Transtension in the Brittle Field: The Coso Region, Southern California

Abstract

Transtension in the upper crustal brittle field, at the lithospheric scale, involves some difficult kinematic problems that result from the non-coaxial component of bulk strain, which generates rotation about vertical axes and the internal deformation of rotating blocks. Plate boundary zone deformation in the non-plane-strain brittle field cannot involve the rotation of a mosaic of very large rigid crustal blocks. In transtension, constriction is achieved by simultaneous arrays of rotating normal faults and wrench faults, partitioned spatially and temporally, producing very complex polyphase brittle deformation. The Coso transtensional region at the southeastern margin of the Sierra Nevada-Great Valley block illustrates the complex synchroneity and polyphase superposition of rotating normal and wrench fault arrays in a constrictional strain field in the brittle regime, which generates a "continuum rubble" of small blocks.

Structure and kinematics of the Taupo Rift, New Zealand

“…This style of deformation leads to complex fault and fracture networks that promote highly localized fluid pathways through the development of dilational shear deformation modes [ Sanderson and Zhang , ], potentially influencing both the location of volcanism and geothermal activity. Furthermore, focal mechanisms from the Matata earthquake sequence show that strike‐slip and normal faults coexist in the same rock volume (attributed to the steeply dipping anisotropic structure of basement fabric dissected by normal faults), which is in contrast to present models for slip partitioning associated with oblique extension [ Dewey et al ., ].…”

“…Changes in fault strike and extension direction along the Taupo Rift result in changes to the obliquity of rifting. The obliquity of rifting, i.e., the extension direction relative to the rift trend, determines the orientation, linkage, and interaction of fault segments, which vary for biaxial (orthogonal rifting) or triaxial (oblique rifting) strain [ McClay and White , ; Dewey et al ., ]. Consistent with previous studies along the Taupo Rift and southern Havre Trough [ Delteil et al ., ; Lamarche et al ., ; Campbell et al ., ], the obliquity of rifting is here defined by the angle α between the extension direction and the rift boundaries: α = 90° corresponds to pure orthogonal extension, α < 90° (clockwise direction) corresponds to a component of right‐lateral shear, and α > 90° (counterclockwise direction) corresponds to a component of left‐lateral shear.…”

Neogene‐Recent Reactivation of Pre‐Existing Faults in South‐Central Vietnam, With Implications for the Extrusion of Indochina

“…As described by Dewey et al (2008) for the Coso region of Southern California, we posit that some of the microblocks of south-central Vietnam are undergoing rotation about vertical axes, as well as potential deformation in a broadly transtensional regime. Dewey et al (2008) described this working model as the "continuum rubble" behavior of small blocks, and we find this term valuable to describe the rotation and "jostling" of the microblocks of Indochina between the RRFZ, EVTZ, and the MPFZ (Figure 1).…”

“…As described by Dewey et al (2008) for the Coso region of Southern California, we posit that some of the microblocks of south-central Vietnam are undergoing rotation about vertical axes, as well as potential deformation in a broadly transtensional regime. Dewey et al (2008) described this working model as the "continuum rubble" behavior of small blocks, and we find this term valuable to describe the rotation and "jostling" of the microblocks of Indochina between the RRFZ, EVTZ, and the MPFZ (Figure 1). Numerical models for the Coso region, for example, by Eckert and Connolly (2007) and Pearce and Dewey (2008) show that both dip-slip and wrench faulting components characterizes such a "continuum rubble" regime, together with the development of significant breakup of a large lithospheric block into what Eckert and Connolly (2007) call "second-order" fractures.…”

A harbinger of plate tectonics: a commentary on Bullard, Everett and Smith (1965) ‘The fit of the continents around the Atlantic’

“…Perhaps the biggest challenge that tectonic and structural geology faces is the detailed relationship between relative plate motion and rock structures and strain in continental plate boundary zones, and as a result of evolving plate boundaries across which the slip vector changes, and triple junctions that change their position and type ( figure 1 ). Analyses have been attempted, fairly successfully, in transtensional zones between blocks with plate boundary zones such as the Eastern California Shear Zone, where Dewey & Taylor [ 51 , 52 ] used GPS, the inversion of large earthquake arrays, palaeomagnetism, volcanic systems and the detailed kinematics of fault systems to deduce the evolution of bulk strain facies patterns in area and depth in the Coso region. This approach can be conducted only in superbly exposed regions and involves a huge amount of field mapping and structural analysis.…”