Polyphase kinematic history of transpression along the Mecca Hills segment of the San Andreas fault, southern California

Abstract

Miocene-Pliocene sedimentary rocks in the Mecca Hills, southern California, were uplifted and deformed by transpression along a restraining bend in the San Andreas fault trace between the Orocopia and San Bernardino Mountains in Pleistocene time. This paper presents field evidence for three stages of structural evolution of a complex, asymmetric wedge-like flower structure, expressed as: (1) subhorizontal en échelon folds and faults oblique to the San Andreas fault; (2) steeply plunging folds subparallel to the San Andreas fault; and (3) folds and thrust faults fully parallel to the San Andreas fault. We argue that the resulting flower-structure deformation formed successively from early distributed transpression through full (?) strain partitioning, rather than from active, synchronous, strike-slip-forming movements, as expected. The model is supported by crosscut relations of major folds and faults and strain estimates from minor conjugate shear fracture sets. The polyphase evolution initiated on a steep right-lateral strand of the San Andreas fault, producing thick fault gouge. Then, the adjacent Neogene strata were folded en échelon outward in a uniformly distributed simple shear strain field. The subsidiary Skeleton Canyon fault formed along a restraining bend that localized right-lateral shearing along this fault, and reshaped the en échelon folds into steeply plunging folds almost parallel to the San Andreas fault in a nascent partly partitioned strain field. The final kinematic stage generated SW-verging folds and thrust faults trending parallel to the San Andreas fault and decapitated the en échelon folds and faults. The switch from early, distributed strike-slip to late-stage regional slip-partitioned shortening (fold-thrust) deformation may have been locally induced by the bending geometry of the fault. The polyphase structures were active in successive order to balance the driving forces in one or more critical-angled transpressional and fold-and-thrust uplift wedges. Fault-related shortening, uplift, and erosion are still controlled in the Mecca Hills by combining and adjusting the wedges with low convergence angle, transpression, and lateral crustal motion in a San Andreas fault plate scenario. Our model, therefore, addresses a more nuanced view of a polyphase flower-structure system and highlights the need to more carefully sort out spatially and temporally different kinematic data as a basis for analog and numerical modeling of transpressional uplift areas. ■ INTRODUCTION Many crustal-scale kinematic models have been proposed to explain strikeslip-dominated transpressional uplifts with flower-structure geometry (Harland,

Tectonostratigraphic record of late Miocene–early Pliocene transtensional faulting in the Eastern California shear zone, southwestern USA

“…The Lost Trigo fault zone changes northward to a NW-striking segment that displays complex anastomosing fault strands, a change in overall dip direction, and variable sense of dip-slip displacement along the trend of the fault zone (Figs. 4, 7), features that are commonly observed in strike-slip fault zones (e.g., Wilcox et al, 1973;Sylvester, 1988;McClay and Bonora, 2001;Bergh et al, 2019) and related sedimentary basins (Christie-Blick and Biddle, 1985;Nilsen and Sylvester, 1995;McNabb et al, 2017). In the absence of fault kinematic data from the Lost Trigo fault in Hart Mine Wash, we speculate that it would have acted as a dextral oblique-normal fault under the same approximately east-west extensional strain regime recorded along the southern segment of the fault zone.…”

Cordilleran Subduction Initiation: Retroarc Timing and Basinal Response in the Inyo Mountains, Eastern California

“…The alternative scenario is that the Last Chance thrust represented the contractional element of strain partitioning during a transpressional tectonic regime prior to continental rupture and subduction initiation. Thrust faults associated with strain-partitioned systems are often oriented~30-45 degrees from the strike of the transform fault (e.g., the San Andreas Fault; [79]), an angle compatible with the relationship between the strike of California-Coahuila transform and folding within the Last Chance thrust [45]. In this scenario, lithospheric rupture along the California-Coahuila transform would have evolved from a partitioned system of shortening structures and strike-slip faults to a continuous subduction zone by ca.…”

Sources of Uncertainty in Remote Stratigraphic Observations

“…The Mecca Hills (Figure 2) are composed of Plio-Pleistocene sediments of the Mecca, Palm Spring, and Ocotillo Formations that were deposited on basement rocks that consist of Mid-Tertiary rhyolite dikes, Mesozoic intrusive rocks, the Mesozoic Orocopia Schist, and the Proterozoic Chuckwalla Complex (Sylvester and Smith, 1987;Sylvester and O'Black-Gans, 2016;McNabb et al, 2017). (Sylvester and Smith, 1987;McNabb et al, 2017;Bergh et al, 2019). Stratigraphic units above basement rock were first mapped by Dibblee (1954) and consist of the Mecca Conglomerate, the Palm Spring Fm., and the Ocotillo Formation (Figure 4).…”