The new tectonic interpretations presented in this paper are based on geologic field mapping and gravity data supplemented by well logs, seismic profiles, and radiometric and earthquake data. The present Caribbean‐South American plate boundary is the South Caribbean marginal fault, where subduction is indicated by folding and thrusting in the deformed belt and a seismic zone that dips 30° to the southeast and terminates 200 km below the Maracaibo Basin. The Caribbean‐South American convergence rate is estimated as 1.9 ± 0.3 cm/yr on the basis of the 390‐km length of the seismic zone and a thermal equilibration time of 10 m.y. The Caribbean‐South American convergence has produced a northwest‐southeast maximum principal stress direction σ1 in the overriding South American plate. The mean σ1 direction for the Maracaibo‐Santa Marta block is 310° ± 10° based on earthquake focal mechanism determinations, and structural and gravity data. On the overriding South American plate, basement blocks have been uplifted 7–12 km in the last 10 m.y. to form the Venezuelan Andes, Sierra de Perija, and the Colombian Santa Marta massif. Crystalline basement of the Venezuelan Andes has been thrust to the northwest over Tertiary sediments on a fault dipping about 25° and extending to the mantle. In the Sierra de Perija, Mesozoic sediments have been thrust 16–26 km to the northwest over Tertiary sandstones along the Cerrejon fault. A thrust fault dipping 15° ± 10° to the southeast is consistent with field mapping, and gravity and density data. The Santa Marta massif has been uplifted 12 km in the last 10 m.y. by northwest thrusting over sediments. The basement block overthrusts of the Perijas, Venezuelan Andes, and the Santa Marta massif are Pliocene‐Pleistocene analogs for Laramide orogenic structures in the middle and southern Rocky Mountains of the United States. The nonmagmatic basement block uplifts along low‐angle thrust faults reveal horizontal compression in the overriding plate over 500 km from the convergent margin. Present‐day east northeast‐west southwest (080°) compression is indicated by earthquake focal mechanisms and strike slip motion on the Bocono fault. These earthquakes are intraplate deformation associated with east‐west (080°) Nazca‐South American convergence.

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A new compilation of gravity data in the region surrounding the Strait of Gibraltar between 2° and 9°W and 34° and 37°30′N is presented in the form of a Bouguer anomaly map. Published data and new data obtained by Princeton University in southern Spain are included. There are four major features of the anomaly map: (1) An arcuate zone of negative anomalies, more than 200 km wide, parallels the trend of the Betic and Rif orogens. The gravity low crosses the Strait of Gibraltar and includes the Atlantic approach to the Strait and the western Alboran Sea. Values as low as −130 mgal in Spain and −150 mgal in Morocco were reduced. (2) Two coastal gravity high zones along the Moroccan and Spanish margins of the western Alboran Sea show steep gradients and closures as high as 110 mgal superimposed on the arcuate negative zone. The gravity highs in part coincide with a belt of outcrop of ultramafic and associated metamorphic rocks. (3) A central high in the Alboran Sea begins about 90km east of Gibraltar and extends eastward into the axial high of the western Mediterranean. (4) Positive anomalies trend WSW from southern Spain and westward from northern Morocco into the Atlantic approaches to the Strait of Gibraltar. All four major gravity features show a rough symmetry about a plane striking E‐W through the Strait of Gibraltar. The most important conclusions that can be drawn from the data are the following: (1) The steep gravity gradients and magnitude of the coastal highs require steeply dipping density discontinuities probably reaching the mantle. The data support recent petrological and structural evidence from Spain of diapiric intrusion of ultramafic rock from the mantle and conflict with previous hypotheses that propose a thin thrust sheet structure of these masses. (2) Continental crust extends across the Strait of Gibraltar in a belt more than 200 km wide, including the western Alboran Sea. The continuity and symmetry of the anomaly pattern across the Strait agree with similar geological observations and correlations of rocks as old as Paleozoic and imply that the Betic and Rif have been part of a single tectonic system including the Alboran Sea probably since Precambrian time. Neither gravity nor geological evidence supports proposals of a major zone of plate discontinuity (transform faulting) between Spain and Morocco as a continuation of the Azores‐‘Gibraltar’ ridge from the Atlantic. (3) The central Alboran Sea high is consistent with either a crustal break or a crustal thinning model. Significant zones of thin crust and possible crustal extension are found also in the Atlantic approaches to the Strait of Gibraltar.

Regional gravity studies in southwesternMontana show a -40 mgal anomaly associated with the Boulder batholith. The anomaly pattern indicates that the batholith is approximately symmetrical about a northeast-trending axis which is parallel to the length of the present intrusive outcrop. The axis of the gravity low, however, lies approximately 25 km northwest of the present outcrop axis of the Boulder batholith but would coincide with the center of the intrusive area if the surficial covering of volcanics were removed. Calculations made to determine the subsurface shape and lateral extent of the batholith indicate that the bottom of the anomalous mass is probably located at depths less than 15 km and is concave upward. Six configurations are postulated for the shape of the intrusive. The eastern, northern, and northeastern limits of the Boulder batholith are well defined by steep gravity gradients which parallel the margins of the batholith outcrop. The western extent of the gravity low is indefinite. Therefore, it is postulated that intrusive rocks underlie a considerable area to the west of the outcrop of the Boulder batholith, in the vicinity of Philipsburg, Montana. There is no geophysical evidence for a sharp structural delineation between the Idaho and Boulder batholiths. The relation of the outlying stocks to the Boulder batholith is discussed on the basis of the geophysical evidence. 401 on June 23, 2015 memoirs.gsapubs.org Downloaded from 402 IGNEOUS AND MET AMORPHIC GEOLOGY CONTENTS

A regional Bouguer gravity anomaly map of Alaska is interpreted in terms of variations in crustal structure and local geology. The crustal results are compared with those determined from seismic measurements in two areas in Alaska and one in Alberta, Canada. These comparisons indicate the importance of including crustal composition in the derivation of crustal thickness from gravity values and suggest that regional isostatic anomalies are probably more indicative of abnormal crustal composition than of abnormal crustal thickness. In the Prince William Sound area of Alaska the seismic measurements show that the positive anomaly associated with the area is related to a thick crust having a high density rather than to a thin crust as might be deduced on the assumption of a normal crustal density. The inverse effect noted in the common association of negative anomalies with granitic intrusions appears to prevail in southern Alaska, where the seismic value of crustal thickness over the Coast Range batholith is less than that which would be deduced from the gravity anomaly values. Although the data are too sparse to permit any extended study of local geologic features several areas of local gravity anomaly are defined. In general, these can be correlated with known geologic features such as sedimentary basins, granite on basic rock intrusions, and the configuration of the buried crystalline rock surface.