Faster seafl oor spreading and lithosphere production during the mid-Cenozoic Clinton P. Conrad Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, Maryland 21218, USA Carolina Lithgow-Bertelloni Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA ABSTRACT Concurrent changes in seawater chemistry, sea level, and climate since the mid-Cretaceous are thought to result from an ongoing decrease in the global rate of lithosphere production at ridges. The present-day area distribution of seafl oor ages, however, is most easily explained if lithosphere production rates were nearly constant during the past 180 m.y. We examined spatial gradients of present-day seafl oor ages and inferred ages for the subducted Farallon plate to construct a history of spreading rates in each major ocean basin since ca. 140 Ma, revealing dramatic Cenozoic events. Globally, seafl oor spreading rates increased by ~20% during the early Cenozoic due to an increase in plate speeds in the Pacifi c basin. Since then, subduction of the fast-spreading Pacifi c-Farallon ridge system has led to a 12% decrease in average global spreading rate and an 18% or more decrease in the total rate of lithosphere production by the most conservative estimates. These rapid changes during the Cenozoic defy models of steady-state seafl oor formation, and demonstrate the time-dependent and evolving nature of plate tectonics on Earth. Keywords: ridge spreading, seafl oor ages, crustal production, plate tectonics, Farallon plate. INTRODUCTION The youngest seafl oor, recently produced at seafl oor spreading allows us to estimate the age The motions of Earth’s tectonic plates rep- spreading ridges, occupies proportionally more distribution of subducted seafl oor, the spreading resent the surface manifestation of convection area than does older seafl oor because it is less counterpart of which is still preserved. We use in the Earth’s mantle, and, over time scales likely to have been subducted since the time of its these additional pieces of information to detect of 50–100 m.y., govern a variety of geologi- formation. By examining the area distribution of changes in spreading rate, ridge length, and sea- cal processes, including mantle heat loss, sea- seafl oor ages, Parsons (1982) and Rowley (2002) fl oor production rates as functions of time and level change (Kominz, 1984; Gaffi n, 1987), observed an approximately triangular shape for between ocean basins. Although these estimates the carbon cycle (Berner and Kothavala, the distribution of area per unit age versus age. become increasingly uncertain with age because 2001), and seawater chemistry (Hardie, 1996). Rowley (2002) noted that this distribution is the subduction is constantly destroying informa- Although the basic history of plate motions expected result of a constant rate of seafl oor pro- tion about past spreading environments, we can has been deduced for the Cenozoic and some duction at ridges combined with consumption observe trends, particularly during the Cenozoic, of the Mesozoic, controversy has emerged of seafl oor with a uniform age distribution. In that deviate from the constant spreading model regarding changes in the global rates of ridge the absence of any other information about past suggested by the seafl oor age distribution alone. spreading and lithospheric production during seafl oor production rates, the present-day sea- As in Rowley’s (2002) study, our estimates are this time period. Tectonic reconstructions sug- fl oor age distribution gives no reason to suspect a based only on seafl oor ages measured from the gest that plate velocities have increased dur- signifi cant change in production rates during the present-day seafl oor and are thus not subject to ing the Cenozoic, particularly in the Pacifi c past 180 m.y. (Rowley, 2002). Although it ignores potential uncertainty associated with reconstruc- (Cogné and Humler, 2004), yet global studies information about past rates that can be gleaned tions of past plate motions. of rates of ridge spreading (Kominz, 1984) or from tectonic plate reconstructions, Rowley’s subduction (Engebretson et al., 1992) suggest (2002) study is appealing because it relies only PAST SEAFLOOR SPREADING that lithospheric production rates at ridges on well-defi ned (Müller et al., 1997) age data RATES AND RIDGE LENGTHS have slowed during the Cenozoic. Rowley from the present-day seafl oor. Other studies that The age of the ocean fl oor at any point (θ, φ) = (2002) used the area-age distribution of the infer faster production rates for past times (e.g., (colatitude, longitude) can be determined by present-day ocean fl oor to suggest that sea- Kominz, 1984; Engebretson et al., 1992) rely on examining the pattern of parallel magnetic fl oor production rates have remained nearly tectonic reconstructions that inevitably carry a anomalies that record Earth’s magnetic fi eld at constant since 180 Ma. This controversy has degree of uncertainty. the time of their formation at a spreading ridge. introduced questions over whether Cenozoic Maps of present-day seafl oor ages (Müller By interpolating between the locations of dated trends such as sea-level lowering (Haq et al., et al., 1997), however, contain additional infor- magnetic anomalies and using additional infor- 1987), increasing Mg/Ca ratio in seawater mation about the spreading systems that origi- mation from fracture-zone traces, Müller et al. (Hardie, 1996), and decreasing atmospheric nally formed this seafl oor. For example, Cogné (1997) determined the seafl oor age on a 0.1° × carbon and associated cooling (Berner and and Humler (2004) recognized that the spacing 0.1° grid (Fig. 1A). We use it to estimate past Kothavala, 2001) are the results of decreasing of isochrons provides a time history of past spreading rates and ridge lengths through analy- lithospheric production rates. spreading rates. In addition, the symmetry of sis of seafl oor age gradients. © 2007 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, January January 2007; 2007 v. 35; no. 1; p. 29–32; doi: 10.1130/G22759A.1; 4 fi gures. 29 Figure 1. Half-spread- North ing rate at time oceanic Atlantic lithosphere was originally pro duced, as determined by applying Equation 1 to sea fl oor age map of West Müller et al. (1997). Light Small Pacific Basins gray indicates regions where ages or rates can- not be determined, middle gray indicates conti nental West East South Indian areas, and dark gray indi- East Pacific Atlantic cates plate boundaries. Indian Dashed black lines are 25 m.y. age contours. We South have defi ned eight dif- Pacific ferent ocean basins, the boundaries of which, shown in black, are drawn through continents and along transform faults. 0 1 2 3 4 6 8 10 15 20 Half spreading rate (cm/yr) (θφ)= +Δ The gradient of seafl oor age, T(θ, φ), is ( ) = T , ttT2 increasingly uncertain for past times because Ltsp∫ dL sp , TtT(θφ, )=−Δ 2 inversely proportional to a rate of spreading, (2) they are based on a progressively smaller frac- θ φ vsp( , ): dA(θφ, ) tion of the original spreading length. where dL = , R sp ΔTv (θφ, ) v (θφ, ) = E = sp CENOZOIC AND CRETACEOUS sp ∇T (θφ, ) and dA(θ, φ) = dθ dφ sin(θ) is the local area. TECTONIC CHANGE ⎡⎛ ∂ (θφ)⎞ 2 Equation 2 determines spreading length by Using half-spreading rates determined from ⎢ 1 T , RE ⎜ ⎟ dividing the local area dA by a distance of Müller et al.’s (1997) data set (Fig. 1), we used ⎢⎝ sin(θ) ∂φ ⎠ (1) ⎣ spreading perpendicular to the ridge axis, given Equation 3 to measure average half-spreading (−12) by ΔTv (θ, φ). Because each spreading ridge rates as a function of time both globally and 2 ⎤ sp ⎛ ∂T (θφ, )⎞ + ⎥ , produces isochrons on either side of its axis, for each of the eight ocean basins defi ned in ⎝⎜ ∂θ ⎠⎟ ⎦⎥ in the absence of subduction the integration of Figure 1. To remove nonphysical slow spread- Equation 2 over all seafl oor of age t yields dou- ing associated with transform faults, we do not where RE is the radius of the Earth. When applied ble the ridge length at that time. The destruction include half-spreading rates slower than 15% to seafl oor age data, inverse age gradients calcu- of isochrons by subduction, however, causes of the average present-day spreading rate for a lated using Equation 1 yield half-spreading rates measured values of Lsp(t) to generally decrease given basin (Fig. 2), as measured by taking the that applied at the time of seafl oor formation with increasing seafl oor age. length-weighted average of full spreading rates θ φ (Müller et al., 1998). We determined vsp( , ) Average rates of spreading, Vsp, can be deter- given by DeMets et al. (1990). We fi nd that this for all points with defi ned ages in Müller mined for times in the past by taking the length- 15% cutoff removes nonphysical slow spread- et al.’s (1997) data set (Fig. 1). Some regions, weighted average of the local half-spreading ing rates associated with transform faults with- particularly within the Pacifi c’s poorly defi ned rate along an isochron: out eliminating spreading associated with past mid-Cretaceous quiet zone (118–83 Ma), show (θφ= +Δ ) lithosphere production. For the present day, ( ) = 1 T , ttT2 (θφ) anomalously fast spreading rates in excess of Vtsp ∫ vdLsp, sp , (3) the average spreading rates measured from the Lt( ) TtT(θφ, =−Δ 2) 20 cm/yr, which we omit from further analysis. sp Müller et al. (1997) age grid (Fig. 2, lines) gen- We also ignore rates determined for seafl oor where dLsp is the local spreading length given erally are within the DeMets et al.