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1. Structural analysis of brittle-plastic shear zones in the Sangre de Cristo Range, southern Colorado, USA: Superposition of Rio Grande rift extension on Laramide contraction

   https://doi.org/10.1130/GES02772.1  

   Sitar, Michael C; Singleton, John S; Rahl, Jeffrey M; Caine, Jonathan Saul; King, Jacob; Kylander-Clark, Andrew; O'Sullivan, Paul (March 2025, Geosphere)

   Abstract The Sangre de Cristo Range in southern Colorado exposes some of the deepest Cenozoic structural levels in the Rocky Mountain region, including mylonitic shear zones associated with both the Laramide orogeny and Rio Grande rift. We investigated the relation between Laramide contraction and Rio Grande rift extension with detailed geologic mapping, kinematic analysis, and geochronometry in a 50 km2 area centered on the Independence Mine shear zone (IMSZ). The 15–100-m-thick IMSZ is one of several shallowly to moderately (~45° ± 20°) W-SW–dipping brittle-plastic shear zones along the western flank of the range. These shear zones display microstructural evidence of initiation as top-NE contractional mylonite zones, consistent with regional Laramide kinematics, which have been pervasively overprinted by shear fabrics indicating top-SW extensional reactivation. Both top-NE and top-SW shear fabrics involve cataclasis and quartz dislocation creep, although top-SW shear is more commonly localized along phyllosilicate-lined shear bands. Shear zones are hosted predominately within Proterozoic gneiss, and contain abundant chlorite and white mica derived from alteration of hornblende and feldspar, which indicates that weakening driven by fluid reactions played an important role in localizing strain. Extensional overprinting appears to be most pervasive along more steeply dipping portions of shear zones and where secondary phyllosilicates form an interconnected weak phase, which suggests that reactivation was primarily controlled by geometry and rheological contrasts inherited from contraction. One top-SW shear zone adjacent to the IMSZ cuts a late Oligocene gabbro stock, and monazite grains synkinematic with top-SW shear in the IMSZ yielded late Oligocene to Early Miocene U-Th-Pb dates that correspond with initiation of the Rio Grande rift. Reactivation of weak reverse faults may represent an important structural control during initial extension in the middle crust, prior to slip along the high-angle Sangre de Cristo normal fault system. 

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2. New Insights into Early Cretaceous Extension in Northern Mongolia

   Pidgeon, J. (October 2022, EOS Transactions American Geophysical Union)

   The late Mesozoic Era was a time of widespread crustal extension in eastern Asia resulting in both rift basin and metamorphic core complex formation. Two of the more recently documented examples of this extensional phase are the Ereendavaa and Buteel metamorphic core complexes (EMCC, BMCC). Both are located in northern Mongolia proximal to the Mongol Okhotsk Suture Zone (MOSZ). The MOSZ is a profound, yet enigmatic structure that formed due to closure of the Mongol-Okhotsk Ocean, a basin that separated the Siberian and North China cratons and intervening terranes of the Central Asian Orogenic Belt. Based on published work by others, the core complexes record NW-SE extension, cooling and deformation from c. 135 to 120 Ma. We present new data as part of a collaborative research project that aims to constrain the evolution of the MOSZ more broadly and its relationship to intracontinental deformation after suturing. Our methods include analysis of satellite imagery and digital elevation models with synthesis of field, (micro)structural, and geochronologic data with published maps and studies. Based on our findings, the EMCC likely extends several 10's of km to the NE. Satellite imagery and DEMs suggest large-scale corrugations along the N-flank consistent with NW-SE extension. To the SW of the EMCC, Early Cretaceous rift basins are associated with strong NE-SW oriented lineaments. We examined the BMCC along its SW mapped extent, an area for which no data were presented in prior publications; we confirmed the presence of a top-to-the-SE detachment fault. The EMCC and BMCC, like the Yagan-Onch Hayrhan MCC in southern Mongolia, have footwall rocks previously mapped as Precambrian that are, in large part, metamorphosed Paleozoic and Mesozoic igneous and sedimentary rocks. All three MCCs exhibit evidence for structural complexity, such as NE-SW trending lineations orthogonal to the NW-SE extension direction. As in S Mongolia, we hypothesize that the NE-SW lineations in the EMCC and BMCC formed during an earlier phase of shortening. The expression of the Early Cretaceous extension (rift basin vs. MCC) appears to be controlled by the inherited structure. 

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3. What you can do with superb rift basin-fill exposures: Recent lithostratigraphic, paleoclimatic, biostratigraphic, and structural studies of the Española Basin, New Mexico, USA

   https://doi.org/10.1130/2026.0077(04)  

   Koning, Daniel J; Spaur, Siânin; Williamson, Garrett; Rugenstein, Jeremy Caves (May 2026, Geological Society of America)

   Wintsch, RP; Drzewiecki, PA; Hobbs, KM (Ed.)

   ABSTRACT This field trip visits exposures of two lithofacies assemblages in 16–11 Ma Santa Fe Group strata of the eastern Española Basin (Rio Grande rift, New Mexico, USA): eastern piedmont slope (lithosome A) and fluvial basin floor (lithosome B). The eastern Española Basin approximates a west-tilted half-graben bounded by the Pajarito-Embudo fault system, whose northern extent (the Embudo fault) serves as an interbasinal transfer structure. The basin has yielded abundant mammalian fauna. Recent refinement of their stratigraphic positions allows establishment of four biostratigraphic interval zones. Both lithosomes A and B progressively coarsen up-section. Prior to ca. 13.5 Ma, there were minor east-west shifts of their mutual interfingering contact. A major progradation of lithosome A occurred at 13.5–11 Ma, and at 13.1 Ma, there was a relatively abrupt coarsening of both lithosomes. The 13.5–11 Ma progradation occurs in the context of upward increasing δ18O and δ13C values in authigenic carbonates, interpreted to indicate more summer-dominated moisture and decreased vegetative productivity in a cooling paleoclimate. Prior to this progradation, paleoclimatic conditions during the Miocene Climatic Optimum (MCO, 17–14.7 Ma) and ensuing Miocene Climate Transition (MCT, 14.7–13.8 Ma) were characterized by warmer temperatures (cooling slightly at the MCT), winter-dominated moisture, and higher plant productivity that resulted in a heterogeneous mosaic of open- and closed-canopy habitats We attribute the 13.5–11 Ma progradation to: (1) more intensive precipitation events and decreased vegetative stability of the paleo-landscape resulting in episodic higher paleodrainage sediment flux and competency; and (2) moderate to high rates of westward tilting of the half-graben that shifted the locus of deposition closer to the central fault system. 

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4. Kinematic Evolution of the Tangra Yumco Rift, South-Central Tibet

   Reynolds, Aislin R; Laskowski, Andrew K (December 2022, AGU Fall Meeting 2022, held in Chicago, IL, 12-16 December 2022, id. T25B-06.)

   We interpret the kinematics of the Tangra Yumco (TYC) rift by evaluating spatiotemporal trends in fault displacement, extension onset, and exhumation rates. We present new geologic mapping, U-Pb geochronology, zircon (U-Th)/He (ZHe) thermochronology, and HeFTy thermal modeling results that are critical to testing dynamic models of extension in Tibet. The TYC rift is bounded by two NNE striking (~N10°E-N35°E) high angle (~45-70°) active normal faults that alternate dominance along strike. Footwall granodiorites show foliation, slip lineation, and fault plane striation measurements indicative of northeast directed oblique sinistral-normal slip. In North and South TYC, hanging wall deposits are cut by a series of active high-angle normal faults which likely sole into a master fault at depth, while in central TYC, hanging wall deposits display synthetic graben structures potentially indicative of low-angle faulting. Analysis of ~50 samples collected across key structural relationships in and around TYC yield 14 mean U-Pb dates between ~59-49 Ma and ~190 single-grain ZHe dates between ~60-4 Ma with spatial trends in ZHe data correlating strongly with latitude. Samples from Gangdese latitudes show a concentration of ~28-15 Ma ages, while those north of ~29.8° latitude yield both younger (~9-4 Ma) and older (~59-45 Ma) ages. We interpret (1) Gangdese Range samples reflect exhumation during contraction and uplift along the GCT peaking at ~21-20 Ma, (2) ~9-4 Ma ages reveal extension timing along fault segments experiencing significant rift-related exhumation, and (3) ~59-45 Ma ages represent un-reset or partially-reset samples from fault segments that have experienced lesser magnitudes of rift exhumation. HeFTy thermal models indicate a two-stage cooling history with initial slow cooling followed by accelerated cooling rates in Late Miocene-Pliocene time (~13-4 Ma) consistent with prior results from TYC and other Tibetan rifts. Our data are consistent with a segment linkage fault evolution model for the TYC rift, with underthrusting of Indian lithosphere likely related to the northward acceleration of rifting. Future work will utilize advanced HeFTy modeling including U-Pb and apatite fission track data to further constrain the exhumation history of TYC and test dynamic models of extension for southern Tibet. 

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5. Paleomagnetic data bearing on vertical axis rotation of the Rio del Oso dike swarm, western Españ ola Basin, New Mexico

   Petronis, M.S. (September 2021, New Mexico geology)

   null (Ed.)

   The Española Basin is one of a series of interconnected, asymmetrical basins in the Rio Grande rift that includes a number of north- and northeast-striking faults that accom- modated block tilting and basin subsidence. The western margin of the Española Basin, in particular, is characterized by a greater than 17-km wide zone of normal and oblique-slip faults. To clarify the involvement of block rota- tion in the tectonic evolution of the Española Basin, we carried out a paleomagnetic study of mafic intrusions (Rio del Oso dike swarm) that are genetically related to regionally exten- sive basalt flows of the mid-Miocene Lobato Formation. The primary hypothesis tested was that these intrusions experienced some degree of vertical axis rotation associated with mid-Miocene to recent continental rifting. In situ paleomagnetic results from forty-two sites yield a group mean declination (D) of 344.0°, an inclination (I) of 41.1°, α95 of 6.1°, and k of 14.1. The group mean result is discordant to the <10 Ma pole of D=356.0°, I=54.4°, α95 = 3.3° with a statistically significant inferred rotation (R) of -12.0°± 7.2° and flattening of +13.3° ± 5.5° relative to the <10 Ma pole field direction. These discordant results indicate that a modest degree of counter-clockwise vertical axis rotation occurred in this region, which is likely associated with Rio Grande rifting north of the Jemez Mountains. It is possible that oblique motion along the Santa Clara fault and/ or the Cañada del Almagre fault facilitated the vertical axis rotation. The results from this study imply that vertical axis rotation is common to extensional rift systems and should be considered when modeling continental extension. 

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