The seafloor spreading happening, because sediments accumulate at the area of spreading
That is not correct. Seafloor spreading occurs due to the movement of tectonic plates. The Earth's lithosphere is broken into several large plates that float on the semi-fluid asthenosphere below. At the boundaries where these plates meet, they can either move apart (divergent boundary), move towards each other (convergent boundary), or slide past each other (transform boundary).
Seafloor spreading occurs at divergent plate boundaries, where plates are moving away from each other. As the plates move apart, magma rises from the mantle to fill the gap. This magma then solidifies to form new oceanic crust. The process of seafloor spreading adds new material to the ocean floor, resulting in the creation of new seafloor. It is not directly related to sediment accumulation.