Detailed tectonics near the tip of the Galapagos 95.5°W propagator: How the lithosphere tears and a spreading axis develops

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Abstract
A model for propagation of an active spreading axis into preexisting lithosphere has been developed based on intensive detailed studies using SeaMARC II, Sea Beam, Deep‐Tow, and Alvin near the tip of the propagator growing westward at ∼52 km/m.y. along the east‐west trending Galapagos spreading axis near 95.5°W. Initial lithospheric rifting appears to be accommodated along reactivated roughly east‐west abyssal hill faults at the “tectonic tip.” Extension leads to downdropping of a keystone block which is pervasively cut by normal faults and fissures. The initial volcanism (“initial volcanic tip”) occurs about 6.5 km behind the tectonic tip as basalt erupts through these fissures, covering most of the keystone block. The “neovolcanic axial tip” occurs another 4.5 km back at the western termination of a well‐defined axial pillow ridge along which volcanism is localized. Spreading along the propagator axis accelerates to the full rate (29 mm/yr half rate) at the “full rate tip,” interpreted to be ∼10 km behind the neovolcanic axial tip, or ∼21 km behind the tectonic tip. A complex pattern of strain in the tip area results from the interaction of plate spreading, transform shear stress, crack propagation, dynamic depression of the western end of the axial valley and the transform zone, and variations in the strength, thickness, heterogeneity, and anisotropy of the lithosphere. Southwesterly curvature of the neovolcanic axis near its tip is inferred to result primarily from the response of young, thin, weak, relatively homogeneous lithosphere to counterclockwise rotation of the maximum tensional stress direction, relative to the spreading direction, due to the influence of shear stress from the transform zone. Similar stress interactions likely occur at the tectonic tip but are inferred to be accommodated in this older, thick, strong, heterogeneous, and anisotropic lithosphere by oblique slip along a set of left‐stepping, reactivated, preexisting east‐west abyssal hill structures. Although on a time scale of ∼106 years the overall process of propagation is mostly continuous, rifting at the tectonic tip appears discontinuous at a scale of ∼1500 m or 30,000 years, based on the east‐west spacing of grabens formed as rifting begins. The scale of discontinuity of propagation of the neovolcanic axis is ∼3–5 km or ∼50,000–100,000 years, based on the distribution of northeast trending features interpreted as beheaded western terminations of paleoneovolcanic axes. The extension involved in rifting is ∼10–20%, based on summing the horizontal normal separation of all normal faults and fissures integrated and averaged over the zone of rifting. These values are approximately doubled if simple shear listric faulting is dominant rather than pure shear normal faulting.