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Home , Aleutian Trench, Denali Fault, Fracture zone

FREDERIC P. NAUGLER 1 Pacific Océanographie Laboratory, National Oceanic and Atmospheric JOHN M. WAGEMAN J Administration, University of Washington, Seattle, Washington 98195

Gulf of : 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 and up to 50 km into the took place just before the generation of ; 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 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 high-amplitude linear magnetic anomalies that will short-circuit the present Juan de Fuca within the continental margin adjacent to the Ridge- 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 ; 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 -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 , 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 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 . (1971) from an aeromagnetic survey. They North of the Aja fracture zone, the Farallon described the shelf anomaly as similar to the ridge was an active spreading center, and at slope anomaly along the Atlantic shelf edge of least until the generation of anomaly 7 (about (Drake and others, 1963). 27 m.y. ago), the youngest identifiable anomaly Over the past few years, several hypotheses adjacent to the continental margin. The bend have been advanced to explain the Atlantic in the Aja fracture zone and the associated slope anomaly and a magnetically smooth zone reorientation of the magnetic anomaly pattern of oceanic crust adjacent to the Atlantic mar- show that the Farallon plate rotated several gin (summarized by Taylor and others, 1968; degrees counterclockwise before disappearing Emery and others, 1970). Taylor and others 6000T . 58°3I N 58°I8'N I5I°53W I38000'W GULF OF ALASKA 1577

Figure 2. and associated magnetic anomaly field (IGRF) along selected tracklines in the Gulf of Alaska (see Fig. 1). i (1968) feel that the slope anomaly reflects an the magnetic smooth zone to a magnetic quiet intrusion emplaced along the continental mar- interval for a number of reasons, including the gin, and that the oceanic smooth zone resulted age of the adjacent oceanic crust, the general from regional metamorphosis that destroyed shape of the zone, and the fact that the the magnetization of the basaltic layer 2. anomalies lose their identity at different Emery and others (1970) relate both phenom- positions along the anomaly sequence. Based on ena to geomagnetic field events recorded on the the above considerations and also continental basaltic layer 2 during its generation by sea- margin relations discussed in the following floor spreading. They believe the slope anomaly paragraphs, the smooth zone appears to rep- reflects spreading during an Early Permian resent Tertiary oceanic crust whose original period of normal polarity that coincided with magnetic character has been distorted. Thus, the initial rifting of the European and North regional metamorphism, invoked by Taylor and American land masses, and the smooth zone others (1968) to explain the Atlantic smooth represents crust generated during the ensuing zone, may find more credence when applied to Kaiman reversed magnetic interval. The paral- the Gulf of Alaska disturbed zone. lelism of these magnetic features and their Also it is highly unlikely that the shelf proposed ages are not inconsistent with the anomalies are in any way related to original geometry and history of spreading in the spreading (extrusive) processes. The Gulf of Atlantic as presently conceived; thus, relating Alaska continental margin has probably under- their origin to fundamental spreading processes gone considerable large-scale deformation be- is an attractive alternative. cause of the vast amounts of oceanic crust sub- The same cannot be said of the relations ducted beneath it (Atwater, 1970). Indeed, the observed along the northeast Gulf of Alaska shelf region here is probably composed almost margin. Here it is virtually impossible to relate entirely of material incorporated into the mar- 1576 NAUGLER AND WAGEMAN

gin during the lengthy periods of . between this eastern limit of underthrusting The anomalies thus appear to represent either and the southern extension of the fault dike intrusion or a basement ridge complex system appears to be largely coupled to the formed during margin deformation. It may be Pacific plate (Richter and Matson, 1971). significant that these anomalies occur only Assuming the rest of Alaska is rigidly attached where the margin is coupled. This suggests that to the Nortn America plate, a transition of they are not the result of plate interaction but the plate boundary from ore of strike-slip dis- were produced after the margin became placement (the Queen Charlotte Islands fault coupled. This argument favors recent dike system) to one of underthrusting (the Aleutian intrusion. arc subduction zone) must be occurring within the continental crust of southeastern Alaska. Recent History of Plate Interaction This results in an unstable situation in which Plate interactions in the Gulf of Alaska have subduction of continental crust is required in been complicated during Tertiary and recent order that a narrow zone of deformation, times by two plates that are no longer present. typifying most plate boundaries, be main- The Farallon plate and the ancient northerly tained throughout the transition. Subduction moving (Pitman and Hayes, 1968; of continental crust is considered by many to Grow and Atwater, 1970; Atwater, 1970) once be physically untenable owing to its buoyancy lay between the Pacific and North America and hyperfusible petrologic make-up (for plates. As these plates were consumed, a com- example, Dietz and Holden, 1970). Thus, to plex sequence of changing plate-boundary allow for the present differential motion be- conditions occurred along the Pacific margin of tween the Pacific plate and :he North America the North America plate (Atwater, 1970) lead- plate, crustal shortening by internal deforma- ing to the present geometry (Fig. 3). tion must be occurring to relieve horizontal In the present model of this region, a great compressive stresses and complete the plate ridge-trench transform fault, or system of boundary transition. This zone of compression transform faults, extends from the Juan de should lie between the eastsrn limit of active Fuca ridge complex to the Aleutian trench subduction along the Aleutian trench and the (Wilson, 1965; Tobin and Sykes, 1968) and system of transform faults farther east. marks the boundary between the Pacific and Based on a late Miocene or early Pliocene dis- North American plates. This system consists of appearence of the northern-moving Kula plate several distinct parts, including the Queen (Atwater, 1970) and recent uplift and deforma- Charlotte Islands fault, the Fairweather and tion in the Alaska area (Stonely, 1967; Plafker, Totschunda faults (Richter and Matson, 1971), 1969), Richter and Matson (1971) propose that and parts of the system. These will the decoupling of the continent along the be referred to collectively as the Queen Denali fault system may have occurred as Charlotte Islands fault system. Theory requires recently as 10 m.y. ago. This resulted in a a transform in this location to accommodate the portion of southeastern Alaska becoming fixed lateral displacement between the Pacific and to the leading edge of the Pacific plate. Richter North America plates. and Matson (1971) further propose that the Convergence of the two plates occurs along Totschunda and Fairweather faults represent the Aleutian (McKenzie and the beginning oi a new transform fault, short- Parker, 1967; Isacks and others, 1968). circuiting the southeast section of the Denali Associated with the is an active fault system in an attempt to better accom- zone of intermediate-depth (70 to modate the present-day plate motions. 170 km) that extends north to the Denali fault Origin of the Disturbed Zone system (Fig. 3; Tobin and Sykes, 1966). Shallow seismic activity occurs between the arc The magnetic disturbed zone paralleling the and its related submarine trench to the south Alaskan continental margin between 143° W. but terminates in an easterly direction at about and 135° W. probably is related to the deforma- 146° W. This location coincides approximately tion occurring in the nearby' continent. Both with the easternmost bathymetric expression of appear to ex:end from the Queen Charlotte the Aleutian trench (Fig. 3), suggesting that Islands fault to the eastern limit of underthrust- active underthrusting is negligible east of 146° ing along the Aleutian trench. Furthermore, W. Consequently, much of southeastern Alaska transmission of compressive stresses acting GULF OF ALASKA 1577

W o °o £ .\©oc

Bering ^

Sea

Totschunda Fault si? 7 Vl Disturbed ÌÌ AMERICAN

Zone -Fairweather \ Fault 6<>

PACI FIC Queen Charlotte St Island Fault PLATE PLATE o ,o

5oc Oc SaHad a

o o >

Figure 3. Sketch map of northwestern North Amer- Sykes, 1968; Richter and Matson, 1971). Arrow in- ica showing major tectonic features along Pacific-North dicates present motion of Pacific plate relative to North America plate boundary (in part after Tobin and America plate. within the continent across a coupled margin compressional stress with a component of right- to the adjacent oceanic crust could be antic- lateral shear. ipated. The location and shape of the dis- The manner in which crustal deformation can turbed zone support this conclusion and suggest lead to destruction of the magnetic anomaly that the crust here is being subjected to a field is difficult to assess at present. Some form 1576 NAUGLER AND WAGEMAN

of regional metamorphism related directly or continental margin. Here, horizontal crustal indirectly to crustal compression may have shortening seems to be accomplished by broad reduced greatly the effective magnetization of compressive deformation as well as under- rock within the magnetized layer. One pos- thrusting and subduction, further complicating sibility is hydrothermal alteration of fractured a region inherently complex because of its rock. Luyendyk and Melson (1967), on lengthy history as a zone of plate convergence. examining rocks dredged from the mid-Atlantic Between about 146° W. and 135° W., where ridge, found that hydrothermal alteration of compressive stresses appear to be transmitted "oceanic" tholeiitic basalts resulted in a break- across the continental oceanic crustal boundary, down of the high-susceptibility magnetic there is no evidence lor subduction in the sur- oxides (titano-magnetite and ilmeno-haema- face of the crust. tite) to sphene and other minerals, thus de- / creasing the susceptibility and remanent Present-Day Plate Motion and the magnetism. Disturbed Zone On examining the orientation of the dis- Eastern Aleutian Trench turbed zone, a conspicuous and perhaps Implied in this discussion is that west of 145° significant relation is noted. The seaward limit W., underthrusting and subduction are re- of apparent deformation conforms with a lieving compressional stresses resulting from straight-line extension of the Queen Charlotte plate convergence. This does not appear to be Islands fault and also encounters the continental wholly correct. In a recent seismic reflection margin in the northernmost Gulf of Alaska study of the continental margin off Kodiak with an orthogonal relation to the presently Island (including the Aleutian trench between active portion of the Aleutian trench. It com- about 152° W. and 145° W.), von Huene pels one to speculate that this represents the (1972) concluded that the presumed mega- eventual plate boundary (transform fault) that thrust producing the Benioff zone of earth- will accomplish a stable transition from the quakes does not come to the surface as a simple Queen Charlotte Islands fault structure to the shear zone at the trench but that there is a Aleutian trench subduction zone, assuming diffuse zone of deformation across the whole present-day plate morions persist. This would continental margin. In his interpretation, the seem a simple solution to the complex situation trench in this region appears relatively unim- that exists today, consequently one might ask portant as a structural feature, with its only why tnis short-circuiting has not occurred significance being that it marks the beginning already. The answer may be tied in with the of a broad zone of compressional deformation; fact that "thin" rigid plates of lithosphere have thus, the northwestern termination of the significant vertical dimension (generally con- disturbed zone and the beginning of shallow sidered to be about 100 km) when compared seismic activity associated with the Aleutian with surface crustal feitures, and processes oc- trench does not mark a prominent boundary curring at depth may not always be reflected in between a coupled margin and a well-developed the surface geology. Far the present geometry subduction zone. The transition is a gradual of the crust (surface of the lithosphere) to exist one, with a broad zone of deformation occur- at all, tectonic activity in this region must be ring along the continental margin perhaps as dominated by motions within the deeper far as the Shumagin transition (von Huene and portions, and probably major bulk, of the Shor, 1969), at about 160° W. The Shumagin lithosphere. The present plate geometry ap- transition marks the location where the pears to result from recent large-scale plate Aleutian arc system narrows significantly as reorganizations that have produced an unstable it leaves the Bering shelf and becomes entirely surface configuration but as yet have not an oceanic feature. become dominated by it. Complications in the form of surface drag from sialic continental West of the Shumagin transition, the crust resisting subduction apparently have not Aleutian arc appears to represent a well-de- become sufficiently critical to require a re- veloped subduction zone with virtually all plate positioning of the plate boundary oceanward. convergence accommodated efficiently by un- This should occur when the compressive stresses derthrusting and resorption of the Pacific plate. acting near the surface reach deep enough into Between 160° W. and 146° W„ the Pacific the lithosphere to impede its motion, resulting plate is apparently partially coupled to the GULF OF ALASKA 1577 in a recoupling of southeastern Alaska to the sea-floor spreading center and strike-slip North America plate. The Fairweather and movement: Jour. Geophys. Research, v. 76, p. 6265-6275. Totschunda fault systems may represent an McKenzie, D. P., and Parker, R. L., 1967, The initial stage of this repositioning. north Pacific: An example of tectonics on a ACKNOWLEDGMENTS sphere: Nature, v. 216, p. 1276-1280. Menard, H. W., and Atwater, Tanya, 1968, We wish to thank David A. Emilia, Barrett Changes in direction of sea-floor spreading: H. Erickson, and Robert E. Burns for many Nature, v. 219, p. 463-467. valuable discussions. Peter, G., and Latimore, R., 1969, Magnetic struc- ture of the Juan de Fuca- area: Jour. Geophys. Research, v. 74, p. 586-593. REFERENCES CITED Pitman, W. C., Ill, and Hayes, D. E., 1968, Sea- Atwater, Tanya, 1970, Implications of plate tec- floor spreading in the Gulf of Alaska: Jour. tonics for the Cenozoic tectonic evolution of Geophys. Research, v. 73, p. 6571-6580. western North America: Geol. Soc. America Plafker, G., 1969, Tectonics of the March 27, 1965 Bull., v. 81, p. 3513-3536. Alaska : U.S. Geol. Survey Prof. Atwater, Tanya, and Menard, H. W., 1970, Paper 543-1, 74 p. Magnetic lineations in the northeast Pacific: Richter, D. H., and Matson, N. A., Jr., 1971, Earth and Planetary Sci. Letters, v. 7, p. Quaternary faulting in eastern : 445-450. Geol. Soc. America Bull., v. 82, p. 1529-1540. Dietz, R. S., and Holden, J. C., 1970, Reconstruc- Silver, E., 1971, Small in the north- tion of Pangea: Breakup and of east Pacific: Geol. Soc. America Bull., v. 82, p. continents, Permian to present: Jour. Geophys. 3491-3496. Research, v. 75, p. 4939-4956. Stonely, R., 1967, The structural development of Drake, C. L., Heirtzler, J., and Hirshman, J., 1963, the Gulf of Alaska sedimentary province in Magnetic anomalies off eastern North America: southern Alaska: Geol. Soc. London Quart. Jour. Geophys. Research, v. 68, p. 5259-5275. Jour., v. 173, Pt. 1, p. 25-57. Emery, K. O., Uchupi, E., Phillips, J. D., Bowin, Taylor, P. T., Zietz I., and Denis, L. S„ 1968, C. O., Bunce, E. T., and Knott, S. T., 1970, Geologic implication of aeromagnetic data for off eastern North America: the eastern continental margin of the United Am. Assoc. Petroleum Geologists Bull., v. 54, States: Geophysics, v. 33, p. 755-780. p. 44-108. Tobin, D. G., and Sykes, L. R., 1966, Relationship Grow, T. A., and Atwater, Tanya, 1970, Mid- of hypocenters of earthquakes to the geology Tertiary tectonic transition in the Aleutian arc: of Alaska: lour. Geophys. Research, v. 71, p. Geol. Soc. America Bull., v. 81, p. 3715-3722. 1659-1667. Haines, G. V., Hannaford, W., and Riddihough, • 1968, Seismicity and tectonics of the north- R. R., 1971, Magnetic anomalies over British east Pacific : lour. Geophys. Research, Columbia and the adjacent Pacific Ocean: v. 73, p. 3821-3845. Canadian Jour. Earth Sci., v. 8, p. 387-391. von Huene, R., 1972, Structure of the continental Heirtzler, J. R., Dickson, G. O., Herron, E. M., margin and tectonism at the eastern Aleutian Pitman, W. C., Ill, and Le Pichon, X., 1968, trench: Geol. Soc. America Bull., v. 83, p. Marine magnetic anomalies, geomagnetic field 3613-3626. reversals, and motions of the ocean floor and von Huene, R., and Shor, G. G., Jr., 1969, The continents: Jour. Geophys. Research, v. 73, p. structure and tectonic history of the eastern 2119-2136. Aleutian trench: Geol. Soc. America Bull., v. Isacks, B. L., Oliver, J., and Sykes, L. R., 1968, 80, p. 1889-1902. Seismology and the new global tectonics: Jour. Wilson, J. T., 1965, Transform faults, oceanic ridges Geophys. Research, v. 73, p. 5855-5900. and magnetic anomalies southwest of Van- Luyendyk, B. P., and Melson, W. G., 1967, couver Island: Science, v. 150, p. 482-485. Magnetic properties and petrology of rocks near the mid-Atlantic ridge: Nature, v. 215, p. 147-149. MANUSCRIPT RECEIVED BY THE SOCIETY SEPTEMBER Malahoff, A., and Handschumacher, D. W., 1971, 21, 1972 Magnetic anomalies south of the Murray REVISED MANUSCRIPT RECEIVED NOVEMBER 11, fracture zone: New evidence for a secondary 1972