Structure and Tectonics of the Galapagos Inner Rift, 86°10'W

Abstract

A near-bottom Deep-Tow investigation of the N-S-spreading Galapagos Rift at 86°10'W shows a tectonic zonation about the axis which includes: (1) A narrow, central zone varying to width from 0.2-1.3 km and composed of discrete elongate highs elevated up to 50 m above the adjacent floor. The central highs (domes) are divided into western and eastern groups. A system of two-meter-wide fissures links the western and eastern groups of domes together. Hydrothermal water emanates near and from these fissures and deposits thin layers of yellow precipitates into the interstices of shattered pillow and pahoehoe basalts. Massive basalts atop the highs flow down to the inner floor. Numerous collapsed lava tubes are aligned along sections of the central high. In other places, lava whorls form where the molten streams meet with the inner floor. (2) Bordering the central zone, northern and southern lava plains (0.2-1.4 km wide) are presumably formed by extensive layers of fissure-erupted lava. Horizontal stresses across this zone exceed vertical stresses creating wide zones of open fissuring. Smooth, ropy, and whorled pahoehoe lavas are the predominant lava forms on the plains. The widest parts of this inner floor occur where central domes are absent. (3) Further away from the center is the E-W trending marginal zone where topographic highs (elevations up to 180 m above the inner floor) alternate with topographic lows (depressions 80 m below the average depth of the inner floor). Peak-to-peak distances between northern marginal highs range from 2.5-4.1 km, while along the southern margin, peak-to-peak distances range from 2.75-5.5 km. Other high-low variations occur perpendicular to the axis where northern marginal lows oppose southern marginal highs and vice versa. Vitreous pillow lavas on top of these highs indicate that basalt is actively extruded at the margins of the inner rift. (4) Adjacent to the highs and lows are two bounding fault-scarp systems composing the inner walls. The northern system is composed of a series of en echelon faults offsetting this crust 200-260 m above the inner floor of the rift valley. The southern inner wall, in contrast, is not continuous from east to west, but plunges from a 200-m uplifted block in the east to a 160-m deep fault-bounded valley in the west. These northern and southern inner walls enclose the inner rift, 3-4 km wide. Structures in the inner rift may be explained by a combination of horizontal, vertical tectonic, and vertical volcanic solutions. Spontaneous overturn of a magma chamber might initiate sinusoidal domal uplift creating the central highs, and marginal highs and lows. Infering from buoyancy experiments, wide magma chambers with a thin central crust tend to maximize the central uplift and minimize the marginal uplifts. Conversely, narrow chambers with thick axial crust encourage the formation of marginal highs which develop at the edge discontinuities between the magma chamber and adjacent cooler crust.