Why do Fold Mountains have enormous thickness of sedimentary rocks?

Fold mountains are characterized by enormous thicknesses of sedimentary rocks because they originate from the accumulation of sediments in a geosyncline. A geosyncline is a large-scale, elongated depression or trough in the Earth's crust that collects very thick deposits of sediments over millions of years [c1, t2]. These sediments, often derived from the erosion of adjacent continental landmasses, can reach thicknesses of many thousands of meters. As these sediments accumulate, they push down the pre-existing crustal rocks due to their weight. When these sedimentary strata are subsequently subjected to intense lateral compressive forces—often during orogenic events—they are folded and deformed to form lofty mountain ranges [c1, c5]. While complex structures like recumbent and nappe folds are common features within these mountains, the primary reason for the sheer volume of sedimentary material is the long-term deposition within the geosynclinal basin [c1, t5].

Sources

1. [1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 10: Types of Mountains > 10.2. Fold Mountains > p. 134
2. [2] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 8: Convergent Boundary > Formation of Fold Mountains (Orogeny) > p. 117