With the advent of plate tectonics 50+ years ago, the growth and demise of mountain belts suddenly seemed to make sense. As rigid plates converge, the overriding plate buckles, breaks, and rises to form mountains; when the stresses driving convergence disappear, erosion and faulting return the high terrain to sea level. By the 1980s, new models of the geologic evolution of western North America emerged showing that the elevation of mountain belts is an expression of the balance between compressional and extensional tectonic forces. An Yin (Fig. 1) arrived from China as a graduate student at the University of Southern California (USA) just in time to witness and participate in this paradigm shift. With the opening of the Chinese hinterlands to international researchers at about the same time, our planet’s most important orographic feature—the Himalayan-Tibetan orogenic system—became accessible to Western geoscientists for the first time in the plate tectonic era. While Professor Yin came to be regarded by many as the premier authority on the tectonic evolution of this remarkable terrane—from the Himalayan foothills to the distal effects of the orogeny in northeastern China—his journey to the literal and figurative top was marked by considerable ambivalence, particularly the Himalayan part.

Yin warily joined a reconnaissance field trip across southern Tibet in the early years of the heroic era of ‘roof of the world’ exploration. His motivation was to test his tectonic model, which appeared to explain a number of curious geologic features across the collision zone (derived spontaneously upon overhearing a conversation at a UCLA social function). The discovery during that trip of the crustal-scale fault, the Gangdese Thrust, which Yin had predicted over a beer, was revelatory and he thereupon shifted his attention fully from Cordilleran studies to Tibet (Yin et al. 1994). He was momentarily lured to the Himalaya in the mid-90s to reexamine the origin of the so-called inverted metamorphism sequences (Harrison et al. 1997), but this only seemed to cement his view that the southern slopes of the collision front were for the less intellectually adventurous. After all, Western geologists had been continuously roaming those hills for over 100 years (e.g., Oldham 1883) and the contemporary geologic debates seemed to him akin to carrying bones from one graveyard to another. Compared with the opportunities for fundamental discovery that lay north of the suture, Himalayan geology seemed in stasis. As such, his early Himalayan research was all north of the range crest, focusing on Tibetan Himalayan structures that informed the post-collisional evolution of the plateau, such as the Great Counter thrust (Yin et al. 1994, 1999; Fig. 1), Himalayan-Tibetan north-trending rifts (Yin 2000), and the Gurla Mandhata detachment system and its links to the Karakoram Fault (Murphy et al. 2002). In writing their review of the Himalayan-Tibetan orogeny (Yin and Harrison 2000), Yin left the description of the former largely to his junior author.

Following the birth of son Danny in 2002, Yin found himself spending a good deal of time at home minding the newborn. There he came across his copy of Heim and Gansser’s classic 1939 Himalaya monograph and, looking for intellectual stimulation during Danny’s nap-times, began to read. Cascading thoughts drove him deep into the Himalayan literature and, in just a few short years, a remarkable synthesis arose pointing to a missing geologic link that suggested a heretofore unprecedented relationship among the major faults. Yin found that the South Tibet detachment, then regarded as one of the world’s most impressive normal faults splendidly exposed along the crest of the Himalaya, had been stunningly misunderstood.

Professor An Yin contributed a plethora of ideas to every field he turned his mind and hand to, but in the Himalayan context, one concept came to have particular prominence—for its shock value, for its insight, for the scholarly magnificence of its communication, and more than anything, for how it changed our understanding of the evolving front of the India-Asia collision. The concept in question was Yin’s recognition that the world’s most spectacular normal fault is, in reality, a backthrust.

By the end of the 1990s, Yin had attracted a remarkable cohort of graduate students. He regularly took them on mapping excursions across the Mojave Desert, including to the Orocopia Mountains, which hosts a particularly well-exposed example of a folded fault—the Orocopia Fault—then understood to be a low-angle detachment. In the processes of these mapping exercises, he came to see that the Orocopia Fault could equally well be interpreted as a passive-roof backthrust. The last line of the abstract describing his model (Yin 2002) proffered a prophetic extrapolation: “This model may also explain a similar structural relationship between the Main Central thrust and the South Tibet detachment fault in the Himalaya”.

Yin had come to recognize that the South Tibet detachment was warped and, overall, had a rather flat-lying geometry. Tracking the fault through multiple generations of published Himalayan maps, he traced it from a branching relationship with the Main Central thrust in the southern reaches of the Himalaya to a linked relationship he had previously noted with the Great Counter thrust along the Indus-Yalu suture (Yin et al. 1994, 1999). The overall geometry required that the South Tibet detachment had been active as a backthrust/passive roof fault, accommodating the development and emplacement of the crystalline core of the orogen along the Main Central thrust.

His first attempts to publish this concept in high-profile journals received uniformly hostile receptions. This was no surprise; many of the leading lights of the then-dominant generation of tectonics workers had made signal contributions developing the idea of the South Tibet detachment as a simple normal fault. Every leading tectonic model for the Himalayan mountain range at that time was predicated on explaining the fault in this way, with the channel flow model (e.g., Beaumont et al. 2001) then seen as the leading contender for a modern paradigm. With each rejection, Yin refined and expanded his review of Himalayan tectonics, such that when his model finally went to print, it appeared as a 139-page book (Yin 2006) somehow contained within the pages of Earth-Science Reviews. The reprint was issued with a spine!

For large swaths of the Himalayan geology community, Yin’s accomplishment remains hidden in plain sight.

Oddly, for large swaths of the Himalayan geology community, Yin’s accomplishment remains hidden in plain sight. The Yin (2006) paper is widely recognized as the dominant synthesis of the Himalayan mountain belt, as it reviews in engrossing detail the lithologies, structures, cooling and depositional histories, as well as models for nearly all imaginable aspects of Himalayan geology. Nonetheless, the tectonic revolution that it contains is somehow commonly missed. To this day, many papers cite Yin (2006) among their first few references, setting the stage of their Himalayan explorations, and yet blithely show the South Tibet detachment as a simple normal fault. This occurs despite his paper demonstrating key geometric constraints both in rich detail and in simple clarity, i.e., the key observation that Tethyan rocks are continuously exposed from the suture zone to the Main Central thrust in the Kashmir and Chamba regions of the western Himalaya. This observation alone precludes the possibility that the South Tibet detachment could surface as a normal fault in these areas. The paper presents a powerful review, but it is in service of a yet greater discovery.

Unlike the High Himalaya range front, the Tibetan Himalaya north of the 8000-m peaks was largely unexplored by geologists until the early 1980s. Consequently, it was the domain of modelers to explain the growth of the Tibetan plateau. Yin recognized the need for targeted geologic mapping to test the predictions of regional models including eastward escape (Tapponnier et al. 1982) and distributed (pure shear) thickening (England et al. 1985). He took note of three key geologic map patterns previously documented in Chinese maps: (1) Cretaceous marine sequences covered the middle section of the Tibetan plateau (establishing that a narrow E–W swath of the plateau was at low elevation just prior to the collision with India); (2) a regionally extensive, flat-lying Paleocene volcaniclastic sequence unconformably overlay deformed Mesozoic strata and the Gangdese arc; and (3) much of this volcanic carapace had been stripped off the arc, implying substantial exhumation. Yin’s observations led to the discovery of the heretofore mentioned Gangdese thrust. On his journey to western Tibet in 1995, he followed in Gansser’s footsteps 60 years earlier by circumambulating Mount Kailas—a feature sacred to multiple Asian religions—to establish that the Great Counter thrust is almost certainly an orogen-wide structure. In doing so, he and his student discovered a spectacular metamorphic core complex south of the Indus-Tsangpo suture at Gurla Mandhata (Murphy et al. 2002). The existence of this core complex indicated that eastward escape was unimportant in the far western range, as one of its central components, the Karakoram fault, likely either terminated or linked with it, rather than extending >1200 km eastward along the suture zone (Murphy et al. 2000). In many respects, the large magnitude of extension absorbed by the core complex led Yin to conceptualize the connection of Tibetan rifts to those in the Himalaya and eastern China, requiring a common mode of extension and continent-scale boundary condition imposed by subducting slabs to the east and south of Tibet. This led to Yin’s first regional synthesis (Yin 2000), which not only provided a testable framework for evaluating the connectivity of active strain within Asia but also contested widely popular models for the origin of Tibetan rifts, including topographic collapse due to mantle lithosphere detachment (Molnar et al. 1993). Yin’s approach of questioning/challenging conventional wisdom through targeted geologic mapping, thoughtful integration of diverse data, and utilization of clever kinematic thinking in map-view and cross-section reconstructions would be repeated in impactful syntheses such as Yin and Harrison (2000), Yin (2006), and Yin (2010), which have provided a vision for the future direction of geoscience research across much of Asia.

In recent years, Yin turned his attention to understanding how the orogen’s geology changes towards its eastern terminus. His seminal work with students detailed the first-order tectonostratigraphy and histories of rock assembly and deformation in the Arunachal Himalaya (Yin et al. 2006, 2010; Webb et al. 2013). He also looked outside the traditional confines of the orogen and correlated key Himalayan rocks and structures with those in the Shillong plateau to the south (Yin et al. 2010) and northern Indo-Burma Ranges to the southeast (Haproff et al. 2019). Focused on the Namche Barwa region, Yin, along with close colleague Ding Lin and others, provided the first detailed constraints on its Cenozoic structural setting and high-grade metamorphic evolution, providing a foundation for subsequent syntaxial studies. Comparing their observations with those from the Nanga Parbat syntaxis in the west, Ding et al. (2001) described how, despite their contrasting structural styles, both experienced remarkably similar metamorphic and cooling histories. Yin and his students used field structural data in the Namche Barwa region to show that Cenozoic overriding-plate deformation was dominated by clockwise crustal flow (Haproff et al. 2018), supporting previously proposed models based largely on other observations.

An Yin’s Himalayan interests continued to diversify. A dire need for neotectonic studies of the eastern Main Frontal thrust led Yin and others to determine the first field-based Holocene shortening rate along the Arunachal Himalaya range front (Burgess et al. 2012). This work showed that active range-front thrusting increases to the east, mirroring the pattern in India-Asia convergence rate that likely controls ongoing growth of the Himalaya. North of the range front, his keen mapping skills led to the identification of several orogen-parallel, active left-slip faults (Li and Yin 2008), refuting the notion that the eastern Himalaya is a simple thrust belt. The existence of these left-slip faults, together with the Karakorum Fault to the west, suggests that oroclinal bending has most recently produced the curved shape of the Himalaya. Yin also turned his focus to Himalayan geomorphology and, specifically, when and how the Yalu River drainage system diverted southward across the range crest, an important but unanswered question first raised over 100 years ago (Burrard and Hayden 1908). By comparing their field and other observations with climate records, Yin and his coworkers showed that the Yalu River once flowed south along the Subansiri River, farther west than its present-day course, likely due to glacial damming during a warm and wet climate period (Zhang et al. 2016).

Professor An Yin passed away suddenly at the age of 64 on July 12, 2023, while instructing the UCLA undergraduate field camp in the White Mountains of eastern California, USA. The sudden loss of this intellectual giant was felt acutely across the geologic world, mitigated only by the model he left us of how the combination of intellectual rigor, originality, and passion can lead to stunning new insights into how our planet works. Our testament to Yin’s key contributions to understanding the geology of the Himalaya, a Sanskrit portmanteau for cold (hima) and dwelling (ālaya), reminds us that this mountain belt is no longer the Medhavinalaya—Sanskrit for the place where our brilliant teacher dwells.