The fate of subducted oceanic slabs can provide important clues to plate reconstruction through Earth history. Since oceanic slabs in continental collision zones are typically not well preserved, ancient subduction zones have rarely been imaged by geophysical techniques. Here we present an exception from the Darbut belt in the Junggar accretionary collage in the southern Altaids of Asia. We deployed a 182 km long magnetotelluric (MT) profile including 60 broadband sounding sites across the belt. Quality off‐diagonal impedances were inverted by a three‐dimensional scheme to image resistivities beneath the profile. The resistivity model along with MT impedance phase ellipses and induction vectors were tested and interpreted in detail. Combining geological and geophysical observations, mineral physical experiment, and geodynamic modeling results, the MT transect suggests a fossil intraoceanic subduction zone during the Late Paleozoic in the western Junggar that has been well preserved due to lack of significant subsequent tecto‐thermal events.

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The western Junggar is situated in the southwestern Central Asian Orogenic Belt, and its origin is believed to be related to either intracontinental arc or intraoceanic arc-related subduction system. However, the mode of subduction is still contentious. In this study, we build a high-resolution 3-D model of the crust of the western Junggar to constrain the subduction mode. By deploying a seismic array consisting of 31 portable broadband seismic stations from September 2013 to December 2013, we determine the Moho depth beneath every station using a H-κ method and the Moho variations along three NW-SE profiles using a common conversion point stacking method. We also construct a 3-D crustal S wave velocity model using ambient noise tomography. Our results reveal significant variations of Moho depths across the study region and a pronounced Moho offset between the Zaire mountains and the western Junggar basin. Ambient noise tomography shows that a high shear velocity layer (>3.9 km/s) is observed in the middle/lower crust of the western Junggar Basin, which is overlaid by a low velocity upper crust. The high-velocity body gradually dips northwestward with depth beneath the Zaire mountains. The speed of this high-velocity body is consistent with the calculated S wave speed of various metamorphic facies of mid-ocean ridge basalt under the P/T conditions of middle/lower crust in the western Junggar. Our seismic observations support a northwest paleosubduction model for the evolution of the western Junggar, in which a remnant oceanic slab is still trapped in the western Junggar.

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