Terraces, uplift and climate, Karakoram Mountains, northern Pakistan.

The Karakoram mountains are situated at the western end of the trans- Himalayan Mountain belt. They are one of the most dynamically-active tectonic and geomorphic areas in the world. The valleys are among the deepest and hold some of the greatest thicknesses of Quaternary and recent valley fill sediments comprising glacial, debris flow, fluvial and aeolian sediments. These have been eroded to form terraces. Their development was controlled by tectonic and climatic factors, recording information about the last few million years of uplift and climatic changes. No simple relationship exists between terrace heights, degree of incision and terrace deformation, and the tectonic and climatic history of the area on the other. Allocyclic processes further complicate the interpretation of terrace formation. Several types of terraces have been differentiated and their sedimentology examined as follows. 1. Morainic terraces. The glacial system dominates with some of the longest glaciers outside the polar regions. These produce large deposits of till, dominantly of supraglacial meltout type. Three extensive glaciations have been recognised during Quaternary time and at least five minor advances during the Holocene. These produced extensive bodies of till scattered throughout the valleys: these have been used to reconstruct the extent and number of glaciations. 2. Glaciofluvial terraces Considerable thicknesses of glaciofluvial deposits infill small palaeovalleys typically of ice-contact facies reflecting deposition by high-gradient streams. Interbedded tills resedimented by debris flow processes are common. 3. Fluvial terraces These form a minor component and are common near the present river level. They were produced mainly by allocyclic processes related to the highly variable discharges of the glacially-fed rivers. 4. Debris terraces These widespread features were produced by failure of steep valley sides or by the resedimentation of debris, freqently till. Processes include debris flow, flowslide, rockslide, debris slide, rotational slide, creep, and slumps. 5. Lacustrine terraces Great thicknesses of silt were deposited rapidly in short-lived lakes. Incision produced terraces after the lakes drained. 6. Fan terraces These are polygenetic landforms comprising the sediments described above, but dominated by debris flow deposits of resedimented till. These formed early in the deglaciation of the area and represent a major phase of deposition which filled the valley bottoms. Fluvial aggradation and small mass movement processes modified their surfaces to produce typical fan geometries with varying surface gradients. Fan-head entrenchment and fan- toe truncation indicates that these are relic features. Tectonically deformed terraces are rare, but active faulting has been recognised near Rakhiot. Glaciotectonic processes, slope processes and dewatering may also deform terraces, and examples are described. Three planation surfaces were recognised and probably represent tectono-climatic cycles, punctuated by uplift and denudation in successive glaciations. A discordant drainage pattern reflects an early Karakoram structural grain modified by differential uplift of the Great Himalaya and the Nanga Parbat- Haramosh massif which produced the concordant drainage of the Indus River.