Geosphere: Polyphase transpression along the San Andreas fault

Steffen Bergh (UiT), Arthur G. Sylvester (University of California), Alula Damte (Central European Petroleum Company Ltd.), and Kjetil Indrevær (University of Oslo, now at NVI) have an accepted article in the journal Geosphere. The article is titled “Polyphase kinematic history of transpression along the Mecca Hills segment of the San Andreas fault, southern California”.

The article focuses on tectonic studies of transpression along the San Andreas Fault in southern California (Mecca Hills). The purpose is to compare processes along an active transform plate boundary with the Svalbard and Barents Sea transform margin, which formed in the Paleocene-Eocene. The article emphasizes the formation, development, segmentation and strain partitioning of active strike slip and reverse dip-slip faults, and provides a more nuanced view of the formation of classic, positive “flower structures” in transpressional mountain chains.

The results are based on detailed structural geological field studies in the Mecca Hills in Southern California, where sedimentary rocks of Miocene-Pliocene age were deposited in pull-apart basins and later on inverted (uplifted) by transpression. The most important results of the project are an increased knowledge of structural development along continental transform plate boundaries, which show great similarities with Svalbard’s (Tertiary fold-and-thrust belt) and the Barents Sea’s western margin. Bergh has for many years provided research on Svalbard and used the San Andreas Fault as a recent analogue. The results are important for ongoing research activities under ARCEX work packages WP1&2 Geology in the Barents Sea / Svalbard.

The work has been carried out in collaboration with researchers at the University of California, Santa Barbara and San Diego State University, USA.

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 echelon 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 echelon 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 echelon 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 echelon 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.

Reference:

Bergh, S.G., Sylvester, A.G., Damte, A., Indrevær, K. (2019): Polyphase kinematic history of transpression along the Mecca Hills segment of the San Andreas fault, southern California. Geosphere, v. 15, no. X, p. 1–34. DOI: 10.1130/GES02027.1. [intranet]