COMPARISON OF SEA-FLOOR TECTONIC FABRIC AT INTERMEDIATE, FAST, AND SUPER FAST SPREADING RIDGES - INFLUENCE OF SPREADING RATE, PLATE MOTIONS, AND RIDGE SEGMENTATION ON FAULT PATTERNS

We, have conducted a comparative study of the tectonic morphology of y oung seafloor using SeaMARC II side scan sonar surveys of the intermed iate spreading Ecuador Rift, the fast spreading East Pacific Rise (EPR ) (8-degrees-30'-10-degrees-N), and the super fast spreading EPR (18-d egrees-19-degrees-S). We find that characteristics of fault population s are not only a function of spreading rate but also vary along axis w ithin individual ridge segments (i.e., with proximity to large- and sh ort-offset discontinuities). We also find that fault azimuths can be u sed to examine plate kinematics on a finer scale than can be obtained using magnetic data alone. Most of the variation in fault populations with spreading rate can be explained by an inverse relationship betwee n spreading rate and thickness of the brittle layer. For example, regi ons of super fast spreading are characterized by the largest numbers o f short faults, the smallest average fault spacing and throw, and the highest fault density. In addition, clusters of short, closely spaced antithetic faults subsidiary to long master inward dipping faults are common within the super fast spreading area, presumably the result of a thinner, weaker brittle layer. Faults facing away from the ridge axi s occur in increasing numbers with increasing spreading rate such that few outward facing faults are found at slow to intermediate rates and approximately equal numbers of inward and outward facing faults are o bserved at the fastest rates. Rapid thickening of the brittle layer wi th distance from the ridge may account for the predominance of inward facing faults at slower spreading rates. Outward facing faults at all spreading rates have shorter mean lengths and lower vertical offsets. These differences may reflect the shorter time outward facing faults a re active owing to increasing strength of the lithosphere with distanc e from the ridge. Fault lengths and spacings in all areas approximate exponential distributions. The extensional strain represented by fault populations is calculated from die displacement and length distributi ons of faults, and strain estimates of approximately 4% are obtained f or each area. Assuming that fault spacing reflects fracture depth exte nt where faults initiate, we infer a brittle layer thickness of approx imately 1 km when faulting begins. Fault populations are examined for ridge segment scale variations in amagmatic extension. We see evidence for greater amagmatic extension associated with long-term reduced mag ma supply along the eastern third of the Ecuador Rift. Evidence for lo cal increased brittle extension is also found within 15 lan of transfo rm faults. Discordant zones left by overlapping spreading centers (OSC s) are characterized by low fault abundances. At OSCs, discrete events of ridge tip propagation may accommodate ''tension taken up elsewhere along the ridge by normal faulting. Fault azimuths do appear to be us eful indicators of plate motion. Within the EPR 8-degrees-30'-10-degre es-N area, fault trends record a recent change in Pacific-Cocos plate motion (3-degrees-6-degrees at approximately 1 m.y.) consistent with m agnetic anomaly and fault lineation data from elsewhere along the nort hern EPR. Within the Ecuador Rift, fault azimuths scatter within 3-deg rees of predicted trends and are consistent with constant spreading ab out one pole for the past 1.5 m.y.