ALBERT

Beschreibung: Understanding the relative motion between the Pacific plate and its neighboring plates in the Paleogene has important consequences for deciphering the relationship between absolute and relative plate motions in the Pacific Ocean basin, the history of circum-Pacific subduction, and the cause of the Hawaiian-Emperor bend (HEB). We quantitatively model the Farallon/Vancouver-Pacific-Antarctic seafloor spreading history from 67 to 33 Ma based on a comprehensive synthesis of magnetic anomaly and fracture identifications. We find a well-constrained increase from 75 ± 5 mm/yr to 101 ± 5 mm/yr in Pacific-Farallon full spreading rates between 57.6 Ma and 55.9 Ma, followed by a stepwise increase to 182 ± 2 mm/yr from 49.7 to 40.1 Ma. The increases in Pacific-Farallon spreading rates are not accompanied by any statistically significant change in spreading direction. The 57.6–55.9 Ma surge of Pacific-Farallon spreading reflects an eastward acceleration in Farallon plate motion, as it precedes west Pacific subduction initiation and is not associated with any significant change in Pacific-Antarctic spreading. We interpret the increase in Pacific-Farallon spreading rates after ca. 50 Ma as a consequence of further acceleration in Farallon plate motion. We find no indication of a major change in Pacific plate absolute motion at this time. Our model suggests that changes in relative motion direction between the Pacific and Farallon and Pacific and Antarctic plates were insignificant around the formation time of the HEB (ca. 47.5 Ma), and the bend is largely a consequence of Hawaiian hotspot motion, which ceased rapid motion after 47 Ma.

Beschreibung: Global oceans are known to have alternated between aragonite and calcite seas. These oscillations reflect changes in the Mg/Ca ratios of seawater that control biomineralization and the composition of marine carbonates, and are thought to be caused mainly by the time dependence of crustal accretion at mid-ocean ridge crests and the associated high-temperature mid-ocean ridge fluid flux. Here we use global ocean basin reconstructions to demonstrate that the fluctuations in hydrothermal ocean inputs are instead caused by the gradual growth and destruction of mid-ocean ridges and their relatively cool flanks during long-term tectonic cycles, thus linking ocean chemistry to off-ridge low-temperature hydrothermal exchange. Early Jurassic aragonite seas were a consequence of supercontinent stability and a minimum in mid-ocean ridge length and global basalt alteration. The breakup of Pangea resulted in a gradual doubling in ridge length and a 50% increase in the ridge flank area, leading to an enhanced volume of basalt to be altered. The associated increase in the total global hydrothermal fluid flux by as much as 65%, peaking at 120 Ma, led to lowered seawater Mg/Ca ratios and marine hypercalcification from 140 to 35 Ma. A return to aragonite seas with preferential aragonite and high-Mg calcite precipitation was driven by pronounced continental dispersal, leading to progressive subduction of ridges and their flanks along the Pacific rim.