Crater-Centered Laccoliths on the Moon: Modeling Intrusion Depth and Magmatic Pressure at the Crater Taruntius

around the crater floor (Schultz 1976). Rarely, in cases of extreme crater modification (e.g., Gassendi), deformation Many floor-fractured craters on the Moon show surface deformation like that seen over terrestrial laccoliths. Consequently, can extend beyond the crater rim to form another annular terrestrial laccoliths provide one model for such crater modifitrough surrounding a narrowed, ridge-like crater rim cation. This model directly relates surface deformation to the (Schultz 1976). growth of a shallow, crater-centered intrusion, and it estimates This sequence of progressive crater floor uplift can be intrusion size and magma pressure. It also yields a minimum attributed to both a viscous relaxation of crater topography estimate for intrusion depth. Maximum intrusion depths cannot (e.g., Danes 1965, Hall et al. 1981) and the emplacement be directly constrained, but a range of likely intrusion depths of crater-centered intrusions (e.g., Schultz 1972, 1976, can be derived with additional assumptions. When the model Brennan 1975). However, several observations appear to is applied to the crater Taruntius, the surface record indicates favor the igneous intrusion mechanism. In particular, the an intrusion p30 km across and 1900 m thick. The calculated frequency of crater-centered volcanism, the equilibration excess magma pressure is p9 MPa (90 bar), and the estimated intrusion depths range from p1 to p5 km. If magma pressure of uplifted crater floors with nearby mare levels, and the reflects a hydrostatic magma column beneath Taruntius, these selectivity of crater modification in some regions are all values suggest a total magma column length of p65 km during more consistent with igneous intrusions than with viscous crater modification.  1996 Academic Press, Inc. relaxation (Schultz 1976). Further, the relative degree of crater shallowing is size dependent in some regions: small craters show proportionally greater floor uplifts than larger INTRODUCTION floor-fractured craters (Wichman and Schultz 1995). Such behavior contrasts sharply with the predicted effects of Over 200 craters on the Moon contain distinctive fracviscous relaxation, where floor uplift preferentially reture patterns in and around their crater floors. These cramoves the longest topographic wavelengths before affectters are often much shallower than pristine craters of the ing smaller craters within a region. same size, are preferentially located near the lunar maria, Although igneous intrusion models qualitatively explain and repeatedly contain minor volcanic units (Schultz 1972, many elements seen in floor-fractured craters, previous 1976, Young 1972, Whitford-Stark 1974). In addition, these studies have provided few quantitative tests for these modicraters apparently record a systematic process of endogenic fication models. Consequently, we have worked to develop crater modification (Schultz 1976). In the least modified a more quantitative model for igneous crater modification craters, fracturing and minor volcanism primarily occur in (Wichman 1993). This model links the apparent sequence the central crater floor, and crater depths are essentially of crater floor fracturing to the emplacement and growth unchanged. At higher levels of modification, however, later of shallow, crater-centered laccoliths (Fig. 1). Failure of failure and volcanism are concentrated into a ring near the central crater floor is ascribed to early flexure over the the edge of the crater floor, and floor uplift begins to intrusion, while moat formation and the majority of floor reduce the crater depth. In many cases, this produces an exceptionally shallow crater with a moat-like depression uplift are attributed to a later stage of piston-like intrusion