Toward a minimum change model for recent plate motions: calibrating seafloor spreading rates for outward displacement

We use seafloor spreading distances derived from reconstructions of more than 7000 crossings of young magnetic anomalies along seven plate boundaries to study outward displacement, a source of systematic bias in estimates of seafloor spreading rates, which is caused by a combination of processes that shift magnetic polarity transition zones away from their idealized locations. Linear regressions of 81 independent sequences of seafloor opening distances as a function of their magnetic reversal ages for anomalies younger than C3n.1 (4.19 Ma) yield 75 positively valued intercepts for zero seafloor age, confirming the ubiquitous outward shift of magnetic reversals reported by previous authors. Grouping these data into 29 locally consistent clusters yields better constrained zero-age intercepts that are uniformly positive and average 2.2 +/- 0.3 km globally. These values, which are 1-3 km at most locations and are significantly larger (3-5 km) along the well-surveyed Reykjanes and Carlsberg ridges, agree well with published magnetic polarity zone transition widths, which are estimated directly from near-bottom seafloor magnetic measurements. Significant variations in outward displacement along the Southeast Indian Ridge are strongly correlated with changes in axial morphology and axial depth; however, a similar correlation is not observed along other ridges. Forward magnetic anomaly modelling suggests that variations in outward displacement can be explained by differences in the magnetic source layers that are assumed to characterize different spreading centres. If not corrected for outward displacement, the implied systematic upward biases in seafloor spreading rates, which are averaged over the width of Anomaly 1-the youngest reversal that is used for plate reconstructions-range from 6 mm yr(-1) along the Reykjanes Ridge, where outward displacement is 4-5 km, to 3 mm yr(-1) along ridges where outward displacement approximates the global average of 2 km.