Refraction Observation Across a Leading Edge ...

Geophysical Monograph Series

The Geophysics of the Pacific Ocean Basin and Its Margin

Vol. 19

10. Project Nariftol III: Refraction Observation Across a Leading Edge, Malpelo Island to the Colombian Cordillera Occidental R. P. MEYER AND W. D. MOONEY 2

Geophysical and Polar Research Center, University of Wisconsin, Madison, Wisconsin 53706

A. L. HALES 3 AND C. E. HELSLEY Institute for Geosciences, University of Texas at Dallas, Richardson, Texas 75080

G. P. WOOLLARD, D. M. HUSSONG, AND L. W. KROENKE 4 Hawaii Institute of Geophysics, University of Hawaii, Honolulu, Hawaii 96822

J. E. RAMIREZ Institute Geoffsico de los Andes Colombianos, Bogotci, Colombia Abstract. Energy critically refracted through the continental crust and mantle as well as the oceanic plate including the zone of subduction has been received by seismographs in the western cordillera of Colombia, South America, from shots fired along a 400-km profile from the Pacific coast to the west. With added control provided by sea gravity data, preliminary results from heat flow, reflection and local refraction measurements at sea, and refraction measurements along the western cordillera, all made during Project Narino, a model crossing the leading edge of the continent at 4°N has been constructed which satisfies the gravity data both at sea and on land with maximum deviation of 22 mgal. The principal results from the study are: (1) the verification of structures expected to be associated with subduction by a controlled seismic source technique in an area of active tectonism; (2) a suggestion of a mechanism for localization of sedimentary basins by continental mantle—oceanic plate interaction; (3) a prediction of the extension of the Bolivar Atrato—San Juan basin to the shelf edge; and (4) a beginning explanation of the source of the unusual positive Bouguer anomaly found in association with the high western cordillera of the Andes of Colombia. In the region from the Yaquina graben to the Malpelo rise, uniform delays in the travel times observed as compared to those computed through the model suggest a soft zone in the oceanic upper mantle in accord with a broad zone of higher than normal heat flow which includes the graben. Multiple observations on land of controlled sources fired at sea appear to be a straightforward and effective method of delineating both oceanic and continental structure.

'The historical perspectives leading to the undertaking of Project Nariflo in 1973, and the objective; and general overview of this, a multinational controlled source seismic investigation in western Colombia and Ecuador, and in the adjacent ocean, have been briefly reviewed by Aldrich et al. [1973] and a general report is being prepared by all the participants. Geophysical and Polar Research Center Contribution Number 320. 3 Now at the Research School of Earth Sciences, The Australian National University, Canberra, A.C.T., Australia 2600. 'Hawaii Institute of Geophysics Contribution No. 702.

Direct seismic examination of the shallow portion of subduction zones including the region of downturn may be possible in a previously untried but straightforward way. We have seen refraction arrivals, most logically associated with paths through the subducting slab from shots crossing the oceanic plate, trench, and continental rise, at seismographs placed along the extension of the shot line on the adjacent continent in Colombia. This paper concerns the techniques and evidence, leading to this conclusion, based on a 400-km line

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of Pacific Ocean shots near 4°N latitude, ob- to continue the search parallel to structure along served principally in the westernmost Andean the cordilleras using timed explosions in lakes, range of Colombia, the Cordillera Occidental. modeling the previous experiments. Further, this technique appears to be generally In the light of plate tectonics and subduction applicable. Its applicability, at least for the east- processes, profiles perpendicular to structure ern Pacific, rests on successful land observa- were planned as well, using the adjacent ocean tions of sea shots, including shots in the region for shots to be monitored by both land and sea seaward of the trench. As a check, test shots off recorders. We attempted this with some trepidaPeru have been subsequently fired near but on tion regarding the land observations. Compresboth sides of the trench and successfully recorded sional energy transmission might well have been in Peru and Bolivia [Ocola et al., 1975]. In 1974, poor across this leading edge, for in addition to a second full-scale experiment with shots cross- the previous experience of poor observations ing the Mid-America trench and receivers on land across structural grain, poor transmission had in Mexico has been observationally successful. been reported for the Sn phase at land observaResults appear at the present stage of analysis tories from offshore earthquakes in some regions as supportive of the procedure's general applica- farther to the south along western South America bility [Mooney et al., 1975; Helsley et al., 1975]. [Molnar and Oliver, 1969]. Fortunately, our exThe Colombian study, where this technique perience with compressional wave arrivals was was initially applied, had its roots in earlier stud- good for stations placed near outcrop even in the ies recording mine blasts and special shots in easternmost cordillera at ranges of over 800 km high lakes of the Andes of Peru, Bolivia, and from 1800 kg shots (L. C. Ocola, personal comnorthern Chile with the hope of understanding munication, 1974). This in part is undoubtedly the structure and tectonic process responsible for due to extremely low ambient noise levels, a conthe mountains. Here, refracted arrivals through dition which seems characteristic of large porthe crust and upper mantle were sought princi- tions of the Andes. The Naririo experiment (Figure 1) covered a pally along strike, as early attempts across structure were unsuccessful [Aldrich et al., 1958, roughly triangular area extending from 4°N 1960; Woo/lard, 1960a, b; Ocola et al., 19711. Fur- to 2°S along its easternmost boundary, the eastther attempts along strike were only partially ern of the three cordilleras of Colombia, and the successful in receiving P, first arrivals. The southern extension of the Andes into Ecuador. results nevertheless supported high basal crustal The western apex of the triangle, 400 km offshore velocities, crustal low-velocity zones, and a very at Malpelo Island (81.5°W), was the juncture of thick crust approximately 65 to 75 km under the two lines of sea shots. One line radiated southmountains [Woo/lard, 1960b; Ocola and Meyer, eastward to terminate near the port of Tumaco 1972], which agrees with estimates by James in southwestern Colombia; the other radiated [1971a] based on earthquake surface-wave dis- eastward toward the port of Buenaventura (77°W) 350 km north of Tumaco. Ten groups of persion studies. investigators from three continents participated, IGPGTCN"UGVVKPI as did three research vessels. Western Colombia, the site of Project Naririo Other analyses being conducted independently (named after a province in southwestern Colombut which we expect will be directly relevant to bia where much of the work was undertaken) was the observations being reported here are those of chosen for a renewed search for an understanding the German Group for Explosion Seismology and of Andean tectonism for a number of reasons. There, to the north from the Ecuadorian border, the University of Hawaii, which have additional the Andes diverge to become three well-separ- perpendicular-to-structure data also within the ated cordilleras having significantly differing western cordillera, and those of the Instituto gravity expressions (both positive and strongly Geofisico del Peru and the Carnegie Institution negative) and are paralleled at sea by a filled, of Washington, which have data from observapossibly fossil, trench. These factors, and a more tions 250 km farther to the east near Bogota:. RTGNKOKPCT["TGUUNVU"HTQO"QVJGT"UVUFKGU moderate level of earthquake activity, suggested an Andean region undergoing less rapid tectonic For this first discussion of the results from obevolution and having more separable parts and servations of the shot line from Buenaventura to processes than those found to the south. A major Malpelo Island, roughly along 4°N latitude, we portion of the Naritio experiment was designed are dependent on initial and preliminary results

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Fig. I. Sketch map of the Narifio project, Colombia and Ecuador, South America [after Aldrich et al., 1973].

from other observers. These have been generously provided to us in advance of publication (Figure 2). At the western apex of the triangle in the Malpelo Island region (Figures 1 and 2) free-floating telemetering hydrophone buoys were set from the research vessel Thomas G. Thompson by the University of Washington to observe shots fired from the research vessel Kana Keoki of the University of Hawaii along the two profiles to the coast. These observations, together with detailed local refraction data taken from the Thompson by the University of Washington in the vicinity of Malpelo Island, have provided estimates of the velocity of the oceanic crust in this region (B. T. R. Lewis, personal communication, 1975). A parallel-to-structure profile shot from the Colombian Navy vessel Gorgona to a University of Texas ocean-bottom seismometer on the shelf near Buenaventura has provided estimates of velocities and thickness of the sediments filling

the basin off Buenaventura, and the velocity of basement rocks underlying it. Equally important to the interpretation of the sea shots are the results of observations of shots made by the Carnegie Institution of Washington at Laguna La Cocha in southwestern Colombia which were recorded to the north along the western cordillera by the University of Wisconsin and the German Group for Explosion Seismology (R. Meissner, personal communication, 1974). This profile crossed the landward extension of the 4°N profile at the parallel-to-structure profile's approximate end, 400 km to the north of the La Cocha shot point [Gettrust et aL, 1973]. Along the western cordillera near the town of Cali and near the intersection of the aforementioned profiles, a partial reverse to the La Cocha profile was provided by three shots from the Yumbo cement quarry of Cementos de Valle, Inc., recorded to the south along the cordillera. These shots, observed by the University of Wisconsin




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108 82° W

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and, more crucially, by energy from KK10. In this information to signal the end of the contithis area, literally a terra incognita, the velocity nental 7.8 km/s slab since it must be unavailable used is a simple continuation of the velocity as- as a critical refractor of energy from these shots, sumed to underlie the basin. This velocity seems for this path, if it existed, would require less time. plausible because the dip of the slab (-15°), Just west of shot G427, shot KK10 is apparwhere modeled by both seismics and gravity, ently the easternmost shot from which refracted enters strongly into calculation of gross times. energy can enter the subducting mantle and arThe dip modeled agrees reasonably with the rive at station 55 via the subducting oceanic plate's dip as can be judged by the hypocenter plate, i.e., the critical point. The position of this distribution (Figure 26). Certainly a small mod- cutoff to the east provides a constraint on the ification in dip could be accommodated by si- plate's dip and depth and the velocity contrast multaneous changes in P, and crustal velocities, between the oceanic mantle and the overlying but gravity fitting in the role of the ultimate material. In a similar way, the modeled 8.35 km/s arbitrator would probably not be very sensitive wedge just above the subducting plate farther to these trade-offs. Without further oceanic data, east under the continent, if correct, should cut additional complications in the model seem arbi- off or greatly modify the direction of energy travtrary. eling to more eastern receiving stations than The travel times based on the scattered and considered here, as these stations should require less complete array data for shots 424 to 427 deeper entrance into the continental section for (Figure 18) imply, on the basis of apparent veloc- the rays either critically refracted along the ity, that the branch observed from these shots is oceanic mantle or arriving by diving wave paths. a continuation of the branch seen from the data of At this time we do not know whether this has shots 411 to 422. However, neither the ampli- affected paths to stations much farther east near tudes at stations 1 and 2 (Figure 22), nor the Bogota, where clear signals were received. One most interesting prediction from the moddivergence of the observed travel times from the synthetic travel times, nor the weak and emer- el is the large seaward extent of the Bolivar bagent character of the arrivals at station 55 (Fig- sin, predicted to deepen slightly toward the ure 16), support this premise. Rather, we have continent's edge. A rationale for the basin's extaken the arrivals from shots 424-427 to repre- istence, its predicted seaward extent, and persent rays critically refracted along the bottom of haps seaward deepening, is provided by the conthe basin and obtained times which fit the ob- tact shown in the model between the 7.8 km/s servations at station 55. Indeed, we have used continental slab and the oceanic plate and the A

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Fig. 26. Hypocenters and the model. Earthquake hypocenters for the years 1965 through 1974, projected to the profile line A-A*"at 37° azimuth or its reciprocal. The direction of the projection was determined from the hypocenter data by examination, rotating projection angle to minimize scatter. For comparison, using Figures 3 and 4, the strike of the western Andes and the Cauca fault is about 24 degrees, the seaward wall of the trench about 36 degrees, and the Yaquina graben about 31 degrees. Thus the azimuth of the projection is closest to the azimuth of the trench. The extent of the area considered is restricted to 100 km north of the profile line, avoiding an area of high seismicity atypical of the profile region. Hypocenters based upon fewer than ten P"or P*"arrivals or held at 33-km depth are not included. The magnitudes are from body wave determinations. Data derived from the Earthquake Data File of the National Geophysical and Solar-Terrestrial Data Center.




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MEYER ET AL. resultant potential eastward and downward motion of the floor of the basin if the continental mantle were moved downward (and eastward) in response to the motion of the descending plate. In short, basin subsidence is postulated as a response to the downward motion of the continental mantle caused in turn by mechanical coupling of it to the subducting plate. This is a checkable hypothesis which could account for the general position of the Bolivar basin and equivalent features elsewhere, as well as the absence of basins or troughs where opportunities for coupling can be shown not to be present. SUMMARY AND CONCLUSIONS

These conclusions arise from but a subset of the data of Project Narino. We expect that they will either be refuted, modified, or strengthened by combination with other results and insights. From shots at sea along 4° N, observed principally in the western cordillera of the Andes mountains in Colombia, refracted first arrivals, by way of the continent crust alone, the continental mantle, or the oceanic mantle subduction zone including the region of downturn, have been identified. This alone is a significant result, as it raises the possibility of closely spaced perpendicular-to-strike measurements potentially providing hitherto unavailable detail concerning the structure of the region of downturn and the shallow area of contact between the continent and ocean lithosphere. Along the 4°N profile, with constraints provided by the sea-to-land refraction data combined with gravity, parallel-to-structure refraction, and heat flow data, a consistent first model arises which is supportive of the structures expected in plate theory for subduction. It should be noted that this is independent evidence separate from the study of natural events. A number of tentative conclusions arise from the model. One concerns a possible soft, partial melt zone in the oceanic mantle east of the Yaquina graben hypothesized to account for 0.7-sec delay between arrivals from shots near to and west of the graben as compared with the present model predictions. This soft zone is supported by higher than normal heat flow, up to 4 HFU in a 100-km band including the graben. A second hypothesis concerns a postulated continuation of the Bolivar basin west to the edge of the contiental shelf along 4°N latitude based on both continental crust and continental mantle refractors and gravity. This hypothesis should be easily checkable by profiling at sea. In regard to the

basin continuation, the model shows contact between the continental mantle and the subducting plate and thus suggests a mechanism controlling basin subsidence—partial coupling of the continental mantle to the downward movement of the subducting oceanic plate. In fitting the Bouguer gravity as well as travel times, the model provides a possible structural configuration satisfying the anomalous combination of positive Bouguer anomalies and high elevation found in the western cordillera of Colombia. The structural configuration is, of course, not unique, but the details of certain portions of it were critical to satisfying the travel times and apparent velocities observed. A finding potentially important to the understanding of details of the structures underlying the Andes is the truly astounding changes in the static station corrections required for oceanic versus continental paths. This suggests that array studies of the areal distribution of station residuals versus apparent velocity for branch identification may be of primary importance in unraveling the structures and tectonic habit of the Andes. Finally, multiple observations on land of controlled sources fired at sea appears to be a straightforward method of delineating both oceanic and continental structure as well as the transition between the two. The model fails to satisfy the observed free-air anomaly in the general region of the trench by as much as 22 mgal. This is but symptomatic of the principal weakness of the data set, lack of knowledge of formation velocities together with independent control of depths. These should be obtained by additional measurements on strike both on land and at sea. The principal strength of the technique lies in providing continuity across structure and in sampling regions unavailable by more local conventional seismic techniques. Agkrs{pihkqirxw1" As the list of collaborators, Figure 2, makes clear, this multinational project required extraordinary international preparation, support, understanding, and enthusiasm, which we gratefully acknowledge. The contribution of the program coordinators, Dr. L. Thomas Aldrich (Carnegie Institution of Washington, D.C.) and one of the authors, J. E. R., is gratefully acknowledged by the other authors. Much of the data reduction and interpretation concerned directly with this study was done at the University of Wisconsin, Madison, in cooperation with the University of Texas, Dallas. We take pleasure in acknowl-




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132 edging numerous contributions. Besides our sincere appreciation for preliminary results cited in the paper, we express our gratitude to the following: Mr. James Luetgert, computer-aided seismicity studies and computer reduction; Mr. Joseph Laurence, station correction techniques; Mr. William Unger, revised bathy-

metric map; Mr. Thomas Riederer, data digitizing; Mr. Lee Powell, overall electronics design; Ms. Dee Kruger, editorial assistance; Mr. James Gallagher, drafting; and Mr. Joseph Nation, preparation of data from the University of Texas stations. Support for the study was from the National Science Foundation.
