8x Verticol Exaggeration 0 — • E 250 — I 500 — * ™ - ^-1000 — IhaSO — a 1500 — 10 20 km SCALE Contour interval 100 meters Figure 2. Bathymetry, sample location and profiles of Northeast Bank. Location of profiles shown by dashed lines. Bathymetry from Coast and Geodetic Survey Chart 1206 N-15, 1968. localities at which volcanic rocks have been which has 0.5 cm glassy rinds, nearly all of it is recovered on the borderland; its shape suggests fresh and unaltered; (2) ragged blocks and that it may be a submerged volcano. In ad- slabs of volcanic breccia and hyaloclastite; dition to the flat-topped roughly conical form these are coated with manganese crusts which it is distinctive (and named for the fact) in that range from a few mm to 1 cm in thickness; (3) it is elongated N. 20° E., in contrast to the subrounded to rounded pebbles (1 to 4 cm northwesterly trending topographic and struc- diameter) of Mn-coated volcanic debris; (4) tural "grain" of the borderland region. How- an angular block of brownish-gray dolomite, a ever, the N. 20° E. trend is also apparent in the rounded dacite porphyry cobble, and a sub- orientation and elongation of features on the rounded graywacke cobble (the three clasts deep-sea floor. During a regional survey in total about 5 kg). The abundance of volcanic June 1969 by the R/V Alexander Agassiz, of material, as well as the angularity of volcanic the Scripps Institution of Oceanography, fragments, indicates that these rocks are volcanic rocks were recovered by dredging and typical of the zone sampled by the dredge. magnetic profiles were recorded by a towed They must represent either material broken magnetometer. The purpose of this report is to from outcrops or from essentially in situ talus describe the rocks recovered, to discuss their of a bank which is interpreted as being formed possible significance in terms of the regional of (covered with?) volcanic rock. The three geology, and to point out some features of rocks listed in category 4 are typical of the general geologic interest. banks on the borderland and they may be part of the platform which underlies the volcanic PETROLOGY OF THE SAMPLES rocks. Introduction Unaltered Basaltic Rock The probable track of the rock dredge A major part of the dredge haul consisted of during sampling operations is shown in Figure remarkably fresh aphanitic basaltic rock. 2. About 100 kg of rocks were recovered from Plagioclase (Anes-vo) is the dominant mineral; the inteival between 1280 and 550 m depth. it occurs as euhedral laths about 0.5 mm long The sample consists of: (1) angular blocks and with strong preferred orientation suggesting keystone shaped fragments of basalt, some of flow alignment. In a few samples there are Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/1/219/3428483/i0016-7606-82-1-219.pdf by guest on 27 September 2021 222 HAWKINS AND OTHERS—PETROLOGY AND HISTORY OF NORTHEAST BANK plagioclase microphenocrysts (Anye). The pla- are probably an alkali feldspar. These patches gioclase laths enclose clusters of colorless to have a diffuse very small 2VT and resemble the faintly pink euhedral to subhedral clinopy- alkali feldspar termed potash-oligoclase (Mac- roxene (2VT = 60°); it probably is augite. Donald, 1942) or "anemousite" (Earth, 1931). Some grains have 2VT as low as 20° suggesting The diffuse character of the optic properties that subcalcic or pigeonitic augite may also be suggests that the positive axial sign may be due present. A few samples have euhedral micro- to overlapping of thin plates of alkali feldspar phenocrysts of augite. Olivine (Foss-g?) is the as discussed by MacDonald (1942). dominant microphenocryst (~8 percent of the One sample has an irregular cluster of rock) and grains with similar optical properties plagioclase grains (ABST) which was partly are also in the groundmass. The micropheno- melted on margins of the cluster and between crysts (0.6 to 1.5 mm) have well-formed individual grains and is spotted with glass. It crystal outlines but many are deeply resorbed; probably is a xenolith. No other evidence for all are fresh. A common form is remarkably contamination was found. elongated along crystallographic X. The A chemical analysis and molecular norm for a opaque minerals have X-ray characteristics of phenocryst poor sample is in Table 1. The magnetite; they are disseminated through the SiC>2, TiC>2 and KaO contents, the Ba, Sr, Y groundmass and less commonly are included by and Zr concentrations, and ratios such as olivine and pyroxene. Irreversible change of the Fe2O3/FeO, Na/K and K/Zr all are typical of magnetization on heating indicates the presence alkali basalts. The normative composition, with of maghemite (M. Ozima, 1970, written 2.6 percent Ne and 10.8 percent Ol would plot commun.). About 10 to 15 percent of the rock on the Ne side of the plane of critical silica is very finely crystalline and isotropic ground- undersaturation in the basalt tetrahedron mass occupying interstices between pyroxene (Yoder and Tilley, 1963).
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