Factors controlling joint spacing in interbedded sedimentary rocks: integrating numerical models with field observations from the Monterey Formation, USA

In addition to bed thickness other factors influence joint spacing. These factors are evaluated through both a review of the Hobbs model for joint spacing and a 2D finite element simulation of a crack confined to a lithology-controlled mechanical unit. The stress reduction shadow increases in length with increasing Young's modulus of the joint bed, though fracture stress, flaw size, flaw distribution and extensional strain all interact with bed thickness and elastic properties ultimately to control joint spacing. One explanation for the observed decrease in joint spacing with increasing Young's modulus in outcrops of the Monterey Formation is that beds with higher Young's moduli fail at lower magnitudes of extensional strain. -from Authors