Polyphase evolution of the East Gobi basin: sedimentary and structural records of Mesozoic–Cenozoic intraplate deformation in Mongolia

Abstract

Mapping and correlation of 2D seismic reflection data define the overall subsurface structure of the East Gobi basin (EGB), and reflect Jurassic–Cretaceous intracontinental rift evolution through deposition of at least five distinct stratigraphic sequences. Three major northeast–southwest‐trending fault zones divide the basin, including the North Zuunbayan (NZB) fault zone, a major strike‐slip fault separating the Unegt and Zuunbayan subbasins. The left‐lateral NZB fault cuts and deforms post‐rift strata, implying some post‐middle‐Cretaceous movement. This fault likely also had an earlier history, based on its apparent role as a basin‐bounding normal or transtensional fault controlling deposition of the Jurassic–Cretaceous synrift sequence, in addition to radiometric data suggesting a Late Triassic (206–209 Ma) age of deformation at the Tavan Har locality. Deposits of the Unegt subbasin record an early history of basin subsidence beginning ∼155 Ma, with deposition of the Upper Jurassic Sharilyn and Lower Cretaceous Tsagantsav Formations (synrift sequences 1–3). Continued Lower Cretaceous synrift deposition is best recorded by thick deposits of the Zuunbayan Formation in the Zuunbayan subbasin, including newly defined synrift sequences 4–5. Geohistory modelling supports an extensional origin for the EGB, and preliminary thermal maturation studies suggest that a history of variable, moderately high heat flow characterized the Jurassic–Cretaceous rift period. These models predict early to peak oil window conditions for Type 1 or Type 2 kerogen source units in the Upper Tsagantsav/Lower Zuunbayan Formations (Synrift Sequences 3–4). Higher levels of maturity could be generated from distal depocentres with greater overburden accumulation, and this could also account for the observed difference in maturity between oil samples from the Tsagan Els and Zuunbayan fields.

How did the Alxa Block respond to the Indo-Eurasian collision?

“…However, Johnson (2004) found that the Zunnbayan fault was not active after the Cretaceous;´although there are many current debates about estimates of displacement of the Altyn Tagh fault (RGAATFS, SSB 1992;Meng et al 2001;Yin et al 2002;Wang 1997;Yue and Liou 1999;Yue et al 2001a, b), the fact that the displacement on the western and central parts of the fault was larger than that on the eastern part has been accepted by most workers, in other words, the activity on the fault became weaker to the northeast. As in the above analysis, the Alxa Block lacks obvious coeval deformation, and the latest deformation (Pleistocene) only occurred along the margins of the block;…”

“…The Alxa Block has undergone strong tectonic deformation since the Mesozoic (Vincent and Allen 1999;Yue and Liou 1999;Yue et al 2001a, b;Lamb et al 1999;Hendrix et al 2001;Ritts 2002, Darby et al 2005;Meng 2003;Meng et al 2003;Webb et al 1999Webb et al , 2006Zheng et al 1996;Wang and Mo 1995;Graham et al 2001;Johnson 2004;Liu 1998;Liu andYang 1997, 2000;Yang et al 1988;Wang et al 1998;Zuo et al 2003;Zheng and Wang. 2005;Wang et al 2005;Chen and Xu 2006;Li et al 2004).…”

Cenozoic Evolution of Hinterland Basins in the Andes and Tibet

“…High-elevation hinterland basins of Tibet ( Fig. 21.2) developed independently of the Himalayan fold-thrust belt to the south, and are distinguished by their structural settings and stratigraphic records from the low-elevation foreland basins and collisional successor basins bordering the plateau, including the Himalayan, Sichuan, Hexi, Tarim, Junggar, Turpan, and various Mongolian basins (Hendrix et al, 1992;Graham et al, 1993Graham et al, , 2001Burbank et al, 1996;DeCelles et al, 1998;Johnson, 2004;Johnson and Ritts, this volume).…”

Surface deformations from moderate-sized earthquakes in Mongolia observed by InSAR

“…Both define the EGFZ as a left-lateral strikeslip fault, but differ in the offset and the timing: ∼400 km offset in Cenozoic and ∼200 km offset mainly in Mesozoic, respectively. Evidence for middle-Cretaceous movement on faults within the EGFZ was identified using seismic reflection data by Johnson (2004). Outcrop data evidenced a sinistral strike-slip motion in the Cenozoic time (Webb and Johnson, 2006).…”