FREDERIC P. NAUGLER 1 Pacific Océanographie Laboratory, National Oceanic and Atmospheric JOHN M. WAGEMAN J Administration, University of Washington, Seattle, Washington 98195 Gulf of Alaska: Magnetic Anomalies, Fracture Zones, and Plate Interaction ABSTRACT A linear anomaly map interpreted from the magnetic data (Fig. 1) was constructed to be Recently acquired magnetic data have al- compatible with the map of Atwater and lowed better definition of the linear magnetic- Menard (1970, Fig. 1) across the 52d parallel; anomaly pattern in the Gulf of Alaska. Based it is also consistent with trackline magnetic on anomaly offsets, the Aja fracture zone has profiles presented by Pitman and Hayes (1968, been located precisely and three additional Fig. 1). The numbering of anomalies follows the fracture zones have been identified. The Aja time scale established by Heirtzler and others fracture zone undergoes a change in trend that (1968, Fig. 3). Deviations from previous inter- reflects a major change in spreading direction pretations were made only where warranted by that occurred about 30 m.y. ago. Along the new data. northwestern margin of the Gulf of Alaska, Salient features revealed are: (1) an abrupt magnetic anomalies can be traced across the bend in the Aja fracture zone that apparently Aleutian trench and up to 50 km into the took place just before the generation of continental margin; whereas to the northeast, anomaly 8; (2) three fracture zones north of the the anomalies lose their identity several tens Aja, two of which were short-lived and of kilometers before encountering the con- terminated by the time of anomaly 13; (3) the tinental margin. This latter zone, paralleling continuation of magnetic anomalies 18 to 21 the continental margin between 135° W. and well into the continental slope in the north- 143° W., probably was caused by compressive western portion of the Gulf of Alaska; (4) a stresses within the continental margin and magnetic "disturbed zone" within the oceanic oceanic crust related to recent plate con- crust adjacent to the continental margin in the vergence along a coupled margin. Also, it may northeastern Gulf of Alaska; and (5) a pair of reflect the future location of a transform fault high-amplitude linear magnetic anomalies that will short-circuit the present Juan de Fuca within the continental margin adjacent to the Ridge-Aleutian trench transform fault and disturbed zone. greatly simplify the plate boundary bordering the Gulf of Alaska. OBSERVATIONS AND DISCUSSION INTRODUCTION Linear Magnetic Anomalies and Fracture Zones During the past several years, National Oceanic and Atmospheric Administration The great east-west fracture zones in the (NOAA) ships have collected magnetic data northeast Pacific were generated from offsets from along 10 east-west tracklines in the north- along the ancient Farallon ridge and represent ern Gulf of Alaska to delineate the hitherto the direction of Farallon plate movement rela- poorly defined magnetic pattern in this region. tive to the Pacific plate; the Aja fracture zone Additional magnetic data were collected by is the most northerly of these previously the NOAA ship Oceanographer in 1971 to sup- described in the literature. Linear magnetic plement the previous trackline information. anomalies generated by sea-floor spreading in a Emphasis was placed on resolving the oceanic reversing geomagnetic field and at right angles magnetic-anomaly pattern near the con- to fracture zones are present throughout much tinental margin. of the northeast Pacific (Atwater and Menard, Geological Society of America Bulletin, v. 84, p. 1575-1584, 3 figs., May 1973 1575 1576 NAUGLER AND WAGEMAN N 155° 150° 145° 140 135 130°W Figure X. Gulf of Alaska magnetic lineations (heavy erence profiles shown in Figure 2. Dash-dot line = solid lines) and fracture zones (double lines). Dashed 1,000-fm isobath. Hachuicd line = Aleutian trench lines = Ships' trackline. Lettered solid lines = ref- axis. 1970, Fig. 1). The new data allow a delineation spreading centers have jumped (see, for ex- of linear magnetic anomalies in close con- ample, Malahoff and Handschumacher, 1971). junction with the Aja fracture zone, thus aiding At approximately 142° W., the Aja fracture in its precise positioning. Also, on the basis of zone experiences a marked change in trend of anomaly offsets north of the Aja fracture zone, about 25° to the north. The offset of the several additional fracture zones have been magnetic anomaly pattern is unaltered through inferred. the bend, though it should be noted that the Between 152° W. and 142° W„ the Aja frac- offset had grown to approximately 200 km by ture zone trends slightly southeast. At the time the time of anomaly 7 because of a slightly of anomaly 19, the Aja represented a ridge off- faster spreading rate north of the fracture. The set of approximately 80 km. By the time of change in spreading direction apparently took anomaly 12, it had absorbed the ridge offsets of place in the interim between the generation of two intermittent transform faults, located 70 anomalies 9 and 8, or about 30 m.y. ago, and 140 km to the north, and attained an offset assuming the bend (as defined) occurred at a of 150 km (see Fig. 1). Because of insufficient position medial to the location of the offset data, the magnetic anomaly pattern associated ridge segments. In o:her words, a point that with the termination of the two intermittent best approximates the bend lies equidistant fracture zones has not been defined. Also, from anomaly 8.5 north of the Aja and its offset where trackline information exists, the mag- equivalent south of the Aja—a relation unique netic signature is poor, which is consistent with to this position in the anomaly sequence. It observations made about other regions where should be pointed out that the bend as shown is GULF OF ALASKA 1577 only an approximation of a feature that prob- beneath the North American plate, perhaps to ably is considerably more complicated; the conform better to the orientation of the nearby magnitude of both the ridge offset and the continental margin. change in spreading direction at the time of the In this region, portions of the Pacific plate bend suggest that several large adjustment younger than anomaly 7 (if ever present) and fractures (Menard and Atwater, 1968) were all vestiges of the Farallon plate have been over- required to accommodate the change. We have ridden by the North American plate, thus shown the best interpretation of the fracture obliterating any oceanic crustal evidence re- zone's position before and after the bend based flecting the final stages of Pacific-Farallon on the available data, and assume that the bend spreading north of the Aja fracture zone. occurs in the center of the undefined portion. Thirty million years ago coincides at least ap- Magnetic Disturbed Zone and Shelf proximately with the termination of the Sila Anomalies fracture zone and possibly with that of the West of about 143° W., magnetic anomalies Sedna and Surveyor fracture zones (see At- 18 to 21 can be traced northward across the water and Menard, 1970, Fig. 1). Aleutian trench and up to 50 km into the con- The Farallon plate started to break up 30 tinental slope (Fig. 1). The anomaly field m.y. ago as the Farallon ridge (eastern margin within this slope region appears to originate of the Pacific plate) encountered the North almost entirely from the underthrusted Pacific American plate (Atwater, 1970). According to plate. As the depth of underthrusting increases models proposed by Atwater (1970, Figs. 6 and beneath the thickening margin, anomaly 8), this interaction started when the Men- amplitudes attenuate rapidly, and over the docino transform fault encountered the North northwestern continental shelf the magnetic American margin, at which time the Farallon field is relatively featureless (Fig. 2). plate broke into two separate units. The bend Along the northeastern margin of the Gulf in the Aja fracture zone and the terminations of Alaska, the magnetic relations differ greatly of the Sila, Sedna, and Surveyor fracture zones from those observed to the west. Here a zone may reflect a large-scale plate boundary read- of oceanic crust up to 50 km wide and closely justment that followed this initial fragmenta- paralleling the continental margin is char- tion. A greatly simplified ridge structure acterized by a relatively smooth magnetic existed between the Mendocino and Aja field. On encountering this zone, extending transform faults for approximately 20 m.y., or from about 143° to 135° W., characteristic up until the approximate time of anomaly 5 oceanic anomalies abruptly terminate or be- (10 m.y. in age). The nature of the ridge during come distorted and drastically reduced in this interval is best reflected in anomaly 6, amplitude. Also associated with the continental which can be identified clearly as a continuous shelf in this region is a pair of broad, high- uninterrupted lineation extending more than amplitude, positive anomalies that closely 2,500 km between the Mendocino and Aja parallels a straight-line segment of the con- fracture zones. Shortly after the generation of tinental margin (Fig. 1). The more western of anomaly 5, the Farallon plate started to frag- these anomalies occurs slightly downslope of ment further because of its diminishing size. the shelf break and is a more continuous feature, The Juan de Fuca complex of rotated and extending from about 143° W. to 138° W.; fractured blocks (Peter and Latimore, 1969; it is shown in its most characteristic form in Silver, 1971) represents the last, and still Figure 2a. The magnetic smooth zone and a active, vestiges of Pacific-Farallon spreading shelf anomaly were noted by Haines and others north of the Mendocino fracture zone.