A New Compilation of Seismic Structure in the Pacific & NW Atlantic

Abstract:

Compilations of crustal seismic structure by Raitt [1963], Shor et al. [1970], and Christensen & Salisbury [1975] have long served as a reference for geologic interpretations, but aside from an assessment of Moho depths by White et al. [1992], there has been no compilation of crustal structure based on modern methods of data acquisition and analysis. This study offers a compilation of the seismic structure of “normal” crust produced at high and intermediate rates and the Pacific Basin – largely in the neighborhood of the Juan de Fuca Plate and the East Pacific Rise – and slow spread crust in the North West Atlantic. The data set comprises 105 profiles from the Pacific and 31 from the NW Atlantic. Layers 2B, 2C, and 3 are found to have mean velocities of 5.2±0.4, 6.2±0.2, and 6.8±0.2 km/sec, in excellent agreement with previous studies. Corresponding depths in the Pacific are 0.5±0.2, 1.0±0.3, and 1.9±0.5 km, while depths in the NW Atlantic are 0.7±0.3, 1.6±0.5, and 3.2±1.0 km. The average velocity gradient in Layer 3 is 0.10±0.03 s^-1. Sonic log velocities in the basalt sections of Holes 418A, 504B, and 1256D are a near-exact match to Layer 2B; hence, the transition from Layer 2A to Layer 2B generally lies within the lava pile. Sonic log velocities at the top of the dikes in Hole 504B match Later 2C, and the gradient is a good match to the velocity gradient between the top of Layer 2C and the top of Layer 3. Layer 2C thus corresponds to the underlying dikes in Hole 504B and 1256D. On average, the crustal strcuture in the Pacific and NW Atlaintic is consistent with the Penrose lithostratigraphy.