Late Oligocene -Early Miocene Kinematics, Volcanism, and Coarse Clastic Sedimentation During the Transition from Margin-Normal Extension to Margin-Parallel Dextral Strike-Slip, Eastern San Emidigo-Western Tehachapi Mountains, Southern California
Abstract
North of the junction of the San Andreas and Garlock faults ≈34-17Ma non-marine coarse-clastic and 22.6±0.6Ma, (Plescia et al., 1994) bimodal volcanic rocks of the Tecuya Fm (Tt) were deposited in a supra-subduction zone basin during the transition to a transform margin. Published plate reconstructions place creation of the Pioneer slab window and Mendocino Triple Junction ≈100km west of the study area between 28.2-24.7Ma. 1:12000 mapping and kinematic analysis of mesoscale structures in Tt exposed over 40sqkm in the Tunis half-graben (Thg) show a detailed record of late Oligocene-early Miocene tectonism, sedimentation, and volcanism. In Thg, Tt consists of 3 units, Tt1 - pre-volcanic exotic conglomerate, Tt2 - syn-volcanic locally-derived conglomerate and breccia, and Tv - volcanic flows, flow breccia, and shallow intrusives that are unconformably overlain by relatively undeformed ≤17Ma coarse clastic rocks (Mc). Thg is bounded on the west by the Tunis fault (Tf, 303±7/70-85). In Tf hanging wall, Tt is ≈2x thicker. East down vertical separation decreases from 800m in Tt1 to 150m in Tt2. Tv shows variable separation from 120m east down to 35m east up. Base of Mc shows vertical separation of ≤20m east down. Abundant mesocale faults in Tt have stratigraphic separation from 0.01m-10m and thin carbonate gouge (≤5cm) and fault breccia zones (≤20cm). 186 predominantly NW-striking mesoscale faults with well developed striae and slip sense were dated by cross-cutting relationships and divided into 4 age groups for kinematic analysis: 1)late Oligocene: n=27, contractional axis (P) 167/72 and extensional axis (T) 33/12; 2)late Oligocene - early Miocene: n=53, P = 200/79, T= 51/9; 3)early Miocene synvolcanic: n=56, P = 309/88, T= 64/1. Orientation of poles to Tv dikes show T = 237/4; 4)early Miocene syn-to-postvolcanic (22-≤17Ma): n=49, two kinematic subsets, P=117/3, T=27/12 and P=56/5, T=310/73. Tf dextrally offsets the pinchout of Tt2/Tv 1.1km along 299/7. Upright, gently NW-plunging, gentle folds deform Tv but not Mc. Kinematic data are interpreted to show a change from NE-SW extension to dextral strike-slip 22-17m.y. ago. Applying line balance methods to a well-constrained cross-section along the azimuth of late Oligocene-early Miocene extension indicates 22% extension of pre-volcanic Tt1 and 4.4% of syn-volcanic Tt2.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.T43E0478T
- Keywords:
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- 8105 Continental margins: divergent;
- TECTONOPHYSICS;
- 8107 Continental neotectonics;
- TECTONOPHYSICS;
- 8109 Continental tectonics: extensional;
- TECTONOPHYSICS;
- 8169 Sedimentary basin processes;
- TECTONOPHYSICS