Early evolution of an extensional monocline by a propagating normal fault: 3D analysis from combined ®eld study and numerical modeling Shawn P. Willsey a , Paul J. Umhoefer a, * , George E. Hilley b a Department of Geology, Northern Arizona University, Flagstaff, AZ 86011, USA b Department of Geology, Arizona State University, Tempe, AZ 85287, USA Received 14 August 2000; revised 23 February 2001; accepted 8 June 2001 Abstract The Nopolo structure is located along the eastern margin of the Baja California peninsula and formed during the early stages 12±6 Ma) of development of the Gulf of California. The Nopolo structure is an ,15-km-long series of two NW-striking, extensional monoclines produced by the upward propagation of normal faults. Where normal faults reach the surface, the footwall contains undisturbed, gently dipping strata, whereas the hanging wall is a highly deformed zone that contains fractured and faulted steep to subvertical strata. Long, narrow grabens with moderate to steep east-dipping strata are present in the hanging wall of the main normal faults. Initial monoclinal folding over blind normal faults produced a minimum of ,300 m of structural relief. Once the faults propagated to the surface, they offset the monoclines ,20±30 m before faulting ceased. We use an elastic dislocation model to invert fault geometry from bedding orientations around the Nopolo structure. The data are best matched by a listric normal fault that soles out at ,5 km depth. The model suggests that the tip line of the fault was located ,1 km below the surface prior to the breaching of the monocline. Because the offset along the main normal faults is minimal ,20 m), the Nopolo structure is a unique example of an extensional faulted monocline and monocline system where faulting ended soon after the monoclines were breached and offset. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Extensional monocline; Upward propagation; Normal fault 1. Introduction Monoclines have long been associated with folding above reverse faults, such as the classic examples on the Colorado Plateau e.g. Davis, 1978; Krantz, 1989; Davis, 1999). The monoclinal shape is dependent on the faults being widely spaced so that the outer limbs can maintain the subhori- zontal geometry of true monoclines. Often, these structures form as drape folds above basement-cored uplifts related to reverse faults Reches, 1978; Friedman et al., 1980; Chester et al., 1988). More recently, contractional monoclines were directly linked to the process of fault propagation Mitra, 1993; Davis, 1999; Tindall and Davis, 1999). Until recently, monoclines associated with extensional normal faults Fig. 1) received much less attention than contractional monoclines e.g. Hancock and Barka, 1987; Walsh and Watterson, 1987; Withjack et al., 1990; Patton et al., 1998; Gawthorpe et al., 1997; Gross et al., 1997; Janecke et al., 1998; Gardner et al., 1999; Sharp et al., 2000). This may be because when extensional folds are breached by normal faults they are commonly called drag folds, with an asymmetric syncline in the hanging wall and an open, asymmetric anticline in the footwall. Many of these drag folds are more likely fault-propagation folds Mitra, 1993; Schlische, 1995) or forced folds Withjack et al., 1990), but the process of fold development above a propagating normal fault is similar. Experiments show that extensional monoclines are expected in layered rocks above the tip line of propagating normal faults Fig. 1; Withjack et al., 1990; Couples et al., 1994; Couples and Lewis, 1998; Patton et al., 1998; Hardy and McClay, 1999; Withjack and Callaway, 2000). However, moderate to large normal fault systems that ceased being active during the initial stages of development are less common in nature. In these cases, the formation of the monoclines and the initial stages of faulting can be examined in detail. In this paper, we describe the style and kinematics of the Nopolo structure near Loreto on the Baja California penin- sula Fig. 2). The Nopolo structure re¯ects the local 3D deformation associated with the upward and lateral Journal of Structural Geology 24 2002) 651±669 0191-8141/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S0191-814101)00120-1 www.elsevier.com/locate/jstrugeo * Corresponding author. Present address: Montgomery Watson, 4525 South Wasatch Blvd., Salt Lake City, UT 84124, USA. Tel.: Harza, 11- 928-523-6464. E-mail address: paul.umhoefer@nau.edu P.J. Umhoefer).