The Irvine Ranch National Natural Landmark, Orange County, CA: A Geologic Record that Preserves The Major Tectono-Stratigraphic Events of The Greater Los Angeles Basin Region And A Natural Laboratory for Neotectonic Research

Cooper, John D.¹ (1) California State University, Fullerton, Fullerton, CA

A major part of the Irvine Ranch Land Reserve in southern Orange County was officially designated as America's newest National Natural Landmark (NNL), October 10, 2006. The ~37,000-acre Irvine Ranch (IR) NNL includes parts of the northern Santa Ana Mountains (NSAM) and San Joaquin Hills (SJH) in the northern Peninsular Ranges. These two parcels lie athwart the structural grain of the NSAM and SJH, thus preserving a relatively complete stratigraphic succession that ranges in age from late Cretaceous to Holocene. Moreover, the IR-NNL's geologic record contains the transition from convergent to transform plate tectonic settings and styles, involving the colossal crustal rotation that created the Los Angeles Basin/Inner Borderland belt, as well as subsequent transtensional and active transpressive deformational phases. Few places can be found where such a compact area, within a seacoast to mountainous transect, contains the record of such a diverse, dynamic, and dramatic long-time span geologic history. The NNL designation is important because diverse geologic data resources will be preserved for future scientific study. Significantly, the pre-Quaternary record provides a framework and foundation for assessing the neotectonic history. Especially significant will be preservation of and hopefully access (including trenches, boreholes, seismic investigations) to active tectonic and seismic hazard data resources such as fill and strath terraces, faults, and Pliocene and Quaternary deposits. Continuation and expansion of ongoing neotectonic studies in the IR-NNL natural laboratory is critical in light of its close proximity to a major metropolitan area with more than four million people.